i

In presenting this thesis in partial fulfillment of the requirements for an advanced degree from Idaho State University, I agree that the Library shall make it freely available for inspection. I further state that permission for extensive copying of my thesis for scholarly purposes may be granted by the Dean of Graduate Studies, Dean of my academic division, or by the University Librarian. It is understood that any copying or publication of this thesis for financial gain shall not be allowed without my written permission. Signature ____________________________ Date ____________________________

ii

THE PETROGENESIS OF QUATERNARY RHYOLITE DOMES IN THE BIMODAL BLACKFOOT VOLCANIC FIELD, SOUTHEASTERN

IDAHO

By Mark T. Ford

A thesis submitted in partial fulfillment

of the requirements for the degree of Masters of Science in the Department of Geosciences

Idaho State University July, 2005

iii

To the Graduate Faculty: The members of the committee appointed to examine the thesis of Mark T. Ford find it satisfactory and recommend that it be accepted. _________________________________ Michael McCurry Major advisor _________________________________ Scott S. Hughes Committee member _________________________________ Lisa Goss Graduate Faculty Representative

iv

ACKNOWLEDGEMENTS First, I would like to thank my advisors Dr. Mike McCurry and Dr. Scott Hughes. Their assistance has greatly improved both the scientific quality and syntax of this work and they helped me navigate the world of igneous petrology. Bobbie, Melissa and Connie along with many others in the main office have smoothed out the rough spots and without them, I might never have gotten paid. Special thanks to Diana Boyack as she has provided invaluable assistance to me, far above and beyond what she needed to do. Many others have impacted my stay here at Idaho State over the past few years. Arron Pope fed me too much coffee and the occasional undercooked meal but let me split some of his wood pile and has been a good friend, conversationalist, and pool shooting adversary. Jamie Blair provided hunting and fishing companionship and introduced me to many new fishing holes and some cool cats here in Pocatello. Duane DeVechio is a great friend and helped to widen my musical horizons plus he always had some Olympia in the fridge. Kate Pickett and Dan Narsavage are also great pals and I wish them continued happiness. Others here at ISU warrant special mention and I’m sure to forget to list a few names: Kaleb Scarberry - disk golf and high adventure, Butch and Diane Wheeler -Snake River fishing and some good eats in Blackfoot, Brian Hennings – Worms and rabbit stew, Mary Hodges – championship canoe rowing and Bannock Pass tufa, Rudy Ganske – petrology discussions and general banter, Songqiao Chen – expert in the lab (but please ware some safety glasses), and Rodger Rapp and Nagendra Singh – office mates over the years. I would also like to thank Dr. Jim Lia and Dr, Jagoda Urban-Klaehn for their support over the years. Thanks to a few other friends from my “living room” including Greg Bary, Dan Dawson, Paul Grayson, Don Lopez, Jim Olsen, Kevin Pitkin and Dennis Pullman. I would be remiss if I didn’t mention some of the folks from New York that have shown continued support over the years including Jason and Walt Strawser and the rest of the “Coon Hollow Gang”, Richard and Ellen Luce and the Clearing Corriedale Sheep and Wool Co., the entire Lynch family and all the old friends from Lamoka Lake and my taxidermist, Dave Fish. Finally, I would like to thank my parents. My father taught me, among other things, that with a good education, hard work and perseverance, one can achieve great things. He was a great fishing partner and my best friend. My mother was one of the happiest people I’ve ever met and, unless you were a snake, was kind and gracious to everyone. I wish I could grow plants as well as she did. I am saddened that they were not able to see me finish this segment of my educational odyssey and I miss them.

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TABLE OF CONTENTS Page # List of Plates vi List of Figures vii List of Tables ix Abstract x Chapter 1: Introduction 1 Section 1.1: Problem Statement and Objective 1 Section 1.2: Location of the Study Area 7 Section 1.3: Regional Geologic Setting 8 Section 1.4: Previous Work in the Blackfoot Volcanic Field 16 Chapter 2: Methods 19 Section 2.1: Mapping and Sampling Field Work 19 Section 2.2: Petrographic Methods 20 Section 2.3: Sample Geochemistry Preparation (ICP, INAA, EMP, and isotope) 20 Section 2.4: ICP-AES Methods and Uncertainties 22 Section 2.5: INAA Methods and Uncertainties 24 Section 2.6: Radiogenic Isotope Methods 25 Section 2.7: Heavy Mineral Separates 26 Section 2.8: Electron Microprobe Methods 29 Chapter 3: Results 30 Section 3.1: Field Relationships 30 Section 3.2: Petrography 43 3.2.1: Petrography of the CDF 44 3.2.2: Petrography of the NDF 49 3.2.3: Petrography of the SIR 51 3.2.4: Mafic Enclaves of the BVF 51 Section 3.3: Geochemistry of the BVF 53 3.3.1: Major-Element Geochemistry 54 3.3.2: Comparison of Major-Element Geochemistry of the BVF to Other Rhyolites 62 3.3.3: Trace-Element Geochemistry 67 3.3.4: Comparison of Trace-Element Geochemistry of the BVF to Other Rhyolites 70 Section 3.4: Electron Microprobe Results on Phenocrysts of the Central Dome Field 77 Section 3.5: Isotope Results 81

vi

Chapter 4: Discussion 82 Section 4.1: Physical Volcanology 82 Section 4.2: Petrography Interpretation 87 Section 4.3: The State of the Pre-Eruptive System 89 Section 4.4: Current Models for Nearby Physiographic Provinces 91 Section 4.5: Patterns of Rock Chemistry in the BVF 95 Section 4.6: MELTS Modeling to Determine Crystallization Sequences and Timing 98 Section 4.7: Isotope Modeling With Bulk Assimilation and EC-AFC Models 99 Section 4.8: NDF Petrogenesis and Regional Implications 115 Chapter 5: Conclusions 118 References: 122

APPENDICES Appendix 1: Summary of Electron Microprobe Standards, Crystals and Integration Times 132

PLATES Plate 1: Map, cross-section and tephra pit stratigraphy of the Central Dome Field back pocket

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FIGURES

Page # Figure 1: Map of southeastern Idaho including the Eastern Snake River Plain (ESRP) and Blackfoot Volcanic Field (BVF) 3 Figure 2: Hillshade DEM of the Blackfoot Volcanic Field 5 Figure 3: Geophysical based cross section of the crust under the ESRP 9 Figure 4: Physiographic province map of and location of topaz rhyolites of the western United States 10 Figure 5: Regional Archean craton map of the Wyoming terrane 11 Figure 6: Regional structure map of southeastern Idaho 12 Figure 7: Photo of lithophysae cavities in the Northern Dome Field 32 Figure 8: Photo of flow banding in the Northern Dome Field 32 Figure 9: Photo of Sheep Island Rhyolite outcrop 33 Figure 10: Photo of Hole in the Rock Lake graben 35 Figure 11: Photo of the graben south of China Cap 35 Figure 12: Photographic overview of the China Hat tephra deposit 37 Figure 13: Photo of palagonitized basalt and rhyolitic tephra in the 38 China Hat gravel pit Figure 14: Photo of block-sized rhyolite in the China Hat gravel pit 38 Figure 15: Photo of the coulee encircling China Hat 41 Figure 16: Overview photo of the Central Dome Field 41 Figure 17: Photo of spines on China Cap 42 Figure 18: Photo of crater between China Cap and North Cone 42 Figure 19: Photomicrograph of strongly embayed quartz 45 Figure 20: Photomicrograph of skeletal sanidine armoring plagioclase 45

viii

Page # Figure 21: Photomicrograph of a blocky magnetite crystal 45 Figure 22: Photomicrograph of a silicic glomerocryst 48 Figure 23: Cross polar photomicrograph of Figure 22 48 Figure 24: Photomicrograph of granophyric textures in a silicic xenolith from the Northern Dome Field 50 Figure 25: Photomicrograph of oscillatory zoned plagioclase within a basaltic magmatic enclave 50 Figure 26: Photomicrograph of boxy cellular plagioclase within a basaltic magmatic enclave 50 Figure 27: Photomicrograph of a zone of hybridization of magmatic rhyolite and basalt 52 Figure 28: Photomicrograph of a pyroxene mantled quartz crystal 52 Figure 29: Photomicrograph of spongy cellular (fingerprint) texture in a sanidine crystal near a basaltic magmatic enclave 52 Figure 30: Major element variation diagram for the Blackfoot Volcanic Field rhyolites 61 Figure 31: Major element variation diagrams incorporating rhyolites from various physiographic regions of the western US 65-66 Figure 32: Spider diagram for rhyolites of the Blackfoot Volcanic Field 69 Figure 33: Trace element variation diagrams incorporating rhyolites from various physiographic regions of the western US 72-74 Figure 34: Chondrite normalized REE diagram comparing igneous rocks from the ESRP and BVF 76 Figure 35: Ternary plots of sanidine, plagioclase, magnetite and Ilmenite compositions 78 Figure 36: Hypothesized cross-section of the timing of the tephra and dome emplacement 85 Figure 37: Hypothesized cross section of crater formation 87

ix

Page # Figure 38: Atomic Mn/Mg concentration diagram for ilmenite

and magnetite equilibrium 90 Figure 39: Spider diagram normalized by primitive BVF basalt 95 Figure 40: Single stage bulk assimilation model 102 Figure 41: EC-AFC model using standard conditions from Table 10 107 Figure 42: EC-AFC model with elevated initial Sr and Nd concentrations 108

TABLES Table 1: Age data and references for the Blackfoot Volcanic Field 6 Table 2: Sample location, major and trace element geochemistry for the Blackfoot Volcanic Field (BVF) rhyolites 55-60 Table 3: Reference list for data given in Figures 31 and 33 64 Table 4: Summary of relative trace element abundances in the BVF rhyolites 68 Table 5: Average concentrations of trace elements from various Physiographic regions of the western US 71 Table 6: Electron microprobe phenocryst chemistry data for the Central Dome Field 79-80 Table 7: Nd and Sr isotopic ratios for the basalts and rhyolites of the BVF 81 Table 8: Pre-eruptive magma chamber properties of the CDF 91 Table 9: Sr and Nd isotopic and elemental concentrations for

xenoliths of the Snake River Plain 100 Table 10: General parameters used in EC-AFC models 105

x

ABSTRACT

Seven Quaternary topaz-style rhyolite domes are present in the bimodal basalt-

rhyolite Blackfoot Volcanic Field (BVF). The high silica, slightly metaluminous

rhyolites average ~ 10% crystals in their glassy carapace (quartz > sanidine > plagioclase

> hornblende ≈ biotite > magnetite > ilmenite) and contain trace phases of zircon >>

apatite, +/- allanite, +/- thorite. As compared to other physio-tectonic rhyolites, the BVF

rhyolites are high in Cs, Ta, Yb, Rb, U, and Th and low in FeO(t), Sc, Sr, LREE, and Eu.

The three, aligned domes of the Central Dome Field (CDF) are especially high in Rb and

low in Sr resulting in Rb/Sr ratios approaching 250. Another distinction of BVF rhyolites

from other rhyolites are very low La/Yb ratios, with the CDF ratio near unity.

The young (~50 ka) CDF was emplaced after localized basalt flows and normal

faulting. It began with hydrovolcanic tephra production and subsequent tephra and

endogenous dome growth overlap. Other features in the CDF related to the rhyolite

emplacement include local tumescence and collapse crater formation and partial dome

collapse resulting in hummocky terrain.

All of the phenocryst phases, except quartz, are in textural equilibrium with the

melt. The CDF magma equilibrated at ~3.5 kbars (13 km deep), at a temperature of

~760oC with a log fO2 of -14.5. The presence of granophyric texture in the three, aligned

domes of the Northern Dome Field (NDF) may indicate a shallower equilibration depth

coupled with a depressurization and volatile loss event.

Fractional melting of either upper or lower crust or a combination of the two has

been ruled out as a possible genesis scenario for the BVF rhyolites based on both isotopic

and trace element constraints. Limited upper crustal assimilation coupled with fractional

xi

crystallization (AFC) models, similar to those models used to generate geochemically

similar Quaternary rhyolite domes in the adjacent eastern Snake River Plain (ESRP)

province (McCurry et al., 1999), can explain both the trace element and isotopic

signatures in the rhyolites using local basalts of the BVF as a parent material. Results

indicate that 9 to 18 percent upper crustal assimilation into the basalt followed by

extensive fractional crystallization can form the CDF rhyolites. Basaltic recharge to the

system that occurred just prior to eruption was very minor and did not affect the trace

element concentrations or isotope ratios in the rhyolitic magma.

The ability to melt the upper crust greatly controls the amount of assimilation in

these AFC systems. The ~1.4 Ma NDF contains ~5 percent more crustal assimilant than

the younger CDF because the crust under the CDF has become more refractory due

additional basalt injections over the past 1.4 Ma. The Quaternary ESRP rhyolites contain

significantly less crustal assimilant as compared to the BVF rhyolites because the crust

under the ESRP is even more refractory. This is a result of voluminous Tertiary rhyolite

eruptions and significantly greater amounts of basalt flooding the crust for the past 2 Ma

under the ESRP.

1

Chapter 1 INTRODUCTION

1.1 Problem Statement and Objective:

This study focuses on the petrogenesis of the rhyolites in the bimodal basalt-

rhyolite Blackfoot Volcanic Field (BVF), located in southeastern Idaho (Figure 1). It

examines their possible genetic association with adjacent and coeval rhyolites of the

ESRP, Basin and Range or a hybrid combination of both provinces. The overlying,

larger issue is: how are rhyolites created in bimodal volcanic systems? A starting point to

answer this question is to determine a hypothetical reservoir for the basalt. The second

step is to determine if there is a petrogenetic link between the basalt and rhyolite of the

volcanic field. If so, the final step is to develop a model as to how this precursor or

parent magma must evolve via combinations of one or more of the following processes:

magma recharge, crustal assimilation, fractional crystallization or fractional melting to

produce the rhyolite. The primary focus of this work will be on the Central Dome Field

(CDF) (Figure 1, Figure 2).

In this study, I evaluate the following hypotheses:

1. Each set of spatially separated domes within the BFV (Figure 2) is chemically

homogeneous. Presuppositions to support this hypothesis include the fact that both sets

of domes, the Central Dome Field (CDF) and Northern Dome Field (NDF), are aligned

(Figure 2) and each dome within a set is within one kilometer of its neighbor, thus

indicating a similar magma source for each set. Radiometric ages (Table 1) imply that

2

domes within each dome field are contemporaneous. Detailed major- and trace-element

geochemistry (Chapter 3) shows homogeneity within each dome field.

2. Rhyolites of the BVF form as a result of extreme fractional crystallization of a parent

magma with limited amounts of crustal assimilation, similar to the Quaternary rhyolites

of the ESRP (McCurry et al., 1999), not as a result of lower crustal melts. The BVF lies

off the southeastern flank of the volcanically active bimodal basalt-rhyolite Eastern

Snake River Plain (ESRP) in the Basin and Range physiographic province (Fiesinger et

al., 1982) (Figure 1). The BVF rhyolites are Quaternary in age (B. Nash, personal

communication referenced in Luedke and Smith, 1983; Leeman and Gettings, 1977;

Armstrong et al., 1975) (Table 1) and overlap in age with Quaternary ESRP (QESRP)

rhyolite domes (e.g. Big Southern Butte, East Butte, Unnamed Butte) (Armstrong et al,

1975; Kuntz et al., 1979; Spear, 1979; McCurry, 1999). Bulk major- and trace-element

geochemistry (Chapter 3) for the QESRP and BVF rhyolites are similar with extremely

low concentrations of Sr and Eu, indicating large amounts of feldspar fractionation. Nd

and Sr isotope ratios of the BVF rhyolites limit the amount of crustal assimilation based

on models using regionally exposed Archean rocks and crustal xenoliths

Figure 1 (overleaf) A hillshade composite DEM (digital elevation model) map of southeastern Idaho including Quaternary rhyolites of the ESRP (e.g. Big Southern Butte, East Butte, Unnamed Butte), presumed cryptodomes along the southern margin of the ESRP (e.g. Ferry Butte and Buckskin Dome) and the Central Dome Field and Northern Dome Field of the BVF. A set of trend lines that run from the Quaternary ESRP rhyolites into the BVF rhyolites and pass through the hypabyssal intrusions roughly approximates the same orientation of a geophysical anomalous zone (Peng and Humphreys, 1998). Dashed lines represent rift zones on the SRP and are roughly parallel to Basin and Range structures. Image modified from McCurry and Morse, (2003).

3

4

(Leeman et al., 1985) present SRP volcanic rocks (Chapter 4). The parental (basaltic)

magmas for both the QESRP and BVF rhyolites have similar geochemical and isotopic

signatures (Pickett, 2004). The lower crust underneath the ESRP and BVF is linked by a

northwest-southeast geophysical anomalous zone, indicative of lower crustal melt (Peng

and Humphreys, 1998), and is roughly represented by the trend lines on Figure 1 and

displayed in cross-section in Figure 3. Lower crustal melting models can not model the

isotopic signatures of the BVF rhyolite and result in multiple subsequent small volume

partial melts of large areas of the lower crust.

3. The BVF rhyolites can be linked to the olivine tholeiite basalts of the BVF by a

petrogenetic model and the BVF basalts are the parent magma. Presuppositions are not

as strong for this hypothesis but isotopic data plotted on mixing hyperbolas indicates less

than 25% crustal component in the CDF rhyolites. Modeling such a large range in

chemistries is difficult due to changes in precipitating phases and the lack of data on

intermediate rocks in the bimodal field, but petrogenetic models like EC-AFC (Spera and

Bohrson, 2001; Bohrson and Spera, 2001) and MELTS (Ghiorso and Sack, 1995) show

that the basalts and rhyolites can be petrogenetically linked (Chapter 4). Partial melts of

Figure 2 (overleaf)

Hillshade DEM of the BVF showing the Quaternary rhyolites of this study (labeled, in black), basalts of the BVF (bright white) and Quaternary Basin and Range style normal faults exposed within the basalts (grey lines within the white basalt lava fields). Note that the dome fields form linear features that do not correspond with localized north-northwest Basin and Range faulting activity. Figure adapted from Oriel and Platt, 1980; Mitchell and Bennett, 1979; Pickett, 2004.

5

6

lower crust, a model suggested by other workers for similar rhyolites (e.g. Christiansen et

al., 1986; Grunder, 1992; Orozco-Esquivel et al., 2002) including topaz rhyolites in the

Basin and Range (Figure 4) is ruled out by a combination of high Rb/Sr ratios and the

isotope systematics of the Archean aged lower crust. Hanna Nekvasil (personal

communication, 2004) has produced rhyolites from experimental melts starting with

Snake River Plain olivine tholeiites. Collectively, these three postulates strongly support

the given supposition.

Table 1. Radiometric ages and associated references for rhyolite domes and outcrops indicated in Figure 2. Note that the samples from the CDF (China Hat, China Cap and North Cone) are significantly younger than samples from NDF or Sheep Island. The nature of the listed uncertainties is unclear.

Sample Age (Ma) Method Source Northern Dome West 1.59 +/- 0.06 K-Ar Luedke and Smith, 1983* Northern Dome Center Not Determined -- Northern Dome East 1.41 +/- 0.15 K-Ar Luedke and Smith, 1983* Sheep Island 1.4 +/- 0.2 K-Ar Luedke and Smith, 1983* China Hat

~0.05 0.04 +/- 0.02 0.08 +/- 0.04

0.061 +/- 0.006

C-14 K-Ar K-Ar K-Ar

Leeman and Gettings, 1977 Armstrong et al., 1975 Armstrong et al., 1975 Pierce et al., 1982

China Cap

0.058+/- 0.007 ~0.05

0.1 +/- 0.1

Ar-Ar C-14 K-Ar

Heumann, 2004 Leeman and Gettings, 1977 Armstrong et al., 1975

North Cone ~0.05 C-14 Leeman and Gettings, 1977 * Referenced as B. Nash, personal communication in Luedke and Smith, 1983

7

1.2 Location of the Study Area:

The study area is located in the Basin and Range province, near the juncture of the

Basin and Range and ESRP physiographic provinces (Figures 1, 2, 4) in southeastern

Idaho. Rhyolites of the BVF are located between 15 and 45 km north of Soda Springs,

Idaho and are contained on three adjacent north to south 7.5’ quadrangle maps: Little

Valley Hills, Henry, and China Hat (Figure 6). Three discrete groups of rhyolite domes

(Figure 2) and their field relationships with local geologic features were studied.

The Northern Dome Field (NDF) consists of three unnamed, locally erosionally

incised dome-like rhyolite outcrops located in the southwest corner of Bonneville

County, to the east of Crooked Creek Flat, 40 to 45 km north of Soda Springs. The three

domes are aligned approximately S60oE and I have informally named them Northern

Dome West, Northern Dome Center and Northern Dome East (labeled on Figure 2).

Sheep Island is a rhyolite outcrop located in the Blackfoot reservoir in Caribou

County and is accessible only by boat.

The Central Dome Field (CDF) contains three very conspicuous rhyolite domes

known locally as China Hat, China Cap (occasionally called Middle Cone in older

literature) and North Cone, located in north-central Caribou County 15 to 20 km north of

Soda Springs, at the southern end of the Blackfoot Reservoir (Figure 2). They are

aligned along a N33oE liniment.

8

1.3 Regional Geologic Setting:

The crust in southeastern Idaho is composed of Archean crystalline basement

(Figure 5) topped by a thick succession of Proterozoic to Mesozoic sedimentary rocks

(Armstrong and Oriel, 1965; Mueller et al., 2002; O’brien et al., 1995). From the

Paleozoic to the early Mesozoic, the study area was in a transitional zone between a

miogeosyncline to the west and the corresponding platform to the east. The thickness of

units deposited in this setting is directly related to the positioning of the transition zone

and thus varies throughout the Paleozoic and early Mesozoic with the total thickness

between 4.5 and 10 km (Armstrong and Oriel, 1965). The thicknesses of the basal

Cambrian unit and Proterozoic rocks are unknown.

Beginning at approximately the Jurassic/Cretaceous boundary, thrust faulting

associated with the Sevier Thrust Belt began in and around the study area. While no

thrust faults are shown to lie directly beneath the study area, the Paris Thrust is located

approximately 15 km south of the CDF, and the Meade Thrust is approximately 5 km to

the northeast of the NDF (Figure 6) (Armstrong and Oriel, 1965; Dorr et al., 1987; Link,

1982). At least five major tectonic movements are recorded along the Paris Thrust

beginning at the Jurassic/Cretaceous boundary and ending in the Coniacian (~88 Ma)

while movement along the Meade Thrust began later (in the Albian ~110 Ma) and ceased

at approximately the same time (DeCelles, 2004). Minimum horizontal movement on

each of these faults was 8 to 16 km (Armstrong and Oriel, 1965; Wiltschko and Dorr,

1983) and may have totaled over 200 km (DeCelles, 2004). Thrust faulting to the east, in

western Wyoming, continued into the Eocene (Armstrong and Oriel, 1965).

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10

Figure 4. The locations of Cenozoic topaz rhyolites (black dots) and general physiographic provinces (dotted lines). The Blackfoot Volcanic Field (BVF) is shaded and location of Big Southern Butte (open circle) on the ESRP is given for reference. The solid line is the 0.706 Sr-isotopic line (inferred to separate accreted terrains from continental terrains with Proterozoic or older basement). Adapted from Finton et al., 1991; Pickett, 2004; Christiansen, et al., 1986).

11

Figure 5. Regional cratonic map. The BVF (labeled with a “B”) overlies Wyoming Archean basement. Other abbreviations are Belt = Proterozoic Belt Supergroup, GFTZ = Great Falls Tectonic Zone, CH = Challis Volcanics, AR = Absaroka Range, Y = Yellowstone, SRP = Snake River Plain, (modified from O’Brien et al., 1995).

South of the BVF, thick successions of miogeosynclinal Paleozoic rocks continue

into the Nevada and western Utah portions of the Basin and Range Province with similar

thrust faulting (Royse, et al., 1975 in Link, 1982). Extensional tectonism started forming

block faulted Basin and Range type mountains in the Oligocene that generally trend north

to northwest in the BVF. The basins within the BVF are filled with Miocene or younger

sediments (Fiesinger et al. 1982; Hughes et al., 1997). Pierce and Morgan (1992)

12

Figure 6. Map showing the locations of major thrust faults and down-dropped graben normal faults in the area. The three quadrangle maps containing the rhyolite domes investigated in this study are shown by the boxes (from north to south: Little Valley Hills, Henry, and China Hat quadrangles). For larger scale normal faults, see Figure 2. (Modified from Link, 1982, Royse, et al., 1975, and Blackstone, 1977)

13

classified the normal faults in the study area as Belt III lesser late Pleistocene faults,

indicating a date of last offset greater than 15 ka with total escarpments of less than 200

m. Seismic activity currently exists along some of these faults. Some of these faults that

the cut the basaltic flows in the BVF are shown on Figure 2.

The Basin and Range province contains bimodal basalt-rhyolite volcanic systems

in addition to predominately basaltic and differentiated alkalic systems (Christiansen and

Lipman, 1972). Fitton et al. (1991) noted that the bulk of more recent basaltic volcanism

occurs at the margins of the Basin and Range province. Basaltic volcanism that is less

than 5 Ma is more mafic than older basalts in the Basin and Range, and is

compositionally similar to the basalts of the BVF (Pickett, 2004). Rhyolites from the

Basin and Range are variable ranging from voluminous oxidized ignimbrites erupted

from calderas to low volume topaz rhyolites emplaced as domes and flows (Christiansen

et al., 1986; Gans et al., 1989). A close spatial relation exists between the location of

topaz rhyolites (Figure 4) and late Cenozoic extensional faulting (Basin and Range, Rio

Grande Rift and the Lewis and Clark line in Montana), which would include the CDF

within the BVF (Christiansen, et al., 1983; Christiansen, et al., 1986). The central and

eastern parts of the Basin and Range Province are underlain by Proterozoic basement

(Gans et al., 1989), in contrast to the Archean rocks under the BVF (Figure 5).

Adjacent to the north of the study area, the Yellowstone - Snake River Plain (Y-

SRP) province is a 600 km long time-transgressive series of rhyolitic caldera forming

eruptions starting in southwestern Idaho at ~15 Ma and proceeding to the Yellowstone

plateau with eruptions from 2 to 0.6 Ma (Armstrong et al., 1975; Pierce and Morgan,

14

1992). These rhyolites, emplaced by ignimbrites and lava flows are exposed along the

flanks of the SRP, in the Owyhee area of southwestern Idaho and on the Yellowstone

plateau (Hughes and McCurry, 2002; Pierce and Morgan, 1992) and may be up to 8 km

thick in places (Sparlin et al., 1982). Petrologic and geochemical data indicate a change

in the genesis of the rhyolites at ~114o west longitude (the Twin Falls eruptive center),

which may represent a change in lithospheric terranes (Hughes and McCurry, 2002).

Tertiary rhyolites in the eastern Snake River Plain (ESRP) are covered by

numerous individual flows of dominantly olivine-tholeiite basalt (OTB), each generally

less than 10 m thick, intercalated in some regions with sediments, that cumulatively form

a layer 0.5 to 1.5 km thick. Basalt flows are ubiquitous on the ESRP with the greatest

thickness found along the Axial Volcanic Zone (Hackett and Smith, 1992; Kuntz et al.,

1992; Whitehead, 1992). They range in age from subsurface basalts at ~4 Ma to the

Holocene such as the 2.1 ka Craters of the Moon (COM) evolved basalts (Figure 1,

Kuntz, et al., 1992) and Wapi-Kings Bowl OTB and, in contrast to the Tertiary rhyolites,

exhibit no time-transgressive characterizations in their spatial distributions. Basalts are

generally erupted from vents aligned along northwest-trending volcanic rift zones, nearly

parallel to the northwest-trending Basin and Range structures. While the basalts are

somewhat variable in chemistry, no correlation exists between location or timing of

eruptions and chemistry (Kuntz et al., 1992).

Quaternary rhyolitic volcanism is expressed only in isolated locations such as Big

Southern Butte, East Butte, Unnamed Butte, and the high silica components of Cedar

Butte (Figure 1) along the Axial Volcanic Zone which have been studied by Leeman

(1982a), Spear and King (1982), Bretches (1984), Hackett and Smith (1992), Kuntz et al.

15

(1992), Hughes et al. (1997), McCurry et al. (1999), and McCurry and Ganske (2005).

These rhyolites are petrologically and geochemically distinct from the voluminous SRP

Tertiary rhyolites and the Quaternary rhyolites associated with the current position of the

proposed hot spot plume located at Yellowstone National Park (McCurry et al., 1999;

Pierce and Morgan, 1992). Petrogenetic studies indicate that these rhyolites formed

primarily by extensive fractional crystallization of mafic parental magma with very

limited crustal assimilation (McCurry, et al., 1999, McCurry and Ganske, 2005).

McCurry et al. (1999) further suggest that there may also be a NW-SE trending rhyolite

association that includes the domes listed above and some cryptodomes along the

southern margin of the ESRP (e.g. Ferry Butte and Buckskin Dome – Figure 1). Peng

and Humphreys (1998) describe an anomalous geophysical zone beneath and southeast of

the ESRP that they interpret as partially molten lower crust (Figure 3). This trend

roughly parallels the trend mentioned above and an extension of this trend to the

southeast would include the rhyolite domes located in the study area (Figure 1). They

place the upper-crust, lower-crust boundary at 21 km under both the ESRP and BVF.

The ESRP also contains intermediate compositions (e.g. COM, Cedar Butte,

Unnamed Butte) that represent varying degrees of magma mixing coupled with fractional

crystallization. The combination of the intermediate composition COM and Cedar Butte

rocks produce smooth, curved trends that link the least evolved COM magmas and

Quaternary ESRP rhyolites (Big Southern Butte type). These trends show the complete

evolution via fractional crystallization between the evolved basalts and rhyolites with

very limited crustal assimilation (McCurry et al., 1999). Unnamed butte and correlated

16

rocks from core hole CH-1 indicate mixing and hybridization of ESRP basalts and

rhyolite formed by fractional crystallization (McCurry et al., 2003, McCurry and Ganske,

2005).

1.4 Previous Work:

While significant work including extensive mapping, petrology, geochemistry,

and radiometric dating has been accomplished on the basaltic rocks in and around the

study area (Armstrong et al., 1975; Fiesinger et al., 1982; Pickett, 2004), rhyolites of the

area have received less attention. Locals and westward travelers along the Oregon Trail

used the conspicuous China Hat as a landmark in the mid to late 1800’s (Carney, 1998).

Mansfield (1927) was the first to document the rhyolites in the study area during

reconnaissance surveys between 1906 and 1912. He noted and roughly mapped both the

size and position of eight rhyolite exposures including the three domes of the CDF,

(China Cap was called Middle Cone by Mansfield), three domes of the NDF (Figure 1,

Figure 2), and two islands in the Blackfoot Reservoir, presumed to be Sheep Island and

Gull Island. He also published a brief petrological characterization of the rhyolites and

hypothesized that basalt flows both pre- and post-date the emplacement of the CDF

rhyolite. Mansfield’s emplacement history involves three rhyolitic eruptive episodes

separated by three basaltic episodes and he attributes many of the faults in the area (Oriel

and Platt, 1980) to collapsed lava tubes.

Since Mansfield’s study, little work outside of some radiometric dating has been

published concerning rhyolites in the study area. Mabey and Oriel (1970) note an

anomalous gravity low near China Hat and hypothesize that one cause could be a granitic

17

intrusion at depth. Dayvault and others (1984) evaluated the CDF as a potential source

for uranium, identified post-emplacement topaz crystals in lithophysal cavities, and

classified the domes as “nonproductive topaz-bearing rhyolites”. Christiansen et al.

(1986) agreed with this classification the CDF rhyolites and noted that the chemistry of

the CDF rhyolite is similar to that of other topaz rhyolites in the western United States

that are enriched in the traditionally incompatible elements. Radiometric dating by

previous workers for the three domes of the CDF, Sheep Island, and two of the NDF

outcrops are given in Table 1. Nd- and Sr-isotope ratios from the China Cap dome were

obtained by Dr. Michael McCurry prior to the initiation of this study and indicate that

there may be more crustal component present in the BVF rhyolites than in the Quaternary

ESRP rhyolites (M. McCurry, personal communication, 2001). Most recently, Heumann

(2004) conducted U-series isotope studies on the CDF to investigate magma residence

times. He concludes that open-system magma evolution has disturbed the Sr isochron

and cannot be used to evaluate magma residence times. He determined that a U-Th

fractionation event occurred shortly before the eruption but that further study of the U-

series disequilibrium and glass–mineral ages would be needed to evaluate the residence

time of the magma.

Additional studies have characterized the basalts in the bimodal BVF. In addition

to Mansfield’s reconnaissance mapping, other workers have refined the extent of the

basalt flows (e.g. Oriel, 1968; Armstrong, 1969; Mitchell and Bennett, 1979; Oriel and

Platt, 1980). Fiesinger et al. (1982) suggested that the basalts in the study area share

many characteristics with the olivine tholeiites of the ESRP whereas Christiansen et al.

(1986) “…prefer to place the development of the Blackfoot (Volcanic) Field in the

18

context of Basin and Range extension”. Pickett (2004) demonstrated that the BVF

basalts share eruptive styles with Basin and Range basalts but have major, trace, and

isotopic signatures that are more similar to ESRP olivine tholeiites. She concluded that

the basalts of the BVF are derived originally from old mantle lithosphere (as are ESRP

basalts – Hughes et al., 2002A) and that they have undergone extensive fractional

crystallization of olivine and plagioclase (and perhaps clinopyroxene). Along with the

fractional crystallization, between 10 and 20 percent bulk assimilation of upper

continental crust component, depending on crustal xenolith isotopic compositions, is

required (Ford and McCurry, 2003) to produce the isotopic signatures of the most

chemically evolved basalts from the most primitive.

19

Chapter 2 METHODS

2.1 Mapping and Sampling Field Work:

Over 55 rhyolite samples were collected for this work. Possible sample collection

locations were determined by finding high points (summits), spines or ridges on the

domes as these areas provided the greatest chance of finding in situ rhyolite as opposed to

colluvium. In the NDF, incised valleys also contain in situ rocks. Only non-weathered,

unaltered, dense and glassy samples were collected for geochemistry. Dense rocks that

contained less pumice-like texture were sampled preferentially over light and frothy

samples. The dense rocks have less surface area available for weathering. Obsidian

without lithophysae or spherulites was considered to be the highest quality sample for

geochemistry. Sample locations were identified using topographic maps and a GPS

receiver, and noted in a field book along with a preliminary hand sample descriptions.

Mapping, including the overall extent of the rhyolite, sample positions and

volcanic features, was done in the CDF using topographic base maps with Mylar overlays

and used to create a geologic map and cross section at a 1:12,000 scale (Plate 1). In

addition to the domes, areas adjacent to the domes including fault scarps, crater-like

depressions and locally exposed tephra were examined to constrain the timing of the

rhyolite emplacement with respect to local basalts and faulting events. Animal burrows

in loess near the domes were investigated to see if they contained any rhyolitic ash or

colluvium. There was no systematic measurement of flow banding attitudes within the

domes. Two areas that received particular attention were the northwest side of China

20

Cap and the south side of China Hat. Both were mentioned by Mansfield (1927) as

places where basaltic activity might have occurred after the dome formation.

2.2 Petrographic Methods:

Twenty-six standard 27 X 46 mm thin sections were produced by two separate

outside labs for petrographic analysis (Spectrum Petrographics, Vancouver, Wash and

University of Utah Thin Section Laboratory, Salt Lake City, Utah). Thin sections from

all three dome sets were examined using a standard petrographic microscope for the

purpose of determining 1. phenocryst populations, 2. textural relationships, 3. basaltic

enclave properties and their relationship to the surrounding rhyolite (where present), 4.

paragenesis , and 5. which phases were in textural equilibrium with each other. Specific

samples that represented the textures and phenocryst populations of the collected rocks,

but without alteration products, were selected for more detailed analysis including point

counts and electron microprobe analysis (see section 2.8).

2.3 Sample Geochemistry Preparation:

A total of 39 dense, non-weathered samples or obsidian, twenty-one from the

CDF, thirteen from the NDF, one from Sheep Island and four “grab” samples from Big

Southern Butte (on the ESRP), were prepared for major- and trace-element geochemical

analysis using standard operating procedures at the Idaho State University Laboratory for

Environmental Geochemistry (ISU-LEG). First, dirt and weathering rind, or other

surface alteration was removed via compressed air, a stiff brush and tap water, or with a

trim saw. Samples were then either air-dried or dried in an oven at ~40oC. A four cm-

21

thick steel plate, used as a base for crushing the sample, was scrubbed with a wire brush,

cleaned with compressed air and wiped with alcohol. The steel plate was then pre-

contaminated by reducing a small (50 to 100 gram) piece of the cleaned sample to pea-

sized gravel with a four pound crack hammer. The resultant rock pieces and dust were

removed using a dedicated synthetic hair brush and discarded, thus increasing the

likelihood that any remaining fragments of rock lodged in the irregular surface of the

steel plate were from the sample of interest.

Following pre-contamination, samples ranging in mass from 550 to 950 g were

first reduced to three centimeter sized pieces or smaller and visually inspected for any

“foreign material”. Any pieces containing xenoliths, basaltic enclaves or remaining

weathering rind were discarded and the sample was further reduced to pea-sized gravel.

The powdered and graveled sample was then removed from the plate with the synthetic

hair brush and passed through a riffle splitter until two separate aliquots of approximately

30 mL were produced from the original sample. For major-element geochemistry (i.e.

ICP-AES), a tungsten-carbide puck mill or shatterbox reduced the sample to a fine

powder (<200 mesh or about 75 microns) in less than one minute. Trace-element

geochemistry (conducted with INAA) required the use of an alumina puck mill for two to

five minutes to obtain a fine powder. Powdered samples were stored in new poly vials to

hold for future processing. Cross contamination between samples was kept to a

minimum by wearing vinyl gloves and by thoroughly cleaning all items used with

compressed air and alcohol wipes between each sample processed.

Samples analyzed at the University of Florida Department of Geosciences (by Dr.

John Chadwick) for radiogenic isotopic analysis, followed a similar process to that

22

outlined for INAA above, that is, using the alumina puck mill. Additional samples sent

to the University of Arizona Department of Geosciences (Dr. Paul Wetmore) were

processed differently. Because they were analyzed for lead isotopes, the samples could

not be broken with steel instruments, cut with a trim saw or otherwise come in contact

with metals. Samples were closely examined to identify old hammer marks or previous

trim saw surfaces and these areas were marked with permanent marker. Samples were

then put into a brown paper bag and placed in a heavy cardboard box and then struck

with heavy wooden dowel rod and broken into one to two cm pieces. These pieces were

then examined and any piece with a hammer marked surface was discarded. A portion of

the broken sample (50 to 100 g) that had never been struck with metal instruments was

forwarded to UA for further preparatory work. At this time, isotopic results from UA

have not been completed but may be obtained from Dr. Michael McCurry at Idaho State

University.

2.4 ICP-AES Methods and Uncertainties:

Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) was

performed at the ISU-LEG following standard operating procedures, briefly outlined

below, on a Jobin-Yvon 70 Plus ICP-AES with both a sequential monochronometer with

a one meter focal length and simultaneous polychronometer with a ½ meter focal length.

This instrumentation and methodology was used to measure concentrations of the

following oxides (wt. percent) and elements (ppm): SiO2, TiO2, Al2O3, FeO, MgO, CaO,

Na2O, K2O, P2O5, Ba, Zr, and Sr.

23

A 0.1000 gram (+/- 0.0003 grams) aliquot of rock powder (prepared with the

tungsten carbide puck mill) was combined with 0.500 grams of Lithium Metaborate

(LiBO2) flux in a graphite crucible and fused in a muffle furnace at 1025oC for 25

minutes. The resultant molten glass was then poured into a beaker containing ~80 mL of

7 percent nitric acid and stirred using a magnetic stir bar until the sample was completely

dissolved. This solution was then transferred to a 100 +/- 0.8 mL class A volumetric

flask, and the beaker and stir bar rinsed repeatedly with 7 percent nitric acid with the

effluent of this rinsing process added to the flask. The total volume of the solution was

then brought to 100 mL resulting in a 0.1 percent solution (0.1 g of rock in 100 mL)

which is used to determine the concentration of Ba, Zr, and Sr. A 10 +/- 0.02 mL aliquot

of this concentrated solution, measured using a class A calibrated pipette, was then

diluted to 100 mL (in a class A volumetric flask) to yield a 0.01 percent solution which

was used to determine the oxide concentrations.

Samples were analyzed using the ICP-AES on several different dates resulting in

slightly variable uncertainties from run to run. The following are the largest analytical

uncertainties for standards, expressed as relative standard deviations (RSD’s) at the 95

percent confidence interval: SiO2, Al2O3, CaO, and Na2O within 2 percent, FeO within 4

percent, and K2O, Ba, and Zr within 10 percent while TiO2, MnO, MgO, and P2O5 were

large due to the very small (<0.1 wt. percent oxide) quantities in the samples. The

uncertainty for Sr was also large as it was under 20 ppm for all samples and under 10

ppm for many samples.

24

2.5 INAA Methods and Uncertainties:

Instrumental Neutron Activation Analysis (INAA) was used to determine the

following oxides (wt. percent) and elements (ppm): FeO, Na2O, Sc, Rb, Cs, La Ce Nd,

Sm, Eu, Tb, Yb, Lu, Hf, Ta, Th, and U. Approximately 0.7 grams (measured to the

nearest 0.0001 gram) of the previously prepared powder (using the alumina puck mill)

was heat sealed in a 2/5-dram reactor safe polyethylene vial which in turn was heat sealed

in a 2-dram polyethylene vial (double encapsulation). Encapsulated samples, along with

appropriate NIST and in-house standards, were sent for activation in the 1 MW Mark II

TRIGA research reactor at Oregon State University where they underwent irradiation for

2 hours at a neutron flux of 3 X 1012 n cm-2 s-1. Samples were then sent back to the ISU-

LEG where gamma emissions were sequentially counted with a high purity Ge (HPGe)

detector over three time intervals. Short-lived nuclides were counted in an initial count

sequence, nuclides with half-lives between ~100 and ~1000 hours were counted in an

intermediate sequence that uses a longer counting time, and long-lived (half lives >1000

hours) nuclides were counted after the activities of the short-lived and some intermediate

activities had decreased, generally 4 to 8 weeks after irradiation. In some cases, more

than one counting sequence yielded acceptable results for a particular decay series (e.g.

Fe). Data was then reduced using in-house software written by Dr. S. S. Hughes of Idaho

State University.

The following were the largest analytical uncertainties for standards, expressed as

RSD’s at the 95 percent confidence interval: FeO, Sc, La Ce, Sm, and Th within 2

percent, Na2O, Rb, Cs, Nd, Tb, Yb, Lu, and Hf between 2 percent and 4 percent, Ta and

Eu between 5 percent and 10 percent and U up to 15 percent. Although INAA can detect

25

Sr and Ba, results were poor and thus the ICP data was used for these elements. For the

oxides FeO and Na2O, results from both techniques gave similar results in most cases and

provided cross-checks on the two techniques.

2.6 Radiogenic Isotope Methods:

Five samples (four from the CDF and one from the NDF) were submitted to Dr.

John Chadwick for 87Sr/86Sr and 143Nd/144Nd analyses at the University of Florida

Department of Geosciences. Approximately 0.1 g of rock powder, prepared following

the same procedure as detailed for INAA powders above, was dissolved in a 100oC HF-

HNO3 mixture in Teflon vials. The dry residuum, left after evaporating the acid mixture

in a class 1000 laminar flow hood, was converted to a chloride salt by dissolving it in 2

mL of HCl and re-drying. These salts were then re-dissolved in HCl and run through a

centrifuge to concentrate any remaining solids so that they could be removed. The

resultant, solid-free liquid was then passed through calibrated cation exchange resin

quartz glass columns to separate Sr and Nd. Re-dried solutions were then loaded onto

filaments where isotopic Sr measurements were collected using a Micromass Sector 54

Thermal Ionization Mass Spectrometer (TIMS) and isotopic Nd measurements were

collected on a Nu Plasma Multiple-Collector magnetic-sector Inductively Coupled

Plasma Mass Spectrometer (MC-ICP-MS). For Sr isotopes, NIST NBS-987 was

measured four times during the suite of analysis in which the BVF samples were

measured and had an average 87Sr/86Sr value of 0.710244 as compared to the accepted

value (the average of this standard over the past year on this instrument) of 0.710243 with

26

standard errors better than +/- 0.000002 (2-sigma). For Nd isotopes, USGS standard

reference material BCR-1 yield 2-sigma values of +/- 0.000011.

2.7 Heavy Mineral Separates:

Mineral separation was performed on three samples so that accessory phases

could be concentrated, made into a grain mount and analyzed on an electron microprobe.

The general process is outlined as follows:

(1) Samples were cleaned and trimmed of weathering rind and the steel plate used to

gravel the sample was pre-contaminated as described in section 2.3 above.

(2) Samples were then reduced using a crack hammer in a stepwise process that involved

lightly crushing the sample, passing it through a 0.81 mm brass sieve, and returning what

did not pass to the steel plate for further reduction. By repeating this process many times,

more material that is of the appropriate size is produced and the production of large

quantities of powder that is too small to use in later processing is avoided. The

percussion method (i.e. hammer and steel plate) for breaking the rock is preferred as it

tends to break the specimen along grain boundaries (Hutchison, 1974).

(3) The smaller than 0.81 mm fraction was then washed to remove the less than 50 µm

fine dust by placing the sample in a large plastic container and using a jet of tap water to

thoroughly mix the sample and separate grains that might adhere to each other. After a

short settling time (about 30 seconds), the murky water containing the small fraction

(dust) of the sample was decanted off the top of the larger, settled fraction. This process

was repeated a number of times until the washing water was nearly clear after mixing.

The removal of the fine fraction is important because if it is not removed, the heavy

27

liquids used later in this process can be hopelessly contaminated as the fine particles will

not settle out of the heavy liquids.

(4) The washed sample was then transferred to disposable aluminum bread pans and

placed in an over at ~50oC to dry.

(5) After drying, a 60 mesh (250 µm) disposable nylon sieve (from the Gilson Co.,

Lewis Center, Ohio) was used to obtain a sample that would range in size from less than

250 µm to greater than approximately 50 µm. This fraction was passed through the

heavy liquids, as detailed below, and the larger than 250 µm fraction was reserved in case

more sample was required at a later date.

(6) Approximately 100 mL of the above prepared sample was passed though a separatory

flask that contained bromoform. The sample was stirred many times and heavy portion

was allowed to settle to the bottom of the flask while the light portion (e.g. glass, quartz,

feldspars) floated to the top. The bromoform was then recovered and the light fraction of

the sample discarded. This process was repeated until a sufficient amount of heavy

minerals were collected.

(7) The heavy minerals portion was collected onto filter paper and rinsed multiple times

with acetone and allowed to dry in a fume hood. It is important to note that vinyl gloves

must not be used in this process as they are permeable to acetone. The author

recommends neoprene.

(8) The clean, dry heavy separate was then spread over a clean sheet of paper and a large

hand magnet, covered with a kimwipe, was passed over the sample to remove the

magnetic fraction which was collected and reserved in a glass vial. If there is sufficient

28

sample available, it can be passed through additional heavy liquids (i.e. methylene

iodide) to form additional separates.

(9) The non-magnetic heavy separate was then passed though a Frantz isodynamic

separator with a forward slope of 15o, a side tilt of 15o and a current of 0.5 amps. This

process resulted in two fractions, a magnetically susceptible fraction (e.g. hornblende,

ilminte, biotite, grains with magnetite inclusions) and a non-susceptible fraction (e.g.

zircon, apatite, thorite). It is important to thoroughly clean the Frantz as there are many

places where particle contamination can occur. Hutchinson (1974) gives a generalized

review of the above process and details for Frantz settings for other minerals.

(10) Mineral grains from all three separates (the magnetic, magnetically susceptible, and

non-susceptible) were placed in holes drilled into a labeled, hard, one inch round, 3/8

inch thick nylon spacer (generally available at hardware stores).

(11) These holes were then filled with Epo Tek 301, a low viscosity epoxy (from Epoxy

Technology Inc, Billerica, Mass). Poking air bubbles with a needle or other sharp device

was effective in removing bubbles from the epoxy.

(12) The grain mount was then placed in a ~50oC oven to cure overnight before getting

sent offsite for a microprobe polish.

The above method produced high quality one inch round mineral grain mounts for

microprobe analysis and there were no problems with the nylon substrate in polishing the

mounts at the University of Utah Thin Section Laboratory (Quintin Sahratian, personal

communication, 2004).

29

2.8 Electron Microprobe (EMP) Methods:

One thin section from the CDF (sample G1) and a heavy mineral separate from

the CDF were examined using a JEOL 8900 R Electron Probe Microanalyzer under the

direction of Karen Wright at the Idaho National Laboratory Research Center (IRC) in

Idaho Falls, Idaho. Beam current was set at 20 nA with a 15 kV accelerating voltage and

a beam diameter 10 microns (plagioclase, sanidine, biotite, hornblende, magnetite, matrix

glass and melt inclusions in quartz) or 5 microns (ilmenite and melt inclusions in

magnetite). Integration times were 20 seconds for major elements, 30 seconds for minor

elements and 10 seconds for background. Both rim and core measurements were

performed on plagioclase, sanidine, hornblende and biotite with single measurements on

magnetite, ilmenite, melt inclusions and matrix glass. Thirty second long energy

dispersive spectroscopy (EDS) profiles were made of selected mineral unknowns in order

to define their major-element composition and mineral identification. Specific EMP

methods are documented in Appendix 1.

30

Chapter 3 RESULTS

This section focuses on the results of field, petrographic and geochemical studies.

Field observations are described and photographs of representative areas and a map (Plate

1) of the CDF are included. Phenocryst assemblages and percentages along with textual

descriptions and representative photomicrographs from the three dome fields are

presented in the petrography section. Geochemical results include major- and trace-

element chemistry as well as isotope results and electron microprobe chemistry on

phenocrysts from the CDF. The geochemistry of the BVF rhyolites is then compared to

rhyolites from other physiographic provinces.

3.1 Field Relationships:

Rhyolite emplacement in the BVF occurs at two distinct times (~1.4 Ma and

~0.05 Ma; Table 1). The two dome sets (NDF and CDF) and Sheep Island Rhyolite

(SIR) are spatially separated (Figure 2). Stratigraphic relationships of the incised domes

of the NDF to surrounding geology are obscured both by loess and weathering products.

Likewise, contacts between the SIR and surrounding rock units are covered either by

water or deep mud. The NDF and SIR areas will be briefly discussed first followed by a

more detailed examination of field relations on the younger CDF.

The NDF, located on the Little Valley Hills 1:24,000 quadrangle map, consists of

three domes, roughly aligned along a S60oE liniment. Samples from Northern Dome

West (NDW) and Northern Dome East (NDE) yield K/Ar dates of 1.59 +/- 0.06 and 1.41

+/- 0.15 Ma respectively (B. Nash, Personal Communication, referenced in Luedke and

31

Smith, 1983; Table 1). Luedke and Smith (1983) did not indicate whether the

uncertainties are at the 1-sigma or 2-sigma confidence interval and thus the domes of the

NDF and SIR could overlap in age. Domes of the NDF have a cumulative volume of

approximately 0.45 km3 of rhyolite with Northern Dome West containing 85 percent of

this volume. It is also the largest in aerial coverage, retaining much of its original glassy

carapace although it is deeply incised by tributaries to Crooked Creek on its southern

side. NDW does contain at least one lobe, expressed to the west and southwest, that

appears to have flowed out onto the Crooked Creek Flat. Between NDW and NDM,

there are large pieces of quartz pebble conglomerate float, some over one meter in size,

that contain slickensides. Reasonable access is available with a high clearance vehicle

only and permission to trespass should be secured from Gentile Valley Land and Cattle

Company, Grace, ID for all visits.

The NDF rhyolite ranges in color from massive white to pink and purple flow

banded sections (Figure 7). Vesicle content ranges from near zero in dense black and

brown obsidian to highly vesiculated pumiceous rhyolite. Lithophysae cavities are

common and some are up to eight cm in diameter (Figure 8). Visible phenocrysts include

quartz, plagioclase, sanidine and biotite in a dense to vesiculated glassy matrix. The

rhyolite also contains sparse basaltic enclaves, some with crenulated contacts with the

rhyolite and some with angular contacts.

The SIR, located in Blackfoot Reservoir on the Henry 1:24,000 quadrangle map,

is similar in age to rhyolites of the NDF (B. Nash, Pers. Com. referenced in Luedke and

Smith, 1983; Table 1). Sheep Island is entirely composed of rhyolite and covers 0.75

km2, reaching an elevation of approximately 57 m above the surface of the reservoir.

32

Figure 7. Typical flow banding in the NDF with alternating layers of pink and purple colored rhyolite. Located on Northern Dome West in an incised valley, representing the interior of the original dome. Four pound crack hammer for scale.

Figure 8. Lithophysae cavities, up to 8 cm, located on the southwest lobe of Northern Dome West near Crooked Creek Flat. These features are more common and larger in the NDF than in the CDF. Four pound crack hammer for scale.

33

Mansfield (1927) reports the existence of two rhyolite outcrops in the Blackfoot

Reservoir. Gull Island, located approximately 600 m to the south-southeast of Sheep

Island is composed of rhyolite and is likely the second outcrop mentioned by Mansfield.

It reaches a height of approximately 12 m above the reservoir covering only 0.03 km2 and

was not investigated. Total volume of both outcrops was not specifically calculated but

is less than 0.03 km3.

The rhyolite at Sheep Island is generally dense and glassy containing few vesicles

and is sparsely porphyritic containing plagioclase and quartz in hand sample. It contains

alternating purple and pink flow-banded layers (Figure 9) inclined at ~45o. While there is

no evidence of enclaves or lithophysae cavities, the island was not thoroughly searched

for them. Access is by boat only and Sheep Island is public land.

Figure 9. Typical dense, glassy flow banded rhyolite from Sheep Island in the Blackfoot Reservoir. There are few outcrops on Sheep Island. Four pound crack hammer for scale.

34

The CDF, the youngest of the three study locations at ~0.05 Ma (Table 1),

contains three domes aligned approximately N33oE located on the China Hat 1:24,000

quadrangle map. Total volume for the CDF is approximately 0.7 km3 with China Hat

containing 0.5 km3, China Cap with 0.12 km3 and the North Cone with 0.08 km3. The

total volume of rhyolite in the BVF, including the NDF and CDF is approximately 1.2

km3. The glassy carapaces are very much intact with essentially no erosion or dissection

of the domes. In addition to the dome formation, a number of other volcanic and local

tectonic events were examined in and around the CDF including graben formation due to

normal faulting, voluminous basaltic flows, rhyolitic tephra production, and the

formation of craters between two of the domes. These are discussed below.

Numerous north and northwest trending normal faults cut Quaternary basalts in

the BVF (Figure 2; Michell and Bennett, 1979; Oriel and Platt, 1980). These faults are

most commonly exposed in the lowest, central part of the large, down-dropped Basin and

Range valley, where the rhyolite domes are located, and often produce small grabens

(Figure 10). Four of these grabens are especially noteworthy including the grabens

containing Hole in the Rock Lake (Figure 10) and Dike Lake, a small graben along the

south-west flank of China Hat, likely an extension of the Hole in the Rock Lake graben,

and a subtle graben directly south of China Cap (Figure 11) and are labeled on Plate 1.

Hole in the Rock Lake partially fills a graben (Figure 10) that has an offset up to

35 m and has a nearly continuous surface expression in excess of eight kilometers. The

northern half of this distance was mapped as a fault by Oriel and Platt (1980) and they

continue the fault to the north-northwest at least 15 km further. The surface expression of

this fault passes the western side of China Cap and extends past the south-west flank of

35

Figure 10. Hole in the Rock Lake partially filling a graben with scarp offsets up to 35 m in the Blackfoot Volcanic Field basalt. This graben has a continuous surface expression over 8 km long. Picture taken from China Cap, looking northwest

Figure 11. Subtle graben south of China Cap (edge of China Hat on the right side of the photo). Thick loess, actively farmed, has muted the surface expression of the graben. Photo taken from China Cap looking south.

36

China Hat, though the apparent offset has decreased (Plate 1). The talus apron around

China Hat obscures any direct evidence of the fault cutting the dome but there is a small

lobe where the rhyolitic lavas appear to have flowed over the edge of a preexisting

precipice or over a fault scarp. Another large fault is mapped directly south of China Hat

(Oriel and Platt, 1980) with offsets up to 35 m does not appear to offset the dome.

Dike Lake also fills a graben (Plate 1), although the offset is not as great as the

Hole in the Rock graben and the linear continuity is hidden by the Blackfoot Reservoir.

The lake filled graben is exposed just to the northeast of China Cap and if the linear trend

of the graben was extended, the fault would pass either under or through the west side of

China Cap and directly into the north side of China Hat. There is no evidence that this

fault offsets either dome but again, the talus apron and dense vegetation on the north side

of China Hat obscure most intact, continuous units. Additionally, unlike the Hole in the

Rock graben, the Dike Lake graben, while visible to within 1000 m of China Cap, does

not have a continuous surface expression directly adjacent or leading into the domes and

therefore may not exist at these points. No evidence of offset tephra beds due to faulting

is evident in the exposed deposit on the north side of China Hat.

A small graben directly south of China Cap (Figure 11, Plate 1) was not mapped

by Oriel and Platt (1980) as a fault. The offset on this graben is approximately 15 m and

its topography is generally muted by an unknown thickness of loess that is actively tilled

and farmed. A laterally continuous band of dense obsidian glass that exists along the

southern edge of China Cap is not offset by this fault. All of the faults described above

are within four kilometers from each other from north to south and within two kilometers

from east to west and presumably cut the same laterally continuous basalt flow. The

37

basalt flows in the area do not show evidence of on lapping the rhyolite domes.

A tephra deposit at least 17 m thick is exposed on the northern flank of China Hat

and (Figure 12, Plate 1). The beds of the deposit have a 5o dip to the north (away from

the dome) and it contains rhyolite and basalt ash, lapilli and block sized constituents.

Some of the larger blocks of rhyolite are up to 0.5 m and contain rounded edges (Figures

12 and 14). Angular, palagonitized basalt fragments (Figure 13) are common, especially

in the bottom four meters of the section. The percentage of basalt clasts appears to

decrease towards the top of the section.

Figure 12. An overview of approximately seven meters of the China Hat tephra deposit with the author for scale. There is substantially less basaltic component to the tephra in this middle portion of the deposit. The pit has been extensively mined since this picture was taken in 2002. Picture by Michael McCurry.

38

Figure 13. Alternating layers of angular palagonitized basalt, both block and lapilli sized rhyolite, some reversely graded and mixed with basalt clasts, and rhyolitic ash in the China Hat gravel pit. Loess tops this section. Wooden staff is 1.6 m long.

Figure 14. Large rhyolite blocks with rounded edges located at the top of the shovel handle. Note the deformed beds around the approximately 30 cm rhyolite clast. This section starts approximately 0.5 m above the top of the section in Figure 11. Wooden staff is 1.6 m long.

39

Near the top of the tephra deposit, there is a layer of block-sized rhyolite at least one

meter thick topped with 0.5 m of very fine ash. On top of this final ash layer is colluvial

talus shed from the dome mixed with loess (Plate 1). While the total section is at least 17

m thick, six meters near the top of the deposit could not be mapped due to overburden.

Since this study, the tephra deposit has been extensively mined for aggregate.

The aerial extent of this tephra deposit is unknown. Mansfield (1927) reports

rhyolitic ash at least eight meters thick along the bank of the Blackfoot River

approximately 3.25 km due east of the China Cap summit. This horizontally bedded fine

grained rhyolitic ash contains a ½ m thick bed of basaltic “ash” about four meters down.

Mansfield indicates that that this deposit might also be the result of erosion and transport

from local sources and that, “local (erosional) conditions varied considerably within short

distances.” Current day local drainages from China Hat flow towards this location (to the

northeast). A hand-dug well, approximately 100 m northwest of China Cap, contained

one meter of rhyolitic ash under approximately 7.5 m of soil (Mansfield, 1927). Also, a

thin bed of ash, of unknown thickness, attributed to the CDF was discovered in mud

cores taken from Gray’s Lake, located 35 km to the north-northeast of the CDF (Ken

Pierce, personal communication, 2003). It is unclear if this is an air fall deposit or fluvial

deposit but the current day smaller scale drainage pattern is to the northwest from the

CDF, not towards Gray’s Lake.

Rhyolite within the CDF shows little variation except in their degree of

vesiculation. The rocks are very light in color, vitric, and crystal poor with phenocrysts

of quartz, euhedral plagioclase, sanidine, biotite and hornblende. The biotite shows

40

variable degrees of weathering indicated by its luster. Fresh, non-weathered crystals are

shiny, mirror-like and black while those that have been weathered are purple, iridescent

or brown. Radiating sprays of post-emplacement topaz have been documented

(Dayvault, 1984) up to two millimeters in length however I looked for them and was

unable to find any. Both spherulitic and lithophysal textures are evident in the field but

are not common with lithophysae generally less than one centimeters where present.

Dense, black obsidian is common and in flowbands and sometimes associated with

devitrification. Unlike the NDF, no brown colored obsidian was found. Two populations

of basaltic enclaves, up to two centimeters, are common but not abundant and were most

easily found on China Cap. One type is typically ovoid and shows crenulated boundaries

with the rhyolite in thin section while the other type has angular margins. These enclaves

will be further detailed in the petrography section.

Other features in the CDF are noteworthy. The three domes of the CDF align

along a N33oE trend and each dome is slightly oblong in shape along a north or northeast

trend. The slopes of the dome margins are consistently steep with 35 to 45 percent

grades with three exceptions. A coulee nearly encircles China Hat resulting in a

noticeable inflection in the slope approximately halfway up the dome (Figure 15). The

southwestern side of the North Cone is the steepest area within the CDF with a shear cliff

over 10 m high while the northeastern side of the dome is manifested by hummocky

terrain and is the least steep area in the CDF (Figure 16, Plate 1). As one approaches the

tops of all three domes, the terrain becomes less steep. All three domes contain spines

with nearly vertical flow banding and block fields. While spines and pressure ridges

exist on all three domes they are particularly common on China Cap (Figure 17). The

41

Figure 15. A coulee nearly encircles China Hat creating a noticeable inflection in the slope of the dome surface. The dome is approximately 300 m tall. Picture taken from Highway 34 looking west.

Figure 16. Three domes (from left to right, China Hat, China Cap and North Cone) showing relative sizes. Note the hummocky terrain on the northeast side of North Cone (right-hand side of the picture) is in stark contrast to the exceptionally steep area on the opposite side of the dome. Picture taken from the Blackfoot River Road looking west.

collapse of pre-existing spines during or soon after their formation may produce the block

fields (Macdonald, 1972), most common on China Hat. Some of block fields have areas

over 0.05 km2 and contain blocks over three meters in size.

Along the same N33oE trend of the domes, two large pits, now containing

Burchett and Gronewall Lakes, exist between China Cap and North Cone. The Burchett

Lake depression (Figure 18) is slightly elongate to the northeast and has a depth of at

42

Figure 17. Many prominent spines exist on China Cap as well as on the other domes. Some of the spines can be 10 m above the local dome topography. Photo taken from China Cap Road looking northwest.

Figure 18. Burchett Lake partially fills a crater between China Cap and North Cone and lies within the linear trend created by the CDF. Note fields on either side sloping away from the crater rim. Photo taken from China Cap looking northeast.

43

least 34 m but this could be considerably greater depending upon the thickness of the

sediments in the bottom of the lake. The Gronewall Lake depression is curvilinear, has a

depth of at least 34 m and forms a moat around the southwestern side of North Cone

(Plate 1). The area between China Cap and North Cone, excepting the craters, is

generally higher in elevation over the surrounding valley by up to 60 m and slopes

downwards along a trend perpendicular to that of the dome alignment (Plate 1) at dips of

five to seven degrees. Basalt flows exposed on the north edge of the Burchett Lake crater

dip a similar direction and amount.

Access to the CDF for all vehicles on paved roads is available along both China

Cap Road and Dike Lake Road and there are other access points that require high

clearance vehicles. The tops of China Hat and China Cap are both public land but

permission for extended periods of access should be secured from David Hubbard of

Soda Springs, ID (China Hat), Lonnie Cellan of Inkom, ID (China Cap), and Joe Elsmore

of Grace, ID (North Cone). Short stops, especially into the gravel pits, should not present

a problem unless the pits are being actively mined.

3.2 Petrography:

A total of twenty-six thin sections were examined using a petrographic

microscope including seventeen from the CDF, eight from the NDF and one from the

SIR. Samples from respective dome sets are all very similar in phenocryst type,

abundance and size as well as textures and generally vary only in the quantity of post

emplacement features such as vapor-phase crystallization, devitrification, spherulitic

aggregate formation, flow banding and vesicle concentration. Additionally, four slides

44

contain enclaves that will be discussed later in this section.

3.2.1 Petrography of the CDF:

The CDF silicic rocks are best described as hololeucocratic hyalo-rhyolites (using

the IUGS classification of LeMaitre et al., 2002) in that they have a color index (M’) of

under ten and contain over eighty percent glass. Common phases include quartz >

sanidine > plagioclase > hornblende ≈ biotite > Fe-Ti oxides (magnetite > ilmenite) and

comprise approximately eight percent of the rock. Accessory phases include zircon >>

apatite > thorite ≈ allanite, with the last two only positively identified in a thin section

made from a heavy mineral separate. As mentioned in the field methods section,

typically dense samples from the glassy carapace were collected and thus the samples

studied in thin section likely under represent the overall crystal content of the domes.

Vesicle concentration ranged from zero to five percent in the thin sections studied,

pumiceous samples should contain a higher proportion of vesicles.

Phenocryst textures:

Quartz: There are two distinct textural types of quartz crystals 1. euhedrally shaped

crystals and 2. strongly embayed crystals. Both types are up to 1 millimeter in size,

occasionally fractured and commonly contain melt inclusions, some of which have

daughter crystals. Many quartz crystals contain small, hourglass shaped embayments

that may be the result of ruptured melt inclusions. Crystals that show embayment-like

features often have some crystal faces that are very straight and otherwise unaltered

(Figure 19). Quartz crystals comprise approximately five percent of the rock on a

45

Figure 19. Strongly embayed quartz likely due to chemical dissolution. Note the unaltered sides retain the very straight edges of the originally euhedral crystal. 10X, crossed polars, long dimension = 0.7 mm.

Figure 20. Square-shaped, skeletal sanidine, indicative of rapid growth, armoring plagioclase. 10X, crossed polars, sanidine is 1 mm on a side.

Figure 21. Blocky magnetite crystal with sharp edges indicating equilibrium with the melt. Note the euhedral quartz along the bottom edge. 10X, plain polarized light, long dimension of the magnetite is 0.4 mm.

46

vesicle-free basis. Euhedral crystals were in textural equilibrium with the melt while

embayed crystals have been resorbed at some point after their original formation.

Sanidine: Sanidine crystals comprise about two percent of the rock on a vesicle-free

basis and the crystals are up to two millimeters on a side. Some crystals show evidence

of inclusion that have leaked along cleavage fracture plains. Euhedral in form, most

crystals are equant and square in cross section while some crystals exhibit evidence of

skeletal growth produced by rapid crystallization. This rapid crystallization can lead to

interesting phenomena where two or three “separate” crystals are in optical continuity

with each other but less than two millimeters apart. It is likely that this is a single

skeletal crystal that was connected in the third dimension. Sanidine crystals occasionally

mantle plagioclase (Figure 20) and were also in textural equilibrium with the melt.

Plagioclase: There are two texturally distinct types of plagioclase in the CDF rhyolites.

The larger-sized crystals, up to two millimeters long, commonly exhibit both Albite and

Carlsbad twinning and comprise approximately one percent of the rhyolite on a vesicle-

free basis. Crystals can be long and slender, mantled by sanidine (Figure 20), broken or

bent and in some cases and show evidence of skeletal growth but all appear to be in

textural equilibrium with the melt. The smaller sized plagioclase crystals are part of the

groundmass and will be described below with the matrix.

Mafic minerals: Hornblende, biotite and Fe-Ti oxide phenocrysts make up far less than

one percent of the CDF rhyolite. Hornblendes often occur as euhedral pleochroic crystals

that exhibit excellent cleavage and have a maximum length of one millimeter. Black,

shiny, euhedral biotite crystals over one millimeter are uncommon but obvious in hand

sample but the biotites preserved in thin section are generally less than 1/2 mm in size.

47

Both magnetite and ilminite are present with the latter always less than 1/5 mm in length.

Magnetite crystals are blocky (Figure 21) and are up to one millimeter in size. All four of

these phases appear to be in textural equilibrium with the melt with no resorption textures

evident.

Accessory minerals: The dominant accessory phase is zircon. Some zircons occur as

inclusions in hornblende while others occur as an intratelluric phase. Apatite is rare and

was only observed as small, less than 1/5 mm acicular crystals in two thin sections. The

existence of thorite and allanite was confirmed with EDS scans from an electron

microprobe (EMP) on a heavy mineral separate and later observed petrographically when

the grain mount for the probe was cut to thin section thickness. Thorite is a tetragonal

mineral with optical properties similar to zircon except that is has a faint apple green

color and is weakly pleochroic.

Matrix: The matrix consists primarily of light brown to colorless glass and vesicles,

some of which appear very long and flat or stretched and varying degrees of spherulitic

aggregate. It also contains microlites of plagioclase crystals, many with swallow tail

forms. These small crystals only occur in long stringers, similar to flow banding that

often appear to flow around larger crystals and sometimes pond around them, perhaps in

previous void spaces. The plagioclase crystals in these stringers are loosely oriented with

the long axis parallel to the length of the stringer (or presumed flow). These

concentrations of microlitic plagioclase can extend across the entire length of the slide

and are found with a very fine grained, less than 0.05 mm, dark, acicular, pleochroic

mineral, likely hornblende, lenticular shaped stretched vessicles and possibly very fine

grained sanidine.

48

Figure 22. Glomerocryst containing quartz, with a melt inclusion, sanidine, and a 1.4 mm long opaque (ilmenite?) mineral. Note this photo shows some of the post emplacement spherulitic aggregate (SA) formation (grey). 10X , plain polarized light.

Figure 23. Same as above but with crossed polars. Some of the crystals are in optical continuity indicating that they may exhibit skeletal growth or are connected in the third dimension. Other cognate pieces contained plagioclase as well.

49

Glomerocrysts: Glomerocrysts containing quartz, plagioclase, sanidine, hornblende and

Fe-Ti oxides make up approximately two percent of the rock. These wall-rock pieces

display intergranular texture (Figure 22, 23) and are in textural equilibrium with the melt

as there are no resorption textures. It appears that the glomerocrysts are similar to the

overall rhyolite in phenocryst assemblage and are predominantly quartz.

3.2.2 Petrography of the NDF:

The NDF rhyolites are also hololeucocratic hyalo-rhyolites and contain the same

major phases as the CDF in nearly the same proportions including quartz (~4.5%),

sanidine (~3%), plagioclase (~1.5%), and hornblende, biotite and Fe-Ti oxides, all far

less than one percent with crystals making up about ten percent of the rock by volume.

Delineation of whether a phase was part of a silicic xenolith or an individual intratelluric

crystal was not made for the NDF rhyolites but they do contain at least some of these

glomerocryst which appear to be in textural equilibrium with the surrounding glass. The

heavy mineral separates EMP mount is dominated with zircon and although allanite or

thorite were not observed, these phases likely exist.

Textures for the two dome sets are very similar for all minerals except for the

following. There were no zones of microlitic plagioclase with swallow tail features and

hornblende found in the NDF rhyolites. Also, unlike the CDF, the NDF did contain some

examples of granophyric texture (the coexistence of graphic and vermicular textures,

Hibbard, 1995) but only in glomerocrysts, not individual phenocrysts (Figure 24).

50

Figure 24. Granophyric texture of glomerocryst in the NDF rhyolites. This texture was not seen in the CDF rhyolite. 20X, crossed polars, field of view ~0.75 mm.

Figure 25. Oscillatory zoned plagioclase within a basaltic magmatic enclave. Note the centers of the crystals exhibit skeletal growth. 10X crossed polars, center crystal is 0.5 mm.

Figure 26. Boxy cellular plagioclase within the basaltic magmatic enclave with rim of later euhedral growth surrounding the quickly formed original crystal. 10X, crossed polars, crystal 1 mm.

51

3.2.3 Petrography of the SIR:

The SIR thin section contains very few crystals, most are smaller than 0.5 mm,

and has substantial post emplacement spherulitic aggregate formation. The thin section

shows flow banding with alternate white and peach colored stripes crossing the slide.

Two textures of plagioclase are present, 1. large euhedral crystals up to 0.5 mm and 2. a

small quantity of microlitic plagioclase. Microlitic sanidine is also present in

small quantities. The slide contains some quartz crystals, at levels less than one percent

by volume.

3.2.4 Mafic Enclaves of the BVF:

While basaltic xenoliths undoubtedly occur within the rhyolite and are ubiquitous,

especially in the lower sections of the tephra deposit, the rhyolite also contains basaltic

enclaves of magmatic origin. A crenulated margin and fine-grained chill texture easily

delineate magmatic enclaves from non-magmatic xenoliths and both the NDF and CDF

contain magmatic enclaves. Some of the margins between the rhyolitic and basaltic

magmas appear to be hybridized.

The mafic enclaves are texturally similar to olivine tholeiites. They contain

olivine crystals that are either unaltered or show evidence of dissolution with crenulated

edges or eroded centers and at least four textures of plagioclase including 1. very fine

grained swallowtail forms, 2. unaltered larger phenocrysts, 3. oscillatory zoned crystals

(Figure 25) and 4. boxy cellular or skeletal crystals rimmed with euhedral plagioclase

growth (Figure 26). In at least one petrographic slide, there is evidence of hybridization

where the basaltic and rhyolitic magmas have begun to mix (Figure 27). This hybridized

52

Figure 27. Zone of hybridization between the rhyolitic magma and basaltic magma. Note small white quartz crystal in the mixed zone (Figure 28). 2.5X, plain polarized light, width of mixed zone ~1.4 mm.

Figure 28. Quartz crystal within the hybridized zone (Figure 27) showing pyroxene- mantled texture with a small gap between the quartz and hornblende. 20X, crossed polars, crystal is 0.25 mm.

Figure 29. Spongy cellular or fingerprint texture in a sanidine crystal approximately 2 cm away from the basaltic magmatic enclave. 40X, crossed polars, width of partially melted zone = 0.05 mm.

53

zone occasionally contains quartz crystals that appear resorbed but does not contain

sanidine (from the rhyolite) or olivine (from the basalt). Quartz within this zone is

pyroxene-mantled (Figure 28). Within the rhyolite, near the basaltic magmatic enclaves,

sanidine crystals exhibit spongy cellular or fingerprint textures (Figure 29). This texture

was not noted in slides that did not also contain a basaltic magmatic enclave.

3.3 Geochemistry of the BVF:

This section first examines the major- and trace-element composition of the BVF

rhyolites, comparing and contrasting the geochemistry of the NDF and CDF to each

other. The BVF rhyolites are then compared to rhyolites in other local provinces

including Quaternary Eastern Snake River Plain (QESRP) rhyolites (e.g. Big Southern

Butte and East Butte), Yellowstone and Tertiary Snake River Plain (YandTSRP)

rhyolites, Cascade rhyolites, Basin and Range rhyolites and topaz rhyolites. Unless

otherwise noted, the major-element data are normalized to 100% on an anhydrous basis

with all iron represented as FeO(t). As noted in the methods section, the thirty-five BVF

rhyolite samples were collected using non-specific criteria that included the degree of

weathering and density/glassiness. Great care was used to remove any weathering rind,

alteration, enclaves or xenoliths and some samples were run multiple times to check data.

Twenty-one samples were analyzed from the CDF, thirteen from the NDF and

one from the SIR while five samples were analyzed from Big Southern Butte (and

included in the QESRP dataset). Major- and trace-element compositions and sample

locations are given in Table 2.

54

3.3.1 Major-Element Geochemistry:

Samples from the both the CDF and NDF are classified as a high-K rhyolite on a

Le Bas, et al. (1986) total alkali silica (TAS) diagram and are slightly metaluminous with

molecular (CaO + Na2O + K2O) > Al2O3 > (Na2O + K2O). Sheep Island is also a high-K

rhyolite but is very slightly peraluminous. Note that there is only one analysis of this

location and that a decrease in Al2O3 content by less than two relative percent (0.3 weight

percent) or increasing the Na2O or K2O by five relative percent, both within analytical

errors for these elements, would shift the SIR into the metaluminous classification.

A major-element variation diagram (Figure 30) gives a graphical representation of

the above data. Assuming a normal distribution about the mean for the two populations,

z-tests at the 99% confidence level indicate that the CDF and NDF differ in their

concentrations of some major elements. The CDF has greater Al2O3, MnO, Na2O while

the NDF has more FeO(t), and K2O with a significantly higher K2O/ Na2O ratio (1.30 vs.

1.07). The remaining major elements are not distinguishable between the two

populations at the 90% confidence interval.

Of special note are the five samples that are above 0.8 weight percent CaO.

These samples are clearly higher than the means of either the CDF or NDF analysis.

Four of the five samples tested positive for carbonate with a 10% HCl solution, likely

from caliche deposition. The one sample with high CaO from the CDF (7160108) did not

react with acid application. Eight other samples that are near the average CaO content

were chosen at random and tested with acid and none showed any reaction, indicating a

lack of caliche.

55

Table 2: Major- and Trace-Element Geochemistry: Central Dome Field; China Cap and China Hat. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample 7290101 8020102 7160105 8020107 7280106 8020101 7290103 7160101Dome CC CC CC CC CC CC CC CHUTM east 451581 451748 451072 451530 451478 451676 451282 450686

UTM north 4741206 4740825 4740919 4740786 4741129 4740872 4741224 4739647

SiO2 76.4 76.5 76.9 76.9 76.8 76.8 77.1 76.6TiO2 0.09 0.08 0.08 0.07 0.06 0.06 0.07 0.08Al2O3 12.6 12.6 12.6 12.5 12.5 12.5 12.5 12.7FeO(t) 0.93 0.90 0.90 0.94 0.86 0.91 0.95 0.94MnO 0.06 0.06 0.05 0.05 0.05 0.06 0.05 0.05MgO 0.08 0.04 0.03 0.02 0.03 0.01 0.03 0.05CaO 0.68 0.56 0.51 0.57 0.59 0.53 0.56 0.55Na2O 4.39 4.48 4.57 4.46 4.20 4.29 4.18 4.47K2O 4.79 4.73 4.34 4.36 4.88 4.72 4.45 4.55P2O5 0.02 0.02 0.01 0.06 0.03 0.05 0.03 0.04

Original total 96.8 97.3 98.2 99.3 97.6 96.1 97.7 97.7

Sc 2.19 2.22 2.18 2.25 2.19 2.28 1.99 2.20Rb 486.5 490.5 517.5 504 500 520 495 479.5Sr 3.59Cs 15.6 15.8 17 16.5 16.2 15.7 17 16.1Ba 152 159 161 179 139 144 161 152La 28.4 18.5 19.3 19.4 24.1 17.1 17.3 19.5Ce 70.6 62.6 69 66.8 77.3 61.6 64.5 69.4Nd 30.1 20.7 29 27.5 24.9 13.8 17.1 22.4Sm 12.4 11.4 12.1 11.8 11.9 11.1 10.8 10.7Eu 0.08 0.07 Tb 3.81 3.55 3.48 3.39 3.94 3.47 2.90Yb 15.5 15.4 16.3 15.7 14.4 14.6 14.4 14.8Lu 2.40 2.27 2.52 2.28 2.42 2.47 2.39 2.30Zr 128 108 126 129 137 131Hf 6.41 6.57 7.04 6.62 6.74 7.02 7.06 6.58Ta 9.81 9.80 10.4 10.1 10.0 10.6 10.1 9.96Th 55.1 52.0 54.3 52.7 53.5 51.8 47.3 50.6U 29.8 31.1 31.7 32.0 30.4 30.5 30.1 29.3

56

Table 2 Continued: Central Dome Field; China Hat. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample 7210103 7280102 7160103 7210102 7160104 7210104 7210101 G1Dome CH CH CH CH CH CH CH CHUTM

easting 450269 451205 450167 450114 450167 450412 449989 451124

UTM northing 4739033 4739494 4739907 4739195 4739822 4738943 4739329 4740456

SiO2 76.4 76.5 77.0 76.6 76.7 76.5 77.0 77.0TiO2 0.10 0.09 0.08 0.06 0.06 0.08 0.07 0.06Al2O3 12.7 12.5 12.6 12.6 12.6 12.6 12.5 12.7FeO(t) 0.99 0.93 0.91 0.90 0.88 0.92 0.88 0.83MnO 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05MgO 0.08 0.04 0.03 0.05 0.02 0.05 0.02 0.07CaO 0.59 0.51 0.51 0.55 0.53 0.55 0.52 0.55Na2O 4.39 4.40 4.44 4.45 4.40 4.43 4.09 4.05K2O 4.73 4.78 4.34 4.74 4.69 4.85 4.87 4.66P2O5 0.02 0.12 0.00 0.03 0.03 0.02 0.03 0.03

Original total 97.2 96.6 98.4 96.9 99.4 97.2 95.2 100.7

Sc 2.22 2.15 2.25 2.23 2.24 2.35 2.23Rb 500 487 492 485 491 492 505Sr 2.22 1.58 2.28 1.97 3.27 1.84 2.26Cs 16.1 15.6 15.9 15.7 16.6 15.8 16.4Ba 163 178 171 156 162 185 172La 18.2 21.7 21.6 23.3 20.8 23.5 21.6Ce 69.8 71.7 70.3 79.2 73 77.1 72.6Nd 27.3 25.0 24.6 25.0 27.7 27.9 25.7Sm 11.3 11.6 11.7 11.9 11.6 12.0 11.7Eu 0.05 0.07 0.08 0.06Tb 3.37 3.54 3.51 3.47 3.48 3.25 3.49Yb 15.8 15.3 15.5 15.6 15.6 15.2 14.8Lu 2.40 2.26 2.24 2.37 2.35 2.27 2.36Zr 129 128 131 141Hf 6.76 6.46 6.49 6.55 6.48 6.57 6.78Ta 10.0 9.62 9.85 9.66 9.96 9.92 10.0Th 55.1 58.6 53.6 56.5 52.7 53.2 51.7U 31.3 32.1 32.0 30.8 30.5 33.8 30.6

57

Table 2 Continued: Central Dome Field; China Hat and North Cone. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample G2 G3 8220101 8220104 8220103 7160108 8220102Dome CH CH NC NC NC NC NCUTM

easting 451124 451124 452659 453139 452460 453304 452482

UTM northing 4740456 4740456 4742710 4742959 4742324 4742702 4742443

SiO2 76.9 76.9 77.0 76.6 76.6 76.5 76.7TiO2 0.06 0.06 0.09 0.08 0.09 0.07 0.06Al2O3 12.5 12.6 12.7 12.6 12.7 12.4 12.7FeO(t) 0.91 0.86 0.92 0.90 0.92 0.97 0.89MnO 0.05 0.05 0.05 0.05 0.05 0.05 0.05MgO 0.04 0.02 0.03 0.04 0.03 0.06 0.03CaO 0.64 0.52 0.52 0.55 0.50 1.09 0.55Na2O 4.19 4.37 4.25 4.32 4.52 4.27 4.33K2O 4.67 4.55 4.48 4.86 4.61 4.60 4.71P2O5 0.01 0.03 0.02 0.04 0.04 0.03 0.03

Original total 99.5 96.9 96.4 97.6 98.1 98.6 95.3

Sc 2.20 2.16 2.18 2.17 2.28 2.30 2.22Rb 500 495 485 476 514 471 500Sr 2.41 2.02 2.71Cs 16.2 16.1 12.0 15.3 16.5 15.6 16.1Ba 159 155 112 156 180 173 157La 18.4 19.1 19.3 22.2 25.2 24.8 18.8Ce 68.7 66.1 65.7 72.2 80.7 81.3 64.6Nd 21.3 24.1 21.0 28.8 33.1 29.9 25.9Sm 11.4 11.2 11.0 11.9 12.7 12.1 11.4Eu 0.08 0.06 0.05 0.06 0.08Tb 3.49 3.45 3.40 3.43 3.28Yb 14.8 14.5 16.4 15.1 16.2 15.1 14.8Lu 2.38 2.33 2.42 2.24 2.36 2.26 2.36Zr 134 133 122 131Hf 6.95 6.79 6.38 6.44 6.76 6.31 6.98Ta 10.0 10.1 13.0 9.62 10.2 9.18 10.1Th 51.2 49.3 50.2 53.8 56.5 63.2 50.8U 31.3 31.0 31.2 30.6 33.3 31.4 30.8

58

Table 2 Continued: Northern Dome Field; eastern most and middle dome. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample 6280204 6280202 6280201 6280205 6110201 9010103 9010105 9010104Dome NDE NDE NDE NDE NDM NDM NDM NDMUTM

easting 457137 456891 456891 457297 456031 455344 455382 455330

UTM northing 4764127 4764467 4764469 4764312 4764626 4764875 4763640 4764655

SiO2 77.0 76.9 77.2 77.0 77.3 76.3 76.7TiO2 0.07 0.07 0.07 0.11 0.08 0.11 0.09Al2O3 12.4 12.6 12.5 12.3 12.5 12.4 12.3FeO(t) 1.03 1.02 1.05 1.19 1.07 1.14 1.05MnO 0.04 0.04 0.04 0.04 0.03 0.04 0.04MgO 0.06 0.03 0.07 0.13 0.05 0.12 0.06CaO 0.64 0.51 0.59 0.63 0.47 0.90 0.54Na2O 3.82 3.83 3.68 3.59 3.72 3.98 4.11K2O 4.74 4.98 4.84 4.95 4.77 4.93 5.04P2O5 0.18 0.03 0.03 0.04 0.03 0.03 0.03

Original total 96.3 97.7 99.8 98.4 102.1 95.8 96.1

Sc 1.78 1.76 1.81 1.71 2.05 1.91 1.80 1.84Rb 345 355 345 375 290 345 306 348Sr 6.30 3.90 9.34 6.79 4.86Cs 9.71 9.41 9.79 10.1 8.70 8.79 8.47 9.56Ba 62.8 78.0 96.4 75.6 71.9 65.3 79.0 80.6La 29.1 32.8 29.1 27.1 28.8 33.1 32.5 31.9Ce 76.3 83.1 78.8 71.5 75.2 83.5 85.3 79.4Nd 23.9 26.4 26.3 23.6 23.5 26.1 26.4 26.4Sm 8.6 9.1 8.7 8.6 8.1 9.3 8.6 8.9Eu 0.10 0.05 0.12 0.15 0.10 0.10Tb 3.28 3.36 3.31 3.20 2.97 3.49 2.05 2.37Yb 10.4 10.0 10.6 10.2 9.0 10.1 9.1 10.3Lu 1.52 1.50 1.56 1.54 1.37 1.51 1.36 1.48Zr 121 141 115 108Hf 4.86 4.99 4.95 4.94 5.60 5.11 4.50 4.89Ta 4.35 4.51 4.43 4.37 3.83 4.47 3.68 4.26Th 52.2 51.0 52.3 49.1 46.2 51.0 49.6 52.6U 13.9 13.4 14.2 13.6 12.8 14.0 12.5 14.6

59

Table 2 Continued: Northern Dome Field; western most dome and Sheep Island. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample 7170201 7110201 7110202 7170203 10070001 7130201Dome NDW NDW NDW NDW NDW SIRUTM

easting 453411 453927 453036 452892 453399 449942

UTM northing 4766554 4766941 4766910 4765950 4767036 4750957

SiO2 77.3 76.9 76.1 76.3 76.7 76.6TiO2 0.07 0.07 0.06 0.06 0.06 0.04Al2O3 12.5 12.3 12.3 12.3 12.4 13.1FeO(t) 1.01 1.01 0.98 1.00 1.00 1.0MnO 0.04 0.04 0.04 0.04 0.04 0.04MgO 0.03 0.04 0.06 0.04 0.02 CaO 0.50 0.51 1.97 1.20 0.51 1.16Na2O 3.58 3.86 3.74 3.73 4.15 3.67K2O 4.90 5.27 4.69 5.34 5.11 4.34P2O5 0.02 0.03 0.03 0.03 0.04 0.03

Original total 98.0 94.6 95.0 96.0 95.3 96.3

Sc 1.75 1.75 1.67 1.72 1.75 1.77Rb 370 335 365 290 345 335Sr 4.30 3.21 7.08Cs 9.52 9.35 8.56 9.02 9.39 8.54Ba 115 77.5 69.7 88.3 96.5 87.0La 28.6 30.6 29.3 28.6 32.4 27.5Ce 74.0 77.9 75.1 73.1 84.4 72.7Nd 23.9 25.6 19.9 23.8 28.4 23.9Sm 8.6 9.1 8.4 8.5 8.9 8.5Eu 0.10 0.12 0.09 0.10 0.08 0.07Tb 3.31 3.20 2.92 2.90 2.35 2.29Yb 9.91 9.97 9.86 10.2 9.00 10.4Lu 1.51 1.56 1.44 1.45 1.50 1.52Zr 122 115 113 Hf 5.13 5.08 4.75 5.10 4.75 4.85Ta 4.37 4.33 4.22 4.16 4.39 4.35Th 50.6 49.5 48.2 49.0 52.8 52.2U 13.7 14.4 13.4 13.3 14.3 13.9

60

Table 2 Concluded: Big Southern Butte Samples. Major elements expressed as wt % oxide, trace elements are in ppm. UTM points NAD 27, zone 12.

Sample 11080301 11080301 11080302 11080303 11080304Dome BSB BSB BSB BSB BSBUTM

easting 335862 335862 Top 334788 334640

UTM northing 4806574 4806574 4807258 4807616

SiO2 76.01 75.71 76.18 76.55 76.15TiO2 0.09 0.09 0.09 0.09 0.09Al2O3 12.61 12.72 12.50 12.32 12.40FeO(t) 1.60 1.63 1.62 1.64 1.60MnO 0.04 0.04 0.03 0.04 0.04MgO 0.04 0.04 0.01 0.00 0.01CaO 0.39 0.38 0.45 0.23 0.19Na2O 4.39 4.58 4.52 4.57 4.78K2O 4.82 4.80 4.59 4.54 4.72P2O5 0.03 0.02 0.01 0.02 0.01

Original total 98.6 96.5 97.9 98.1 97.0

Sc 0.14 0.14 0.18 0.19Cr 19.4 19.2 17.2 20.5Rb 301 294 302 291Sr 3.47 2.95 0.06 0.81 0.96Cs 2.78 2.41 2.71 2.14Ba 64.7 55.4 53.0 52.9La 58.1 64.6 50.7 77.1Ce 128 126 113 149Nd 38.6 45.8 41.1 57.0Sm 12.4 14.0 12.5 17.3Eu 0.25 0.25 0.27 0.35Tb 3.1 3.46 3.14 4.43Yb 17.6 18.9 16.3 20.8Lu 2.35 2.52 2.08 2.60Zr 297 306 316 304 308Hf 15.8 16.2 15.5 16.6Ta 18.6 18.8 19.0 18.6Th 20.1 19.1 19.1 20.0U 13.0 13.8 13.1 13.4

Table 2 Notes:

Major elements reported are normalized to 100% anhydrous. P2O5, where italicized, was estimated at 0.03% for normaliza-tion purposes only. All major elements determined using ICP-AES and trace elements determined with INAA except FeO and Na2O measured with both techniques with disputed measurements redone and Sr, Ba, and Zr determined with ICP-AES. Abbreviations as follows: Central Dome Field: CH = China Hat CC = China Cap NC = North Dome Northern Dome Field: NDE = eastern most dome NDM = middle dome NDW = western most dome Other rhyolites SIR = Sheep Island BSB = Big Southern Butte No data indicates the analyte was not measured for that sample or was below detection limits (MgO and Eu only).

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62

The contribution of calcium from caliche was not calculated. It is likely that most

of the “extra” calcium above 0.6 weight percent can be attributed to caliche formation in

the NDF rhyolites. These five samples also have lower SiO2 concentrations as a result of

both increased CaO concentrations and the loss of CO2 in the dissolution process. The

high CaO in the one sample from the CDF remains unexplained. The data from the ICP

analysis is robust and a non-quantitative examination of the hand sample and thin section

give no indication of significant differences from other samples analyzed (i.e. increased

plagioclase percentage or the existence of other calcium-bearing phenocrysts). The SIR

also has a high CaO percentage and contains at least some caliche but it is unclear how

much calcium came from the caliche. There are no apparent systematic variations of

other common caliche constituents such as Mg and Sr in the five samples with high CaO

concentrations.

3.3.2 Comparison of Major-Element Geochemistry of the BVF to Other Rhyolites:

The CDF, NDF and SIR are similar but differentiable in their major-element

constituents (Figure 30 – note scales). Their differences are indistinguishable when

plotted on diagrams that contain a wide span of rhyolite compositions and thus are

generically referred to as Blackfoot Volcanic Field (BVF) rhyolites for comparison to

rhyolites from other tectonic settings. As the major-element variation diagrams in Figure

31 show, the rhyolites of the BVF lie within the boundary of or near the edge of the area

defined by Yellowstone and Tertiary Snake River Plain (YandTSRP) rhyolites, except

that they are lower in FeO(t). Likewise, data from the Basin and Range province (which

include Long Valley, Death Valley and the Latir Volcanic Field) often overlap with the

63

BVF data. This relationship, where the major-element compositions of the BVF rhyolites

are similar in nature to both YandTSRP and Basin and Range rhyolites, mimics the

results seen in the BVF basalts (Pickett, 2004).

It is important to again clarify the distinction between the YandTSRP and

Quaternary Eastern Snake River Plain (QESRP) rhyolites. While the two groups of

rhyolites occupy the same physiographic province, they are distinct in many of their filed,

compositional and mineralogical characteristics. The YandTSRP (“hot-spot track”)

rhyolites are produced by massive, caldera forming eruptions, generally emplaced by

ignimbrites and contain significant Proterozoic or Archean crustal component, up to 35

percent (McCurry et al., 2002) while the QESRP rhyolites are emplaced as low volume

domes (and cryptodomes in the case of East Butte, Fairy Butte, Buckskin Dome, etc.) and

are formed by extreme fractionation of a mafic parent with very little crustal component,

as little as one percent (McCurry et al., 1999). Several papers (e.g. Spear, 1979; Hildreth

et al., 1991; Leeman, 1982a; Leeman, 1982b; Pierce and Morgan, 1992; McCurry et al.,

1999; McCurry et al., 2002; McCurry and Ganske, 2005) cover the formation of these

two highly distinguishable rhyolite suites.

From Figure 31 note that both QESRP and topaz rhyolites have similar major-

element compositions to the BVF rhyolites. On some variation diagrams (e.g. FeO(t) and

TiO2), two groups of QESRP data points can be delineated. These represent data points

for the two largest domes exposed on the surface, East Butte (typically higher iron) and

Big Southern Butte, with points from other QESRP (e.g. Unnamed Butte, INL Core Hole

1 and Cedar Butte) scattered and more variable. In comparison to the BFV rhyolites, the

QESRP rhyolites are somewhat higher in FeO(t) and correspondingly lower in SiO2 and

64

Table 3. References for data represented for major-element variation diagrams (Figures 31) and trace-element variation diagrams (Figure 33). Quaternary Eastern Bretches, 1984 Snake River Plain Ganske, in preparation Rhyolites Hayden, 1992 (QESRP) Leeman, 1982a Morse, 2002 Spear, 1979 This study Tertiary Snake River * Binderman and Valley, 2001 Plain and Yellow- Eckren et al., 1982 stone rhyolites Hildreth et al., 1991 (YandTSRP) Honjo, 1990 * Kellogg and Marvin, 1988 Morgan et al., 1984 Parker, 1996 Watkins, 1998 Wright, 1998 Basin and Range * Asmerom, 1994 Rhyolites * Binderman and Valley, 2002 * Heumann and Davis, 1997 * Johnson and Lipman, 1988 Cascade rhyolites * Bacon, 1985 * Brophy et al., 1996 * Drophy and Dreher, 2000 * Bullen and Clynne, 1990 * Condie and Hayslip, 1975 * Gerlach and Grove, 1982 * Grove and Donnelly-Nolan, 1986 Topaz rhyolites Christiansen et al., 1986

* From the PLUTO Geochemical database (Baedecker et al., 1998)

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67

East Butte is higher in TiO2. The topaz rhyolite points come from eight temporally and

spatially separated sets of domes resulting in more scatter in the data than seen with the

QESRP of BVF data sets. References for Figure 31 are given in Table 3.

3.3.3 Trace-Element Geochemistry:

While the CDF and NDF rhyolites are similar, but distinguishable, in their major-

element compositions, more substantial differences occur in some of their trace-element

characteristics. These differences are distinguished on a primordial mantle normalized

multi-element spider diagram (Figure 32). The average CDF rhyolite contains

significantly (at least 25%) more Cs, Rb, Ba, U, Ta, Hf and Sm and they are moderately

higher (10 to 25% more) in Zr and Tb than the NDF. The NDF rhyolite only has

significantly higher La and Sr with moderately higher Ce. Th and Nd are similar (less

than a 10% difference). All of the major elements have less than a ten percent difference.

The multi-element spider diagram does not show all of the elements from Table 2 and the

remainder of the trace elements are represented on a chondrite-normalized REE diagram

(Figure 34). Yb and Lu are significantly higher in the CDF while Sc is moderately higher

while Eu appears to be higher in the NDF (Table 4).

Four elements warrant special consideration. P2O5, as stated in the major-element

chemistry section, is estimated at 0.03 for a number of compositions for normalization

purposes. Also, there were a few less samples analyzed for both Zr and Sr calculations

(as indicated in Table 2). Finally, the Eu concentration in both dome fields was near

detection limits and thus there is also less data for this analyte. None of the above

68

conditions altered the overall trends of the data represented in the spider diagram (Figure

32). Data comparing the two BVF dome sets is summarized in Table 4.

Table 4. Summary of the comparison of trace element abundance between the average CDF and NDF of the Blackfoot Volcanic Field. Specific sample values can be found in Table 2 and averaged values are listed in Table 5. Graphical representations are provided, along with additional physiographic province data in Figures 33 and 34. The CDF rhyolite contains significantly to moderately higher concentrations (greater than ten percent more) of nearly all the trace elements. >25% more

in CDF 10-25% more in CDF

>25% more in NDF

10-25% more in NDF

Nearly the same in both

Sc X

Rb X

Sr X

Cs X

Ba X

La X

Ce X

Nd X

Sm X

Eu X

Tb X

Yb X

Lu X

Zr X

Hf X

Ta X

Th X

U X

69

Figure 32. Primordial mantle-normalized multi-element spider diagram of all samples from the CDF (23 samples – black lines) and NDF (13 samples – grey lines). This figure illustrates some of the obvious differences in the trace-element composition of the two dome sets. Normalization values are from Taylor and McLennan (1985), except for P from Sun (1980), and are consistent with those values used by Pickett (2004) for the basalts of the BVF.

0.1

1.0

10.0

100.0

1000.0

10000.0

Cs Rb Ba Th U K Ta La Ce Sr Nd P Hf Zr Sm Ti Tb

70

3.3.4 Comparison of Trace-Element Geochemistry of the BVF to Other Rhyolites:

While the BVF rhyolites are similar to a number of other rhyolites from various

tectonic and physiographic settings (e.g. YandTSRP, Basin and Range, QESRP and topaz

rhyolites) in their major elements, the concentrations of many of the trace elements lie

outside the ranges of most other groups of rhyolites, except for topaz rhyolites. These

include U, Th, Cs and Rb which are present in concentrations that are significantly higher

than rhyolites from other physiographic provinces examined in this study (Table 3). The

BVF and QESRP rhyolites both have higher concentrations, as compared to other

rhyolites, in Tb, Yb, Lu, and Ta. Additionally, both the BVF and QESRP rhyolites are

lower than other rhyolites in Sr, Eu and Ba. Eu and Sr concentrations in the BVF,

especially the CDF, merit special mention as both show extreme depletion with only Big

Southern Butte of the QERSP having less Sr. Table 5 summarizes the average values for

the various tectonic and physiographic provinces for trace elements. Some trace-element

variation diagrams are shown in Figure 33 and help to visualize the data in Table 5.

Both Dayvault et al. (1984) and Christiansen et al. (1986) have classified the CDF

as a topaz rhyolite based upon both the chemical makeup of the rocks and on the presence

of post-emplacement topaz formation. Both dome sets in the BVF, despite their trace

element differences (Table 4), are more similar to topaz rhyolites than to rhyolites from

the afore mentioned suites (Table 5). Further similarities between the topaz rhyolites of

the western United States and the BVF rhyolites will be presented with chondrite

normalized REE data next and in the Discussion section.

71

Table 5. Average concentrations, in ppm, of trace elements from various physiographic regions with the BVF delineated into the CDF and NDF, topaz = topaz rhyolites of the Western United States, B and R = Basin and Range rhyolites, and the QESRP delineated into EB = East Butte and BSB = Big Southern Butte. Sources are the same as in Table 3 except East Butte is Ganske (2005, in preparation) and Big Southern Butte is this study and Spear (1979). Sr for Big Southern Butte, marked with the asterisk, shows an average of eights sample. Dr. Michael McCurry has further refined the value to be ~0.6 ppm (personal communication, 2001). Ba values for topaz rhyolites are widely variable and REE element data was only represented only in chondrite normalized graphs by Christiansen et al. (1986).

CDF NDF topaz B and R Cascades YandTSRP EB BSB

Sc 2.2 1.8 2.2 2.6 5.2 5.6 0.65 0.16

Rb 495 335 600 135 100 175 210 295

Sr 2.3 5.7 12.6 100 225 98 11 1.8*

Cs 16 9.2 30 4.7 5.7 3.3 1.7 2.5

Ba 160 82 625 815 970 115 64

La 21 30 37 22 80 110 63

Ce 71 78 66 42 150 220 140

Nd 25 25 20 19 64 89 51

Sm 12 8.7 3.2 3.4 12 23 15

Eu 0.06 0.10 0.52 0.68 1.61 1.3 .26

Tb 3.5 2.9 0.49 0.55 1.8 4.4 3.5

Yb 15 9.9 1.9 1.9 6.1 18.3 20

Lu 2.3 1.5 0.27 0.35 0.85 2.1 2.4

Zr 130 120 120 170 175 460 380 185

Hf 6.7 5.0 6.8 4.2 4.7 14 16 16

Ta 10 4.3 14 1.7 0.83 3.3 12 19

Th 53 50 52 16 10 27 23 20

U 31 14 18 5.1 3.2 6.6 7.1 13

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75

The chondrite-normalized (using Taylor and McLennan, 1985 values) rare earth

elements (REE) patterns of the BVF are unique, along with the topaz rhyolites as

compared to other rhyolites (Figure 34). Both the CDF and NDF show relative depletion

of LREE and enrichment of HREE with the CDF having a nearly 1:1 ratio of LaN to YbN

and the NDF a 2:1 ratio with those for topaz rhyolites commonly falling between 1:1 and

3:1 (Christiansen et al., 1986). Rhyolites of the BVF have an exceptionally deep Eu

anomaly with Eu/Eu* = 0.014 for the CDF and 0.026 for the NDF, again, within the

range of topaz rhyolites. The REE trend for Big Southern Butte (QESRP) is nearly

identical to the NDF with normalized values approximately twice those for the NDF. In

contrast, the YandTSRP rhyolites have a La/YbN ration of 10 and a Eu/Eu* approaching

0.5 with an overall REE pattern that closely approximates those of A-type granites

(Collins, 1982). The Cascades are generally depleted in all REE, with only a slight Eu

anomaly. Basalts from the Blackfoot Field are similar to Snake River Plain basalts

(Pickett, 2004) and have a Eu/Eu* slightly above 1 on average (Figure 34). Sources for

the aforementioned data are given in Table 4 except as listed in Figure 34 and Big

Southern Butte (this study and Spear, 1979).

As presented earlier, the BVF rhyolites have many similarities to the QESRP

rhyolites in their trace-element geochemistry. Some similarities are also present in the

REE data. While normalized LREE values vary, all are relatively enriched in HREE and

all show strong Eu anomalies. Christiansen (1986) indicates that La/LuN and Eu/Eu*

values are inversely proportional to uranium content in topaz rhyolites. This holds true

for both the BVF and QESRP rhyolites, including Big Southern Butte, East Butte and

Unnamed Butte.

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77

3.4 Electron Microprobe Results on Phenocrysts of the Central Dome Field:

Thirty-five EMP analyses on sample G1 of the CDF were completed at the IRC in

Idaho Falls, Idaho. These included core and rim analysis on a plagioclase, three sanidine,

two biotite, two amphibole (“hornblende”), two magnetite and two ilmenite crystals as

well as four melt inclusions and two matrix glass areas. Complete anaylses are given in

Table 6 and the specific EMP methods are given in Appendix 1.

Feldspars: Only the plagioclase crystal showed evidence of compositional zoning. The

plagioclase crystal is anothite14 with an orthoclase component of seven percent (on

average) with a slightly more sodic and less calcic rim (normal zonation). On average,

sanidine phenocrysts are orthoclase65 with anorthite contents less than one percent

(Figure 35).

Fe-Ti oxides: Magnetite phenocrysts have an average of ulvospinel26 and contain just

over one weight percent MnO and Ilmenite averages hematite10 with 2.5 weight percent

MnO (Figure 35).

Amphibole: The average amphibole is classified as a ferro-edenite.

Mica: The mica is a biotite with approximately annite60 and phlogopite40 with molar

aluminum content slightly higher than these two end members. The extra molar

aluminum may exist in the octahedral sight resulting in up to 15 percent siderophyllite

component.

Melt inclusions: The four melt inclusions are somewhat give variable results in the

original analyses but consistent when normalized to 100 percent anhydrous. Three of the

analyzed melt inclusions are located in quartz crystals while one is in a magnetite crystal

(sample MI 5). The analysis of the melt inclusion in magnetite appears to over-represent

78

concentrations of FeO and TiO2 indicating potential contamination from the host crystal.

This is also potential host crystal contamination in sample MI 1, hosted in quartz.

Sample MI 3 has a low total of oxides attributed to one of two phenomena; either the

inclusion are tight (or sealed) and the majority of the missing mass, approximately six

percent, is water or the microprobe beam spot also incorporated some epoxy used in the

slide preparation or some other foreign substance during the analysis. Analysis of sample

MI 4 clearly clipped the edge of a bubble, possibly filled with epoxy, and had a total of

only 83.4 percent. The average melt inclusion analysis is similar to the bulk rock

analyses except that Na2O is low (Figure 30).

Matrix glass: Two matrix glass measurements are similar to each other and give original

totals near 100 percent. The matrix glass analyses are similar to the bulk rock chemistry

for nearly all major elements, except Na2O. Complete analyses are given in Table 6 and

melt inclusion and matrix glass average values are plotted with bulk chemistry on Figure

30.

Figure 35. Ternary diagrams for the microprobe analyses of sanidine, plagioclase, magnetite and ilmenite. The Fe-Ti oxides are connected by a dotted tie line.

79

Table 6. Normalized, to 100% anhydrous unless otherwise indicated, individual phenocryst chemistries from EMP for both rims and cores (shown with original totals listed). Abbreviations as follows Sa = sanidine, MI = melt inclusions, Mt = magnetite, Ilm = ilmenite. MI 5 is in a magnetite crystal while other melt inclusions are located in quartz. Average values for MI and MG are plotted with bulk chemistry on Figure 30. Mineral standards and integration times are given in Appendix 1.

Sample Sa 1 Sa 1 Sa 2 Sa 2 Sa 3 MI 1 MI 3 MI 4 MI 5 core/rim core rim rim core na na na na

SiO2 65.80 66.13 66.01 66.04 66.13 80.57 78.09 77.96 78.95 Al2O3 18.43 18.57 18.69 18.57 18.51 10.81 13.09 12.44 12.28 TiO2 0.02 0.01 0.03 0.08 FeO 0.08 0.14 0.12 0.08 0.12 0.75 0.82 0.99 1.91

Cr2O3 MgO 0.00 0.00 0.03 0.00 MnO 0.06 0.07 0.01 0.00 NiO ZnO CaO 0.15 0.17 0.20 0.19 0.15 0.44 0.47 0.49 0.47 Na2O 4.26 3.88 3.95 4.09 3.99 3.14 2.65 3.15 1.91 K2O 11.28 11.11 11.03 11.03 11.10 3.98 4.46 4.64 4.11 SrO P2O5 0.01 0.00 0.00 0.00

F 0.16 0.24 0.13 0.24 Cl 0.16 0.25 0.24 0.18

SO3 0.00 0.00 0.01 0.01 orig. total 99.35 100.60 100.80 100.64 100.58 98.96 93.28 83.43 99.19

Sample Mt 1 Mt 1 Mt 2 Mt 2 Ilm 1 Ilm 2 Ilm 2 Ilm 2 core/rim rim core rim core na na na na

SiO2 0.04 0.09 0.07 0.08 1.02 0.02 0.01 0.00 Al2O3 0.74 0.76 0.80 0.76 0.26 0.03 0.05 0.03 TiO2 9.50 9.88 9.95 10.61 47.35 48.07 48.35 47.67 FeO 88.16 87.77 87.67 87.03 48.51 49.02 48.78 49.28

Cr2O3 0.00 0.01 0.03 0.00 0.01 0.01 0.03 0.00 MgO 0.13 0.11 0.14 0.16 0.32 0.31 0.30 0.33 MnO 1.14 1.22 1.13 1.21 2.52 2.45 2.37 2.60 NiO 0.00 0.02 0.00 0.02 ZnO 0.29 0.17 0.21 0.14 0.00 0.07 0.09 0.06 CaO 0.00 0.00 0.00 0.00 Na2O K2O SrO P2O5

F Cl

SO3 orig. total 97.25 97.07 96.69 97.61 96.70 98.69 99.49 97.79

80

Table 6 Concluded. Abbreviations as follows: Plag = plagioclase, MG = matrix glass, Bt = biotite (not normalized), Hlb = hornblende (not normalized). Analytes for which there is no data were not tested for in respective phases (e.g. TiO2 in plagioclase).

Sample Plag 1 Plag 1 Plag 1 Plag 1 Plag 1 Plag 1 MG 1 MG 3 core/rim rim core rim rim rim core na na

SiO2 65.61 65.35 64.80 64.83 65.73 64.69 77.75 77.68 Al2O3 21.02 21.53 21.77 21.62 21.08 21.93 12.32 12.42 TiO2 0.02 0.01 FeO 0.15 0.12 0.16 0.13 0.11 0.17 0.84 0.85

Cr2O3 MgO 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 MnO 0.04 0.00 NiO ZnO CaO 2.42 2.77 3.07 3.01 2.38 3.06 0.53 0.50 Na2O 9.41 9.02 9.13 9.23 9.34 9.18 3.69 3.64 K2O 1.38 1.21 1.08 1.18 1.36 0.97 4.51 4.59 SrO P2O5 0.01 0.00

F 0.23 0.28 Cl 0.17 0.16

SO3 0.01 0.01 orig. total 101.14 100.45 100.89 100.74 100.92 100.49 100.86 101.30

Sample Bt 1 Bt 1 Bt 1 Bt2 Bt2 Hbl 2 Hbl 2 Hbl 3 Hbl 3 core/rim core rim rim rim core core core core rim

SiO2 36.43 36.45 36.21 35.95 36.28 41.64 40.82 42.12 42.12 Al2O3 12.26 12.05 12.04 12.23 12.14 7.81 8.29 7.41 8.04 TiO2 3.49 3.64 3.59 3.52 3.75 1.37 1.57 1.13 1.30 FeO 28.97 29.36 28.57 28.70 29.09 30.07 30.19 27.53 27.71

Cr2O3 MgO 6.47 5.54 6.29 6.82 6.66 3.37 3.64 5.17 4.99 MnO 0.65 0.58 0.58 0.60 0.60 1.15 1.13 1.03 1.01 NiO ZnO CaO 0.04 0.00 0.03 0.01 0.00 10.23 10.37 10.05 10.26 Na2O 0.39 0.45 0.42 0.37 0.41 2.06 2.04 2.02 2.11 K2O 8.78 8.79 8.81 9.18 8.61 1.15 1.27 1.07 1.17 SrO P2O5

F 0.17 0.17 0.19 0.26 0.26 Cl 0.27 0.29 0.27 0.30 0.31 0.33 0.37 0.31 0.30

SO3 orig. total 97.78 97.18 96.86 97.74 97.92 99.10 99.59 97.77 98.95

81

3.5 Isotope Results:

Nd and Sr isotopes on four samples from the CDF and one sample from the NDF

were analyzed by Dr. John Chadwick at the University of Florida Department of

Geosciences and are presented in Table 7 and plotted on Figure 39 (Chapter 4). Also

included in the table are basalts from the BVF (Pickett, 2004) and a rhyolite sample from

China Cap of the CDF (Michael McCurry, personal communication, 2001). The BVF

rhyolites are slightly more evolved (higher 87Sr/86Sr and lower σNd) than the high-silica

rhyolites of the QESRP (McCurry et al., 1999) but are closer to the QESRP rhyolites than

to Basin and Range rhyolites (Gans et al., 1989, not plotted). The isotope ratios of the

basalts of the BVF are more similar to QESRP basalts than to Basin and Range basalts

(Pickett, 2004).

Table 7. Nd and Sr isotopic ratios for rhyolites and basalts from the BVF. The rhyolite data are initial concentrations corrected using the given Sr and Nd concentrations (Table 2) except for the China Cap MM sample for which average Sr and Nd concentrations were used. Time corrections used were 50 ka for the CDF and 1.41 Ma for the NDF. The basalt values listed are the measured values. Plus or minus values are X 10-6. China Cap MM from M. McCurry (personal communication, 2001) and KEP basalts from Pickett (2004).

Sample Rhyolites Sr

(ppm) 87Sr/86Sr

initial +/- Nd

(ppm) 143Nd/144Nd

initial +/- Sigma

Nd 7160101 CDF 2.2 0.710591 6 22.4 0.512080 6 -10.9 8220101 CDF 2.02 0.709523 25 21 0.512083 7 -10.8 G1 CDF 2.23 0.7097306 7 25.7 0.512130 40 -9.9 G2 CDF 2.2 0.709595 8 21.3 0.512074 5 -11 China Cap MM CDF 0.710341 10 0.512075 5 -11 9010104 NDF 4.86 0.709354 12 26.4 0.512023 4 -12 Basalts (measured) (measured) KEP 004 "primitive" 350 0.706646 5 26.8 0.512358 30 -5.5 KEP 027 "evolved" 350 0.708905 4 81 0.512241 17 -7.7 KEP 029 Willow Cr 375 0.707929 5 16.8 0.512374 26 -5.1

82

Chapter 4 DISCUSSION

This chapter focuses on the CDF and starts with a discussion of the physical

volcanology and the emplacement history for the igneous rocks of the BVF followed by

petrography interpretation. I then examine the state of the pre-eruptive system including

the phenocryst assemblages and intensive variables (e.g. pressure, temperature and fO2).

Current models for creating topaz rhyolites with fractional melting and the QESRP

rhyolites with assimilation and fractional crystallization are then evaluated as analogs to

the BVF system. The patterns of rock chemistry in the BVF are evaluated to link the two

members of the bimodal field with fractional crystallization. Because there are no

intermediate rocks in the BVF, MELTS modeling (Ghiorso and Sack, 1994) is attempted

to help establish the paragenesis for these intermediate melt compositions during the

preferred fractional crystallization evolution model for the BVF system. These results

could better constrain the timing of phase initiation within the evolving magma to help

determine the bulk distribution coefficients. Assimilation is first modeled with via single

stage bulk mixing and then with EC-AFC models (Spera and Bohrson, 2001; Bohrson

and Spera, 2001). These models are designed as plausibility studies, can we produce the

CDF rhyolite from basalts in the BVF with reasonable constrains? Finally, I give a brief

discussion on the NDF and on the regional implications of my results.

4.1 Physical Volcanology:

Within the vicinity of the domes in the CDF, the basalts were emplaced and cut

by faults before the domes formed. While small normal faults in the valley center are

83

likely present but hidden by still younger basalt flows, many of the surface basalts of the

Blackfoot Lava Field are cut by numerous north to northwest trending normal faults

(Oriel and Platt, 1980) (Figure 2). Some of these fault traces intersect the rhyolite domes

(Figures 10 and 11) but there is no evidence of offset in either obsidian bands or tephra

units on the domes. Also, there is a small but noticeable lobe on the southwestern side of

China Hat where the rhyolite flowed over the edge of a pre-existing fault scarp (Plate 1).

Additional evidence of rhyolite production after localized basalt flows is the

covering of in-situ tephra with loess on the north side of China Hat at an elevation of

approximately 1875 m (6150 feet) (Figure 13), and not basalt while there is a basaltic

vent less than 0.5 km to the north from the tephra deposit with an elevation exceeding

1896 m (6220 feet) (Plate 1). It is possible that this local basaltic vent predates the dome

formation while other vents in the lava field postdate dome formation. No evidence of

basalts on-lapping the domes is apparent, thus any basaltic eruptions occurring after

dome emplacement must be localized and away from the domes.

Dates of basaltic vents from Luedke and Smith’s (1983) map range from 0.01 to

1.0 Ma while the most recent sanidine Ar-Ar age for the CDF is 0.057 ± .008 Ma

(Heumann, 2004). Valley wide, it is clear from the dates above that basalt flows, faulting

and rhyolite dome formation are coeval. Field work in this study favors rhyolite

formation after localized basalt emplacement and faulting.

A small mafic dike was described by Mansfield, 1927 on the north side of China

Cap that may have been emplaced after the rhyolites. I interpret this feature as a slab of

formerly flat-lying surface basalt that was pushed up or rafted and broken by the rhyolite.

Glassy rhyolite coats some of the large, greater than one meter pieces of basalt in a layer

84

that is brittle easily removed with a hammer. A piece of basalt over 100 cm found in this

vicinity likely weathered out of this feature. Xenoliths of basalt in rhyolite are common

near this dike-like feature and resemble a similar area that is rich in xenoliths on the

southwest side of China Cap just above the gravel pit.

The rhyolite eruption began with tephra production and had a hydrovolcanic

component as evidenced by the palagonitized basalts (Figure 13) in the exposed tephra

cone section on China Hat. The tephra beds are largely made up of both pyroclastic

surge and fallout deposits with some larger bocks of rhyolite, up to 0.5 m, indicative of a

crumble breccia (Figures 12 and 14, Plate 1). The lower sections of the tephra deposit

contain more basalt fragments, from pre-existing surface flows, than do the upper

portions of the deposit and likely represent a vent cleaning and widening phase early in

the eruption. Only one tephra deposit is exposed, on the north side of China Hat while

other tephra deposits, if produced in the formation of the other two domes of the CDF,

are covered by the dome or are present but covered by loess. Another unlikely

alternative, due to the good preservation of the deposit around China Hat, is that they

have been eroded away and no evidence of them is present.

Dome growth began before tephra production had stopped as there are pieces of

crumble breccia within the mid- to upper-portions of the tephra deposit (Figure 14).

Dome growth continued, mainly by endogenous processes and resulted in the formation

of many spines (Figure 17) and breccia (block) fields (Plate 1) created by spine collapse

(Macdonald, 1972). Field evidence indicates only a single, nearly continuous

emplacement of the domes, not multiple eruptions separated by considerable time as

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86

suggested by Mansfield (1927). Geochemical evidence also backs this theory as there is

no discernable chemical variation within the domes or within the whole of the CDF.

Partial dome collapse resulted in the region of hummocky terrain or run out feature on the

northeastern side of the North Cone (Figure 16, Plate 1). A four-part schematic dome

emplacement timetable is given as Figure 36.

Two hypotheses may be invoked to explain the depressions, now partially filled

with Burchett Lake (Figure 18) and Gronewall Lake, between China Cap and North Cone

(Plate 1). The Burchett Lake depression is oblong in the same direction as the trend of

the three domes while the Gronewall Lake depression is reniform in shape. The area

adjacent to these two depressions is 60 m (200 feet) higher than the surrounding valley

floor and slopes down to the valley floor at ~5o along a northwesterly trend,

perpendicular to the dome alignment trend. One possible way to produce these structures

is as explosion craters, an idea promoted by Mansfield (1927) or maars. Evidence to

support this includes local hydrovolcanics (i.e. in the China Hat tephra pit). Evidence

against this interpretation is the lack of palagonitized basalts within the craters or along

the rims, distal, columnar jointed basalts within the craters and the exposed basalt dips

away from the crater center.

Another hypothesis is that the pits are produced by one of two similar phenomena,

either by localized extension above a dike (Scenario 1, Figure 37) or by collapse of the

surface after magma is withdrawn from a dike that fed the three domes (Scenario 2,

Figure 37). In either the above situations or the Mansfield (1927) hypothesis, the local

tumescence between China Cap and the North Cone is caused by the dike that fed the

rhyolitic eruptions. The formation of the depressions is contemporaneous with the dome

87

Figure 37: Hypothetical, diagrammatic cross-section between China Cap and the North Cone of the CDF perpendicular to the N33oE trace of the domes showing dike injection causing tumescence and two related possibilities for the formation of depressions (i.e. Burchett Lake and Gronewall Lake). Not to scale.

formations as the North Cone likely filled a more circular hole that now contains the

moat-like Gronewall Lake (Plate 1).

4.2 Petrography Interpretation:

All phenocrysts, with the exception of quartz and those crystals in or near

basaltic magmatic enclaves, appear to be in textural equilibrium with the surrounding

melt. Some quartz crystals are embayed (Figure 19) while others are euhedral (Figure

21). Since quartz generally has a uniform chemical composition, these different textures

likely represent one set of crystals that has been destabilized, possibly due to

depressurization of the melt (Hibbard, 1995), and one set that formed after this

depressurization incident.

88

Within the basaltic enclaves, the boxy cellular or skeletal plagioclase crystals

rimmed with euhedral plagioclase growth (Figure 26) indicate initial rapid under cooling

due to the transfer of heat to the cooler rhyolitic magma followed by growth of the non-

cellular rim zone as the enclave approached equilibrium (Hibbard, 1995). Swallowtail

forms also indicate rapid crystal growth (due to cooling). Both forms are consistent with

the introduction of hot basaltic magma into a rhyolitic system (Hibbard, 1995).

Oscillatory zoned crystals, perhaps due to localized diffusion-supersaturation interplay

(Vance, 1962 referenced in Hibbard, 1995), also indicate localized cooling with a change

in calcium concentration.

Some of the basaltic enclaves show localized evidence of hybridization with

rhyolitic magma (Figure 27). Quartz within this zone is pyroxene-mantled with the space

between the pyroxene and quartz being caused by minor resorption or dissolution of the

quartz (Figure 28). Some olivine crystals within the enclaves also show evidence of

resorption. These features are compatible with magma mixing (Hibbard, 1995). Within

the rhyolite, near the basaltic magmatic enclaves, there is evidence of some partial

melting of sanidine crystals as they exhibit spongy cellular or fingerprint textures (Figure

29). Heat from the basaltic magma is likely responsible as this texture was not evident in

slides that did not contain a basaltic magmatic enclave.

Alignment of the microlitic swallowtail plagioclase crystals coupled with long,

flattened or stretched vesicles are indicative of shear during and shortly after

emplacement (M. McCurry, personal communication, 2005). The silicic glomerocrysts

contain the same phases as the rhyolite and are in textural equilibrium as they do not

exhibit resorption textures. The Fe-Ti oxides within these glomerocysts can be

89

significantly larger than those seen in the rhyolite (Figure 22). These factors indicate that

the glomerocrysts are magma chamber wall rock pieces that were quarried just prior to

eruption.

Section 4.3 The State of the Pre-Eruptive System:

Four analyses of hornblende crystals (ferro-edenites) by electron microprobe

(Table 6) were input into the program AMPHCAL (Yavuz, 1996) to obtain the

geobarometry of the crystals. Both the Johnson and Rutherford (1989) and the

Hammarstrom and Zen (1991) geobarometers, contained with the AMPHCAL program,

gave similar results for the four analyses with a total range of 3.47 ± 0.41 kbars and an

average of 3.5 kbars. This puts the depth of formation for the hornblende crystals, and

thus the magma source of the rhyolite, at 12.7 to 13.4 km using average crustal densities

of 2.8 g/cc and 2.67 g/cc respectively. This places any restite formed by crystallization at

a depth that is equivalent to the top of the mid-crustal sill located under the ESRP (Peng

and Humphreys, 1998; Figure 3).

Petrographic interpretation of the CDF shows that phenocrysts in the rhyolite are

in textural equilibrium, except where localized conditions are altered by the incorporation

of a basaltic magmatic enclave. As a partial check to this interpretation, the Mg/Mn

partitioning into the Fe-Ti oxides, concentrations of which are obtained from EMP data

(Table 6), are plotted on Bacon and Hirschmann’s (1988) graph of empirical data on

fresh volcanics rocks. The results show that the ilmenite and magnetite are in

equilibrium (Figure 38). This allows for the use of QUILF (Andersen et al., 1993), a

program to determine the temperature, fO2 and ∆FMQ (or DFMQ – the log fO2 relative to

90

Figure 38. Log / log plot of atomic Mg/Mn concentrations in ilmenite and magnetite crystals that appear to be in textural equilibrium (small dots) and the CDF, compositions from microprobe analysis (square, covering the range of analyses). While this does not conclusively prove the two phases are is equilibrium, many rocks for which the two phases are not in equilibrium plot outside the 2-sigma lines. Adapted from Bacon and Hirschmann (1988).

91

the FMQ buffer at a given temperature and pressure) of the melt. QUILF results, using a

pressure of 3.5 kbars, show that the temperature of the melt is 758oC ± 10oC and is

insensitive to reasonable changes in pressure. The hematite component of ilmenite was

varied by plus or minus ten percent and the temperature was varied by ± 20oC to check

the robustness of the oxygen fugacity results. The range for log fO2 is -14.5 ± 0.6 and

0.68 ± 0.14 for the DFMQ (see Table 8) for the variations stated above.

Table 8. Magma chamber properties for the CDF. Uncertainty values for pressure are the total range given by a number of different geobarometers Uncertainties for depth are the maximum and minimum range using 2.8 g/cc and 2.67 g/cc crustal densities and the average pressure. Uncertainties in temperature are calculated by QUILF are an indication as to how much the input parameters would need to change in order to get an exact solution. Ten degrees is indicative of a fully determined system and represents the uncertainty in the model reactions used (Anderson et al., 1993). Uncertainties for the log fO2 are determined by changing input parameters, temperature by ± 20oC and hematite component of ilmenite by ± 10 percent. Pressure Depth at given

pressure Temperature Log fO2 DFMQ

Value 3500 bars 13 km 758 degrees C -14.5 units 0.68 log units

Uncertainty ± 400 bars ± 0.4 km ± 10 degrees ± 0.6 0.14 log units

Section 4.4 Current Models For Nearby Physiographic Provinces:

This section examines two existing models for the formation of the BVF

rhyolites, 1. the rhyolites are a product of partial melting or 2. the rhyolites are a result of

fractional crystallization coupled with limited upper crustal assimilation.

There are a number of models put forth for topaz rhyolite formation in the Great

Basin and Basin and crustal extension areas of Mexico. Christiansen et al. (1986) has

92

four models of which only one refers to topaz rhyolite domes with the absence of caldera

structures. This model suggests partial melting of felsic granulites in the lower crust,

caused by basaltic magma ponding at the crust – mantle boundary, followed by either

extensive crystallization as the partial melts pass through colder crust or extensive

crystallization in a shallow magma chamber. They did not have access to Nd isotopes for

their 1986 study.

Orozco-Esquivel et al. (2002) report on “topaz rhyolites” from the Mesa Central

(MC) of the Sierra Madre Occidental Volcanic Province in Mexico. The rhyolites they

studied span a small range of SiO2 compositions (76 to 78.2 %) and their analysis show a

trend opposite to what would be produced by fractional crystallization with Rb, Nb, Ta,

and Th decreasing and Sr and Ba slightly increasing with increased SiO2 concentration.

Their model for the formation of these rhyolites involves both chemical and isotopic

disequilibrium partial melting of granulitic lower crust. This involves dehydration

melting of hydrous phases and buffering of the melt by residual feldspars.

Unlike some of the topaz rhyolites in the Great Basin, the rhyolites described by

Orozco-Esquivel et al. (2002) do not occur in a basalt – rhyolite bimodal field. There are

two sequences of rhyolites, a lower sequence (NOT a topaz style rhyolite) formed by

fractional crystallization of a mafic parent with some crustal assimilation and the upper

sequence, modeled above. The Nd isotopes, at εNd values less than -3 for all their

samples, are more radiogenic than those for the BVF basalts and the crustal rocks are of

Grenville age and thus the model described by Orozco-Esquivel et al. (2002) may not be

analogous to the BVF system with its much older crust. Additionally, they assert that the

geochemical trends they see are counter to fractional crystallization mechanisms. Their

93

range in SiO2 concentration is nearly within analytical error at 2-sigma if reported as

RSD (error reported only as ± 2%) on the eight topaz rhyolite samples analyzed. It is

also unclear that if the feldspars act as residual, buffer phases and biotite is the primary

phase melted, how SiO2 concentrations reach rhyolitic levels. They further state that

isotopic disequilibrium melting of the lower crust is “ …still a matter of discussion” and

that little is known about the mineral composition and physical conditions in the lower

crustal rocks under the MC.

Halliday et al. (1991) modeled the petrogenesis of high Rb/Sr silicic magmas

although not topaz rhyolites specifically. They favor fractional crystallization over

crustal partial melting to achieve high Rb and low Sr concentrations. Using a lower crust

bulk distribution coefficient (D) for Sr of 0.05 (as suggested by Bohrson and Spera,

2001), and a starting Sr content of 60 ppm (one of the lowest of the SRP xenoliths, from

Leeman et al., 1985), the Sr increases with batch melting. Even using a very high DSr (to

represent feldspar buffering) of 3, the Sr content in the CDF rhyolites can not be modeled

with a single stage of melting. A minimum of three partial melting episodes of less than

10 percent are required with the subsequent melt coming from the previous melted and

segregated crust. Some Leeman et al. xenoliths would require up to six of these partial

melting episodes and the high DSr values also rule out melting of upper crust.. Rb values

are equally unrealistic with reasonable DRb values.

Another reason crustal melts could not be the source of the BVF rhyolites is based

on isotopic signatures. The lower crust could not have been partially melted to produce

the BVF rhyolites because the Sr isotopes ratios are too low in the Wyoming age rocks.

Any combination of lower crustal melts and assimilated upper crust would produce Nd

94

isotopes that are too evolved (εNd < -25 based on the Leeman et al., 1985 xenoliths). The

partial melting of upper crust for the BVF rhyolites can be ruled out based upon the work

of Halliday et al (1991). Additionally, Christiansen (personal communication, 2004),

with new research (and given the ~2.2 Ga age of the crust in the eastern Great Basin) is

currently working on new models involving extensive fractionation of a mafic parent

coupled with minor upper-crustal assimilation, not partial melting, for topaz rhyolites.

Halliday et al. (1991) favor fractional crystallization with or without minor crustal

contamination and do not rule out rhyolite magma formation by fractional crystallization

of basalts. As Dsr increases with increasing SiO2 content, the percentage of fractional

crystallization decreases. This is the model favored by McCurry et al., (1999) for the

formation of the QESRP rhyolite domes (e.g. Big Southern Butte, Cedar Butte and East

Butte). The BVF and QESRP rhyolites have more similarities in their geochemistry than

compared to rhyolites from other tectonic or physiographic province, excepting topaz

rhyolites. Pickett (2004) showed a genetic link between the basalts of the two provinces

and Peng and Humphreys (1998) indicate a zone of partial melt running beneath both

areas (Figure 3) indicating the mafic parent material is similar for both regions. The

QESRP rhyolites have Sr and Nd isotopic ratios that are closer to the parent magma than

the BVF rhyolites and since parent magma is similar, the QESRP rhyolites must have less

crustal assimilation (Figure 40; McCurry et al., 1999). Hanna Nekvasil (personal

communication, 2004) has also produced rhyolites from the sequential crystallization of

SRP olivine tholeiite experimentally, showing that this range in chemistry is possible

utilizing only fractional crystallization.

95

Section 4.5 Patterns of Rock Chemistry in the BVF:

The relationships between the trace-element chemistry of the basalts and rhyolites

of the BVF can be examined to see if it is possible to link the two types of rocks using

fractional crystallization models. Figure 39 is spider diagram illustrating evolved basalt

(KEP-027) and the BFV rhyolite compositions, normalized to the most primitive BVF

basalt (KEP-004). This diagram, coupled with estimated partition coefficient (Kd’s) for

common igneous minerals can delineate plausible fractionating phases and possibly

timing.

Figure 39. Spider diagram of the CDF (23 samples – black lines), NDF (13 samples – grey lines) and KEP-027, the most evolved BVF basalt (triangles) using the most primitive basalt (KEP-004) for normalization. There were no uranium analyses of the basalts of the BVF however the CDF rhyolite is 20 to 60 times enriched in U as compared to the most primitive SRP basalts (Hughes et al., 2002a), similar to Th.

0.001

0.01

0.1

1

10

100

Cs Rb Ba Th K Ta La Ce Sr Nd P Hf Zr Sm Ti Tb

Roc

k / K

EP-0

04

96

The evolved basalt (KEP-027) is enriched over the most primitive basalt as the

phenocryst phases present in the basalt, olivine and plagioclase, are incompatible with

most of the elements listed. For REE and high field strength (HFS) elements, this

enrichment is uniformly about 3 to 1, except for Ti. This indicates that ilmenite was

produced (Ti Kdilmenite ≈ 200) at some point between these two end member

compositions. Likewise, the amount of plagioclase produced must be enough to raise the

bulk distribution coefficient (D) to unity in order to keep Sr concentrations equal for both

samples. The production of plagioclase may be enough to slightly increase bulk D for the

large ion lithophile (LIL) elements as plagioclase has higher Kd values for these elements

than for the REE or HFS elements listed. The Kdolivine is less than 0.05 for all the listed

elements. MELTS modeling (Pickett, 2004) indicate that cpx might fractionate at

moderate to high pressures (middle crust or greater) and fractionation of this phase could

explain the anomalous Sc pattern observed in the basalts. CPX however is not observed

as a phenocryst in the basalts.

While 50 to 60 percent of the phenocrysts in the CDF rhyolite are quartz (Kd

essentially zero for all listed elements), it also contains ~25 percent sanidine, ~15 percent

plagioclase and at least one percent hornblende and biotite and some magnetite and

ilmenite. The accessory phases (zircon >> apatite > thorite ≈ allanite) while minor in

abundance, have very large Kd values for some elements.

The rhyolites of the BVF show greater enrichment over the evolved basalt in most

of the LIL elements and slight depletion of most of the REE and HFS elements. Ba

shows marked depletion amongst the LIL elements and is indicative of alkali feldspar

fractionation. This fractionation is also responsible for the depletion of K, Sr and the

97

large Eu anomaly (Figure 34). The light REE are only slightly depleted compared to the

evolved basalt and the trace phases, especially allanite which has massive Kd’s over 2000

for the light REE, are likely the cause. The middle REE (Sm and Tb) plot near the

evolved basalt while the heavy REE (not shown) are enriched over the evolved basalt.

Another line of evidence for allanite fractionation is the 1:1 La to Yb ratio observed

(Figure 34) as allanite preferentially incorporates the light REE. Phosphorous depletion

may be a result of apatite fractionation and while apatite is an accessory phenocryst phase

in the rhyolite, it is possible that more significant apatite fractionation took place when

the magma was more mafic (and MELTS modeling, described later, support this

scenario). The HFS elements are slightly depleted as compared to the evolved basalt and

those depletions are attributed to zircon fractionation. The depletion of Ti is again mostly

due to ilmenite but hornblende and magnetite might also reduce Ti. Th is still

substantially enriched despite the fact that thorite, which has Th as a stoichiometric

constituent is an accessory phase. This phase may have nucleated late in the evolution of

the magma chamber and thus only small quantities were able to fractionate out before the

eruption. Fractionation of allanite would also remove Th from the melt. Typical allanite

analyses show 10 to 20 fold enrichments of La over Th (Deer et al., 1986) and thus large

quantities of the light REE elements are removed with allanite fractionation while

substantially small amounts of Th are removed. Approximate Kd’s used above were

taken from the Geochemical Earth Reference Model (GERM) (http://earthref.org/) and

Rollinson (1998).

98

Section 4.6 MELTS Modeling To Determine Crystallization Sequences and Timing:

In this section, MELTS modeling (Ghiorso and Sack, 1994) is used to help

constrain the timing of phase formation so that bulk distribution coefficients for the

magma can be estimated. I failed to mimic the phases or bulk chemistry of the BFV

rhyolites with numerous runs invoking both equilibrium and fractional crystallization

with reasonable variation in other parameters (e.g. participating phases and intensive

variables). Resultant chemistries however were rhyolitic. Christiansen et al. (1983)

indicate that high fHF stabilizes biotite over fayalite in topaz rhyolites but it is unclear if

MELTS accounts for these unusual magmatic conditions.

Pickett (2004) had poor results modeling the basalts of the BVF with MELTS.

Ghiorso and Sack (1994) state their model is only applicable above 900oC and I could not

model the rhyolites from the evolved basalts using MELTS. MELTS is unable to model

accessory phases (e.g. allanite, thorite) which play substantial roles in determining the Kd

values for many trace phases and the magma chemistry of this system (Figure 39 and

Patterns of Rock Chemistry in the BVF discussion). I was unable to reproduce the

phenocryst assemblage of the rhyolites resulting in less robust control of calculated

magma distribution coefficients used in EC-AFC modeling (next section). This

represents possible limitations within the MELTS program, due at least in part to the

unusual chemistries of topaz rhyolites and extensive fractionation to temperatures below

900oC, and does not indicate problems with the liquid line of decent (fractional

crystallization) hypothesis that links the basalts and rhyolites of the BVF.

99

Section 4.7 Isotope Modeling With Bulk Assimilation and EC-AFC:

After describing the upper crustal rocks under the ESRP, two approaches are used

to model the isotopic evolution of the CDF magma 1. single stage bulk mixing models

and 2. EC-AFC (Energy-Constrained Assimilation and Fractional Crystallization - Spera

and Bohrson, 2001; Bohrson and Spera, 2001). Both of these models have a given set of

assumptions, some of which are poorly constrained, and thus results shown in this section

are intended to only show some consistency between petrographic, geochemical and

model results. This is designed as a plausibility study; can we produce CDF rhyolites

from basalts in the BVF with reasonable constraints?

The crust beneath the BVF consists of Wyoming Terrane cratonic rocks (Figure

5) (O’Brien et al., 1995). The age of these rocks, approximately 2.7 Ga, results in upper

crustal isotope signatures that are in stark contrast to those of the mantle reservoirs

(Faure, 2001; Zindler and Hart, 1986). Actual isotope measurements from crustal

xenoliths from the northern margin of the Snake River Plain (SRP), brought to the

surface by Quaternary volcanism, and the Archean Albion Range (along the southern

margin of the SRP) are documented by Leeman et al., (1985) and given in Table 9.

Leeman et al. (1985) states that the “least modified” xenoliths equilibrated at ~5 kbars

which corresponds to a depth of 18.5 ± 1 km, right at the upper crust – lower crust

boundary (Sparlin et al., 1982) or just a few km above it (Peng and Humphreys, 1998;

Figure 3). Matty (1984) calculated pressures greater than 5 kbars for most xenoliths.

The assertion by Leeman et al., (1985) and Matty (1984) that most of his xenoliths are

representative of the deep crust is in contrast to the elevated Sr isotopic signatures of

100

these 2.7 Ga or older rocks which would place the xenoliths in the upper crust (Faure,

2001). While Leeman et al., (1985) does investigate some xenoliths from

the lower crust, based upon 87Sr/86Sr ratios of less than 0.706, none of those were

included in this study as they have lower 87Sr/86Sr values than those of the rhyolites and

indeed the basalts of the BVF. Assimilation of lower crustal xenoliths could not produce

an increase in the 87Sr/86Sr ratios and in fact would make the rhyolites appear more

primitive, isotopically, than the basalts. A working assumption is that the crustal rocks

that underlie the BVF are similar to those under the SRP as volcanics from both areas

must pass through the Wyoming Terrane rocks.

Table 9. 87Sr/86Sr and 143Nd/144Nd isotopic ratios for selected crustal xenoliths from the Snake River Plain and Albion Range (south-central Idaho) with abbreviations as follows: SK = Spencer-Kilgore; COM = Craters of the Moon; SM = Square Mountain and ALB = Albion Range. Errors for Sr isotope data are ± 0.00004 or better while errors for Nd isotopes are ± 0.000030 or better and average ± 0.000018. As stated in the text, all of these xenoliths are proposed to be from the upper crust based on their age and evolved Sr isotopic signatures. All data from Leeman et al., 1985.

Location Sample Sr

(ppm) 87Sr/86Sr Nd

(ppm) 143Nd/144Nd sigma Nd SK 73-68X 175 0.72550 22.3 0.510895 -34.0 COM CKI-1 211 0.73359 13.5 0.510499 -41.8 COM 70-40 324 0.71525 9.8 0.510454 -42.6 COM SI-1 70 0.71795 85.0 0.510590 -40.0 COM COM-1 97 0.81728 9.2 0.510228 -47.1 SM SM-2G 559 0.72092 7.1 0.509973 -52.0 SM DM-103 310 0.73878 25.7 0.510853 -34.9 ALB YAG-799 115 0.8060 19.0 0.511278 -26.6 ALB YAG-800 181 0.89260 59.0 0.510675 -38.3

101

Simple single-stage mixing hyperbolas between the most primitive SRP

compositions (Hughes et al., 2002) and selected Leeman et al., (1985) xenoliths and

Albion range rocks are graphed in Figure 40. The hyperbolas show that in order to

achieve the isotope ratios in CDF rhyolites, between 30 and 45 percent bulk assimilation

of crustal component is required if the most primitive SRP magma is used as a parent. If

the most primitive BVF basalt is used as the parent, 20 to 35% crustal component is

required (Ford and McCurry, 2003). The mixing hyperbolas indicate that more crustal

component must be added to the magma to form the isotopic signature of the parent of

the BVF rhyolites than the parent of the QESRP domes (prior to the bulk of fractional

crystallization).

Figure 40 (overleaf) Simple one-stage mixing models using isotope ratios of the most primitive basalt from the SRP (Hughes et al., 2002) and crustal xenoliths from Leeman et al., 1985 (see Table 9). Each tick mark represents an additional five percent bulk assimilation of crustal rocks to the hypothetical basaltic parent magma. For example, the first tick mark along mixing hyperbola COM SI-1 (located on the left hand side of the graph) represents five percent bulk addition of xenolith COM SI-1 to the parent magma, the second tick mark represents ten percent xenolith assimilation and so on. Pickett, 2004, established an isotopic and chemical link between the BVF basalts and those of the SRP and indeed, the most primitive BVF sample (Table 7) plots in the SRP field. Additional evidence that links the two basaltic fields is a contiguous zone of inferred partial melt at the crust – mantle boundary (Peng and Humphreys, 1998 – Figure 3). Quaternary rhyolites of the Eastern Snake River Plain (e.g. Big Southern Butte, East Butte and the silicic compositions of Cedar Butte) plot in or very near the SRP basalt field and contain less than five percent crustal component, perhaps much less depending on the starting parent basalt isotopic ratios (McCurry et al., 1999). The CDF data points (one point with large Nd isotope errors is not represented here – see Table 7) lie along lines represented by crustal xenoliths COM 70-40 and SK-73-68X. Between 30 and 45 percent assimilation of custal rocks with these isotope signatures into the most primitive SRP basalt is required to produce the isotope signatures in the CDF rhyolites while only 20 to 35 percent crustal assimilation is needed if the parent magma has the isotopic composition of the most primitive BVF basalt. The one isotope point from the NDF rhyolites (Table 7) requires approximately five percent more crustal assimilation than the CDF rhyolites.

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103

The curves also indicate that the chemically evolved basalts in the BVF need to

have assimilated some crust to achieve their isotopic signatures. To produce the isotopic

composition of most evolved basalts (KEP-027) from the most primitive (KEP-004)

requires 6 to 18 percent bulk assimilation, depending on the xenolith (Ford et al., 2004).

While this model can reproduce the isotopic signature of the BVF rhyolites, the resultant

hybrid composition would be andesitic and requires fractional crystallization to produce

the chemical signature of the BFV rhyolites. This bulk assimilation model will be

contrasted against the EC-AFC model.

EC-AFC modeling (Spera and Bohrson, 2001; Bohrson and Spera, 2001) allows

for simultaneous assimilation and fractional crystallization of a magma and incorporates

fractional melting of country rock, the latent heats of crystallization and heat transfer to

country rocks using an EXCEL spreadsheet form. The theoretical basis for an extended

model incorporating magma recharge (EC-RAFC - Spera and Bohrson, 2002; Bohrson

and Spera, 2003) has been published, but was not yet included in the EXCEL macro,

therefore I used the program without magma recharge (EC-AFC).

Magma recharge does not affect the late-stage evolution of the magma chamber.

Petrographic evidence does indicate the addition of olivine tholeiite-like basaltic magma

to the rhyolitic chamber (i.e. magmatic recharge) with at least some localized

hybridization (Figure 27) just prior to eruption. Field work indicates that such enclaves

are rare and trace element calculations show that the rhyolite to basalt ratio must be at

least 100:1 (i.e. less than one percent basalt composition magma added and mixed with

the rhyolitic magma). The best example of this is Sr. Even if the rhyolitic magma started

104

with absolutely no Sr, an unlikely scenario, an addition of one percent typical BVF basalt

(Pickett, 2004 at 350 ppm) would raise the Sr content in the magma to 3.5 ppm, higher

than the average for the CDF rhyolites. Magmatic recharge may or may not have

happened at earlier stages in the magma chamber evolution but the recharge that certainly

did occur shortly before the eruption, as evidenced by the enclaves, was very minor and

can be ignored for modeling purposes.

A number of parameters for the EC-AFC model are briefly detailed below. Each

parameter is adjusted to best represent the local magma chamber conditions. For

example, the assimilant trace element distribution coefficient (Da) for Sr can be given

either typical upper crustal or lower crustal values, depending upon where the user thinks

assimilation is occurring.

Thermal parameters:

- (Tlm) magma liquidus temperature: can be estimated using MELTS runs or by some

other means. This parameter helps to constrain the depth of the system.

- (Tm0) magma initial temperature: often the same as Tlm.

- (Tla) assimilant liquidus temperature: estimated based on type of assimilant.

- (Ta0) assimilant initial temperature: Based on a number of factors including depth of

assimilation, the geotherm of the area and local conditions (i.e. the possibility that

recent basaltic intrusions have increased the local country rock temperature).

- (Ts) assimilant solidus temperature: an estimate that must be hotter than the Ta0.

- (Teq) equilibration temperature: With the above thermal input, the user has the option

of choosing an equilibration temperature from a pull-down menu. Spera and

Bohrson (2001) suggest this temperature be set below the eruptive temperature of

105

the most evolved rocks in a volcanic system that contains a spectrum of

compositions. This increases mass of country rock affected by the heat from the

magma body. If Teq < Ts, no anatexis will take place.

- (∆T) normalized temperature change, set to -0.005 for all runs in this study.

Geochemical parameters:

This includes magma elemental concentrations (bulk rock) of Sr and Nd as well as the

87Sr/86Sr and 143Nd/144Nd isotopic ratios and the trace element (i) bulk distribution

coefficients for the magma (Dmi) and assimilant (Dai). Maximum and minimum bulk

distribution coefficients for the basaltic magmas of the BVF were calculated using the

equation: Ai

Ai KdWD *∑=

where W is the weight fraction of phase A, and i is the element of interest. Resultant Dmi

values are given in Table 10 and were calculated using phenocryst crystallization

percentages calculated by Pickett (2004) and approximate Kd’s from the Geochemical

Earth Reference Model (GERM) (http://earthref.org/) for olivine and plagioclase in

basaltic rocks and specific gravities for these phases.

Table 10: General parameters used in EC-AFC modeling for the basalts of the BVF. The Tlm corresponds to a depth of approximately 22 km and the Ta0 is higher than the geothermal gradient to indicate local country rock heating by basalts flooding the region over time. The Di’s for the magma were figured using the method stated above. Starting Sr and Nd isotopic and trace element data for KEP-004 are given in Table 7. Linear melting functions were used for all runs. Tlm Tm0 Tla Ta0 Ts Teq

value DmSr min.

DmSr max

DmNd min

DmNd max

1300 1300 1100 950 1000 minimum 1.07 1.45 0.029 0.036

Other parameters:

106

Other options include assigning different crystallization or fusion enthalpies,

changing specific heats of the magma and assimilant, assigning linear or user-defined

non-linear melting functions and including additional isotope and trace elements in the

modeling. None of these parameters were altered for these runs. There are also various

ways to change the output format.

The first step in using EC-AFC model was to try to produce the most evolved

basalt in the BVF (KEP-027) from the most primitive basalt in the BVF (KEP-004).

Bohrson and Spera (2001) suggest using DaSr values of 1.5 if in the upper crust, 0.05 for

Sr in the lower crust, and 0.25 for Nd in either the upper or lower crust. Runs using

lower crustal values of Dai are not reported as all crustal xenoliths used in modeling have

upper crustal isotopic signatures. Of the possible assimilants given in Table 9, if upper

crust Dai values are used, only xenolith SM-2G has a chance of elevating the Sr content

back to 350 ppm in the evolved basalts after some fractional crystallization of plagioclase

from the most primitive basaltic composition (Section 4.5 and Pickett, 2004). While Nd

and Sr composition data can not be simultaneously modeled with any possible assimilant

or Dmi values in the range given in Table 10 and upper crustal Dai values, the isotopic

data can be modeled and indicates crustal assimilants in the 5 to 12 percent range (Figure

41), slightly less but similar to the bulk assimilation model discussed earlier.

Modeled Nd concentrations can reach expected levels for those assimilants with

high concentrations of Nd (e.g. SK-73-68X, SM-DM-103, YAG-800) however the Sr

concentrations in these assimilants is too low and thus as soon as assimilation begins, the

modeled Sr content drops rapidly. Using the minimum DmSr (1.07), the Sr content just

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109

during the fractional crystallization, before assimilation begins, drops about 25 ppm.

This is represented by approximately 58 percent of the magma chamber being

crystallized, before the country rock begins to assimilate, at the thermal parameters giver

in Table 10. Thus, if the starting value for the most primitive magma was increased to

375 ppm, a value that is certainly plausible, using compositions from Pickett (2004) and

with the expected low percentages of assimilation, only the assimilants with the lowest Sr

contents and/or the least evolved Sr-isotope values (e.g. COM-SI-1, SK-73-68X) fail to

model the expected Sr content. A similar statement is true for Nd concentrations. If the

stating Nd is increased by three to six ppm, the model can more accurately predict the

expected Nd concentrations and assimilation percentages range from 4 to 12 percent

(Figure 42).

Adjustments to the poorly constrained Ta0 (assimilant initial temperature) can

change the Sr and Nd concentrations. A lower value of Ta0 (colder crust) results in

assimilation being retarded and allows for more fractional crystallization before

assimilation. Higher values of Ta0 result in assimilation happening earlier in the

evolving magma chamber. A number of runs indicate that a value of 930oC (slightly less

than was used in the above work) yields the best results using minimum Sr and Nd Dmi

values with xenolith SM-2G (the high Sr xenolith). While Sr concentrations modeled

with this run are still slightly low (330 ppm), the isotope (87Sr/86Sr = 0.70871 and

143Nd/144Nd = 0.51224) Nd results (80 ppm – compare to KEP-027, Table 7) nearly

mimic the evolved BVF basalt values with 6.2 percent assimilation and approximately 66

percent crystallization. Pickett (2004) suggests that between 64 and 72 percent

crystallization is required to model the La and Th values in the basalts and MELTS runs

110

also indicate 70 percent crystallization is required to obtain the proper MgO

concentration. Neither of Pickett’s calculations incorporated assimilant geochemistry and

Hughes et al. (2002b) suggest that this level of crystallization results in rocks that are

more differentiated that what is seen in the BVF basalts.

The overall sensitivity of the model for the BVF basalts is more dependent on the

concentrations of the analytes and Ta0 values than on the expected variations in Dmi

values. For example, a change in the DmNd value within the limits given above barely

changed the resultant Nd concentrations. A minimum DmNd value of 0.001 was tried and

resulted in slightly higher Nd concentrations at similar Sr concentrations but again, the

change was not enough to correctly model the Nd concentrations (using the temperature

constraints from Table 10). Using the maximum value for DmSr (Table 10) only widened

the gap between the observed data and the modeled data and given that there is no trend

between SiO2 and Sr in the basalts (i.e. no decline in Sr as the basalt increases in SiO2 as

a result of assimilation and fractional crystallization), a value closer to 1 is more

appropriate for DmSr.

Bohrson and Spera (2001) give upper crustal values of DaSr = 1.5 and DaNd =

0.25. Changes in DaSr to values less than one could increase the Sr content of the magma

chamber after assimilation begins for those assimilants with low Sr concentrations (i.e.

less than the starting magma). Such low bulk distribution coefficients for upper crustal

assimilant are unwarranted given the modal analysis of Snake River Plain xenoliths

(Matty, 1984; Leeman et al., 1985). All of their xenoliths have modal plagioclase plus

alkali feldspar plus interstitial glass greater than 50 percent. Many of the xenoliths have

more than 40 percent modal plagioclase plus alkali feldspar which would yield DaSr

111

values greater than 1.2 (for Sr Kdplagioclase = 3 and Kdalkali feldspar = 4). Additional evidence

that DaSr values should be greater than unity is electron microprobe analysis of the

interstitial glass in xenolith DM-103 reported by Matty (1984). The bulk rock Sr values

are 310 ppm while the interstitial glass has 85 ppm indicating Sr prefers the crystalline

phases over the melt. Other typical “feldspar compatible” elements such as Ba and Eu

also have lower concentrations in the glass than in the bulk rock.

Trying to model the rhyolites from the most evolved basalt (KEP-027) is

somewhat more difficult for a number of reasons. First, the basalt sample in question has

no phenocrysts, as does another sample with similar chemistry (Pickett, 2004) and thus it

is not clear what phases are precipitating in the evolved basalts. Pickett’s MELTS runs

indicate that two types of clino-pyroxene and ortho-pyroxene, in addition to spinel,

olivine and plagioclase fractionate at 5 kbar runs. There is no petrographic evidence in

any of the BVF basalts for pyroxene phenocryst development, and pyroxene is very rare

in the similar SRP rocks as well (Hughes et al., 2002b). With no phenocrysts present in

the rocks and inconclusive MELTS results indicating fractionating assemblages that do

not exist in the rocks, the Dmi values can not be accurately calculated.

A second problem is the great span in chemistry between the starting composition

(KEP-027 at 51.5 % SiO2) and the BVF rhyolites (76.7 % SiO2) with no surface rock

samples from this composition gap. It is unclear when many phases, such as alkali

feldspars, will start to form and significantly change the Dmi values. While Cedar Butte

from the SRP spans this compositional gap, it is a poor analog because it’s silicic rocks

do not contain the same phase assemblage as the BVF rhyolites and the most chemically

112

evolved rocks (highest SiO2) have isotopes that are more primitive than KEP-027 from

the BVF (McCurry et al., 1999). Additionally, most topaz rhyolites come from basalt-

rhyolite bimodal fields as well (e.g. Thomas Range, Spor Mountain, Wah Wah

Mountains, Sheep Creek Mountains, and others) and while topaz rhyolites have similar

phases (Christiansen et al., 1986), they also lack the intermediate rocks required to show

the timing for phase development.

The problems associated with the calculation of the magma bulk distribution

coefficients are accentuated by the high volatile (fluorine and chlorine) percentages in the

BVF rhyolites. Topaz rhyolites with similar bulk, trace, and volatile chemistries are the

only rhyolites in a suite of compositions that do not fit models designed to calculate Kd’s

for a number of elements (Ren et al., 2003) in plagioclase. Ren et al. (2003) go on to

state that high F and Cl values, “…might inhibit Sr from entering plagioclase.” His study

indicates plagioclase KdSr values of between 2 and 3 for topaz rhyolites with similar

plagioclase An contents, slightly more CaO and slightly less SiO2 as compared to BVF

rhyolites.

Ren’s (2004) experiments with sanidine Kdi values also indicate that topaz

rhyolites are harder to model than other rhyolites but indicate that KdSr is inversely

proportional to Al2O3 content and increases slightly with SiO2. The BVF rhyolites have

slightly higher SiO2 and lower Al2O3 than the topaz rhyolites in Ren’s (2004) study

indicating that sanidine KdSr is about 4 for the BVF rhyolites, slightly higher than the

topaz rhyolites sited Ren’s work.

One final issue is the low percentage of phenocrysts in the BVF rhyolites.

Calculating relative proportions of the fractionating phases where phenocrysts make up

113

less than ten percent of the rock introduces large errors in calculating Dmi values

especially when trying to account for uncommon trace phases with very high Kd’s (i.e.

Nd Kdallanite ≈ 1600). The unknown starting assemblage in the BVF evolved basalts,

unknown timing of the beginning phase fractionation (e.g. sanidine, zircon, allanite,

hornblende, biotite), changes in Kdsan and Kdplag with changes in bulk chemistry as

fractionation continues (Ren et al., 2003; Ren, 2004), and poor constraints on the relative

proportions of trace phases with high Kd’s for Nd all conspire to make calculation of Dmi

values difficult. Even with all the uncertainty described above, it is unlikely that the

DmSr value would be greater than three in the BVF magma.

Assimilation of crustal rocks must occur earlier in the magma evolution rater than

later because if any assimilation occurs after large amounts of fractional crystallization,

the small volume of resultant magma is changed enormously by the additions of even

small volumes of partially melted assimilant. For example, using evolved basalt

composition KEP-027 and an estimated DmSr of 3 (indicative of adding alkali feldspars at

some point in the fractionation process) to drop the Sr concentration in the magma below

5 ppm requires 89% crystallization of the magma body. Addition of even one percent

COM-SI-1 assimilant, one of the most primitive in Sr isotopic ratios and lowest

concentration of Sr of any of the assimilants, results in magma isotopic ratios that are too

high using upper crustal DaSr values. Other assimilants that are higher in Sr

concentration also increase magma Sr contents to unreasonable levels. Increasing DmSr

above three requires less crystallization of the magma chamber to achieve the same

magma Sr concentration and thus decreases the affects of small amounts of assimilant on

114

the overall isotopic ratios of the magma. Values of DmSr above 3 seem unwarranted

based on the work by Ren et al. (2003) and Ren (2004) which place the Sr Kdsan at

approximately 4 and the final phenocryst assemblage is only about 25 percent (volume)

sanidine based on petrography, while intermediate (un-erupted) compositions may

contain even less sanidine.

If assimilation occurs early in the evolution of the magma chamber, then the total

assimilation required using EC-AFC modeling to get from the most primitive basalt to

the rhyolite isotopic signatures is between 9 and 18 percent, less than the 20 to 35 percent

suggested by bulk assimilation (Figure 40). The percentage of assimilation to produce

the isotopic signature in the rhyolites is only slightly more than required to produce the

evolved basalts. To model the Sr and Nd concentrations, the most primitive magma must

undergo 60 to 70 percent fractional crystallization and the evolved basalt must undergo

an additional 80 to 90 percent crystallization (a total of 92 to 97 percent total fractional

crystallization). Production of high (~50 ppm) concentrations of Th in the rhyolite

requires at least 98 percent fractional crystallization of the most primitive parent and

greater than 95 percent from the more evolved basalts. These fractionation percentages

are higher than those modeled for Sr and Nd concentrations.

In summary, I was unable to produce the rhyolite phase assemblage and chemistry

from the basalts with MELTS models (Ghiorso and Sack, 1994), perhaps because the

model is not well calibrated for the unusual topaz rhyolite chemistry or fractionation at

such low temperatures. Bulk assimilation is a good initial isotopic model but over

estimates the amount of assimilant required. EC-AFC models (Spera and Bohrson, 2001;

Bohrson and Spera, 2001) indicate that assimilation must have occurred early in the

115

evolution of the magma chamber and totaled between 9 and 18 percent. Slight changes to

the original starting composition for Nd and Sr and minor changes in Ta0 can accurately

model the resultant Nd and Sr concentration and isotopes in the basaltic rocks, not

reasonable changes in Dmi values. The model also indicates that the most primitive

basalt must be crystallized between 60 and 70 percent to produce the evolved basalt,

similar to results discussed by Pickett, 2004 to model some trace elements. This evolved

basalt then requires an additional 80 to 90 percent crystallization to reach proper rhyolite

Sr concentrations. Recharge to the system by basaltic magma was not modeled and may

take place early in the evolution of the magma chamber but the recharge that did happen

just prior to eruption (as evidenced by the mafic magmatic enclaves) was very minor.

Section 4.8 NDF Petrogenesis and Regional Implications:

This section first examines some of the similarities and possible differences

between the petrogenesis of the CDF and the NDF within the BVF. I then briefly discuss

the implications and possible causes for the differences between the QESRP and BVF

rhyolites.

All three domes from the CDF are chemically and petrographically identical as

are all three domes from the NFD. The two dome sets are similar in many respects (i.e.

major element chemistry, phenocryst assemblage, the presence of basaltic magmatic

enclaves, emplacement in a basalt-rhyolite bimodal field) but do contain the following

differences: 1. the NDF is ~1.4 million years older than the CDF, 2. the NDF has

evidence of granophyric texture (Figure 24), 3. possible accessory phase differences, 4.

differing enrichments in trace element compositions (Table 2, Table 5, Figure 39) and 5.

116

slightly different isotope ratios (Table 7). Some of these differences may result in slight

changes to the petrogenetic model developed for the CDF.

Granophyic texture is present in silicic glomerocrysts in the NDF but not the CDF

rhyolite. This texture forms as a result of rapid crystallization of alkali feldspar and

quartz due to a rapid loss of volatiles or depressurization (change in PH2O) and occurs at

depths less than ~10 km (Mason, 1985 referenced in Wilson, 1989). Granophyric texture

is also found in East Butte of the SRP in plagioclase crystals, not in glomerocrysts (R.

Ganske, personal communication, 2005) again indicating magma chamber residence at

less than 10 km. This indicates that the NDF magma chamber may have resided at more

shallow depths than the CDF. Conversely, the presence of this texture may indicate

higher volatile partial pressures in the NDF magma chamber. The exact consequence of

finding this texture in a glomerocrysts is unknown.

The heavy mineral separates EMP mount for the NDF is dominated with zircon

and although allanite or thorite were not evident, it is possible these phases exist (no EDS

scans were performed on this sample nor has the probe mount been made into a thin

section). Although there is two to three times as much heavy separates produced from a

nearly identical amount of rock processed, it is unclear if the NDF sample contained a

higher percentage of phenocryst. While zircons were not represented in the point count

data on the thin sections examined, they appear to be more common in the NDF rhyolite

slides. This may indicate greater amounts of zircon fractionation resulting in lower

concentrations of Zr, Hf, Ta, and U (Figure 39, Figure 32; Table 5), all with high Kdzircon

values. More work is needed to assess the affects of other accessory phases on the trace

element compositions (i.e. the slightly higher La to Yb ratio for the NDF). While there

117

are some differences in the trace elements, it is important to point out that both dome sets

have highly elevated “incompatible” elements (i.e. Rb and Cs) as compared to other types

of rhyolite, other than topaz rhyolites, and the formation of both requires large amounts

of fractional crystallization.

The NDF has slightly different isotope ratios than the CDF indicating

approximately 5 percent more crustal assimilation in the NDF (Figure 40). One possible

hypothesis for this additional assimilation of crustal component in the BVF is that the

middle crust under the NDF was less refractory and therefore easier to melt than the

middle crust under the CDF because the crust had not been receiving basaltic injections

for as long (i.e. lower the Tla or Ts values). Another hypothesis is that the parent basalt

had a different isotopic ratio although the one isotope point from the Willow Creek Lava

Field (Table 7) is similar to that of the most primitive BVF sample (KEP-004) used in the

assimilant calculations. More complete isotope work on the basalts of the BVF, which is

forthcoming, will elucidate this question.

The ESRP crust has received more voluminous basalt injections over a longer

period of time. Additionally, the voluminous Tertiary rhyolites have stripped the crust

under the ESRP of mobile LIL elements. The well defined and refractory mid-crustal sill

(Peng and Humphreys, 1998; Figure 3) is the position of restite material from the partial

melts that produced the Tertiary rhyolites and products of fractional crystallization of the

more recent basalts (McCurry et al., 2002). Thus, the crust under the SRP maybe more

difficult to assimilate and contain lower concentrations of the LIL elements as compared

to the crust under the BVF.

118

Chapter 5 CONCLUSIONS

Both sets of spatially separated rhyolite domes in the BVF are chemically and

petrographically homogeneous. This, coupled with the alignment of the dome sets and

associated features (i.e. craters in the CDF) indicates that the each set is likely connected

to an intrusive dike system. The domes in the CDF were produced after local basalt

emplacement and faulting although other faults or basalt flows located away from the

domes might have occurred after dome formation. The CDF eruptions began with vent-

clearing, hydrovolcanic tephra production before the onset of simultaneous tephra

production and largely endogenous dome growth. The higher elevations between China

Cap and North Cone in the CDF are caused by tumescence above the intruding dike with

the craters (e.g. Gronewall and Burchett Lakes) forming as a result of extensional faulting

above the dike, collapse after magma withdrawal from the dike, or a combination of

these. I was not able to find any post emplacement vapor phase topaz mineralization

(Dayvault et al., 1984).

The amphibole geobarometry indicates the CDF magma equilibrated at ~3.5 kbars

which corresponds to depths or 13 ± 0.4 km, depending on the density of the upper crust.

This depth roughly corresponds to the position of the mid-crustal sill under the adjacent

ESRP (Peng and Humphreys, 1998; Figure 3). The temperature of the magma is tightly

constrained at 758o ± 10o C and the log fO2 is -14.5 units, calculated by QUILF (Andersen

et al., 1993). All phenocryst phases, except for quartz, in the CDF are in textural

equilibrium with the magma. The strongly embayed quartz crystals are likely the result

of a drop in volatile partial pressures which can destabilize quartz (Hibbard, 1995).

119

Previous models for the formation of topaz rhyolites invoke the partial melting of

lower crust (Christiansen et al., 1986, Orozco-Esquivel et al., 2002). These models are

rejected on the basis that unlikely small portions of multiple, sequential partial melts are

required to produce the low Sr concentrations (less than 3 ppm) and correspondingly high

Rb/Sr ratios (over 200), even with incongruent, non-modal melting. Additionally, low

εNd values in the Archean crust (Zindler and Hart, 1986, Faure, 2001) under the BVF

preclude partial melting of either the upper or lower crust or a combination of the two as

a principle mechanism for forming the CDF rhyolites.

Assimilation and fractional crystallization (AFC) evolution of the basalts is the

preferred model for the formation of the CDF rhyolites. This model has been used in the

adjacent ESRP province to model the development of Quaternary rhyolite domes (e.g.

Big Southern Butte, Cedar Butte, East Butte) from basalt by McCurry et al., (1999) and

rhyolite compositions have been produced experimentally by fractionally crystallizing

SRP olivine tholeiites (H. Nekvasil, personal communication, 2004). E. Christiansen

(personal communication, 2004) is also working on a new model utilizing AFC for topaz

rhyolites in the Great Basin.

The BVF and ESRP are linked by a geophysically anomalous zone at the crust –

mantle boundary inferred to be partial melt (Peng and Humphreys, 1998; Figure 3).

Also, the basalts, or parent material, of the bimodal volcanic ESPR are geochemically

and isotopically similar to the basalts of the BVF (Pickett, 2004). Fractionation of the

rhyolite phenocryst phases can produce the trace element patterns of the rhyolites from

the basalts although required percentages were not calculated. Even the low

concentrations of Sr and Eu in the BVF rhyolites can be modeled with extensive

120

fractional crystallization (Halliday, et al., 1991). High concentrations of Th require 95

percent fractional crystallization of the evolved basaltic parent. I was unable to model

the phenocryst assemblages in either the evolved BVF basalt or the rhyolite with MELTS

models (Ghirso and Sach, 1994) and thus the timing of phase development or loss in the

magma is unknown. MELTS models did produce a rhyolitic magma but not of the

compositions seen in the BVF.

About 20 to 30 percent bulk assimilation of upper crust is required to form the

isotopic signatures in the BVF rhyolites when starting with the most primitive BVF basalt

although additional magma processing (e.g. fractional crystallization) is required to create

a “rhyolitic” composition. When Energy-Constrained Assimilation Fractional

Crystallization models are used (Spera and Bohrson, 2001, Bohrson and Spera, 2001), 4

to 12 percent upper crustal assimilation coupled with 60 to 70 percent fractional

crystallization is required to form the evolved basalts. A total 9 to 18 percent upper

crustal assimilation with an additional 80 to 90 percent fractional crystallization is

required to form the rhyolites from the most primitive BVF basalt. The percentage of

assimilant is largely dependent upon the stating xenolith composition and how close the

country rocks (assimilants) are to their solidus temperature. Slight and acceptable

changes in the model parameters can model the Sr and Nd elemental concentrations with

little effect on the total amount of assimilant required to model the isotopes. All of the

assimilation occurred early in the evolution of the magma.

Magma recharge into the system was not modeled and may present a problem for

future workers. The recharge event that did occur just prior to the eruption, as evidenced

by basaltic magmatic enclaves, was very minor and did not affect significantly affect the

121

element concentrations or isotopic ratios in the rhyolite. Although there is no evidence to

suggest more substantial magma recharge earlier in the evolution of the magma chamber,

it is possible.

122

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Appendix 1 Electron Microprobe Methods: Crystals LiF PET TAP LDE1 LDE2 LDEB Specrometer 1: (1) PET (1) TAP (1) LDE1 (1) LDEB Specrometer 2: (2) LiF (2) PET Specrometer 3: (3) TAP (3) LDE2 Specrometer 4: (4) LiF (4) PET Specrometer 5: (5) LDE1 (5) LDE2

Feldspar crystal count time probe diam STD (seconds) (microns) SiO2 Albite (1) TAP 30 10 Al2O3 Kyanite (1) TAP 30 10 FeO* Almandine (2) LiF 30 10 MgO Diopside (3) TAP 30 10 CaO Wollastonite (2) PET 30 10 Na2O Albite (3) TAP 30 10 K2O Orthoclase (4) PET 30 10 SrO Celestite (4) PET 30 10

Melt Inclusions* crystal count time probe diam STD (seconds) (microns) SiO2 Diopside (1) TAP 30 10 TiO2 Rutile (4) LiF 30 10 Al2O3 Almandine (3) TAP 30 10 FeO* Hematite (4) LiF 30 10 MnO Spessartite (2) LiF 30 10 MgO Diopside (3) TAP 30 10 CaO Diopside (1) PET 30 10 Na2O Albite (3) TAP 30 10 K2O Orthoclase (4) PET 40 20 P2O5 Apatite (1) TAP 40 20 S Anhydrite (2) PET 40 20 Cl KCl (2) PET 40 20 F Fluorite (5) LDE1 60 20 * also for biotite and hornblende

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Appendix 1: concluded Olivine crystal count time probe diam STD (seconds) (microns) SiO2 Diopside (1) TAP 30 10 FeO* Almandine (2) LiF 30 10 MnO Spessartite (2) LiF 30 10 MgO Olivine (3) TAP 30 10 CaO Diopside (4) PET 30 10 NiO Ni-metal (4) LiF 30 10

Pyroxene crystal count time probe diam STD (seconds) (microns) SiO2 Diopside (1) TAP 30 10 TiO2 Rutile (2) LiF 30 10 Al2O3 Almandine (1) TAP 30 10 Cr2O3 Cr-metal (2) LiF 30 10 FeO* Almandine (2) LiF 30 10 NiO Ni-metal (4) LiF 30 10 MnO Spessartine (4) LiF 30 10 MgO Diopside (3) TAP 30 10 CaO Diopside (4) PET 30 10 Na2O Albite (3) TAP 30 10

Oxides crystal count time probe diam STD (seconds) (microns) SiO2 Almandine (3) TAP 30 10 TiO2 Rutile (2) LiF 30 10 Al2O3 Almandine (1) TAP 30 10 Cr2O3 Cr-metal (2) LiF 30 10 FeO* Hematite (4) LiF 30 10 MnO Spessartine (4) LiF 30 10 NiO Ni-metal (2) LiF 30 10 ZnO Sphalerite (4) LiF 30 10 MgO Periclase (3) TAP 30 10 CaO Diopside (1) PET 30 10