Highly siderophile element systematics of the

early Archean mantle: Evidence from 3.3 Ga

Ruth Well komatiites, Pilbara Craton,

Western Australia

Jairus Slagle

Advisor: Igor S. Puchtel

Department of Geology, University of Maryland College Park, MD, 20742

GEOL 394 – Final Paper

November 21, 2018

i

Abstract While the heterogeneous nature of the mantle in terms of Os isotopic composition

and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) abundances is well

established based on komatiite research of the past two decades (Maier et al., 2009,

Puchtel et al., 2016, 2018), the origin, magnitude, and longevity of this heterogeneity are

still little understood. This is particularly true for the early Archean mantle, where the

available komatiite record is very limited. The main goal of this study is to place

constraints on the absolute HSE abundances, Os isotopic composition, and time-

integrated Re/Os ratio of the early-mid Archean mantle using ~3.3 Ga Ruth Well

komatiites from the Pilbara Craton in Western Australia.

Major-, lithophile trace element-, and HSE abundances, as well as Re-Os isotopic

data are reported for a set of drill core and surface outcrop komatiite samples that

intersected several differentiated lava flows. Regression of the lithophile trace element

and HSE abundance data against the MgO content indicate limited element mobility

during post-magmatic processes and olivine control over the entire range of komatiite

compositions during lava differentiation. The komatiites are depleted in both light and

heavy REE [(La/Sm)N = 0.70 ± 0.03, (Gd/Yb)N = 1.37 ± 0.05]. These features indicate

derivation of the komatiite lavas from a deep (>450 km), melt-depleted mantle source, in

equilibrium with majorite garnet.

In the set of samples analyzed, Os isotopic compositions exhibit poor correlation

with the Re/Os ratios, likely due to Re mobility during metamorphism that occurred

hundreds of millions of years after the lava emplacement. The calculated total Pt and Pd

abundances in the Ruth Well komatiite source are 55±4% of those in the estimates for the

modern bulk silicate earth (BSE). This estimate is similar to the source characteristics of

several other early Archean komatiite systems, such as the 3.48 Ga Komati and 3.26 Ga

Weltevreden (Puchtel et al., 2014). This observed deficit in HSE is likely due to the Ruth

Well source not receiving the full complement of late accreted component; however, the

exact mechanisms and timing for creating this heterogeneity are yet to be established.

ii

Table of Contents ABSTRACT ............................................................................................................................................................... i

TABLE OF CONTENTS .............................................................................................................................................. ii

TABLE OF FIGURES ................................................................................................................................................ iii

1. INTRODUCTION .................................................................................................................................................. 1

2. HYPOTHESES ...................................................................................................................................................... 1

2.1. HYPOTHESIS 1. ....................................................................................................................................................... 1 2.2. HYPOTHESIS 2. ....................................................................................................................................................... 1 2.3. HYPOTHESIS 3 ........................................................................................................................................................ 1

3. ANALYTICAL TECHNIQUES .................................................................................................................................. 2

3.1. SAMPLES ............................................................................................................................................................... 2 3.2. SAMPLE PREPARATION ............................................................................................................................................. 3 3.3. MAJOR, MINOR, AND TRACE ELEMENT ABUNDANCES ...................................................................................................... 4 3.4. OS ISOTOPIC COMPOSITION AND HSE ABUNDANCES ...................................................................................................... 5

4. RESULTS ............................................................................................................................................................. 6

4.1. MAJOR AND TRACE ELEMENT ABUNDANCES ................................................................................................................. 6 4.2. RE-OS ISOTOPE SYSTEMATICS .................................................................................................................................. 11 4.3. HSE ABUNDANCES ................................................................................................................................................ 11

5. DISCUSSION ..................................................................................................................................................... 16

5.1. RUTH WELL ARCHEAN OCEANIC CRUST? ................................................................................................................... 16

6. SUMMARY ....................................................................................................................................................... 17

ACKNOWLEDGEMENTS ........................................................................................................................................ 19

REFERENCES ......................................................................................................................................................... 19

iii

Table of Figures

FIGURE 1. ............................................................................................................................................................... 2

FIGURE 2. ............................................................................................................................................................... 3

FIGURE 3. ............................................................................................................................................................... 4

TABLE 1. ................................................................................................................................................................. 7

TABLE 2. ................................................................................................................................................................. 8

FIGURE 4. ............................................................................................................................................................... 9

FIGURE 5. ............................................................................................................................................................. 10

FIGURE 6. ............................................................................................................................................................. 10

FIGURE 7. ............................................................................................................................................................. 12

TABLE 3. ............................................................................................................................................................... 13

FIGURE 8. ............................................................................................................................................................. 14

FIGURE 9. ............................................................................................................................................................. 15

FIGURE 10. ........................................................................................................................................................... 16

FIGURE 11. ........................................................................................................................................................... 18

1

1. Introduction Osmium isotope and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd)

abundance systematics in Archean komatiites provide information about early earth processes

that occurred between 4.5 and 2.5 Ga (Walker 1989, 2009).

The observation that the HSE occur in roughly chondritic relative proportions in the BSE,

and that the absolute HSE abundances are higher than would be expected from metal-silicate

equilibration, have led to the concept of late accretion. Late accretion is envisioned as a process

whereby at least 0.5% of earth’s mass was added to the mantle through continued accretion of

planetesimals after core formation was completed (Chou et al., 1983; Morgan, 1985). Issues

related to late accretion are still debated, including the composition of late accreted materials and

the timeframe within which they were delivered to earth and homogenized within the mantle

(e.g., Maier et al., 2009; Walker, 2014; Puchtel et al., 2018). Some of the uncertainties regarding

these issues stem from the fact that the absolute HSE abundances in the early earth’s mantle are

not well constrained and the causes of their variations in abundance are little understood. For

example, Maier et al. (2009), on the basis of measured platinum contents in Archean komatiites,

argued for a gradual increase in HSE abundances in their presumed deep mantle sources between

~3.5 and ~2.9 Ga due to slow downward mixing of a “late veneer” of chondritic materials. In

contrast, Puchtel et al. (2016a; 2016b; 2018) demonstrated that HSE variations in the sources of

Archean komatiites were highly variable and were due to slow homogenization within the mantle

of: (1) post-magma ocean domains characterized by variably-fractionated siderophile element

abundances; and (2) silicate reservoirs containing poorly mixed material of large accreted

differentiated planetesimals (late accreted component), the bulk of which was delivered to earth

prior to 4.40 Ga. The diversity of views is caused by the limited HSE abundance and osmium

isotopic data for the early Archean mantle. The main goal of this study is to place additional

constraints on the absolute HSE abundances and time-integrated Re/Os ratio of the early

Archean mantle using ~3.3 Ga Ruth Well komatiites from the Pilbara Craton in Western

Australia.

2. Hypotheses

2.1. Hypothesis 1.

Ruth Well komatiites were derived from deep mantle melting in the majorite stability field,

which resulted in depletions in Al and heavy REE. This source was also depleted in highly

incompatible lithophile trace elements.

2.2. Hypothesis 2.

The mantle source of the Ruth Well komatiites evolved with long-term chondritic Re/Os.

2.3. Hypothesis 3

The mantle source of the Ruth Well komatiites was deficient in the HSE relative to modern

estimates for BSE as a result of incomplete homogenization of late accreted component within

the mantle domain from which it was derived.

2

3. Analytical Techniques

3.1. Samples

The komatiite samples (both drill core and surface outcrop) were collected by Prof. Euan

Nisbet, University of London, in late 1970s in the Pilbara Craton of Western Australia (Figure

1). They were donated to the Sedgwick Museum of Earth Sciences at the University of

Cambridge, UK. Igor S. Puchtel visited his long-term collaborator Prof. Nisbet at Cambridge in

2017 and selected the appropriate samples for this study. The criteria used included (1) visible

preservation of primary textural and mineralogical features, and (2) attainment of the largest

possible range of chemical compositions, which was achieved via selecting samples from

spinifex- and cumulate parts of lava flows. The drill core and outcrop samples represent several

differentiated spinifex-textured komatiite lava flows.

Figure 1.

Generalized map of Archean cratons

of Western Australia (after Lambert et

al., 1998). Circle area indicates the

location of the Pilbara Craton and the

Ruth Well komatiite system selected

for this study.

3

3.2. Sample Preparation

The drill core samples were cut length-wise using the diamond rock saw into slabs for both

petrographic and chemical analysis (Figure 2). The ~50 g slabs were abraded on all sides using

SiC sandpaper to remove saw marks, rinsed with Milli-Q water, dried, crushed in an alumina jaw-

crusher, and then powdered in an alumina ceramic dish followed by an alumina disk mill (Figure

3). The powders were used for the geochemical studies. The slabs labeled with ‘TS’ (Figure 2)

were shipped to IGEM in Moscow for preparation of thin sections which were then used for

petrographic studies.

Figure 2.

Example of one of the Ruth Well drill core

samples analyzed in this study. Black

sharpie marks indicated with TS beside

them (in blue pen) were sent out for thin

sections used for petrographic analysis.

The remaining areas were made into

powders for chemical analysis and

isotopic studies.

4

1.

2.

3.

4.

Figure 3.

Equipment used to process the slabs into fine powders for chemical analysis and isotopic studies.

1. Diamond rock saw. 2. Alumina jaw-crusher. 3. Alumina ceramic dish. 4. Alumina disk mill.

3.3. Major, minor, and trace element abundances

The 2.5 gram aliquots of sample powders were sent to Franklin and Marshall College,

Lancaster, PA, for major element analysis. The analyses were performed on fused glass discs

using a Phillips 2404 XRF spectrometer following the protocol of Mertzman (2000). The typical

accuracy of the analyses was ~2% relative for major elements present in concentrations greater

than 0.5% and ~5% relative for the rest of the major elements estimated on the basis of replicate

analyses of USGS GRM BIR-1 and BCR-1 as unknowns (Puchtel et al., 2016a).

The trace element abundances were determined using the standard addition solution

inductively coupled plasma mass spectrometry technique (SA-ICP-MS) following the protocol

outlined in Puchtel et al. (2016b). Approximately 100 mg of sample powder was weighed out in

5

15 mL screw-cap Savillex Teflon vials with 0.5 mL of double-distilled concentrated HNO3 and 3

mL of double-distilled concentrated HF. The vials were tightly sealed with Teflon tape and kept

on a hotplate at 180°C for 48 hours. The vials were opened, and the sample solutions evaporated

to dryness. Approximately 0.5 mL of concentrated HClO4 (SeaStar) was added to the dry residue

to convert fluorides into perchlorates. The vials were resealed and placed on a hotplate at 180°C

for 24 hours. The vials were reopened, and the sample solutions dried down at 230°C on a hot

plate. The residue was then re-dissolved in 2 mL of 6M HCl, which converted it to chloride

form. This process was repeated twice. The dry residue was re-dissolved in 10g of 0.8M HNO3,

and the stock solution was used for preparing spiked aliquots for ICP-MS measurements.

Two standard addition spikes were prepared by precisely diluting a mixture of

commercially available Alfa Aesar calibrated ICP-MS standard solutions, one containing

concentrated mixed solutions of transition metals V, Sc, Ga, Cu, Y, and Zr, and the other

containing trace elements Nb, Hf, Th, U, and REE. Three aliquots of each sample containing 1.0

g of sample stock solution (with the weight recorded), were prepared for each of the two groups

of elements to be analyzed, one containing no spike, one with the amount of spike containing the

estimated amount of element present in the sample aliquot, and one with the amount of spike

containing twice the estimated amount of element present in the sample aliquot, with the weights

of the spikes recorded. One total analytical blank (TAB) was prepared and measured with every

batch of six samples. Approximately 100 mg (with the weight recorded) of 500 ppb In solution

was added to each sample aliquot and the TAB (Total Analytical Blank) solutions to monitor and

correct for signal drift during analysis. Ultramafic lavas, such as komatiites, are depleted in In,

and the added In greatly overwhelmed any native In present in the samples. Subsequently, the

one sample- and two sample-spike solutions for each sample were diluted to 10 g of 0.8 M

HNO3.

The sample solutions were analyzed on a ThermoFisher Element2 ICP-MS at the Plasma

Laboratory (PL), University of Maryland. The intensities of selected isotopes of each element

were measured in either low resolution (trace elements) or medium resolution (transition metals)

modes. The raw data were reduced using an in-house Microsoft Excel macro previously created

by geology graduate student Ming Tang and graduate assistant Valentina Puchtel (Puchtel et al.,

2016).

3.4. Os isotopic composition and HSE abundances

In order to obtain Re-Os isotopic and HSE abundance data, ~1.5 g of whole-rock powder,

5 mL of purged, triple-distilled concentrated HNO3, 4 mL of triple-distilled concentrated HCl,

and appropriate amounts of mixed 185Re-190Os and HSE (99Ru, 105Pd, 191Ir, 194Pt) spikes were

sealed in double internally-cleaned, chilled 25 mL PyrexTM borosilicate Carius Tubes (CTs) and

heated to 270°C for 96 hours. Osmium was extracted from the acid solution by CCl4 solvent

extraction (Cohen and Waters, 1996), back-extracted into HBr, and purified via microdistillation

(Birck et al., 1997). Osmium isotopic measurements were performed via isotope dilution

negative thermal ionization mass spectrometry (N-TIMS: Creaser et al., 1991). Samples were

analyzed using a secondary electron multiplier (SEM) detector on a ThermoFisher Triton mass

spectrometer at the Isotope Geochemistry Laboratory (IGL), University of Maryland.

The measured isotopic ratios were corrected for mass fractionation using 192Os/188Os =

3.083. The internal precision of measured 187Os/188Os for the samples was between 0.04% and

0.06% relative. The 187Os/188Os ratio of 500 pg loads of the in-house Johnson-Matthey Os

standard measured during the two-year period leading up to the current analytical sessions

6

averaged 0.11376±10 (2SD, N = 64). The value characterizes the external precision of the

isotopic analyses (0.10%), which was used to estimate the true uncertainty on the measured 187Os/188Os ratio for each individual sample. The measured 187Os/188Os ratios were further

corrected for instrumental mass bias relative to the average 187Os/188Os = 0.11379 measured for

the Johnson-Matthey Os standard on the Faraday cups of the IGL Triton (Puchtel et al., 2016b).

The correction factor of 1.00026 was calculated by dividing this value by the average 187Os/188Os

measured for the Johnson-Matthey Os standard on the SEM of the same instrument.

The average chondritic Os isotopic composition at the time of the assumed lava

emplacement was calculated using the 187Re decay constant = 1.666×10-11 year-1, an early Solar

System initial 187Os/188Os = 0.09531 at T = 4558 Ma, and 187Re/188Os = 0.40186 (Smoliar et al.,

1996; Shirey and Walker, 1998).

4. Results

4.1. Major and Trace Element Abundances

The major and lithophile trace element data are presented in Tables 1 and 2 and plotted

on the MgO variation diagrams in Figure 4 and as BSE-normalized abundances in Figure 5. The

MgO abundances in the komatiite samples range from 26.6 to 35.42 wt %. One sample contains

14.2 MgO wt % and is classified as komatiitic basalt. The field evidence implies the komatiitic

basalt and komatiites erupted either in rapid succession from one vent or simultaneously from a

closely juxtaposed vent (Nisbet & Chinner 1981).

In Figure 4, strong negative correlations between Al, Ti, and MgO are displayed. The

regression lines are consistent with olivine control of element abundances over the entire range

of lava composition sampled, similar to other Archean komatiites (e.g., Puchtel et al. 2004b).

The data for the komatiitic basalt plots on the same trend line as the other komatiite samples,

indicating that these samples are likely genetically related. Ruth Well komatiites are depleted in

Al2O3 and have high Gd/Yb ratios relative to Al-undepleted komatiites, such as those from the

Alexo and Pyke Hill localities (Puchtel et al., 2004b).

Ruth Well komatiites are depleted in both light rare-earth elements (LREE), and heavy

rare-earth elements (HREE) ((La/Sm)N = 0.73 ± 0.08, (Gd/Yb)N = 1.39 ± 0.05). This suggests

that LREE and HREE were depleted during formation of the Ruth Well komatiites, albeit likely

via different processes. The samples have positive Th and U anomalies (Figure 5).

The ratios of Al2O3/TiO2 and (Gd/Yb)N are parameters used to estimate mantle source

characteristics, such as fractionation or retention of garnet in the mantle residue, and, as a result,

the depth of separation of a komatiite melt from the source. For example, the BSE-like

Al2O3/TiO2 and Gd/Yb ratios (Munro-Type) of the Alexo and Pyke Hill komatiites indicate they

formed at a depth less than 450 km in equilibrium with majorite-free residue (Nisbet et al. 1979,

Puchtel et al., 2004b). Komatiites with low Al2O3/TiO2 ratios (~11), are depleted in Al and

HREE (Figure 6). These komatiites belong to the Al-depleted (Barberton-Type) type of Nisbet et

al (1979) and are thought to have formed at >450 km depth in the majorite stability field

(Herzberg, 1995). This conclusion is consistent with Hypothesis 1.

7

Table 1.

Major (wt %) element abundance data for the Ruth Well komatiites

Sample 138308-1 158694-2 158694-1 158693-2 158693-1 158692-3 158692-2 158692-1 158691-2 158691-1 148259- 140258- 138239-1 137638 137644

SiO2 45.53 45.76 46.43 46.22 45.67 46.12 45.10 45.65 46.21 51.51 44.39 46.84 47.07 46.48 45.36

TiO2 0.37 0.36 0.39 0.32 0.34 0.36 0.30 0.35 0.36 0.61 0.35 0.35 0.40 0.37 0.25

Al2O3 4.09 4.00 4.33 3.58 3.64 3.88 3.43 3.80 3.85 6.55 3.80 4.01 4.44 4.04 2.72 Fe2O3 12.82 13.14 12.94 12.55 12.95 11.91 12.45 13.78 13.74 13.47 14.06 11.89 12.50 12.10 13.55 MnO 0.19 0.19 0.14 0.17 0.18 0.17 0.21 0.18 0.15 0.23 0.22 0.20 0.14 0.29 0.20 MgO 29.03 28.38 27.81 29.76 31.02 29.08 32.49 30.08 28.68 14.18 29.89 28.84 26.62 29.61 35.42 CaO 6.83 7.02 7.03 6.33 5.21 7.42 5.10 5.14 5.95 11.22 6.44 6.86 7.91 6.25 1.55 Na2O 0.43 0.45 0.27 0.38 0.31 0.36 0.27 0.29 0.29 1.46 0.11 0.30 0.27 0.20 0.17 K2O 0.11 0.11 0.05 0.08 0.06 0.08 0.06 0.12 0.17 0.40 0.15 0.10 0.04 0.07 0.02 P2O5 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.06 0.03 0.03 0.03 0.03 0.06

LOI 7.66 7.22 5.65 6.79 7.22 10.70 8.57 7.31 5.94 1.09 6.93 6.97 7.26 7.50 0.96

Al2O3/TiO2 10.91 11.11 11.16 11.22 10.71 10.81 11.43 10.83 10.69 10.82 10.83 11.43 11.01 10.9 10.9

Note. Analyses were recalculated on an anhydrous basis, but not renormalized to 100% in order to preserve information on the

quality of the analyses.

8

Table 2.

Trace element abundance data (ppm) for the Ruth Well komatiites

Sample 138239_1 140258 158691_1 158691_2 158692_1 158692_2 BSE

Th 0.162 0.140 0.163 0.150 0.206 0.194 0.0813

U 0.0652 0.0407 0.0418 0.0393 0.0778 0.0649 0.0203

Nb 0.573 0.531 0.627 0.539 0.656 0.626 0.6175

La 1.02 0.742 0.936 0.769 1.29 1.05 0.6139

Ce 2.71 2.01 2.61 2.12 3.28 2.85 1.601

Pr 0.426 0.324 0.415 0.334 0.503 0.457 0.2419

Nd 2.36 1.84 2.25 1.83 2.65 2.48 1.1890

Sm 0.884 0.721 0.829 0.671 0.902 0.951 0.3865

Hf 0.625 0.547 0.609 0.520 0.645 0.624 0.2676

Zr 19.6 19.6 21.8 20.4 23.1 21.4 9.7140

Gd 1.20 1.02 1.13 0.917 1.21 1.28 0.5128

Tb 0.204 0.173 0.198 0.157 0.203 0.223 0.0940

Dy 1.34 1.13 1.26 1.03 1.30 1.44 0.6378

Ho 0.267 0.228 0.252 0.208 0.263 0.293 0.1423

Y 6.20 5.99 6.47 5.99 6.94 7.32 3.94

Er 0.753 0.628 0.713 0.603 0.745 0.816 0.4167

Tm 0.105 0.0880 0.102 0.0866 0.105 0.116 0.0643

Yb 0.688 0.567 0.665 0.576 0.690 0.747 0.4144

Lu 0.101 0.0819 0.0965 0.0845 0.101 0.105 0.0637

Ti/Zr 108 107 99.1 103 77.8 100 112

Nb/Nb* 0.511 0.596 0.581 0.576 0.460 0.502 1.00

(Gd/Yb)N 1.41 1.46 1.37 1.29 1.41 1.38 1.00

(La/Sm)N 0.725 0.648 0.711 0.721 0.902 0.698 1.00

9

Al 2

O3

Wt

%

0

2

4

6

8

10

12

14

2.72 Ga Pyke Hill

2.72 Ga Alexo

Ruth Well

MgO Wt %10 20 30 40 50

TiO

2 W

t %

0.0

0.2

0.4

0.6

0.8

Figure 4.

Abundances of selected major elements (wt %) obtained in this study plotted against MgO (wt

%) in the Ruth Well komatiites compared to the data for Alexo and Pyke Hill from Puchtel et al.

(2004).

10

Figure 5.

BSE-normalized abundances of lithophile trace elements in the Ruth Well komatiites arranged in

decreasing order of incompatibility during mantle melting. The normalizing values are from

Hofmann (1988).

Figure 6.

Diagram illustrating Al/Ti and Gd/Yb data for Al-depleted and Al-undepleted komatiites from

various cratons and of various ages compiled from Puchtel et al. (2009a, b; 2013; 2016a,b;

11

2018). The data for the Ruth Well komatiites from this study plot in the field for the Al-depleted,

or Barberton-type, komatiites.

4.2. Re-Os isotope Systematics

The Re-Os isotopic data for the Ruth Well komatiites are listed in Table 3 and plotted on

the 187Re/188Os versus 187Os/188Os diagram in Figure 8 and on MgO versus HSE variation

diagram in Figure 9. Seven samples define a regression line with a slope corresponding to an

ISOPLOT model 3 age of 3179 ± 27 Ma a near chondritic initial 187Os/188Os = 0.10210±0.00082

(γ187Os = -2.9 ± 0.8), which is consistent with Hypothesis 2. This age is consistent, within

uncertainty, with a previous estimated age of ~3.2 Ga based on SHRIMP dating of felsic tuff

layers (Maier et al., 2009, and references therein). The low MSWD (19) indicates limited scatter

by the data points.

4.3. HSE abundances

HSE abundance data for the Ruth Well komatiites and the komatiitic basalt are listed in

Table 3 and plotted on the MgO vs HSE variation diagram in Figure 9. The trends for Pt and Pd

are consistent with olivine control indicating that these elements behaved immobile during

alteration, and that olivine was the only liquidus phase that controlled the concentrations during

differentiation of the Ruth Well lavas. The negative trends of Pt and Pd indicate Ruth Well lava

had a negligible amount of magmatic sulfide content (Keays 1982).).

In order to calculate the absolute HSE abundances in the mantle source of Ruth Well, the

projection technique of Puchtel et al. (2004b) was used. One of the pre-requisites for this

protocol is to have complete exhaustion of lower-temperature sulfides harboring Pt and Pd in the

mantle source during partial melting. This is established by Pt and Pd being incompatible shown

by Figure 9. An instant where this projection technique will not be useful is when Pt and Pd

show a positive slope indicating cumulates plus sulfides, which gives the high concentration of

Pt or Pd vs high MgO wt %. The calculated total Pt and Pd abundances in the Ruth Well

komatiite source is 55±4% of those in the estimates for the modern BSE, which is consistent

with Hypothesis 3. This estimate is similar to the source characteristics of several other early

Archean komatiite systems, such as the 3.48 Ga Komati (60±7%) and 3.26 Ga Weltevreden

(65±7%) (Puchtel et al., 2014). The Pt and Pd abundances and MgO content of the Pyke Hill

komatiitic olivine from Puchtel et al. (2004) were used based on the assumption that a komatiitic

magma similar in MgO content to that at Pyke Hill was likely the parental magma to Ruth Well

(Puchtel et al. 2018). Puchtel et al. (2018), references therein, states that since Pt and Pd are

highly incompatible in olivine and the small variations in abundances in the olivine would have

had negligible effect on the calculated Pt and Pd in the Ruth Well mantle source. The calculated

Pt and Pd abundances from the source of Ruth Well are 55±4%. In other early Archean

komatiites systems, the Pt and Pd abundances span a substantial range. The 3.55 Ga

Schapenburg characterized by lower Pt and Pd are 29±5%, but two other early Archean

komatiite systems vary from 60±7% for 3.48 Ga Komati to 65±7% for 3.26 Ga Weltevreden, of

the total Pt and Pd present relative to the estimates for modern BSE. (Puchtel et al. 2009a;2014).

The HSE abundances in early Archean komatiite sources were low, relative to the BSE, they

were variable, and show no trend of increasing HSE abundances.

Osmium concentrations have a general pattern of decreasing abundances with increasing

MgO content. This type of behavior is similar to that in the 3.55 Ga Schapenburg komatiite

system, although the abundances are higher in the Ruth Well komatiites. It appears that Os falls

12

on the olivine control line (Figure 9). This is typical of early Archean komatiites, such as those

from the 3.55 Ga Schapenburg or 3.48 Ga Komati komatiite systems (Puchtel et al., 2014; 2016).

This type of behavior indicates that Os carriers in the mantle, such as high-T sulfides and alloys,

were completely exhausted during partial melting that produced the Ruth Well komatiites, and

there was no fractionation of an Os-Ir phase, unlike in the case of the late Archean komatiites,

such as those from Alexo and Pyke Hill (Puchtel et al. 2004). The available data for Ru, indicate

minimal variance during olivine fractionation.

If assessing whether a specific element has been mobile, the element is characterized with

other elements in a known system. Most elements, in komatiites, should define trends that

extrapolate to the composition of olivine on MgO variation diagrams (see Figure 9). Any

indication of falling off the trends indicate mobility and/or involvement in other processes

(Puchtel et al. 2004). Lambert et al. (1998) suggests post recrystallization may occur during or

after deformation. It could be interpreted that post-recrystallization caused Re mobility. One

thing, the mobility of Re must have took place shortly after emplacement, with the Re-Os system

remaining undisturbed since then. The post-recrystallization would have not mobilized any major

oxides or Pt and Pd.

The calculated HSE abundances in the Ruth Well lavas are normalized to the average

values of CI chondrite Orgeuil from Horan et al. 2003 and plotted in Figure 10. Ruth Well lavas

are depleted in IPGEs relative to PPGEs, with (Pd/Ir)N = 1.90 to 2.48. Rhenium, Pd, and Pt are

more strongly partitioned into komatiite melts relative to chondrites than Ru-Ir-Os. The

enrichment order observed is Re>Pd>Pt>Ru>Os>Ir (Keays 1982). During partial melting, most

of the Ir and Os remains in the residue, while Pd is partitioned in the melt (see Figure 10).

Age, Ga

0.0 1.0 2.0 3.0 4.0

Pt+

Pd

in t

he

sou

rce,

% o

f B

SE

20

40

60

80

100

120 3.18 Ga Ruth Well

2.06 Ga Lapland

2.41 Ga Vetreny

2.69 Ga Belingwe

2.72 Ga Pyke Hill

2.72 Ga Alexo

2.72 Ga Boston Creek

2.82 Ga Kostomuksha

2.88 Ga Volotsk-Kam

3.26 Ga Weltevreden

3.48 Ga Komati

3.55 Ga Schapenburg

BSE

Figure 7.

Calculated total Pt and Pd abundances in the sources of Ruth Well and other Archean komatiite

systems plotted as per cent of the total Pt and Pd abundances in the estimates for

modern BSE of Becker et al. (2006). Uncertainties are 2SD. (Puchtel et al. 2004b, 2009, 2014,

2016a, b, 2018).

13

Table 3.

HSE abundances (ppb) and Re-Os isotopic data. Highlighted area indicates the komatiitic basalt.

Sample Re Os Ir Ru Pt Pd 187Re/188Os ±2SE 187Os/188Os ±2SE γ187Os(T)

158691_1 0.539 2.080 1.925 4.406 7.264 5.951 1.2559 0.008 0.17079 0.00017 -2.54

158691_2 0.570 1.995 1.830 4.458 6.644 5.810 1.3848 0.008 0.17671 0.00018 -3.58

158692_2 0.426 1.775 1.699 4.167 5.967 4.947 0.87205 0.003 0.15024 0.00013 -2.23

138239_1 0.221 2.073 1.910 4.102 6.981 5.959 0.61457 0.004 0.13507 0.00014 -3.34

140258 0.293 2.301 2.039 4.428 6.827 5.761 0.87205 0.005 0.15024 0.00015 -2.23

158692_2 0.404 2.237 2.102 4.621 7.253 6.358 10.984 0.066 0.69960 0.00070 -2.84

140259 0.325 0.153 0.256 0.848 3.636 4.819 0.5955 0.004 0.13394 0.00013 -3.43

137638 0.273 2.209 2.119 4.503 6.456 5.144 1.2559 0.008 0.17079 0.00017 -2.54

14

187Re/188Os

0 2 4 6 8 10 12

18

7O

s/1

88O

s

0.2

0.4

0.6

0.8

RW komatiites

RW komatiitic basalt

T = 3179±27 Ma187Os(T) = -2.9±0.8

MSWD = 19

Figure 8.

Re-Os diagram for 7 samples from Ruth Well analyzed in this study. The age defined by the

seven data points correspond to the age Ruth Well. See text for additional details.

15

Os

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pt

0

2

4

6

8

10

12

14

MgO

10 20 30 40 50

Re

0.0

0.2

0.4

0.6

0.8

1.0

Ir

0.0

0.5

1.0

1.5

2.0

2.5

MgO

10 20 30 40 50

Ru

0

2

4

6

Pd

0

2

4

6

8

10

RW komatiite

RW komatiitic basalt

Pyke Hill olivine

Figure 9.

Abundances of the HSE (ppb) in Ruth Well obtained in this study plotted against MgO contents

(wt %).

16

Os Ir Ru Pt Pd Re

Org

ue

il-n

orm

aliz

ed

0.001

0.01

0.1

RW komatiites

Figure 10.

HSE abundances in the Ruth Well komatiite normalized to the CI chondrite Orgueil (Horan et

al., 2003). Left to right, the sequence of elements follows the increasing incompatible behavior

during mantle melting.

5. Discussion Determining the absolute and relative abundances of the HSEs in the early terrestrial mantle

is vital for understanding early planetary processes. Planetary processes refer to earth’s primary

separation of metallic core from silicate mantle (Righter 2003), continued accretion following

core formation (Morgan 1985), differentiation of the mantle (Walker et al. 1989), and possibly,

inner core crystallization and core-mantle exchange (Walker et al. 1995).

5.1. Ruth Well Archean Oceanic Crust?

Arndt et al. (2001) suggested that Ruth Well are approximately contemporaneous with

the Warawoona Group. The Warawoona group has slightly enriched LREEs, and small negative

Nb anomalies and appears to have interacted with continental crust. Ruth Well lacks any

indication that it reacted with continental crust. Ruth Well have a depletion in LREE instead of

enrichment in LREE, indicating that the source of Ruth Well experienced previous episodes of

melt extraction. Chase and Patchet (1988) argued that the complimentary reservoir to the

depleted mantle was mafic or ultramafic oceanic crust. This could indicate that Ruth Well

constitute for being normal oceanic crust that formed at an ocean spreading center and was

subsequently accreted to the growing Pilbara continent (Ohta et al. 1996). In order to prove that

17

Ruth Well is part of the existence of Archean oceanic crust would be challenging, although it

may exist (Arndt et al. 2001).

6. Summary Ruth Well komatiites are some of the oldest (~3.18Ga) mantle-derived lavas on earth. The

komatiites differentiated after eruption into upper sections of olivine and spinifex rocks and

lower sections of olivine cumulate rock. These rocks are altered, and few primary minerals are

present, the shape and distribution of relict grains indicate that olivine was the principal mineral

in both spinifex and cumulate lavas (Arndt et al. 2001). In this study, we report new Re-Os

isotopic and HSE abundance data, for Ruth Well komatiites from the Pilbara Craton, Western

Australia.

Ruth Well komatiite melt formed deep (>450km) in the mantle in the majorite stability

field which resulted in the Al-depleted signature of the parental melt. The alumina-depleted

komatiites are characteristic of the Barberton-Type and have an estimated age of 3.3 Ga. This

contrasts with Alexo and Pyke Hill komatiites that formed in equilibrium with majorite-free

residue forming above 450 km depth. Ruth Well was depleted in LREE and HREEs [(La/Sm)N

= 0.70 ± 0.03, (Gd/Yb)N = 1.37 ± 0.05]. The Ruth Well komatiites were derived from a depleted

source interpreted by the depletion of LREE and the depletion of HREEs from melting in

equilibrium with garnet.

The mantle source of Ruth Well, that evolved with long term chondritic Re/Os due to the

late accreted component. The data provides meaningful geochronological information, which

define a regression line with a slope corresponding to an ISOPLOT model 3 age of 3179±27Ma

and a near chondritic initial 187Os/188Os = 0.10210±0.00082 (γ187Os = -2.9 ± 0.8) which is

consistent with SHRIMP dating of felsic tuff layers, which were dated at ~3.2 Ga.

Ruth Well komatiites were deficient in HSE abundances compared to BSE (Figure 10).

The differentiation of the history of the Ruth Well magma is poorly constrained prior to

emplacement by Os, Ir, and Ru fractionation, and the mobile post-emplacement behavior of Re.

The abundances of Pt and Pd in Ruth Well have incompatible and immobile behavior and the

total Pt and Pd abundances calculated in the Ruth Well komatiite source are 55±4% of those in

the estimates for the modern bulk silicate earth (BSE), relatively close to other early Archean

komatiite systems such as Komati (60±7%) and Welevreden (65±7%), but substantially more

than Schapenburg (29±5%).

Ruth Well komatiites are providing insights on early earth processes and understanding

the relative and absolute HSE abundances of the Archean mantle. Ruth Well provide information

on the HSE abundances from the Ruth Well mantle source. This study helps constrain the

absolute and relative HSE abundances from the Archean.

18

35

40

45

50

55

Wt % MgO

10152025303540

Wt

% o

f o

xid

es

0

2

4

6

8

10

12

14

16

K2O

Na2O

CaO

Fe2O3

Al2O3

SiO2

Figure 11.

Fenner diagram: Major element oxide abundances have negative correlations with wt. % MgO

abundance, indicating that the fractionation of olivine controlled the chemical differentiation of

the Ruth Well samples, and that all major elements were not significantly affected by alteration.

19

Acknowledgements I would like to thank Robert William Nicklas for assisting with the SA ICP-MS procedure, and

collecting the trace element data.

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