Platinum group element fractionation in a komatiitic basalt lava lake

Geochimica et Cosmochimica Acta, Vol. 65, No. 17, pp. 2979 –2993, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 ⫹ .00

Pergamon

PII S0016-7037(01)00642-1

Platinum group element fractionation in a komatiitic basalt lava lake IGOR S. PUCHTEL1,* and MUNIR HUMAYUN1 1

Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA (Received July 1, 2000; accepted in revised form April 5, 2001)

Abstract—Precise PGE (Os, Ir, Ru, Pt, Pd) abundance data for bulk rock samples, and mineral separates, from a deep differentiated komatiitic basalt lava lake in the Vetreny Belt (Baltic Shield) were obtained using an ICP–MS ID technique. The composition of the erupted liquid (MgO ⫽ 15%) is characterized by a highly fractionated PGE pattern, (Pd/Os)N ⫽ 48, with subchondritic (Os/Ir)N ⫽ 0.43. Analysis of separated olivine and chromite revealed that Ru, Os, and Ir were slightly compatible to moderately incompatible in olivine (D ⫽ 1.7–0.8), and were compatible with chromite (D ⫽ 100–150). Platinum and Pd were highly incompatible with olivine (D ⫽ 0.08–0.03), and moderately compatible with chromite (D ⫽ 1.6–3.3). Bulk solid–liquid partition coefficients for PGEs, Ni, Cr, and Cu, estimated from the whole rock regressions, were: Os 7.9, Ir 6.6, Ru 4.5, Pt 0.53, Pd 0.09, Ni 6.2, Cr 4.4, and Cu 0.01. The highly incompatible behavior of Cu and Pd indicates that there was no separation of FeNiCu immiscible sulfide liquid during the differentiation of the lava lake. Comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included ⬃10⫺5⫾1 mass fraction of pyrrhotite. The data obtained provide new constraints on the effects of fractional crystallization on the PGE compositions of mafic– ultramafic magmas. Copyright © 2001 Elsevier Science Ltd attempt to reconstruct the original PGE compositions of erupted komatiite magmas and, thus, their mantle sources. The fractionation processes in komatiite lavas have mainly been addressed by studies of major and lithophile trace element distributions in layered flows (e.g., Arndt, 1986a; Barnes et al., 1988b). Data on the PGE fractionation in komatiites have been essentially limited to abundances of Pd, Ir, and Pt (e.g., Crocket and MacRae, 1986). Bru¨gmann et al. (1987) reported Os, Ir, Ru, and Pd abundances for several flows from Alexo and Gorgona Island komatiites, and proposed that olivine and chromite controlled Os, Ir, and Ru abundance variations based on covariation of these elements with MgO and Ni. Rehka¨mper et al. (1999) reported Ir, Ru, Pt, and Pd on six komatiite samples, including some of those from the work of Bru¨gmann et al. (1987). Barnes and Picard (1993) presented Ir, Rh, Pt, and Pd data for komatiitic basalts from the Cape Smith fold belt and concluded that Ir abundance in the melt was controlled by olivine and chromite fractionation. They also found that Pt and Pd behaved incompatibly until the very last stages of magma differentiation, during which S saturation was achieved, and proposed that Pt and Pd were scavenged by an immiscible FeNiCu sulfide phase. Puchtel and Humayun (2000) reported Os, Ir, Ru, Pt, and Pd abundances on a number of komatiite flows from the Kostomuksha greenstone belt, and showed that these had undergone little PGE fractionation despite olivine accumulation. There have been many determinations of PGEs in mineralized lavas due to their economic significance (e.g., Ross and Keays, 1979; Keays et al., 1981; Barnes and Naldrett, 1986; 1987). In these mineralized lavas, PGE abundances were mainly controlled by the removal of an immiscible FeNiCu sulfide liquid. There remains substantial ambiguity with respect to the ultimate cause of the PGE fractionation observed in nonmineralized komatiites. Removal of Os, Ir, and Ru is usually ascribed to olivine (⫾chromite) settling (Crocket and MacRae, 1986; Bru¨gmann et al., 1987). Other frequently cited phases

1. INTRODUCTION

Platinum-group element (PGE) abundances of the mantle have seen increasing application to studies of Earth’s accretion, core formation, mantle differentiation and core-mantle interaction (Rehka¨mper et al., 1997; Brandon et al., 1998; 1999; Puchtel and Humayun, 2000; Righter et al., 2000). The PGE abundances of the mantle are difficult to infer from basalts, since these exhibit a complex history of PGE differentiation (Rehka¨mper et al., 1999). In contrast, komatiites are highly magnesian lavas that have been used to derive the siderophile element content of the mantle (Sun, 1982; Bru¨gmann et al., 1987; Puchtel and Humayun, 2000). This is possible because of the specific conditions of komatiite formation, including high degrees (30 –50%) of partial melting either in deep mantle plumes (Campbell et al., 1989; Storey et al., 1991) or in Archean subduction zones (Grove et al., 1999), sulfur undersaturation, very rapid adiabatic ascent and eruption at temperatures close to the liquidus. These conditions ensure little or no fractionation prior to eruption. One of the most fascinating features of komatiite flows is their layered structure (e.g., Pyke et al., 1973). Following eruption, some lavas differentiate into an upper spinifex and a lower cumulate layer with a sharp boundary between the two. This differentiation is manifested by regular changes in mineralogy and chemical composition throughout the layered units, including an increase in modal olivine (⫾chromite) abundances in the cumulate layers, accompanied by an increase in MgO, Ni (⫾Cr) and a decrease in incompatible lithophile trace element contents (e.g., Arndt et al., 1977). Differentiation of komatiite lavas is also accompanied by PGE fractionation. Therefore, understanding the differentiation processes is important in any

* Author to whom correspondence should be addressed (ipuchtel@ geosci.uchicago.edu). 2979

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include FeNiCu sulfides, Os–Ir alloys and platinum-group minerals such as laurite RuS2 (Keays, 1982; Barnes et al., 1985; Bru¨gmann et al., 1985; Crocket and MacRae, 1986; Barnes et al., 1988a; Rehka¨mper et al., 1999). Particularly problematic has been the relative fractionation of Os, Ir, and Ru from Pt and Pd observed in magmatic rocks. Experimental sulfide liquidsilicate liquid partition coefficients for Ir and Pd are equal, D ⬇ 10 4 (Peach et al., 1990; Fleet et al., 1996), and thus phases that control Os, Ir, and Ru such as Os–Ir alloys and laurite have been proposed to explain this fractionation (Keays, 1982; Rehka¨mper et al., 1999). Clearly, PGEs behave distinctly under different conditions of magma differentiation, and the role of various major or minor mineral phases has yet to be established conclusively. A highly differentiated, well preserved komatiitic basalt lava lake (Puchtel et al., 1996) afforded the opportunity to study the effects of fractional crystallization during magma differentiation. Puchtel et al. (1997) found evidence for crustal contamination prior to eruption of this originally komatiitic magma. Crustal contamination has a pronounced effect on the sulfur fugacity and eruption temperatures of komatiitic magmas, and, thus, on the PGE behavior (e.g., Sun et al., 1991). As crustally contaminated komatiites are rather common in Precambrian continental settings (e.g., Arndt, 1986b), better understanding of the effects of crustal contamination on PGE abundances in komatiites is required prior to deducing mantle PGE compositions. In this study, precise PGE abundances for whole rock samples and mineral separates of olivine, chromite, and sulfide were obtained and used to (1) reconstruct the PGE composition of the erupted magma and estimate the siderophile/chalcophile element mass balance for the lava lake; (2) infer the PGE composition of the fractionating mineral assemblage and estimate the PGE partition coefficients for the liquidus phases involved in the differentiation of the lava lake; (3) assess the impact of assimilation-fractional crystallization (AFC) processes on PGE fractionation in komatiites. 2. GEOLOGICAL AND GEOCHRONOLOGICAL BACKGROUND

The ⬃250 km long Vetreny Belt is one of the largest Paleoproterozoic volcano-sedimentary belts in the SE Baltic Shield (see Fig. 1 in Puchtel et al., 1997). It was interpreted to have formed in a continental rift setting during the interaction of a mantle plume with the Archean continental crust of the Karelian granite– greenstone terrane. Impingement of a mantle plume head beneath the continental lithosphere ⬃2.45 Ga ago resulted in its thinning, stretching and rifting, and catastrophic eruption of vast volumes of mafic and ultramafic magmas over a short period of time (Puchtel et al., 1997). Trace element and Nd–Pb isotope studies, and petrological modeling, showed that magmas parental to the Vetreny Belt lava sequences were komatiitic in composition (⬎18% MgO) and were derived from a long-term LREE-depleted mantle source with ␧143Nd(T) ⫽ ⫹2.6 (Puchtel et al., 1997). Based on the Nb–Th–La relative abundances and Nd–Pb isotope data, it was inferred that the chemical evolution of the Vetreny primary komatiite magmas en route to the surface was controlled by fractional crystallization and 4 –15% contamination

with ⬃3.2 Ga old felsic crustal rocks. Puchtel et al. (1997) reported Sm–Nd internal isochron ages of 2449 ⫾ 35 and 2410 ⫾ 34 Ma and a bulk-rock Pb–Pb isochron age of 2424 ⫾ 178 Ma for the uppermost Vetreny suite komatiitic basalts. They also obtained a U–Pb zircon age of 2437 ⫾ 3 Ma for the lowermost Kirichi suite andesites. A Re–Os isochron age of 2384 ⫾ 57 Ma, with a chondritic initial ␥187Os ⫽ 0.3 ⫾ 1.0, was reported for the lava lake by Puchtel et al. (2001b). Petrology of Victoria’s lava lake was described in detail by Puchtel et al. (1996). The lake is a 110 meter-deep pond of komatiitic basalt erupted with ⬃15% MgO, which filled a large topographic depression soon after eruption. After emplacement, the lake underwent substantial differentiation, and developed a prominent internal layered structure. It is comprised of four main units (from top downward): scoria, upper chilled margin, spinifex zone (spinifex subzone proper and a lower layer of fine-grained basalt), and cumulate zone (Fig. 1). The rocks are characterized by a superb state of preservation and, except for the scoria and upper chilled margin, represent almost completely unaltered mineral assemblages, containing in places primary volcanic glass. The metamorphic grade of the scoria did not exceed the prehnite–pumpellyite facies. A set of 12 samples from a complete section through the lake (Fig. 1), representing the full range of observed variations in MgO, Cr, and Ni abundances, was selected for PGE analysis. Also analyzed were four olivine separates from the cumulate samples, and chromite and sulfide mineral separates from cumulate sample 91105. 3. ANALYTICAL TECHNIQUES

3.1. Mineral Separation Pure mineral separates were obtained at the Institute of Geology, Karelian Research Center, Petrozavodsk, using heavy liquid and electromagnetic separation techniques, followed by handpicking. The two sulfide fractions of 91105 were separated on the basis of differing magnetic susceptibility. Analyses of S, Fe, Ni, and Cu in these sulfide fractions were performed on separated grains using a JEOL JSM5800LV scanning electron microscope (SEM) at The University of Chicago. Operating conditions were 15 kV accelerating voltage and 4 –5 nA beam current, in low vacuum mode on uncoated grain mounts. Olivine and chromite compositions were determined by electron microprobe (Puchtel et al., 1996; unpublished data). 3.2. Analysis of MgO, Ni, Cr, Cu, Zn by XRF Concentrations of the elements Cr, Ni, Cu, and Zn in bulk rocks were determined on pressed powder pellets, and MgO was determined on fused glass discs, by x-ray fluorescence using a Philips PW-1404 spectrometer at the Johannes–Gutenberg Universita¨t in Mainz. Puchtel et al. (1996) presented MgO, Ni, and Cr data for these samples, while Cu and Zn abundances are reported here. Precision was 2% for MgO, and ⬃5% for Cr, Ni, Cu, and Zn. 3.3. PGE Analysis by NiS Fusion The NiS fire assay technique was utilized for extraction of the PGEs from bulk rock samples (⬃3.5 g of sample powder)

Platinum group element fractionation

Fig. 1. Detailed profile through Victoria’s lava lake (after Puchtel et al., 1996). Locations of the samples are marked in the section.

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Fig. 2. The external reproducibility of Ru, Pd, Ir, and Pt isotope ratio measurements of the in-house 0.54 ppb PGE-standard solution carried out in the course of this study (10/99 – 01/00).

and an olivine separate from sample 91105 (⬃1.7 g). The experimental protocol for the isotope dilution PGE analysis was modified from that of Puchtel and Humayun (2000) as described below. After dissolving the NiS bead and filtering the solution, the cellulose membrane containing the PGE sulfides and trace element impurities was divided into two parts. One part was used for Ru, Pd, Ir, and Pt abundance determinations, as before. The second part of the membrane was used to extract and purify Os by adapting the microdistillation procedure of Birck et al. (1997) with modifications (A. D. Brandon, personal communication). A piece of membrane was placed into the cap of a 5 ml conical Savillex™ PFA beaker and oxidized using ⬃100 ␮l of chromic acid (CrO3:12N H2SO4 ⫽ 1:1) at 60°C for 16 –18 hours. The microdistilled Os was trapped in 20 ␮l of conc. HBr, and then introduced into the ICP torch via an MCN6000 desolvating nebulizer from a reducing solution.

dissolved at ⬃120°C in 15 ml closed Savillex™ PFA beakers using a mixture of ultrapure concentrated HF ⫹ HNO3 ⫹ HClO4 acids, followed by inverse aqua regia. This procedure did not allow Os determinations for these olivine separates. The sulfide separates (0.2–1.2 mg) were spiked and dissolved in 2 ml closed Savillex™ PFA beakers in hot inverse aqua regia. All solutions prepared above were dried down, taken up in 0.15 N HCl and loaded on a cation exchange column where anionic PGE chlorocomplexes were eluted in the first 1.5 mL of 0.15 N HCl, while Fe, Cr, Ni, Cu, etc., were retained on the column. The procedure was repeated two to three times and the final fraction obtained was used directly for ICP–MS analysis. Since these minerals lack incompatible trace elements, isobaric interferences of Zr, Lu, Hf, and Ta oxides (e.g., Ely et al., 1999) were not observed. 3.5. Isotopic Measurements

3.4. PGE Analysis by Acid Digestion Acid digestion was used for olivine, chromite, and sulfide separates. For chromite, Carius tube dissolutions following the method of Shirey and Walker (1995) were used. Approximately 40 mg of pure chromite separate from sample 91105, mixed PGE spike, and 6 ml of inverse aqua regia (3 ml HNO3 : 1 ml HCl) were placed into a frozen quartz glass Carius tube and sealed immediately. The tube was allowed to warm up to room temperature, was wrapped in aluminum foil, placed in a stainless-steel jacket, and heated at 220°C for 48 hours. After cooling to room temperature, the tube was chilled to approximately ⫺20°C, broken open, and the clear sample solution removed. For olivine separates, ⬃130 mg sample was spiked and

Isotopic measurements of PGEs were performed on a singlecollector, magnetic sector, high-resolution ICP–MS, the Finnigan Element™ (Puchtel and Humayun, 2000). Typical count rates were 105–106 cps (for Os ⬇ 103–105 cps), and internal precisions of individual runs were between 0.2– 0.5% for Ir, Ru, Pt, and Pd and 0.4 –1.0% for Os (2␴aver). Long-term reproducibility of a 0.54 ppb in-house PGE-standard solution, which characterizes the external precision of the analysis, was between 0.3% and 1.0% (2␴pop) on all isotope ratios (Fig. 2). 3.6. Procedural Blanks and Reproducibility Analytical blanks for the NiS fusion, Carius tube and acid digestion procedures are reported in Table 1. Total proce-

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Table 1. PGE data for analytical blanks and blank corrections applied. Elements

Os

Blanks (pg) NiS fire assay Quartz Carius tube Teflon Blank corrections (%) Bulk rocks Olivine by fusion Olivine by acid digestion Chromite Sulfide

Ir

17

17 2.7 0.5

0.5–7 2

0.2–4 2 1–5 0.1 1.0

0.2 3.5

Ru 130 16 41 1–5 3 4–6 0.1 0.9

Pt

Pd

253 15 4.4

657 4.6 25

0.5–1.5 20 1–3 0.1 3.7

1–2 120 20–60 0.6 0.1

4. RESULTS

Concentrations of PGEs in the rocks and mineral separates from the lava lake are listed in Table 2 and shown as chondritenormalized abundances in Fig. 3. The scoria and upper chilled margin samples (91110 and 91112, ⬃15% MgO) have nearly identical PGE abundances and are characterized by sloping patterns (Os ⬍ Ir ⬍ Ru ⬍ Pt ⬍ Pd) with a (Pd/Os)N ⬇ 48 and a subchondritic (Os/Ir)N ⬇ 0.43. This is in marked contrast with the PGE distribution patterns in other komatiites, which exhibit much less fractionated patterns [(Pd/Os)N ⫽ 5–9] and chrondritic Os/Ir ratios (Bru¨gmann et al., 1985; Puchtel and Humayun, 2000). Spinifex-textured komatiitic basalts and olivine cumulates from the lava lake have Pt and Pd abundances roughly similar to those in the chilled samples. However, Os, Ir, and Ru concentrations are substantially lower in the former [(Pd/Os)N ⬇ 200] and much higher in the latter [(Pd/Os)N ⬇ 12]. The (Os/Ir)N ratio in the scoria and cumulates remains nearly constant (0.42– 0.53), but increases in the spinifex-textured rocks [(Os/Ir)N ⫽ 0.63– 0.80]. The (Ru/Ir)N ratio shows a systematic increase from 1.9 –2.4 in the cumulates through 3.1–3.3 in the scoria and upper chilled margin to 7–10 in the spinifex-textured rocks. Olivine separates analyzed in this study comprised lightgreen grains 100 –200 micron in size with rare visible inclusions of glass and products of its devitrification. The olivines have low PGE abundances with a spike at Ru (Fig. 3). The Os

dural blank of the NiS fire assay technique of the present study was determined from two individual fusions using ⬃200 mg of low-PGE basalt #9443 of known PGE abundances, and compares favorably with that reported by Puchtel and Humayun (2000). Blank corrections applied are also listed in Table 1, and were negligible except for Pt and Pd in the NiS fusions of the olivine separate. Reproducibility of the NiS fire assay procedure, assessed by replicate analyses of three samples from the lava lake (Table 2), was 1– 6%, and was mainly determined by uncertainties from sample powder heterogeneity.

Table 2. PGE and selected major and trace element data for the lava lake. Sample

MgO

Cr

Ni

Scoria and upper chilled margin 91110 14.7 1619 408 91112 13.7 1570 348

Cu 84 81

Ol and Px spinifex-textured komatiitic basalts 91101 8.85 688 119 95 91101* 91117 10.1 898 174 92 91119 9.94 836 193 96 91122 9.16 642 141 96 91108 7.55 545 70 96 Olivine–chromite cumulates 91103 20.4 2317 91104 21.6 2139 91104* 91105 26.1 3451 91105/1 22.2 2922 91105/1* 91106 22.6 2066 Mineral separates 103 Ol(t) 47.1 104 Ol(t) 45.4 105 Ol(f) 46.3 (f) 105 Ol * (t) 105 Ol 106 Ol(t) 47.3 105 Crt(ct) 2.69 (t) 105 Sulf (t) 105 Sulf *

Zn

Os

Ir

76 77

0.209 0.211

0.453 0.508

85

0.041 0.042 0.054 0.102 0.046

0.819 0.655 0.679 0.813 0.499 0.493 0.764

83 82 84 86

801 874

65 67

75 77

1138 918

54 63

73 76

952

62

75

513 469 710

2499 2240 2280

629 46.8

2428 272 37.0 36.8

0.215 0.285

0.6 0.3

32.2 11.8 17.7

Pt

Pd

(Ru/Ir)N

(Os/Ir)N

2.19 2.31

9.63 10.6

11.9 11.5

3.3 3.1

0.46 0.41

0.060 0.061 0.084 0.127 0.063 0.072

0.861 0.945 0.957 1.18 0.729 0.747

11.0 11.0 9.88 10.3 10.1 10.5

13.8 13.8 11.7 12.1 11.4 13.2

9.8 10.5 7.7 6.3 7.9 7.0

0.68 0.68 0.63 0.80 0.73

1.68 1.60 1.57 1.51 1.05 1.04 1.73

4.67 4.60 4.69 5.32 3.60 3.50 5.52

8.16 9.26 9.96 7.85 8.11 8.26 8.89

9.37 12.1 12.6 7.35 8.27 8.59 11.8

1.9 1.9 2.0 2.4 2.3 2.3 2.2

0.48 0.41 0.43 0.53 0.47 0.47 0.44

0.239 0.181 0.362 0.409 0.335 0.121 47.9 39.9 67.1

Ru

4.43 4.89 2.79 3.21 5.48 4.28 340 1922 3130

1.39 0.695 0.354 0.747 1.25 0.243 33.5 48.9 156

0.488 0.261 0.239 0.326 0.341 0.316 18.7 14450 21160

13 18 5.2 5.3 11 24 4.8 33 32

0.59 0.69 0.67 0.29 0.26

* Different digestions of the same sample. Analyses recalculated on an anhydrous basis. PGE in ppb, MgO in wt. %, Cr, Ni, Cu, Zn in ppm (bold face values in wt. %). MgO, Cr, and Ni data from Puchtel et al. (1996; ). Ni and Cu data for sulfide from this study. (t)Digestion in Savillex beakers, Digestion in quartz Carius tube, (f)Digestion using NiS fireassay.

(ct)

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Fig. 3. CI-chondrite normalized (Anders and Grevesse, 1989) PGE abundances in the lava lake rocks and mineral separates. Composition of the primitive mantle (dashed line) is calculated assuming an Ir content of 3.3 ppb (Morgan, 1986) and CI-chondrite relative abundances of Anders and Grevesse (1989). Scoria composition is shown for reference purposes on panels (a)–(c).

abundance of the olivine separates from 91105 (0.2 ppb) is higher than that reported by Puchtel et al. (2001b) for separated olivine from 8950 (0.05 ppb), but is similar to that reported by Walker et al. (1999) from a Gorgona komatiite (0.2 ppb). The chromite separate consisted of solid euhedral opaque grains up to 50 micron across. The PGE abundance pattern in the chromite (Fig. 3) is similar to that of the olivine, though the concentrations in the chromite are almost two orders of magnitude higher. The Os abundance of the chromite separate from 91105 (32.2 ppb) is consistent within 2% with that reported by Puchtel et al. (2001b) for the same chromite separate measured by NTIMS (31.6 ppb). These values are closer to the higher end of a range of chromite Os abundances (4 –54 ppb) reported from Re–Os isotopic studies (Walker et al., 1999; Puchtel et al., 2001a; 2001b). The sulfide separates consisted of 20 to 180 micron subsequant grains of pentlandite (37% Ni, 31% Fe, 31% S), with minor (⬃10%) pyrrhotite and pyrite, and are characterized by a distinct PGE pattern, with high abundances of the light PGEs, Ru and Pd, and much lower concentrations of Os,

Ir, and Pt. No substantial differences in PGE distribution patterns were revealed between the two fractions, though the more magnetic fraction had a higher overall PGE content. The Os/Ir ratio in all mineral fractions is subchondritic and varies between 0.3 (sulfide) and 0.7 (olivine and chromite). 5. DISCUSSION

5.1. PGE Content of the Erupted Liquid The volcanic breccia (scoria) and upper chilled margin with olivine microspinifex texture formed almost instantaneously after emplacement of the lava. This part of the lake did not experience any differentiation after eruption and therefore was interpreted by Puchtel et al. (1996) to be representative of the major and trace element composition of the liquid from which it formed. It follows that the PGE abundances in the samples of the scoria and upper chilled margin (91110 and 91112) also represent those of the erupted komatiitic basalt. This was con-

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Table 3. Results of mass-balance calculations for the lava lake. Interval, m Length, m Rock type Samples averaged MgO, wt. % Cr, ppm Ni Cu Zn Os, ppb Ir Ru Pt Pd a

0–21

21–39

39–79

79–104

104–110

0–110

21 Scoria ⫹ chill 91110 91112

18 Upper spx 91117 91119 91101 9.63 807 162 94 83 0.068 0.090 1.01 10.4 12.5

40 Lower spx 91122 91108

25 Upper cum 91105/1 91105

110 Bulk lake

8.36 594 106 96 85 0.046 0.068 0.74 10.3 12.3

24.2 3187 1028 59 75 0.654 1.28 4.44 8.02 7.89

6 Lower cum 91103 91106 91104 21.5 2174 876 65 76 0.750 1.67 4.95 8.89 11.2

14.2 1595 378 83 77 0.210 0.481 2.25 10.1 11.7

14.0 1495 419 83 80 0.257 0.513 2.14 9.68 11.2

% deva

⫺1 ⫺6 10 0 5 22 7 ⫺5 ⫺4 ⫺4

Calculated as the percent difference between the composition of the scoria ⫹ chill and that of the bulk lake.

firmed by performing a mass balance of the various differentiated units. Shown in Table 3 are the results of mass-balance calculations for MgO, Cr, Ni, Cu, Zn, and PGEs in the lava lake. To calculate compositions of various subunits within the lava lake, compositions of appropriate samples were averaged as specified in Table 3. The results indicate that the excess or deficit of MgO, Cu, Zn, Ru, Pt, and Pd in the cumulate portion is consistent, within 5%, and of Ni, Cr, and Ir within 10%, with the deficit or excess of these elements in the spinifex portion. This implies that the fractionation in the lake proceeded in an essentially closed system and that no tapping or replenishment of the magma had taken place after the lake ponded in a topographic depression. A larger than average deviation for Os (22%) is attributable to an insufficient coverage of the upper part of the cumulate zone, represented by two samples, one of which is relatively enriched in Os. 5.2. Fractional Crystallization of the Lava Lake The effect of fractional crystallization processes on PGE composition of the erupted magma are considered below. The fractionating minerals used in modeling were either observed as cumulus phases or were inferred on the basis of trace element distribution in the lava lake. Elemental variations at the whole-rock scale for Cr, Ni, Cu, and Zn vs. MgO are shown in Fig. 4 and PGEs vs. MgO and Cr are shown in Fig. 5. The general implications of the inferred fractionation processes for komatiitic magmas are considered, as well. The role of olivine and chromite. Petrographic observations (Puchtel et al., 1996) indicated that the major fractionating phases were olivine (Fo ⫽ 87– 89%) and chromite (# ⫽ 0.69 – 0.77). This is supported by strong positive correlations between Ni, Cr, and MgO in Fig. 4, from which the ratio of olivine to chromite was inferred to be 50:1. Regression of the data in the MgO vs. Ni plot shows that the regression line intersects the Ni axis at ⬃2400 ppm at the estimated composition of the cumulus olivine, MgO ⫽ 48 wt.%. This is consistent with EMP analysis of olivine cores (Table 2) and indicates that Ni variation in the lava lake was controlled by olivine. From the correlations of Cu and Zn vs. MgO (Fig. 4), bulk partition coefficients of 0.01 and

0.76, respectively, were obtained, indicating that Cu was highly incompatible, and Zn moderately incompatible, in the fractionating assemblage. Horn et al. (1994) reported a chromite-liquid partition coefficient for Zn of 4.5 which, combined with the fraction of chromite in the crystallizing assemblage (⬃2%), implies that chromite had a negligible effect on Zn abundances. Thus, the behavior of Zn in the lava lake was controlled by olivine fractionation, as well. Bougault and Hekinian (1974) obtained olivine/glass distribution coefficients for Cu ⫽ 0.11 and for Zn ⫽ 0.86 in MORB lavas, which are similar to those estimated above, except that Cu was slightly more incompatible in the lava lake than in the MORB lavas. The Os–Ir abundances in the lava lake vary by nearly two orders of magnitude, and the Ru abundances vary by an order of magnitude (Fig. 5). These elements show a substantial decrease in the spinifex-textured lavas (Pd/Os)N ⬇ 200, and an increase in the olivine cumulates, (Pd/Os)N ⬇ 12, compared to the composition of the erupted liquid (Pd/Os)N ⬇ 50. The compatible behavior of Os, Ir, and Ru in the lava lake is qualitatively similar to that observed by Bru¨gmann et al. (1987) in Alexo komatiites, and for Ir by Crocket and MacRae (1986) in Pyke Hill komatiites and Fred’s flow komatiitic basalts. However, it differs from that in the Kostomuksha komatiites, in which Os and Ir were moderately incompatible (D ⬃ 0.7) during olivine fractionation (Puchtel and Humayun, 2000). Based on the covariation of Os, Ir, and Ru abundances with MgO and Ni, Bru¨gmann et al. (1987) concluded that these were controlled by olivine fractionation, and inferred bulk D ⬇ 2 for Os, Ir, and Ru. These conclusions were supported by Crocket and MacRae (1986) and Barnes and Picard (1993). Here, the effects of bulk olivine ⫹ chromite fractionation on PGE compositions in the lava lake are modelled on the basis of PGE compositions of mineral separates. PGE analyses of separated olivine fractions from four cumulates are given in Table 2, and shown in Fig. 5. These olivine separates are characterized by low PGE abundances, with the exception of Ru. The olivine PGE pattern is similar to that of the chromite, but the abundances in the former are lower by almost two orders of magnitude. The olivine data cannot be accounted for by assuming ⬇1% contamination of the olivine

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Fig. 4. Variation diagrams of selected chalcophile/siderophile elements (ppm) vs. MgO (wt. %) for the lava lake rocks and mineral separates. The calculated olivine composition was obtained from regression of the bulk rock data, assuming the olivine maximum MgO content of 47.9% from Puchtel et al. (1996).

separate with chromite inclusions, as the latter were not detected during thorough inspection of the separates under the microscope. The olivines exhibit D Os ⫽ 1.2, D Ir ⫽ 0.8 ⫾ 0.2, D Ru ⫽ 1.8 ⫾ 0.4, with Os and Ru being moderately compatible, and Ir being slightly incompatible. These D values are similar to those of Bru¨gmann et al. (1987), with the exception of Ir, which is a factor of 2 lower. Platinum is moderately incompatible in olivine, with D Pt ⫽ 0.08 ⫾ 0.05, while Pd is highly incompatible, with D Pd ⫽ 0.03 ⫾ 0.01. The olivine D Ir and D Pd obtained here are consistent with the bulk partition coefficient inferred from fractionation trends in the Kostomuksha komatiites by Puchtel and Humayun (2000); the D Pt is significantly lower. It should be noted that these D values decrease as Ru ⬎ Os ⬎ Ir ⬎ Pt ⬎ Pd, while the oxidation potentials of the respective PGEs increase in the same order (O’Neill et al., 1995), qualitatively consistent with partitioning into the lattice structure of olivine. Chromium behavior in the Vetreny komatiitic basalt was very similar to that in Fred’s Flow, Munro Township, which had about the same MgO content in the erupted magma (⬃15%, Crocket and MacRae, 1986). In contrast, Cr was incompatible with the liquidus mineral assemblage in the Kostomuksha and Pyke Hill komatiites, where the erupted liquid contained 26 –27% MgO (Crocket and MacRae, 1986; Puchtel

et al., 1998). Komatiite studies showed that chromite normally did not crystallize during the differentiation of komatiite magmas until the MgO content of the liquid dropped below ⬃24% (Arndt, 1986a; Barnes, 1998). This maximum corresponds to the point where fractionating komatiite liquids meet the chromite saturation surface at the QFM buffer. The erupted liquids from the lava lake and Fred’s flow had a MgO content well below this threshold, while the most evolved spinifex-textured komatiites from Pyke Hill and Kostomuksha barely reached this threshold (Crocket and MacRae, 1986; Puchtel and Humayun, 2000). Two samples from the lowermost part of the basal subzone (91104 and 91106) show an inverse correlation between MgO and Cr indicating that there was less chromite accumulation relative to olivine in the lower 2 to 3 meters of the lava lake. PGE analysis of a separated chromite fraction from cumulate 91105 indicates that this mineral controls an important part of the PGE budget of the lava lake (Fig. 3). The PGEs are compatible in chromite with D Ru and D Os ⬇ 150, D Ir ⫽ 100, D Pt ⫽ 3, and D Pd ⫽ 1.6, qualitatively consistent with the order of the oxidation potentials of the PGEs, as observed above for the olivine separates. Righter and Downs (2001) showed experimental evidence in favor of incorporation of Os, Ir, and Ru into the MgFe2O4 spinel lattice, qualitatively consistent with

Platinum group element fractionation

Fig. 5. Variation diagrams of PGEs (ppb) vs. MgO (wt. %) and Cr (ppm) for the lava lake rocks and mineral separates. Composition of the Kostomuksha lavas (Puchtel and Humayun, 2000) are shown for comparison.

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Parameter

MgO

Concentrations C erupt.liq 14.2 46.5 C Ol* C Crt 2.69 C Sulf** Partition coefficients D Ol D Crt D OlCrt† D bulk††

Cr 1595 580 46.8

0.36 201 4.4 4.4

Ni 378 2362 272 37 6.2 0.72 6.1 6.4

Cu 83

Zn 77

0.5

0.01

0.76

Os

Ir

Ru

Pt

Pd

0.210 0.250 32.2 14.7

0.481 0.369 47.9 53.5

2.25 3.83 340 2526

9.60 0.782 33.5 102

11.7 0.302 18.7 17804

1.7 151 4.7 4.5

0.08 3.3 0.14 0.53

1.2 153 4.2 7.9

0.77 100 2.7 6.6

0.03 1.6 0.06 0.09

a PGEs in ppb, MgO in wt. %, Cr, Ni, Cu, Zn in ppm (bold face values in wt. %). The partition coefficients (D) were derived by dividing the concentrations in the respective mineral phases in Table 2 by those in the erupted liquid. * Average of three analyses of olivine from sample 91105 (Table 2). ** Average of two analyses from Table 2. † Calculated assuming co-precipitation of olivine and chromite in the proportion 50:1. †† Calculated from regressions in Figs. 4 and 5.

the observed PGE partitioning in the chromite. Capobianco and Drake (1990) determined Ru, Rh, and Pd incorporation into the MgAl2O4 spinel lattice, and found that Ru was strongly partitioned into spinel (D ⫽ 22–25) relative to Pd (D ⫽ 0.02). Capobianco et al. (1994) determined Ru, Rh, and Pd incorporation into the FeFe2O4 spinel (magnetite) lattice, and found that both Ru (D ⫽ 110–450) and Pd (D ⫽ 0.4–1.2) were more strongly partitioned into FeFe2O4 spinels, than into MgAl2O4 spinels. There are currently no data on FeCr2O4 spinels to directly compare the present results with, but Capobianco et al. (1994) found that increasing Cr contents of magnetites resulted in lower D values for Ru. Thus, the present chromite PGE determinations are in reasonable agreement with experimental partitioning results for Ru and Pd in spinels with compositions intermediate between those of MgAl2O4 and FeFe2O4 spinels. From the observed whole-rock fractionations in Fig. 5, regression calculations yielded bulk solid–liquid partition coefficients D Os ⫽ 7.9, D Ir ⫽ 6.6, D Ru ⫽ 4.5, D Pt ⫽ 0.53, and D Pd ⫽ 0.09. Bulk partition coefficients for the olivine ⫹ chromite (50:1) assemblage are listed in Table 4 and calculated fractionation trends for this assemblage are shown in Fig. 6. As can be seen from these diagrams and Table 4, chromite ⫹ olivine settling in the lava lake can entirely account for the observed Ru and Pd variations (D ⫽ 4.7 and 0.06). The two basal cumulates that exhibit less chromite accumulation, plot off the trend towards olivine. It is evident, however, that olivine ⫹ chromite assemblage can account for only part of Os, Ir, and Pt fractionation in the lake. Thus, an additional phase is required to explain these trends. The role of sulfide. Sulfide is considered to be an important phase that fractionates chalcophile/siderophile elements, including Ni, Cu, and PGEs, in mafic– ultramafic magmas (Peach et al., 1990; 1994; Barnes and Picard, 1993). Therefore, sulfide may have also played an important role in fractionating Os, Ir, and Pt in the lava lake. The highly incompatible behavior of Cu (Fig. 4) implies that an immiscible Cu-bearing sulfide liquid did not segregate in the lava lake. The sulfide separates analyzed in this study do not provide an adequate endmember for the trends in Fig. 5, being

highly enriched in Ru and Pd. Evidently, fractionation of a sulfide of this composition would have decreased Ru and Pd concentrations in the spinifex zone and increased those in the cumulate zone, compared to the composition of the erupted liquid. This is contrary to what is observed in Figs. 4 and 5, which show that the olivine ⫹ chromite assemblage was deficient in Os, Ir, and Pt, but not in Ru or Pd. This implies that the olivine ⫹ chromite assemblage must have been accompanied by a phase rich in Os, Ir, and Pt to fully account for the Vetreny magma evolution. Apparently, this phase was either too small, or too scarce, to be detected during petrographic and intensive SEM examination of thin sections of the cumulates. Chemically, it must have been depleted in Ru and Pd. This phase is tentatively identified as pyrrhotite (Pyr). Pyrrhotite is the most common high-temperature (⬃stable up to 119°C, Jensen, 1942; Kullerud et al., 1969; Arnold, 1971) sulfide phase in mafic– ultramafic magmas (Naldrett et al., 1967; Ebel and Naldrett, 1996). Pentlandite is a lower-temperature phase that exsolves from pyrrhotite and is stable up to 610°C (Kullerud, 1963). As the separated mineral fractions contained mostly pentlandite and only a small proportion (⬍10%) of pyrrhotite, it is possible that the mineral separation process was less efficient at concentrating pyrrhotite. The composition of this phase was calculated by mass balance as follows. The bulk D for each PGE was determined from the trends in Fig. 5, and is given in Table 4. The individual mineral D’s were determined from the mineral separate data, assuming these to have been in equilibrium with the erupted liquid. The difference between the observed bulk D and the calculated D for the olivine ⫹ chromite assemblage yields ( f ⫻ D) Pyr, where f is the mass fraction of pyrrhotite in the fractionating assemblage (Table 4). Figure 7 shows the concentration of Ir in this phase vs. the mass fraction required, together with compositions of magmatic sulfides and PGE minerals. Assuming that Pyr was correctly identified as a magmatic sulfide, the mass fraction required would be 10⫺4 to 10⫺6. The presence of 0.001% of sulfide (Pyr) in the fractionating assemblage can explain the observed variations of PGEs in the whole range of the lava lake rock compositions. The deviations of the three lowermost cumulate samples from the trends (Fig. 6) can be accounted for by the lower proportion of

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Fig. 6. Expanded portion of Fig. 5 featuring the bulk fractionation curve and calculated olivine ⫹ chromite fractionation trend for Cr (ppm) vs. Ni (ppm) and PGE (ppb) abundances in the lava lake assuming that the composition of scoria and upper chilled margin represents the starting liquid composition. Partition coefficients used are from Table 4. The lines represent batch equilibrium fractionation trends with ticks drawn in 5% increments.

chromite in the fractionating assemblage. The fact that this decrease in the proportion of chromite is not accompanied by a decrease in Os and Ir abundances implies that the sulfide phase was likely to coprecipitate with olivine, but not with chromite. The role of other minor phases. Other potential minor phases that are often cited as having crystallized from komatiite magmas include PGE minerals such as OsIr alloys, PtFe3 and laurite. Figure 7 shows that a mass fraction of 10⫺8, or less, of these minerals would account for the PGE fractionation observed in Fig. 5. Such minerals commonly occur as inclusions in chromitites, but their origin has been argued to be secondary (Stockman and Hlava, 1984). Mathez (1999) provides an extensive discussion of the origins of OsIr inclusions in chromitites. The crystallization of OsIr alloys has been proposed to explain the decoupling of the compatible PGEs (Os, Ir, Ru) from Pt and Pd during partial melting and fractional crystallization of mafic– ultramafic magmas (Keays, 1982; Barnes et al., 1985; Barnes and Picard, 1993; Rehka¨mper et al., 1999). Experimental determinations of the solubility of PGE metals in haplobasaltic compositions have indicated extremely low solubilities for Os and Ir of the order observed in natural melts

leading to the conclusion that natural melts are near saturated with PGE metal (Borisov and Palme, 1995; O’Neill et al., 1995; Borisov and Walker, 2000). In contrast, Peach and Mathez (1996) showed that the solubility of Ir metal in Fe- and Sbearing natural melts is too large to allow OsIr alloys a role in magmatic PGE fractionation. They argued that Os–Ir alloys are low-temperature exsolution products from sulfides and chromites (Peach and Mathez, 1996; Mathez, 1999). A similar conclusion was reached by Ballhaus and Ryan (1995) for laurite. Thus, experimental evidence does not support a role for OsIr alloys in determining magmatic evolution of the Vetreny lava lake. The fact that olivine ⫹ chromite fractionation can fully account for the Ru variations also indicates that a role for laurite (RuS2) is not required, here. 5.3. Implications for the use of Komatiites as Probes of Mantle PGE Composition In calculating the relative differences between Alexo and Kostomuksha sources, Puchtel and Humayun (2000) showed that the abundances of the compatible Os, Ir, and Ru were most

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abundances. Before any attempts are made to reconstruct mantle source PGE composition from komatiites and other mantlederived rocks, these must be thoroughly screened for crustal contamination using such sensitive indicators as Nb/Th and Nb/La ratios coupled with Pb–Nd isotope compositions. 5.4. Composition of the Vetreny Erupted Magma: Boninite or Komatiitic Basalt?

Fig. 7. Inferred Ir abundances (ppm) in the minor phase calculated from the inverse relationship between f and D, and the Ir content of the erupted liquid (0.48 ppb). The range of Ir abundances reported for mantle FeNi sulfides by Alard et al. (2000) is shown for comparison. Also shown is the range of Ir contents for Ir-bearing PGE minerals (osmiridium, irarsite, etc.).

diagnostic of differences in mantle source compositions. These three elements were shown here to be particularly prone to being fractionated during AFC processes. Two criteria are proposed here to ensure that PGE abundances in a komatiite magma have not been compromised as a result of AFC processes. First, to retain its full complement of mantle PGE and chalcophile element abundances, a komatiite magma must be sulfur undersaturated, a condition, which was not met for the Vetreny lava lake. Second, in order to escape fractional crystallization and, therefore, the loss of PGEs, komatiite magmas must remain at temperatures above or close to the liquidus prior to emplacement. These conditions are manifested by the scarcity of olivine (and chromite) phenocrysts in the chilled zones of erupted lavas. As high-temperature komatiitic melts are extremely voracious of felsic crust (Nesbitt, 1986), the amount of consumed crustal material can be as high as 40% during magma ascent (Huppert and Sparks, 1985b) and up to 10% during emplacement (Huppert and Sparks, 1985a). Digestion of such amounts of continental crust inevitably results in cooling and fractional crystallization of komatiites to form high-Si komatiitic basalts. This fractional crystallization, in turn, leads to the appearance of chromite on the liquidus and to extensive PGE fractionation. Unlike with lithophile element abundances, though, the effect of AFC processes on PGE abundances in mantle magmas was to remove PGEs by the accompanied fractionation processes rather than to add the PGEs from the assimilant. This is due to the extremely low PGE abundances (e.g., Ir ⫽ 30 ppt) in continental crust (Schmidt et al., 1997; Peucker-Ehrenbrink and Jahn, 1999) compared with primary mantle magmas (e.g., Ir ⫽ 1500 ppt). The strong Os, Ir, and Ru fractionation recorded in the Vetreny Belt komatiitic basalts provides an important, cautionary tale in the use of komatiites as probes of mantle PGE

The question of whether the Vetreny Belt parental magmas were boninitic (Sharkov et al., 1997) or komatiitic in composition (Puchtel et al., 1997) has been recently debated (see also discussion in Alapieti et al., 1990). As boninites are normally regarded as being diagnostic of formation in an oceanic arc environment (e.g., Murton, 1989; Sun et al., 1989; Falloon and Crawford, 1991; Taylor et al., 1994), this question has important implications for the tectonic setting of not only the Vetreny Belt, but the whole 2.45 Ga large igneous province in the Baltic Shield. The PGE and Cu data for the Vetreny Belt lavas allow new constraints to be placed on the origin of the parental magmas. Unique chemical characteristics of boninites (high MgO, Ni, and Cr coupled with high SiO2 and low Ti) have been used to argue that these magmas are partial melts of a severely depleted peridotite mantle source (e.g., Sun and Nesbitt, 1978). In contrast, “normal” basaltic magmas such as MORBs, are considered to originate from the mildly depleted mantle. As a result of this difference between the source regions of the two types of magmas, they have distinct patterns of chalcophile elements (Hamlyn et al., 1985). Magmas generated in mildly depleted source regions by low degrees of partial melting (e.g., MORBs), are S saturated and are depleted in PGEs relative to Cu and Ti compared to the primitive mantle as a result of both sulfide retention in the source (low degrees of melting) and the fractionation of immiscible sulfide liquid during ascent (DSulf–sil for Pd ⫽ 104 and for Cu ⫽ 103, Peach et al., 1990; 1994). During the formation of boninites, remelting of a strongly depleted source would result in the complete dissolution of the residual sulfide component in the mantle. The resultant magmas will be initially S undersaturated and enriched in PGEs relative to Ti and Cu (Hamlyn et al., 1985). One prediction of such a behavior would be that Cu/Pd and Ti/Pd will be lower than primitive mantle values in boninite-type magmas and higher in MORB magmas. In contrast, komatiite-type magmas formed at high degrees of melting of a mildly depleted source, will have primitive mantle ratios of Cu/Pd or Ti/Pd. To test this hypothesis, primitive mantle values of Cu/Pd ⫽ 7500 and Ti/Pd ⫽ 2.9 ⫻ 105 were calculated from PGE data on mantle xenoliths assuming Cl-chrondrite relative abundances of Anders and Grevesse (1989), an Ir content of 3.3 ppb (Morgan, 1986), and the primitive mantle estimates of Hofmann (1988) for Cu and Ti. In the Vetreny Belt magma, these ratios are 7470 and 3.1 ⫻ 105, i.e., very close to the primitive mantle estimates. Figure 8 shows that, on average, boninites have Cu/Pd ⫽ 1500 and Ti/Pd ⫽ 0.8 ⫻ 105 (Hamlyn et al., 1985), a factor of 4 lower than the average Gorgona komatiites (Bru¨gmann et al., 1987) and the lava lake komatiitic basalts. In contrast, MORBs have up to three orders of magnitude higher Cu/Pd and Ti/Pd ratios. These results imply that (1) the Vetreny Belt magmas were komatiitic, not boninitic in chalcophile element and PGE be-

Platinum group element fractionation

2991

Fig. 8. Cu/Pd vs. Ti/Pd for komatiites and boninites. Vetreny Belt, this study; Gorgona, Bru¨gmann et al. (1987); boninites, Hamlyn et al. (1985); MORBs, Rehka¨mper et al. (1999). Primitive mantle is represented by the intersection of lines. Komatiites plot close to PM values, whereas boninites are depleted in Cu and Ti relative to Pd as a result of previous episodes of low-degree melting that left most of the sulfide in the mantle source.

havior, and (2) the Vetreny Belt magmas were high-degree partial melts, so that the sulfide in the mantle source region was completely consumed during partial melting. Melting models of Barnes et al. (1985) and Hamlyn et al. (1985) suggest that 20% to 25% partial melting of the mantle is required for all of the sulfide to be consumed. The concentrations of moderately incompatible elements Ti, Y, and Yb in the lava lake melt indicate that at least 30% partial melting of a primitive mantle source would be required to generate such a melt, thus, implying complete removal of sulfide from the source. As olivine crystal fractionation en route to the surface could have substantially increased Ti, Y, and Yb in the melt, this is a conservative estimate of the degrees of partial melting for the Vetreny primary komatiite magma. 6. CONCLUSIONS

(1) Platinum was moderately incompatible, and Cu and Pd were highly incompatible, during differentiation of the lava lake (bulk Ds ⫽ 0.53, 0.01, and 0.09, respectively). This implies that separation of FeNiCu immiscible sulfide liquid did not take place during the differentiation of the lava lake. (2) Nickel, Cr, Os, Ir, and Ru displayed moderately to highly compatible behavior (bulk D ⫽ 6.2, 4.4, 7.9, 6.6, and 4.5, respectively) and, thus, fractionated significantly during the differentiation of the lava lake. (3) Analysis of separated olivine and chromite fractions showed that Ru, Os, and Ir were slightly compatible to moderately incompatible in olivine (D ⫽ 1.7–0.8), and were compatible with chromite (D ⫽ 100–150). Pt and Pd were highly incompatible with olivine (D ⫽ 0.08–0.03), and moderately compatible with chromite (D ⫽ 1.6–3.3). The partitioning behavior for chromite observed is consistent with the existing experimental partitioning of Ru and Pd in spinels. It is also consistent with the relative order of

oxidation of the PGEs, lending credence to the notion that these are incorporated in mineral lattices. (4) Comparison of the bulk partition coefficients with those calculated from olivine and chromite compositions imply that the fractionating mineral assemblage included ⬃10⫺5⫾1 mass fraction of a sulfide, possibly pyrrhotite. (5) Assimilation of continental crust by primary komatiite magmas leads to their cooling and fractional crystallization and, as a result, to a substantial decrease in Os, Ir, and Ru abundances in the komatiitic basalt melts. No detectable addition of PGEs occurred during assimilation. (6) The Vetreny erupted magma had Cu/Pd and Ti/Pd ratios of 7470 and 3.1 ⫻ 105, similar to those in the primitive mantle (7500 and 2.9 ⫻ 105). These data indicate that the Vetreny erupted magma formed at ⬎30% partial melting from a source that had not been extensively melt depleted. These characteristics are typical of komatiites, but not of boninites. Acknowledgments—We wish to thank our colleagues from the Karelian Research Center V. Kulikov and V. Kulikova for help during field work. Special thanks are due to Nora Groschopf who was responsible for performing the XRF analyses and to V. Kevlich for obtaining the mineral separates. We are indebted to Alan Brandon for his help and advice with Os microdistillation. Thanks also go to Andy Campbell for assistance with the ICP–MS measurements and SEM work, and to Denton Ebel for useful comments. This paper benefitted greatly from thorough reviews by E. A. Mathez and G. Bru¨gmann. Editorial handling by M. A. Menzies is acknowledged. This study was supported by NSF EAR 9601478 to M. H. and by funds from The University of Chicago, for which we thank David B. Rowley. Associate editor: M. A. Menzies REFERENCES Alapieti T. T., Filen B. A., Lahtinen J. J., Lavrov M. M., Smolkin V. F., and Voitsekhovsky S. N. (1990) Early Proterozoic layered intrusions

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