24. LITHOLOGY, PETROGRAPHY, AND MINERALOGY OF BASALTS … · 2007. 5. 7. · B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE Heat Flow Age Magnetic Anomaly Distance 90 80 70 60 50 40

24. LITHOLOGY, PETROGRAPHY, AND MINERALOGY OF BASALTS FROM DSDP SITES 482,483, 484, AND 485 AT THE MOUTH OF THE GULF OF CALIFORNIA1

Brendon J. Griffin, Electron Optical Center, University of Adelaide, Box 498, G.P.O. Adelaide, South Australia 5001and

Rolf D. Neuser and Hans-Ulrich Schmincke, Institut für Mineralogie, Ruhr-Universitàt, 4630 Bochum,Federal Republic of Germany

ABSTRACT

The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flowsranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are in-terlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of thesection. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered aremore phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Pla-gioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclasemegacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletalmicrophenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and composi-tions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystalsgrown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particu-lar, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Feg in the cores of phenocrysts to Ca^óMg^Fef< in the groundmass; and olivine ranges from Fo86 to Fo8,.

The wide range in mineral compositions, together with evidence of disequilibrium based on textures and compari-sons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal frac-tionation and magma mixing.

INTRODUCTION

During Leg 65 of the Deep Sea Drilling Project(DSDP), six major holes were drilled at three principalsites (482, 483, and 485) at the mouth of the Gulf ofCalifornia (23 °N) (Figs. 1 and 2), recovering a total ofabout 420 meters of basalt (Figs. 3 and 4). The holeswere drilled in crust ranging in age from 0.5 m.y. (Site482) to 2.0 m.y. (Site 483) at distances ranging from 12km (Site 482) to 52 km (Site 483) from the spreadingaxis. One additional site (Site 484) was drilled in thenearby Tamayo Fracture Zone (Fig. 1). One of the mainpurposes of this leg was to study magma chamber evolu-tion and crustal construction processes at a ridge with amoderately high spreading rate (about 5 cm/y.) in ayoung ocean basin with a high sediment influx for com-parison with slow spreading ridges (2-3 cm/y.) of theAtlantic type with very low sedimentation rates. Thepresent chapter presents and briefly evaluates lithologic,petrographic, and mineral composition data for basaltsfrom these sites. Major and trace element data are dis-cussed in a companion paper (Flower et al., this volume)and volcaniclastic rocks and glass alteration are discussedby Schmincke (this volume). A fuller discussion of thedata will be presented elsewhere.

GENERAL LITHOLOGYTwo principal types of basalt were recovered in the Leg

65 drill sites: massive basalt (Plate 1, Figs. 1-4; Plate 2,

30° N -

25° N

¥s'an ' * V- JpDiego X

•

Pacific Oceπn

U.S. "*5?5

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A ^ Nj^Guaymas

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f Pacific Rise ^ ^

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A,f~~ Spreading axis and^ ^ transform faults

• Leg 65 Sites

\

Mexico

VEMazatlán

485 ̂ Nv te •:•TamayoX;:;; .*

WijSSTransform/j/.••.;

110°W

Lewis, B. T. R., Robinson, P., et al., Init. Repts. DSDP, 65: Washington (U.S. GovtPrinting Office).

Figure 1. Index map of the Gulf of California showing the main sitesdrilled during Leg 65.

Figs. 1-3) and pillow basalt (Plate 2, Fig. 4; Fig. 5; Table1). Most, if not all, of the massive basalts are believed tohave been emplaced as sheet flows rather than as sills.This interpretation is based on several lines of evidence,

527

B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE

Heat Flow

AgeMagneticAnomaly

D i s t a n c e90 80 70 60 50 40 30 20 10 0 10 20 30 40 50

I • I • I • I i I • i(km)

Figure 2. Cross section from Baja California through the East Pacific Rise showing main drill sites, sedi-ment basins (black), heat-flow values, and magnetic anomalies. (Simplified from Lewis, this volume.)

482C

0

10

20

•2 30

2 40

50

60

70

482D100 m-

D

Figure 3. Stratigraphic correlation of main lithologic and chemicalunits between three holes drilled at Site 482. (A, B, E, and F repre-sent chemical (or magma) types defined by Flower et al., thisvolume.)

including the following: (1) the presence of a distinctlyvesicular zone below the top of the massive basalts(Plate 2, Figs. 1-3); (2) the absence of intrusive relation-ships; (3) the absence of baked contacts (the minor in-duration observed in the sediments immediately overly-ing several of the massive basalt units is attributed tosubsequent diagenetic reactions near the sediment/ba-salt contact) (Plate 1, Figs. 1, 2); (4) the presence in onehole of a bedded hyaloclastite which chemically resem-bles the underlying massive basalt, suggesting derivationfrom the glassy crust of a surface flow (Schmincke, thisvolume); (5) the stratigraphic continuity over distancesof several hundred meters of flows which are verticallyseparated from other massive units by a few centimetersto a few meters of sediment (Fig. 3); (6) the imprint ofbroken vesicles in the sediments lying in contact with theunderlying basalt; and finally (7) the presence of ophi-mottled textures in the basalts themselves (Robinson etal., 1980).

Other lines of evidence, however, such as internal dik-ing in Unit 3, Hole 483 (Plate 1, Fig. 4) and chemical dif-ferences within Unit 5, Hole 485 (Flower et al., this vol-ume) suggest intrusive processes. In lithologic Unit 3

(Hole 483) the diking could have occurred as autointru-sion during the sagging of a thick, liquid-cored lava sheetafter it was emplaced on soft sediment. The chemical di-versity within Lithologic Unit 5 (Hole 485A), however,is probably the result of the intrusion of magma batchesof slightly differing chemistry. Thin stringers of sidero-melane extending into the sediment from the top ofLithologic Unit 8 in Hole 485A also suggest intrusion(Plate 1, Fig. 3). The sites drilled are thus intermediatebetween an environment with a high sedimentation rate,such as the Guaymas Basin, in which many if not all ofthe basalts are emplaced as intrusives (Curray, Moore,et al., in press) and one with a low sedimentation rate,such as the Mid-Atlantic Ridge, where nonintrusive sheetflows are common (e.g., Robinson et al., 1980). A fur-ther indirect but powerful argument for the interpreta-tion of the massive basalts as extrusives is the successionof flows observed in Holes 483 and 483B in which mas-sive flows of more evolved chemistry and mineralogyare underlain by pillow lavas of more mafic chemistryand higher phenocryst content (Flower et al., this vol-ume). The massive basalt flows are from 3 to 7 metersthick in the upper part of Holes 483 and 483B above thepillows, but are much thicker in Hole 485A where theaverage thickness is about 10 to 14 meters (excludingUnit 5, which is 26 m thick). We are not certain whetherthick sheet flows and intrusives grade into each otherwhere surface flows have burrowed into soft sediments.In any case, thick massive flows appear to dominateduring the late and probably waning stage of volcanism.They represent single extrusive events separated fromeach other by a significant hiatus in time. The sheetflows interbedded with pillow lavas in the lower part ofHoles 483 and 483B (Fig. 4) are thinner and morevesicular than the flows higher in the section and areseparated from each other and the overlying pillows bymuch thinner sediment layers. The pillow sequences arealso thin (about 15m). Although sheet flow and pillowunits could represent the distal edges of eruptive units,the similarity in the thickness of the units between Holes

528

110.0-

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O O α>O OOC

483

io 2 — "°

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JD W W

125.0-

150.0-

175.0-

200.0-204.5-

13

III14

15

ΠT16

17

18 III

19

— m20

21 lU

22

2 3 üJ•

24 m

26 li

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ITT \5•"/ \ v \ 41

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TST3

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1

2

3

4

5

b

7

—

—

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Sediments

483B

LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS

485A

cπ σ>

o o α> •t; .r _ç s

225.0-

250.0-

267.0

• = • = • s a>

153.5

175.0-

200.0-

225.0-

250.0

275.0-

300.0-

12

34

38

39

7-

Figure 4. Stratigraphic columns for Holes 483, 483B, and 485A showing core recovery, main igneous lithologic units, and posi-tion of samples studied. Extent of igneous units defined by logs (Hole 483) and drilling rates (Hole 485A) is greater thanrecovered portion.

Table 1. Comparison of characteristic features of massive and pillowbasalt flows drilled on Leg 65.

Basalt

Massive (sheet) flowsPillow (tube) flows

Thickness(m)

1.0-10.00.4-0.5

Pheno-crysts

(vol.%)

25-15

Contacts

FlatCurved

Thicknessof Glassy

Rinds(cm)

0.51-2

Fractures

StraightIrregular

483 and 483B, about 100 meters apart (Fig. 4), as well asbetween Holes 482B, C, and D where the holes areabout 200 meters apart (Fig. 3), suggests that the erup-tive units are relatively thin in this environment. Inother words, the height of sheet flow and pillow volca-noes during terminal activity at the mouth of the Gulf ofCalifornia appears to have been low.

One of the factors determining volcano height ap-pears to be spreading rate; volcano heights range from

529


about 250 meters in the FAMOUS area to 50 meters inthe Galapagos Rift (Ballard et al., 1979). Eruptive units,as well as eruptive cycles combining several petrologicallyrelated eruptive units, appear to be thicker in the Atlan-tic than in the Pacific, judging from the deepest holesdrilled on Legs 37 and 51-53 (Flower and Robinson,1979, 1981; Robinson et al., 1980).

Robinson et al. (1980) restrict the term "flow" to cool-ing units more than 3 meters thick. While such thick-nesses hold for flows in the upper part of Holes 482, 483,483B, and all of Hole 485A, several massive basalt unitsinterlayered with pillows in the lower part of Holes 483and 483B have been interpreted as flows even thoughthey are less than three meters thick. We have observedflows of similar thickness to be common in the Troodosextrusive section (Schmincke et al., in press).

The reason why submarine lavas erupt as pillow andsheet flows is not clear, but high eruptive rates arethought to be an important factor (Ballard, 1979; Rob-inson et al., 1980). The lateral continuity of individualflows over at least 200 meters at two sites (482 and 483)also suggests that these flows represent high eruptiverates.

PETROGRAPHYMETHODS

About 60 thin sections, representing all holes and all major basalttypes, were first examined qualitatively using a petrographic micro-scope. Twenty thin sections, chiefly of glassy to tachylitic basalts,were then studied quantitatively using the semi-automatic VIDEO-PLAN picture-analysis system, which allows precise measurement ofthe volume, size, and form of phenocryst phases. Two methods ofmeasuring crystal dimensions were employed in this study. In the firstmethod, thin sections were projected onto a digitizing tablet and thephenocryst perimeters were then traced with an electronic pen. In thesecond method, the light from a light-emitting diode (LED) attachedto an electronic cursor is projected into the measuring field of themicroscope. Thus the movements of the cursor on the digitizing tabletcan be followed while observing the thin section. The longest andshortest dimensions were then calculated for each crystal from the ma-jor (A) and minor (B) axes of the ellipse with the same moment of in-ertia as that of the measured grain. In each thin section, up to six areasabout 0.8 cm in diameter were measured, covering from 50% to 80%of the area.

Pillow basalts were selected for more detailed examination in thisstudy because they tend to be more phyric than massive basalts andbecause phenocrysts can be more easily identified in rocks with aglassy or fine-grained groundmass. The data from this analysis arereported in Table 2 and in Figures 5 through 9.

General Petrographic OverviewThe basalts drilled on Leg 65 show the entire range of

submarine basalt textures, from thick, fresh, glassy pil-low rinds (Plate 2, Fig. 4) and thin, mostly devitrifiedmargins of massive flows (Plate 1, Figs. 1, 2) to coarse-grained gabbros in the 10- to 20-meter-thick cooling unitsrecovered at Site 485. Most of the basalts are aphyric tosparsely phyric, but phyric basalts with 5% to 15% totalphenocrysts are common in the pillowed units of Holes483 and 483B (Table 2) and in the lowermost massiveflows in Hole 483. Like most ocean floor basalts, themost common phenocryst phase is plagioclase, whichgenerally composes from 60% to 90% of the phenocrystassemblage. Olivine is generally less abundant ( 3% in

volume), and clinopyroxene is usually sparse (< 1%) toabsent. Clinopyroxene is more common than olivine,however, in Cores 483B-22 through 25, where it occursprincipally in glomerophyric intergrowths with plagio-clase, indicating more advanced crystallization of themagma prior to eruption.

Vesicles are rare to absent in the chilled margins ex-amined but increase in size and abundance toward theinteriors of pillows and the tops of sheet flows, averag-ing less than 2% by volume. Segregation vesicles, on theother hand, are very common in both types of basalt(Plate 2, Figs. 1-3). These become increasingly filled to-ward the interiors of pillows and may be rimmed withplagioclase.

Pervasive alteration is generally restricted to the fill-ing of vesicles (Plate 2, Figs. 1-3) and fractures, and tothe replacement with smectite of groundmass glass andmost of the olivine in the more coarse-grained basalts.Higher-grade alteration occurs near a hydrothermal veinin Hole 482C and in the interior of several thick coolingunits at Site 485 (Morrison and Thompson, this volume).

Phenocrysts

PlagioclasePlagioclase occurs in four texturally and composi-

tionally distinct forms in the Leg 65 basalts. These con-sist of megacrysts, normal phenocrysts, micropheno-crysts, and microlites.

Plagioclase I (megacrysts): (Plate 3, Figs. 1,2). Theplagioclase megacrysts observed occur almost exclusive-ly in the phyric pillow basalts of Holes 483 and 483Bwhere they generally constitute less than 1 % of the rock.They are classified as megacrysts not because of theirabnormal size (54 mm) but because they are larger thanthe other plagioclase types, occur mostly as single crys-tals, are moderately to strongly rounded, may have coresfilled with, or delineated by, devitrified melt inclusions,and show complex, in part oscillatory zoning with oneor more major breaks in composition. The crystals varyin area from about 5 to 20 mm2 with a mean value of10.5 mm2 and have an aspect ratio, A/B, which is al'ways less than 3 (Figs. 6, 7).

Plagioclase II (normal phenocrysts): (Plate 3, Fig. 3).Normal phenocrysts constitute between 6% and 8% ofthe most phyric basalts and range in length from 0.1 to0.3 mm, with most averaging 0.7 mm. The phenocrystsvary in area from 0.3 to 10 mm2 with a mean of 1.5 mm2

and have a maximum aspect ratio of 8 (Figs. 6 and 7).The crystals are invariably euhedral and commonly dis-play synneusis twinning. Zoning, however, is not pro-nounced. The plagioclase in this group may occur inglomerocrysts with olivine but crystals intergrown withclinopyroxene are also included in this group, thoughthey are mostly sub- to anhedral. In some basalts, phe-nocrysts grade irregularly into microphenocrysts.

Plagioclase III (microphenocrysts or pre-emptionmicrolites): Plate 3, Fig. 4; Plate 4, Fig. 1). The plagio-clase crystals belonging to Group III are generally eu-hedral and range up to 1.2 mm in length, with most fall-ing between 0.2 and 0.25 mm in length. The crystals are

530

Table 2. Textures and phenocryst abundances in Leg 65 basalts.

Sample(interval in cm)

RockType

LithologicUnit

CoolingUnit

Chemicala

Type

Phenocrysts(vol. Vo, modal)

Glomerocrysts(vol.% modal)

Ol I Ol II Cpx I Cpx II PI I PI II PI III Total Ol-Pl Ol-Pl-Cpx Pl-Cpx VesiclesCounted (C);Estimated (E) Predominant Texture

Hole 482D

11-1, 24-28

Hole 483

21-1,49-5521-1, 57-6121-1, 78-8221-3, 6-921-2, 68-7222-2, 85-8822-2, 88-9322-2, 94-99

Hole 483B

20-2, 37-4220-2, 50-5920-2, 62-6522-2, 63-6722-2, 77-8225-2, 39-4527-1, 12-1830-3, 13-1632-1, 1-632-1, 27-3132-1, 44-5032-1, 90-94

Hole 485A

11-3, 57-6011-3, 116-12112-1, 10-1612-1, 76-8213-1, 110-11730-3, 38-4130-4, 8-1336-3, 65-6939-1, 26-32

PT

PTPCPBPBPTPCPCPB

SBSBSBPCPBSTSTPBPTPCPCPB

PTPC

PC?PC?scscsc

PB?SC

2626265858788394

104104104104

— 1.3 — — — 0.8 4.8 6.9

FFFFFF

FF

HHHJ

JKKLMMMM

AABCEIIK

L

0.20.30.8

1.01.00.70.20.5

0.30.3

4.0

1.02.01.51.42.0

_

_

0.3

1.20.90.40.80.50.21.20.6

0.50.7

0.51.0

1.82.03.02.02.23.21.11.6

3.32.5

1.2

0.60.2_

—

0.1

0.7

1.3

0.2

0.3

0.1

—

—

0.10.10.20.2—

0.2

0.1

—

_

3.02.74.03.01.00.40.40.4tr

_

—

tr

0.31.01.51.03.13.0

1.51.0

0.4

1.01.02.01.01.02.51.21.11.51.2

—

—

0.8

6.55.54.83.05.53.24.05.0

1.5

1.61.26.06.27.05.04.03.82.65.23.9

—

—

2.2

2.12.42.4

1.01.71.82.7

2.9

—

—

1.00.91.01.01.02.02.1

2.32.8

0.70.22.7

3.1

11.010.410.1

7.0

10.29.1

10.411.3

2.33.2

7.0

12.013.615.013.011.611.610.911.912.5

4.02.72.7

7.6

ddddrrrrrrrr

rr

rrdddd

d

0.1

mmmmdd

dd

dddd

mmmm

0.30.60.20.10.4

2.30.5

1.5

0.20.3

0.50.32.01.52.01.02.40.7

1.0

0.2

Vitrophyric-variolitic

C Variolitic with segregation vesiclesC Tachylitic with segregation vesiclesC Vitrophyric-tachyliticE Vitrophyric-varioliticC Vitrophyric-tachyliticC Tachylitic with filled segregation vesiclesC Tachylitic with open segregation vesiclesC Tachylitic with filled segregation vesicles

E IntergranularE IntergranularE Vitrophyric-intersertalE TachyliticC Tachylitic-varioliticE IntersertalE IntersertalE Vitrophyric-varioliticC Vitrophyric-varioliticC Intersertal with segregation vesiclesC TachyliticC Variolitic

C Vitrophyric-varioliticE IntersertalC Tachylitic-ophimottled

IntersertalIntersertalOphiticOphitic

C Tachylitic-vitrσphyricSubophitic

Note: PT = pillow top; PC = pillow center; PB = pillow bottom; ST = sheet flow top; SC = sheet flow center; SB = sheet flow bottom; d = Dominant type of glomerocryst; ma After Flower et al. (this volume).

Minor type of glomerocryst.


Olivine I (•)Ol iv ine II (•)0 1 2 3 4

50 F"~

Clinopyroxene I (•)Clinopyroxene II ( )

0 1 2 3

Megacrysts I (•)Phenocrysts 11 ( )

Microphenocrysts III (O)0 2 4 6 8

Vesicles0 2

0

,O


80

60

II 40

20

PI I (6)PI II (203)PI III (2303)

\

0.01 0.1 1

Phenocryst Area (mm2)

Figure 6. Relative frequency vs. cross-sectional area of phenocrystsbelonging to different generations of plagioclase found in Leg 65basalts. (PI I, megacrysts; PI II, phenocrysts; and PI III, microphe-nocrysts. Numbers in parentheses represent numbers of measure-ments.)

0.01 100.0

Figure 8. Aspect ratio vs. cross-sectional area of clinopyroxene crys-tals in Leg 65 basalts. (Note the relative abundance of small crys-tals with high length/width ratios. The low ratios shown forcrystals with small areas result from crystals with long axes in-clined to the plane of the thin section.)

PI l (7)Pi l l (203)PI III (2286)

6 9 12Aspect Ratio of Phenocrysts (A/B)

15

Figure 7. Relative frequency vs. aspect ratio of phenocrysts belongingto different generations of plagioclase found in Leg 65 basalts. (PII, megacrysts; PI II, phenocrysts; and PI III, microphenocrysts.The peaked distribution for PI I indicates that the megacrystsgenerally have low length/width ratios while the flatness of the PIIII curve reflects the high length/width ratios of acicular plagio-clase. The plagioclase phenocrysts (PI II) show an intermediatedistribution of aspect ratios. Numbers in parentheses representnumbers of measurements.)

ly rounded, and occur as rare, single crystals up to 12mm in length. The more common euhedral phenocrystsmay be present in clots with plagioclase and, more rare-ly, with clinopyroxene phenocrysts.

Olivine II (microphenocrysts): The crystals belongingto Group II are generally less than 2 mm long, showskeletal growth forms (Plate 3, Fig. 4; Plate 4, Fig, 1)

0.01 100.0

Figure 9. Aspect ratio vs. cross-sectional area of blivine crystals in Leg65 basalts.

and occur in glomerophyric clots with plagioclase or,more rarely, as single crystals.

Olivine III (groundmass olivine): Groundmass oliv-ine is common in the highly crystallized interiors of mas-sive basalts, but it is generally replaced by alterationproducts.

Plagioclase/Clinopyroxene Glomerophyric ClotsThree types of plagioclase/clinopyroxene clots with

gradations between all types are recognized in the Leg65 basalts. These include: (1) small clots, generally 1.0mm in length, of strained anhedral clinopyroxene andacicular plagioclase (Plate 4, Fig. 4); (2) intergrowths ofmoderately coarse-grained, subhedral to euhedral clino-

533


pyroxene and plagioclase (Plate 4, Fig. 3), and rare clotsof rounded clinopyroxene and plagioclase; and (3) sub-ophitic to ophitie clots of anhedral clinopyroxene andplagioclase ranging up to 5 mm in length. These are dis-tinct from the clinopyroxene/plagioclase clots that formin situ in the zone between the chilled margin and thefully crystallized interior of the thicker cooling units.

Petrographic RelationshipsA number of conclusions may be drawn from the ob-

servations that have been presented. For example, it isclear that plagioclase phenocrysts are much more abun-dant in the Leg 65 basalts than are olivine and clinopy-roxene phenocrysts but that a wide spectrum of assem-blages is observed, including plagioclase + olivine, pla-gioclase + clinopyroxene, plagioclase + olivine + clino-pyroxene, and olivine + spinel. It is also evident thateach of the major phenocryst phases may appear in sev-eral different forms, such as megacrysts, normal pheno-crysts, microphenocrysts, and microlites. Finally, it isapparent that the pillow units at Site 483 are distinctlymore porphyritic than the massive basalts but that intra-cooling-unit variation in phenocryst volumes and pro-portions within the pillow basalts are negligible.

The occurrence of different assemblages and amountsof phenocrysts is clear evidence that crystal fractiona-tion is likely to have played a major role in the evolutionof these magmas. However, the diversity in texturaltypes within a given rock indicates that single-stage,closed-system fractionation is not a realistic model fortheir generation. We interpret the phenocryst texturesand different types of clots in terms of several differentsites and dynamic conditions of crystallization. Of these,the cooling history appears to be the most importantcontrolling factor. Crystals grown at slow cooling ratesare larger and more equant (Figs. 7, 8), a physical rela-tionship that is accentuated by the relatively high degreeof resorption of these crystals occurring under disequi-librium conditions subsequent to their crystallization.Possible mechanisms for the resorption of early formedcrystals include either a decrease in pressure accompa-nied by contraction of the primary plagioclase field oran influx of hot magma into a more evolved, coolermagma and the selective concentration of plagioclase asa result of flotation (see also Flower et al., this volume).The last generation of microphenocrysts, on the otherhand, commonly displays a skeletal habit (Plate 4, Fig.1), suggesting crystallization under rapid cooling condi-tions high in the section.

Apart from the high-pressure(?) megacrysts, the ma-jority of phenocrysts are interpreted as having crystal-lized in the central magma chamber under the ridge. Theoccurrence of gabbroic clots indicates the disruptionand incorporation of slowly cooled parts of the chamberinto ascending magma. Although the phenocryst assem-blages, amounts, and textures reflect a wide range in thedegree of crystallization, the highly porphyritic varietiesencountered in several holes in the Atlantic were notfound, possibly indicating more rapid replenishment ofthe magma reservoirs.

The general lack of significant variation in pheno-cryst abundance within the pillows (Fig. 5) indicates theintrusion of well-mixed, homogeneous magmas. Evenwhere such variation is significant, as in the base of sev-eral of the massive sheet flows in Hole 483B, it is inter-preted as being the result of posteruptive crystal settling.

The more porphyritic nature of the pillows as a wholeis not unique to Site 483 but has also been observed inseveral holes in the Atlantic. A systematic relationship isalso suggested by the observation that thin sheet flowsof a more fractionated, but related chemical composi-tion underlie the pillow lavas (Flower et al., this vol-ume). In such eruptive cycles, the sheet flows possiblyrepresent the upper part of zoned magma chambers. Pil-low sequences have also been observed to overlie sheetflows in the Galapagos Rift zone (Ballard, 1979) and agradation toward more mafic composition is commonin the eruptive cycles observed in the Atlantic (Flowerand Robinson, 1979, 1981).

GEOCHEMISTRYSAMPLE SELECTION AND METHODS

After the petrographic studies had been completed, 15 representa-tive basalt samples were chosen for detailed microprobe studies ofmineral composition and three additional samples were chosen forglass analysis. About 600 analyses were obtained on these samples atthe Max-Planck Institute für Chemie in Mainz using a KEVEX 5100energy dispersive analytical system attached to an ARL-SEMQ micro-probe. An additional 80 analyses were obtained using an automatedCAMEBAX microprobe. Glass analyses were carried out using a de-focused beam. Particular attention was paid to compositional varia-tions within single crystals. Representative mineral and glass analysesare given in Tables 3 through 9, and the average mineral and glasscompositions are summarized for selected basalt samples in Table 10.

Mineral Chemistry

OlivineThe olivine phenocrysts and microphenocrysts ana-

lyzed from Holes 483 and 483B are chemically homoge-neous within each sample. The compositions range fromFo813 for the microphenocrysts examined in Section483-21-3 to Fo86 2 for the phenocrysts in Section 483B-30-3 (Table 3). These olivine crystals are neither in equi-librium (Roedder and Emslie, 1970) with the glass in thegroundmass (where present) (Table 10), nor with pos-sible "primary" magma compositions. This indicatesthat at the time of olivine crystallization, these lavas hadalready undergone some degree of fractionation.

ClinopyroxeneMost of the clinopyroxenes examined consist of augite

(Tables 4, 5). The more primitive phenocryst cores fall inthe endiopside field while the groundmass crystals rangefrom endiopside toward ferroaugite and subcalcic augitein composition (Tables 4, 5; Fig. 10). The phenocrystcore compositions are notably uniform in their majorelement chemistry (100 Mg/Mg + Fe2 += 84.86; Ca41Mg51Fe8); in accordance with their relatively primitivemajor element chemistry, they contain minor Cr2O3 (1.0-1.3%) and relatively little TiO2 (0.3%).

534


Table 3. Composition of selected olivine phenocrysts.

Component

Major oxide (wt.(

SiO2FeO«a

MnOMgOCaONiO

Total

483-21-3,6-9

39.2917.570.22

42.410.25n.a.

99.73

Cation proportions

SiFeMnMgCaNi

Total

Fo (mole


Table 4. Composition of selected clinopyroxene phenocrysts, Holes 483, 483B, and 485A.

Component

483-21-3,6-9

Major oxide (wt.%)

Siθ2TiO 2A1 2O 3FeO* a

MnOMgOCaONa 2 OC r 2 O 3

Total

53.930.321.467.740.13

20.0316.32

0.25

100.18

Cation proportions"

SiAlΣTiFeMnMgCaNaCrΣ

Total

M g N o .

19.5940.627

20.2210.0882.3520.039

10.8456.352

0.07219.747

39.968

82.2

Normative composition

NACJdCCRCTiCATsWoEnFs

0.000.000.720.881.90

30.1154.3911.99

483-21-3,6-9

51.660.993.777.22

17.1819.460.140.40

100.81

18.8331.621

20.4540.2722.201

2.3347.6010.0990.115

19.622

40.076

80.9

0.980.000.162.705.27

33.6546.3210.92

483B-20-2,16-21(rim)

53.690.212.344.83

18.0421.14

0.73

100.98

19.3730.994

20.3670.0581.456

9.7048.175

0.20919.601

39.968

86.9

0.000.002.020.583.40

38.0248.68

7.30

483B-20-2,37-42(core)

53.570.232.194.77

18.1320.58

0.170.74

100.38

19.4260.936

20.3620.0621.445

9.7977.9960.1210.213

19.635

39.997

87.1

1.210.000.920.623.60

37.4348.99

7.23

483B-20-2,37-42

53.570.441.289.560.20

20.5414.34

0.11

100.02

19.5600.552

20.1120.1202.9200.061

11.1775.609

0.03119.917

40.029

79.3

0.000.000.311.201.40

26.5155.7214.86


483B-20-2,37-42

50.021.152.81

11.900.14

16.1416.10

98.26

18.9811.256

20.2370.3293.7750.0969.1316.545

19.825

40.062

70.7

0.000.000.003.272.97

29.4045.3718.99

483B-20-2,50-59(core)

52.010,423.635.21

17.3219.990.141.30

100.01

18.9911.560

20.5510.1161.591

9.4247.8210.0970.375

19.189

39.98

85.6

0.970.002.791.165.26

34.6047.24

7.98

483B-20-250-59(rim)

50.960.933.938.39

16.2319.10

99.53

18.8821.715

20.5970.2602.598

8.9637.582

19.403

40.000

77.5

0.000.000.002.605.97

33.6244.8112.99

483B-20-2,50-59

52.960.372.556.08

17.9119.64

0.57

100.09

19.3181.098

20.4160.1021.854

9.7397.676

0.16319.534

39.950

84.0

0.000.001.641.033.67

35.4148.94

9.32

483B-20-2,50-59(core)

53.070.423.036.03

17.5020.32

0.65

101.03

19.2031.292

20.4950.1151.826

9.4407.880

0.18719.447

39.942

83.8

0.000.001.881.164.40

35.9147.47

9.18

483B-20-2,50-59(rim)

49.651.303.89

13.090.17

14.1117.550.30

100.05

18.6961.726

20.4220.3674.1230.0537.9197.0820.217

19.760

40.182

65.8

0.002.130.003.603.81

31.0738.8820.51

Note: NAC = NaCr (Si 2O 6); Jd = NaAl (Si 2O 6); CCR = CaCr (AlSiO6); CTi = Tp = CaTi (A12O6); CATs = CaAl (AlSiO6).a FeO* represents total iron as FeO." Proportions shown on a basis of 60 oxygen atoms.

the calcic plagioclase and magnesian clinopyroxene ob-served in ocean floor basalts. Such an origin requiresmagma mixing at oceanic spreading ridges. Rhodes andDungan (1979) discuss this latter question in some detailbased on residual glass compositions and compositionalvariations in olivine and plagioclase phenocrysts. Theysuggest that mixing and crystal contamination betweenupwelling primary magmas and evolved magmas in asteady-state magma chamber beneath the ridge give riseto a "buffered" ocean floor basalt magma. However,since none of the residual or included glass composi-tions match the composition suggested by Green et al.(1979) a primary magma origin is considered unlikelyfor the calcic phenocrysts considered in this study. It isalso interesting to note that while Rhodes and Dungan(1979) found plagioclase values as high as An85 in liq-uidus runs on Leg 45 basalts, they did not find plagio-clase as calcic as the natural phenocryst compositions.

The disparities observed between experimental workand natural rocks demonstrate the existence of a funda-mental problem with respect to the petrogenesis ofocean floor basalt. Clearly an understanding of the

genesis of highly calcic plagioclase phenocrysts is vitalto determining the origin and evolution of ocean floorbasalts. The Leg 65 results demonstrate the existence ofa continuum of feldspar compositions to values as highas An92 within samples from one area. The presence ofthis compositional continuum is inconsistent with theexistence of a uniform steady-state magma chambersince it demands wide variations in the conditions ofcrystallization. Since experimental studies on primitivebasalts have failed to reproduce the range of phenocrystcompositions seen in submarine basalts and studies ofnatural glass inclusions (Rhodes and Dungan, 1979) failto support the existence of magmas other than those ob-served on the ocean floor, it is clear that the experimen-tal results must be used with caution until the methodol-ogy of experimental petrology has been improved.

ACKNOWLEDGMENTS

We are grateful to the Deutsche Forschungsgemeinschaft for pro-viding financial support for this research (Schm 250/24 and 26). Wealso wish to thank G. Pritchard for providing a number of mineralanalyses; the Max Planck Institut für Kosmochemie (Mainz) and K.Abraham (Ruhr-Universitàt Bochum) for allowing us to use their

536


Table 4. (Continued).

483B-22-1,28-32(core)

52.770.422.846.15

17.5720.16

0.61

100.52

19.2071.218

20.4250.1151.872

9.5327.863

0.17619.557

39.982

83.6

0.000.001.781.154.07

35.8947.78

9.38

483B-22-1,26-36

52.190.953.787.17

16.8519.47

0.28

100.70

19.0021.620

20.6220.2612.184

9.1947.583

0.08219.264

39.886

80.7

0.000.000.832.645.14

34.1046.2511.05

483B-22-2,63-67

53.670.251.536.33

18.7318.34

0.33

99.78

19.6150.659

20.2740.0682.118

10.2057.181

0.09519.667

39.941

82.8

0.000.000.960.682.15

34.2251.3310.65

483B-22-2,63-67

52.000.512.538.55

17.2018.27

99.06

19.3051.109

20.4140.1432.655

9.5187.268

19.583

39.997

78.2

0.000.000.001.434.12

33.5847.6013.28

483B-22-2,77-82

51.690.843.358.070.25

16.3719.320.28

100.17

19.0381.454

20.4920.2332.4860.0788.9897.6240.200

19.610

40.102

78.3

0.001.980.002.313.90

34.6344.4912.69


483B-22-2,77-82

53.210.471.877.080.20

18.0318.470.22

99.55

19.5380.809

20.3470.1302.1740.0629.8707.2660.157

19.569

40.006

81.9

0.001.570.001.301.96

34.6849.3211.17

483B-25-2,39-45

50.381.442.90

11.950.24

14.3418.85

100.11

18.9121.282

20.1940.4083.7520.0778.0267.583

19.845

40.039

68.1

0.000.000.004.062.32

34.5739.9719.07

483B-25-2,39-45(core)

52.890.473.296.27

17.6220.08

0.70

101.31

19.1011.399

20.5000.1281.893

9.4837.768

0.20019.471

39.971

83.4

0.000.002.011.284.73

34.9447.55

9.49

483B-25-2,39-45

52.860.311.798.91

18.7416.67

0.11

99.39

19.4910.777

20.2680.0862.747

10.2976.587

0.03219.750

40.018

78.9

0.000.000.320.862.86

30.8651.3913.71

483B-27-1,12-18(core)

52.060.553.336.15

17.1320.080.240.67

100.22

19.0381.436

20.4740.1531.882

9.3377.8700.1700.193

19.605

40.079

83.2

1.690.000.231.525.49

35.4246.32

9.34

483B-30-3,13-16

53.440.421.796.83

19.7417.05

0.60

99.86

19.4590.766

20.2250.1152.079

10.7126.653

0.17319.732

39.957

83.7

0.00

o.oo1.741.151.82

31.0553.7910.44

483B-32-1,1-6

(core)

54.180.251.856.23

19.7118.12

0.77

101.10

19.4770.785

20.2620.0671.873

10.5586.977

0.21919.693

39.955

84.9

0.000.002.200.672.17

32.5253.039.41

microprobe facilities; A. Fischer and K. Bauszus for drafting andphotography; and V. Helmke for typing the manuscript,facilities; A. Fischer and K. Bauszus for drafting and photography;and V. Helmke for typing the manuscript.

REFERENCES

Ballard, R. D., 1979. The Galapagos Rift at 86°W: 3. Sheet flows,collapse pits, and lava lakes of the rift valley. J. Geophys. Res.,84:5407-5422.

Flower, M. F. J., and Robinson, P. T., 1979. Evolution of the FA-MOUS ocean ridge segment: Evidence from submarine and deepsea drilling investigations. In Talwani, M., Harrison, C. G., andHayes, D. E. (Eds.), Deep Drilling Results in the Atlantic Ocean:Ocean Crust. Sec. Maurice Ewing Series: Washington (Am. Geo-phys. Union), 314-330.

! , 1981. Basement drilling in the western Atlantic Ocean 2. Asynthesis of construction processes at the Cretaceous ridge axis. /.Geophys. Res., 86:6299-6309.

Green, D. H., Hibberson, W. O., and Jaques, A. L., 1979. Petrogen-esis of Mid-ocean Ridge Basalts. In McElhinny, M. W. (Ed.), TheEarth: Its Origin, Structure and Evolution: London (AcademicPress) pp. 265-299.

Kirkpatrick, R. J., 1979. Processes of crystallization in pillow basalts,Hole 396B, DSDP Leg 46. In Dmitriev, L., Heirtzler, J., et al., In-

it. Repts. DSDP, 46: Washington (U.S. Govt. Printing Office),271-282.

Kuo, L.-C, 1980. Morphology and zoning patterns of plagioclase inphyric basalts from DSDP Legs 45 and 46, Mid-Atlantic Ridge[M.A. dissert.]. University of Illinois.

Lofgren, G., 1974. An experimental study of plagioclase crystal mor-phology: Isothermal crystallization. Am. J. Sci., 274:243-273.

Natland, J. H., 1978. Crystal morphologies in basalts from DSDPSite 395, 23°N, 46°W, Mid-Atlantic Ridge. In kelson, W. G.,Rabinowitz, P. D., et al., Init. Repts. DSDP, 45: Washington(U.S. Govt. Printing Office), 423-445.

Rhodes, J. M., and Dungan, M. A., 1979. The evolution of ocean-ridge basaltic magma. Deep Drilling Results in the Atlantic Ocean:Ocean Crust. Sec. Maurice Ewing Series: Washington (Am. Geo-phys. Union), pp. 262-272.

Robinson, P. T., Flower, M. F. J., Swanson, D. A., et al., 1980. Li-thology and eruptive stratigraphy of Cretaceous oceanic crust,western Atlantic Ocean. In Donnelly, T., Francheteau, J., Bryan,W., Robinson, P., Flower, M., Salisbury, M., et al., Init. Repts.DSDP, 51, 52, 53, Pt. 2: Washington (U.S. Govt. Printing Office),1535-1556.

Roedder, P. L., and Emslie, R. F., 1970. Olivine-liquid equilibrium.Contrib. Mineral. Petrol., 29:275-289.

Schmincke, H.-U., Rautenschlein, M., Robinson, P. T., and Me-hegan, J., in press. Accretionary processes inferred from theTroodos Extrusive Series, Cyprus. Earth Planet. Sci. Lett.

537



Component

483B-32-1,1-6

(rim)

Major oxide (wt. Vo)

SiO 2TiO 2A12O3FeO* a

MnOMgOCaONa 2 OC r 2 O 3

Total

51.670.593.835.37

16.6520.890.281.39

100.68

Cation proportions"

SiAlΣTiFeMnMgCaNaCrΣ

Total

MgNo.

18.8321.645

20.4770.1621.638

9.0488.1580.1970.401

19.604

40.081

84.7

Normative composition

NACJdCCRCTiCATsWoEn

Fs

1.950.002.021.615.54

35.8844.88

8.12

483B-32-1,1-6

(core)

51.710.643.865.78

16.9320.26

0.87

100.05

18.9171.665

20.5820.1761.717

9.2317.942

0.25119.367

39.949

83.9

0.000.002.521.775.34

35.1046.39

8.88

483B-32-1,1-6

(rim)

51.510.934.556.13

17.2420.03

0.291.20

101.89

18.5671.935

20.5020.2531.849

9.2647.7340.2020.342

19.644

40.146

83.4

1.990.001.382.496.35

33.0145.66

9.11

485A-12-1,76-82

51.021.163.737.460.20

15.9019.930.21

99.64

18.8911.628

20.5190.3232.3100.0638.7777.9600.151

19.584

40.103

79.2

0.001.500.003.214.12

35.8443.5611.78

485A-13-1,110-117

52.020.732.797.520.22

17.1018.760.23

99.37

19.2241.215

20.4390.2032.3240.0699.4217.4280.165

19.610

40.049

80.2

0.001.640.002.023.20

34.3546.8811.91


485A-3O-3,38-41

52.800.631.598.680.27

17.5017.790.21

99.46

19.5310.693

20.2290.1752.6850.0859.6517.0510.151

19.798

40.022

78.2

0.001.510.001.750.96

33.8248.1513.82

485A-30-4,8-13

53.030.551.916.340.19

17.2120.28

0.22

99.73

19.3510.837

20.1880.1541.9270.0609.5428.0810.159

19.923

40.111

83.2

0.001.570.001.521.83

38.2047.08

9.80

485A-30-4,8-13

53.860.841.72

10.890.33

16.0118.040.25

101.94

19.6030.738

20.3410.2303.3150.1028.6877.0350.176

19.545

39.886

72.4

0.001.780.002.330.52

34.1643.9417.28

485A-30-4,8-13

51.780.711.699.720.16

15.8118.75

98.62

19.4740.749

20.2230.2013.0570.0518.8647.555

19,728

39.951

74.4

0.000.000.002.021.74

36.0844.5415.62

485A-39-1,26-32

50.751.083.778.03

15.5920.38

0.33

99.94

18.7991.647

20.4460.3022.488

8.6068.088

0.09719.581

40.027

77.6

0.000.000.973.014.72

35.9842.9112.41

485A-39-1,26-32

52.470.652.01

11.070.27

17.6316.46

100.49

19.3370.871

20.2080.1813.3920.0849.6816.501

19.839

40.047

74.1

0.000.000.001.802.53

30.1948.1817.30

538


Table 5. Composition of selected clinopyroxene groundmass crystals.

Component483B-20-2,

16-21

Major Oxide (wt.%)

SiO2T1O2A12O3FeO*a

MnOMgOCaONa2OCr 2O 3

Total

50.561.152.02

15.570.25

12.7417.73

100.0

Cation Proportions

SiAl

tTiFeMnMgCaNaCr£Total

Mg No.

19.2460.906

20.1520.3284.9570.0807.2277.230

19.821

39.973

59.3

Normative Composition

NACJDCCRCTiCATsWoEnFs

0.000.000.003.291.25

33.9836.2325.25

483B-20-2,16-21

50.881.232.52

13.380.21

15.6416.02

99.88

19.0931.113

20.2060.3464.2000.0668.7456.442

19.798

40.004

67.6

0.000.000.003.462.10

29.4143.7021.32

483B-20-2,50-59

50.101.154.378.37

16.8917.780.150.38

99.18

18.6161.916

20.5320.3202.601

9.3547.0790.1050.112

19.570

40.102

78.2

1.040.000.073.176.28

30.2846.2912.87

483B-20-2,50-59

52.620.683.246.29

17.6719.740.170.51

100.93

19.0751.383

20.4580.1861.908

9.5467.6680.1210.146

19.576

40.034

83.3

1.210.000.251.854.91

34.7047.57

9.51

483B-20-2,50-59

50.581.263.05

14.490.22

14.3316.76

100.69

18.9601.346

20.3060.3554.5410.0718.0076.731

19.706

40.012

63.8

0.000.000.003.553.18

30.2639.9923.03

483B-20-2,50-59

48.822.185.67

11.460.13

19.9817.33

100.58

18.1332.482

20.6150.6103.5580.0428.2956.896

19.401

40.016

70.0

0.000.000.006.096.30

28.2341.4117.97


483B-22-2,63-67

49.451.434.778.840.12

15.6119.070.290.11

99.68

18.4202.094

20.5140.4002.7530.0398.6667.6110.2070.031

19.707

40.221

75.9

0.301.720.003.915.47

32.5442.3913.66

483B-22-2,63-67

50.581.183.55

10.360.27

17.7715.42

0.10

99.23

18.8231.557

20.3800.3303.2230.0859.8566.149

0.03119.673

40.053

74.4

0.000.000.313.284.31

26.6349.0216.45

483B-27-1,12-18

50.321.062.10

15.010.35

14.3016.280.27

99.69

19.1310.941

20.0720.3034.7730.1148.1036.6310.196

20.120

40.192

62.9

0.001.920.002.970.68

30.7039.7523.97

483B-27-1,12-18

50.291.443.99

10.20

15.0918.760.220.21

100.20

18.6971.749

20.4460.4023.170

8.3617.4710.1620.062

19.630

40.076

72.5

0.620.990.003.994.19

32.9841.4915.73

485A-39-1,26-32

50.790.882.819.14

16.2318.38

0.35

98.60

19.0671.244

20.3110.2502.869

9.0837.393

0.10419.698

40.009

76.0

0.000.001.042.503.20

33.5645.3714.33

485A-39-1,26-32

53.211.461.28

11.350.21

18.0815.91

100.50

19.5820.555

20.1370.1283.4940.0659.9166.273

19.875

40.012

73.9

0.000.000.001.281.49

29.9449.5217,77

485A-39-1,26-32

51.481.021.89

15.070.34

15.6415.260.17

100.87

19.2390.831

20.0700.2874.7100.1088.7136.1110.120

20.049

40.119

64.9

0.001.190.002.840.68

28.4443.0523.81

Note: NAC = NaCr (Si2O6); Jd = NaAl (Si2C>6); CCR = CaCR (AlSiO6); CTi = Tp = CaTi (A12O6); CATs Ca* FeO' represents total iron as FeO.

Proportions shown on a basis of 60 oxygen atoms.

539


Table 6. Composition of selected plagioclase phenocrysts, Holes 483, 483B, and 485A.

Component

Major oxides (v

SiO2TiO 2A12O3FeO* a

M n O

MgO

C a O

Na 2 OK 2 O

SrO

483-21-3,6-9

(rim)

rt.%)

48.15

33.510.52

0.1816.55

1.92

483-21-3,6-9

(core)

52.44

30.010.52

0.2513.223.940.05

483-21-3,6-9

(core)

49.69

32.500.40

0.2415.822.510.07

483B-8-1,24-27

47.830.04

33.320.38

0.1817.17

1.74

483B-8-1,24-27

. 47.52

32.990.39

0.1717.11

1.740.01

483B-13-1,60-64

48.030.04

32.630.520.020.17

16.492.040.01


483B-13-1,60-64

49.350.04

31.960.520.020.11

15.602.560.010.06

483B-20-2,16-21

(rim)

53.62

28.570.88

0.2711.674.900.08

483B-20-2,16-21

48.13

34.090.29

0.1717.34

1.68

483B-20-2,16-21(core)

49.75

31.710.37

0.1314.753.02

483B-20-2,37-42(core)

50.41

32.510.45

0.2615.582.650.06

483B-20-2,37-42(core)

52.40

30.380.44

0.3013.563.950.06

483B-20-2,37-42(rial)

58.73

25.760.66

0.168.067.220.12

Cation proportions

Si

ΛIΣTiFé

MgCaNaKSrE

Total

An(mole%)

8.7632.188

15.951

0.079

0.0493.2260.678

4.031

19.982

82.6

9.4946.404

15.898

0.078

0.0672.5641.3820.012

4.103

20.001

64.8

8.9856.926

15.911

0.061

0.0663.0650.8810.017

4.089

20.000

77.3

8.7317.169

15.9000.0050.058

0.0493.3580.616

4.086

19.986

84.5

8.7417.152

15.893

0.060

0.0473.3720.6210.002

4.102

19.995

84.4

8.8247.065

15.8890.0060.0800.0030.0473.2460.7270.0020.0044.115

20.004

81.7

9.0186.883

15.9010.0050.0790.0030.0303.0540.9070.0020.0064.086

19.987

77.1

9.7386.116

15.845

0.134

0.0732.2711.7260.019

4.223

20.077

56.5

8.6907.255

15.945

0.043

0.0463.3540.587

4.031

19.976

85.1

9.1086.842

15.950

0.057

0.0352.8941.071

4.057

20.007

73.0

9.0446.875

15.919

0.067

0.0692.9940.9230.014

4.067

19.986

76.2

9.4356.448

15.883

0.066

0.0802.6161.3780.130

4.153

20.036

65.3

10.4715.412

15.883

0.098

0.0431.5402.4940.027

4.201

20.084

37.9

a FeO* represents total iron as FeO.Proportions shown on a basis of 32 oxygen atoms


Component

Major oxides (v

SiO2TiO2A12O3FeO aMnOMgOCaONa2OK 2 OSrO

483B-25-2,39-45

rt.%)

55.49

27.460.87

0.1410.265.700.09

483B-25-2,39-45(core)

48.77

31.540.32

0.2615.142.67

483B-25-2,39-45

52.06

30.210.49

0.2313.373.880.06

483B-27-1,12-18

50.15

31.190.41

0.2814.603.040.06

483B-27-1,12-18

48.22

32.530.31

0.1315.912.09

483B-27-1,12-18

52.20

30.350.54

0.2713.463.840.06


483B-30-3,13-16(core)

46.33

33.890.33

0.3517.54

1.25

483B-30-3,13-16

51.61

30.480.59

0.2913.843.66

483B-3O-3,13-16(core)

48.62

32.640.21

0.2115.992.36

483B-32-1,1-6

51.99

30.170.56

0.3413.593.710.10

483B-32-1,1-6

(core)

47.26

33.290.32

0.2217.16

1.53

483B-32-1,1-6

48.63

32.340.31

0.2215.772.25

483B-32-1,1-6

(rim)

53.67

28.860.83

0.4512.494.550.18

Cation proportions''

SiAlETiFeMnMgCaNaKSr

Total

An(mole%)

10.0295.849

15.878

0.135

0.0371.9871.9960.022

4.177

20.055

49.6

9.0346.885

15.919

0.050

0.0713.0040.959

4.084

20.003

75.8

9.4446.458

15.902

0.074

0.0622.5991.3650.013

4.114

20.016

65.4

9.1806.72915.90

0.063

0.0762.8631.0780.014

4.093

20.002

72.4

8.8957.071

15.966

0.048

0.0373.1440.748

1977

19.943

80.8

9.4426.449

15.891

0.082

0.0722.6081.3460.015

4.123

20.014

65.7

8.5517.373

15.924

0.051

0.0973.4690.448

4.063

19.987

88.6

9.3636.517

15.880

0.089

0.0772.6891.287

4.142

20.022

67.6

8.8987.039

15.937

0.032

0.0593.1350.838

4.064

20.001

78.9

9.4266.446

15.872

0.085

0.0932.6391.3020.023

4.142

20.014

66.6

8.6997.221

15.920

0.049

0.0623.3850.547

4.043

19.963

86.1

8.9387.005

15.943

0.048

0.0613.1050.802

4.017

19.960

79.5

9.6696.128

15.797

0.125

0.1202.4111.5890.041

4.285

20.082

59.7

540



483B-20-2,50-59(rim)

53.39

29.410.71

0.2512.564.40

483B-20-2,50-59

51.38

30.420.49

0.3013.833.48

483B-20-2,50-59

46.58

33.590.33

17.091.15

483B-20-2,50-59

49.68

32.420.39

15.412.63

483B-20-2,50-59

50.29

31.570.40

0.2714.962.910.06

483B-20-2,50-59

47.52

33.040.35

0.2416.621.820.06

483B-20-2,50-59(core)

49.10

32.400.45

0.2015.942.27


483B-20-2,50-59(rim)

58.70

25.330.70

7.637.260.12

483B-22-1,26-32(rim)

53.76

28.490.72

0.3211.984.650.05

483B-22-1,26-32

50.95

30.060.55

0.2913.503.430.06

483B-22-1,26-32

48.47

31.920.35

0.1815.432.340.05

483B-22-2,63-67(rim)

54.46

29.100.86

0.3012.254.790.08

483B-22-2,63-67

49.65

31.850.34

0.2515.262.600.06

483B-22-2(63-67(core)

51.37

30.540.51

0.2313.853.650.10

483B-22-2,77-82

52.140.08

29.570.700.020.20

13.643.570.010.06

99.99

9.6286.521

16.149

0.107

0.0662.4261.540

3.68

20.017

61.2

9.3646.533

15.897

0.075

0.0822.7001.231

4.088

19.985

68.7

8.6547.354

16.008

0.052

3.4010.413

3.867

19.875

89.2

9.0296.945

15.974

0.059

3.0010.927

3.987

19.961

76.4

9.1436.766

15.909

0.061

0.0722.9141.0260.013

4.085

19.994

73.7

8.7537.174

15.927

0.053

0.0673.2800.6490.015

4.064

19.991

83.2

8.9566.965

15.921

0.068

0.0553.1140.805

4.043

19.964

79.5

10.5495.366

15.915

0.105

1.4702.5280.028

4.131

20.046

36.5

9.7566.093

15.849

0.109

0.0872.3301.6360.012

4.173

20.022

58.6

9.3836.525

15.908

0.085

0.0792.6631.2240.014

4.065

19.973

68.3

8.9776.969

15.946

0.054

0.0503.0620.8390.013

4.018

19.964

78.2

9.7176.119

15.836

0.128

0.0812.3411.6560.019

4.225

20.061

58.3

9.0706.857

15.927

0.051

0.0692.9870.9190.014

4.041

19.968

76.2

9.3446.547

15.891

0.077

0.0612.6991.2860.023

4.143

20.034

67.3

9.4976.348

15.8450.0110.1070.0030.0542.6621.2610.0020.0064.006

19.951

67.8


485 A-11-3,68-73

50.160.08

31.080.59

0.2015.093.04

0.04

100.28

9.1566.686

15.8420.0110.090

0.0542.9511.076

0.0044.186

20.028

73.3

485A-12-1,76-82

51.820.08

29.550.670.020.25

13.873.690.010.06

100.02

9.4526.353

15.8050.0110.1020.0030.0682.7111.3050.0020.0064.208

20.013

67.5

485A-13-1,110-117

51.870.06

30.070.630.020.18

14.243.46

0.07

100.60

9.4076.427

15.8340.0080.0960.0030.0492.7671.217

0.0074.147

19.981

69.5

485A-13-1,110-117

56.150.15

27.121.100.020.11

11.065.360.010.06

101.14

10.0575.725

15.7820.0200.1650.0030.0292.1221.8610.0020.0064.208

19.990

53.2

485A-3O-3,38-41

52.060.06

29.080.72

0.0913.903.850.01

99.77

9.5206.268

15.7880.0080.110

0.0252.7231.3650.002

4.233

20.021

66.6

485A-30-4,8-13

53.340.11

29.010.760.030.22

12.794.320.020.07

100.67

9.6406.179

15.8190.0150.1150.0050.0592.4771.5140.0050.0074.197

20.016

62.0


485A-30-4,8-13

51.220.06

30.160.600.020.17

14.143.570.020.06

100.02

9.3516.490

15.8410.0080.0920.0030.0462.7661.2640.0050.0064.190

20.031

68.6

485A-30-4,8-13

50.11

29.830.55

14.462.74

97.69

9.3466.558

15.904

0.086

2.8900.991

3.967

19.871

74.5

485A-39-1,26-32

50.89

30.850.45

0.2814.193.34

100.0

9.2766.628

15.904

0.069

0.0752.7721.181

4.096

20.000

70.1

485A-39-1,26-32

57.06

26.560.62

0.119.286.44

100.07

10.2615.629

15.890

0.094

0.0291.7872.247

4.157

20.047

44.3

485A-39-1,26-32(core)

45.38

34.660.26

0.1818.270.99

99.74

8.3917.553

15.944

0.040

0.0503.6190.355

4.065

20.009

91.1

485A-39-1,26-32(rim)

53.71

28.970.80

0.1912.124.69

100.49

9.7046.169

15.873

0.120

0.0502.3471.644

4.161

20.034

58.8

485A-39-1,26-32

48.69

32.710.37

0.2416.172.14

100.32

8.8907.037

15.927

0.057

0.0663.1630.757

4.043

19.970

80.7

485A-39-1,26-32

53.36

29.160.67

0.2212.344.630.05

100.43

9.6526.217

15.869

0.101

0.0592.3911.6260.012

4.189

20.058

59.3

541


Table 7. Composition of selected plagioclase groundmass crystals.

Component

Major Oxides (\

SiO2A12O3

F eO aMgOCaONa2OK2O

Total

483B-20-2,16-21

vt.%)

53.8429.030.780.22

11.864.67

100.40

Cation Proportions

SiAlΣFeMgCaNaKE

Total

An mole%

9.7216.179

15.9000.1170.0602.2951.636

4.107

20.007

58.4

483B-20-2,16-21

56.2927.070.77

9.798.84

99.76

10.1635.761

15.9240.116

1.8932.044

4.054

19.978

48.1

483B-20-2,37-42

52.7729.590.810.27

12.734.320.07

100.56

9.5506.313

15.8630.1230.0722.4691.5150.0164.196

20.059

61.7

Sample(interva

483B-20-2,50-59

56.7127.47

1.050.059.905.950.07

101.20

10.1175.775

15.8920.1560.0221.8912.0570.0124.138

20.030

47.8

il in cm)

483B-20-2,50-59

52.7229.38

1.100.35

12.634.18

100.34

9.5656.281

15.8460.1660.0942.4551.469

4.183

20.029

62.6

483B-20-2;50-59

54.1928.190.830.17

12.154.47

100.73

9.7546.133

15.8870.1260.0462.3421.559

4.072

19.959

60.0

483B-2O-2,50-59

57.2427.040.790.089.636.170.09

101.04

10.2095.683

15.8920.1180.0221.8402.1330.0214.134

20.026

46.1

483B-27-1,12-18

52.3929.610.590.26

13.084.10

100.04

9.5256.346

15.8710.0900.0712.5481.446

4.154

20.025

63.8

a FeO* represents total iron as FeO.Proportions shown on a basis of 32 oxygen atoms.

Table 8. Composition of selected Fe/Ti-oxides.

Component

Major oxides

Siθ2Tiθ2A12O3FeO* a

FeOF e 2 O 3MnOMgOCaO

V 2 O 3

Total

483-22-2,89-83

(wt.%)

0.5222.93

5.0868.34

(51.13)(19.13)

0.730.650.330.20

98.78

(100.7)

Cation proportions"3

SiAlT i oF e 2 +

Fe 3 +

MnMgCaV

Total

0.1872.1576.211

15.4035.1960.2230.3490.1270.058

29.911


483-22-2,89-83

23.945.16

67.48(52.57)(16.57)

0.480.480.17

97.99

(99.37)

2.2306.601

16.1194.5720.1490.2620.067

30.0

483-22-2,89-83

21.776.67

70.47(51.87)(20.67)

0.420.410.110.37

100.22

(101.92)

2.7885.807

15.3865.5170.1260.2170.0420.105

29.988

485A-30-4,8-13

0.5521.15

1.9971.28

(49.74)(23.93)

0.820.53

96.32

(98.71)

0.2070.8815.972

15.6216.7620.2610.297

30.001

Note: Recalculated values shown in parentheses.j* FeO* represents total iron as FeO.k Proportions shown on a basis of 40 oxygen atoms.

542


Table 9. Selected glass compositions.

Major Oxides(wt.%)

SiO2TiO2A12O3FeO*b

MnOMgOCaONa2OK2OP2O5SO3

Total

483-21-3,6-9a

50.512.04

14.3511.51

—4.39

12.602.580.120.140.45

98.70

483-21-3,6-9

50.102.00

13.9111.400.177.04

11.222.850.080.200.55

99.53

483-21-3,6-9

51.692.11

14.0712.18—6.90

11.190.670.080.140.53

99.57


483B-22-1,26-32

50.471.94

14.0011.470.137.11

11.372.910.090.170.48

100.16

483B-22-2,77-82

51.951.90

13.3811.430.236.45

10.862.720.07——

98.99

483B-3O-3,13-16

50.241.86

13.9811.37

—

7.1511.392.630,08—

0.29

99.00

483B-32-1,1-6

50.791.73

14.3510.48

7.6211.962.850.120.110.47

100.48

a Glass inclusion in olivine." FeO represents total iron as FeO.

Table 10. Average mineral and glass compositions for selected basalts from Holes 483 and 483B.


Hole 483

21-3, 6-9

22-2, 85-88

26-1, 39-40

Hole 483B

20-2, 16-2120-2, 37-42

20-2, 50-59

20-2, 50-59

22-1, 26-3222-2, 63-6725-2, 39-4527-1, 12-18

30-3, 13-16

30-3, 18-2232-1, 1-6

39-1, 26-32

Olivine

CrystalType

MP

P

—

—

-

—

P——

1

PP

MP, P

—

Mg No.a

81.3 (10)

83.3 (7)

—

—

-

-

81.6 (7)———

86.2 (4)82.9 (6)84.5 (2)84.5 (11)

—

CrystalType

Gl

P

—

MPMP

GL, MP

MMP

MP, GLMP, GLMP, GLMP, GL

MP, GL

—MP, GL

MP, GL

Phenocrysts

Clinopyroxene

Mg No.a

82.8 (4)

85.6 (6)

—

86.9 (4)87.1 (5)

85.6 (9)

84.1 (7)84.7 (7)84.0 (14)83.1 (15)84.7 (23)83.1 (4)

84.2 (8)

—84.9 (12)

77.7 (6)

Ca:Mg:Fe

37:52:11

41:51:8

—

42:50:842:51:7

42:50:8

40:50:1041:50:940:50:1040:50:1040:50:941:49:10

42:49:9

—36:54:10

42:45:13

Plagioclase

CrystalType

IMP

IMPP

pP

IM\P

P

PPPP

!GPLPP

| M

IGL

An(mole


• Megacrysts

o Phenocrysts, groundmass

Ca

Figure 10. Quadrilateral diagram showing compositions of clinopy-roxene megacrysts, phenocrysts, and microphenocrysts in Leg 65basalts.

100

90-/'

• Plagioclase phenocrysts .

+ Plagioclase microlites jL

80-/

40 -

6V

50 -/.

10

20

30-/10

Or

20

Figure 11. Ternary plot showing selected plagioclase compositions forLeg 65 basalts.

544


Plate 1. Massive basalts recovered on Leg 65. 1. Sample 483B-20-2, 62-65 cm; thin layer of sandstone adhering to the devitrified base of a massivelava flow (Lithologic Unit 6) (see also Fig. 2.). 2. Sample 483B-20-2, 62-65 cm; basal contact of massive sheet flow(?) (Unit 6) against underlyingsiltstone (light-colored); glass shows dark alteration against silt and on both sides of straight fracture filled with smectite; glass rinds on massiveflows are thinner than on pillows (width of photomicrograph, 5 mm). 3. Sample 485A-38-2, 5-10 cm; cross section through top of deepestmassive (intrusive?) basalt drilled in Hole 485A (Lithologic Unit 8) showing devitrified glass stringers in siltstone (width of photograph, 2mm). 4. Sample 483-16-2, 64-70 cm; fine-grained basalt intruding (self-intruding) medium-grained, massive basalt (Unit 3) (width of photomi-crograph, 5 mm).

545


35 rW

Plate 2. Pillow basalts recovered on Leg 65. 1. Sample 483B-18-2, 35-40 cm; highly vesicular basalt 1.5 meters below top of 3.5-meter-thick massiveflow (Lithologic Unit 6) (see also Fig. 3). 2. Sample 483B-26-2, 42-47 cm; massive, vesicular basalt with a coarse-grained, subophitic to interser-tal groundmass (Lithologic Unit 8); vesicles to right and center lined with carbonate and filled with smectite; vesicle on left filled with chlorite(?);vesicle to the upper right is a filled segregation vesicle (width of photomicrograph, 5 mm). 3. Sample 483B-18-2, 18-22 cm; partially filledsegregation vesicle subsequently lined with smectite(?) showing contraction cracks; note plagioclase laths aligned tangential to vesicle (see Fig. 1)(width of photomicrograph, 5 mm). 4. Sample 482D-11-1, 24-28 cm; cross section through thick, fresh glassy margin of porphyritic pillow lava(50 cm thick) (see also Plate 4, Fig. 1 for photomicrograph of olivine and plagioclase microphenocrysts) (width of photomicrograph, 2 cm).

546


Plate 3. Plagioclase phenocrysts. 1. Sample 483B-7-1, 7-12 cm; plagioclase megacryst in massive flow (Lithologic Unit 3) showing inclusion-rich,partially resorbed core (light gray), an oscillatory zoned mantle, and a dark outer rim separated from the mantle by a sharp compositional break(width of photomicrograph, 5 mm). 2. Sample 483-23-1, 10-16 cm; glomerocryst in pillow basalt (Unit 6) consisting of three partially resorbedplagioclase xenocrysts with numerous glass inclusions and two subhedral plagioclase phenocrysts (width of photomicrograph, 5 mm). 3. Sample483-21-1, 49-55 cm; euhedral to subhedral clot of regularly zoned plagioclase phenocrysts in tachylitic matrix of pillow basalt (Unit 6) (width ofphotomicrograph, 5 mm). 4. Sample 485A-36-3, 65-69 cm; clot of skeletal olivine and weakly skeletal plagioclase microphenocrysts in fine-grained pillow basalt(?) (Unit 7) with a tachylitic groundmass containing skeletal plagioclase microlites (width of photomicrograph, 2 mm).

547


~/ r

IP

Plate 4. Phenocrysts and clots. 1. Sample 482D-11-1, 24-28 cm; clot of skeletal plagioclase and olivine microphenocrysts in a matrix of fresh sidero-melane from the margin of a pillow; microphenocrysts show minor terminal in situ growth (width of photomicrograph, 2 mm). 2. Sample483-22-2, 68-72 cm; partially resorbed clinopyroxene megacryst enclosing euhedral plagioclase phenocrysts in a tachylitic pillow margin(Lithologic Unit 6) (width of photomicrograph, 5 mm). 3. Sample 483-22-2, 77-82 cm; clot of subhedral to euhedral clinopyroxene and plagio-clase crystals in a tachylitic pillow margin (Lithologic Unit 6) (width of photomicrograph, 5 mm). 4. Sample 483B-29-1, 55-57 cm; clot ofstrained anhedral clinopyroxene and plagioclase in an intersertal pillow basalt matrix (Lithologic Unit 9) (width of photomicrograph 5 mm).

548

Documents

24. LITHOLOGY, PETROGRAPHY, AND MINERALOGY OF BASALTS … · 2007. 5. 7. · B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE Heat Flow Age Magnetic Anomaly Distance 90 80 70 60 50 40