SUPPLEMENTARY METHODS Petrology and mineral chemistry … · SUPPLEMENTARY METHODS Petrology and mineral chemistry ... 190Os, 191Ir, 194Pt), and 6 mL of inverse ... analysis, as observed

SUPPLEMENTARY METHODS

Petrology and mineral chemistry Four polished thick- and thin-sections of GRA 06128 (sub-sections 42 and 51) and

GRA 06129 (22 and 25) were obtained from the Meteorite Working Group (MWG) for petrographic and mineralogical characterisation. GRA 06129, 22 and 25 have surface areas of ~1.6 cm2 and ~0.9 cm2, whereas GRA 06128, 42 and 51 have smaller surface areas (~0.6 cm2; Fig. S1). Mineral modes in three sections (GRA 06128, 42; GRA 06128, 51; and GRA 06129, 22) were obtained using the Feature-Scan Phase Distribution software package of an Oxford instrument energy dispersive spectrometer interfaced to a Cameca SX50 electron microprobe at the University of Tennessee (Ref. S1). Modal data are presented in Table S1. Major and minor element analyses were performed on four sections of the meteorites using a CAMECA SX50 electron microprobe analyser at the University of Tennessee. Average compositions of mineral phases are presented in Table S2. Mineral compositions were determined in wave-length dispersive spectral mode using an accelerating potential of 15 keV, a 20 nA beam current and the beam was focussed to 1µm. Peak and background counting times of 20-30 s and standard ZAF (PAP) correction procedures were used. Glass and plagioclase compositions were determined using a 10nA beam current, a 5-10 µm beam size and longer counting times to avoid mobilisation of Na, K, or Cl. Similar analyses have also been reported from other laboratories (See Refs. S2-S9). A combination of natural and synthetic standards were used for calibration and were measured periodically within analytical sessions to ensure data quality. Drift was within counting error throughout

every analytical session. The detection limits (3σ above background) are <0.03 wt.% for SiO2, TiO2, Al2O3, MgO, CaO, Na2O and K2O, <0.05 wt.% for Cr2O3, MnO, FeO, P2O5, and <0.05-0.1 wt.% for all other oxides and elements listed (Ref. S10).

Laser ablation ICP-MS analysis Concentrations of minor- and trace-elements were determined in minerals using a

New Wave Research UP213 (213 nm) laser-ablation system coupled to a ThermoFinnigan Element 2 ICP-MS at the University of Maryland. Olivine, pyroxene, plagioclase, phosphate and sulphide were analysed using individual spots with a 15-80

µm-diameter, a laser repetition rate of 7 Hz and a photon fluence of 2 to 2.5 J/cm2. Th/ThO production was ca. 0.07% for all analytical sessions. The ablation analysis took place in a 3 cm3 ablation cell. The cell was flushed with a He gas flow of 1 L/min to enhance production and transport of fine aerosols and was mixed with an Ar carrier gas flow of 0.4 L/min before reaching the torch. Each analysis consisted of ~60 s of data collection. Backgrounds on the ICP-MS sample gas were collected for approximately 20 s followed by approximately 40 s of laser ablation of the sample. Washout time between spots was in excess of 2 minutes. Data were collected in time-resolved mode so that effects of inclusions, mineral zoning, and possible penetration of the laser beam to underlying phases could be evaluated for each analysis. Plots of counts per second versus time were examined for each analysis, and integration intervals for the gas

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background and the sample analysis were selected manually. Each LA-ICP-MS analysis was normalized to a major-element oxide, measured previously by electron microprobe, as an internal standard to account for variable ablation yield (Fe for sulphides and Ca for silicates and glass). This internal normalization is supported by the electron microprobe data, which allowed no more than ~2% relative difference in Fe or Ca

content over a 50 µm spot. Furthermore, LA-ICP-MS time-resolved patterns show that the ablated volumes were generally homogeneous. For all data, the NIST 610 glass standard was used for calibration of relative element sensitivities. Replicate LA-ICP-MS analyses of the BIR-1g glass standard, run at intervals during the analytical session,

yielded an external precision of better than 3% (1σ relative standard deviation) for all measured trace- and major-element compositions in silicates and phosphates. Replicate LA-ICP-MS analyses of the University of Toronto JB Sulphide standard run at intervals

during analytical sessions of sulphide, yielded an external precision of better 1% (1σ relative standard deviation) for highly siderophile element compositions. Representative data for silicate and phosphate minerals in GRA 06128/9 are graphically represented in Figure S2 and sulphide data is graphically presented in Figure 3a.

207Pb -

206Pb age data

207Pb-206Pb ages were obtained using the same laser and mass spectrometer settings as those for minerals and glasses analyzed by LA-ICP-MS (Table S3). The high concentrations of U (0.1 to 3 ppm) in chlorapatite and merrillite make them suited to LA-ICP-MS 207Pb-206Pb dating. Phosphates were identified for dedicated Pb analyses using the SX50 electron microprobe analyser at the University of Tennessee and the JEOL-8900 Super-probe at the University of Maryland. All data reduction was made offline using Microsoft Excel. Background Pb signals were taken on mass, and subtracted from each isotopic measurement during ablation. Each ratio was determined using the background corrected Pb isotopic measurements. The average and 2σmean of

the background corrected ratios, after ratios outside 3σ were discarded, were used to determine the age and error for each phosphate. An exponential fractionation law was used to correct for mass fractionation by means of bracketing the phosphate analyses with standard reference materials (SRMS: NIST 610, NIST 612, BCR-2g). Ratios of 207Pb/206Pb for each SRM were used to calculate the fractionation factor (α) (Ref. S11). Differences in α between the three SRMS had a negligible effect on calculated ages. The 207Pb-206Pb ages were calculated using Isoplot/Ex (Ref. S12). Corrected Pb ratios and ages are presented in Table S3.

Whole-rock geochemistry Aliquots of GRA 06128 (sub-aliquot 22; 1.71 g) and GRA 06129 (9; 2.18 g) were

provided by the Meteorite Working Group, for whole-rock geochemical studies. The samples were crushed and homogenised using a high-purity agate pestle and mortar under clean laboratory conditions. Ultra-pure quartz powder was ground in the agate pestle and mortar, followed by >24 hr leaching in a ~0.5M HCl solution and subsequent extensive rinsing using ultra pure water (Milli-Q) prior to each use of the pestle and mortar. Blank contributions from the agate (from measurement of the ground quartz powder) show them to be negligible for the HSE and other trace elements. Whole-rock powder aliquots (~50 mg) were fused into two separate glass beads for each meteorite,

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using a Mo-strip resistance heater, in a nitrogen atmosphere. Fused bead major element concentrations were analysed using the University of Tennessee CAMECA SX50 electron microprobe and protocols for glass analyses outlined above. The average of >25 spot analyses are reported on a Mo-free basis. Minor- and trace-element concentrations were measured on the same beads using the University of Maryland LA-ICP-MS protocol outlined above, with 150 µm raster paths and obtaining 20 s of background and ~60 s of analysis. Three analyses of each bead were integrated to give the average composition of the bead (Table S4). The beads were found to be relatively homogenous during raster analysis.

Os isotopic and platinum-group elemental analyses were performed at the University of Maryland, College Park using homogenised powders. The samples were digested in sealed borosilicate Carius tubes (after Ref. S13), with an isotopically enriched multi-element spike (99Ru, 106Pd, 185Re, 190Os, 191Ir, 194Pt), and 6 mL of inverse Aqua Regia. Samples were digested to a maximum temperature of 260ºC in an oven for >36 hours. Os was triply extracted from the inverse Aqua Regia using CCl4 (Ref. S14) and then back-extracted into HBr, prior to purification via micro-distillation (Ref. S15). Re and the PGE were recovered and purified from the residual solutions using standard anion exchange separation techniques. Isotopic compositions of Os were measured in negative ion mode on a Triton thermal ionisation mass spectrometer (N-TIMS) instrument for GRA 06128/9 and an NBS-type N-TIMS for Brachina. Offline corrections for Os involved an oxide correction using 16O/18O and 17O/18O values from Ref. S16, an iterative fractionation correction using a 192Os/188Os ratio = 3.08271 (Ref. S17), a 190Os spike subtraction, and finally, an Os blank subtraction. External precision for 187Os/188Os, determined via measurement of standards bracketed with the meteorites samples was better than 2.5‰ (2σ). Re, Pd, Pt, Ru and Ir were measured using an Aridus desolvating nebuliser coupled to an Element 2 ICP-MS in low-resolution mode. Measured Re, Ir, Pt, Pd and Ru isotopic ratios for sample solutions were corrected for mass fractionation using the deviation for the standard average run on the day over the natural ratio for the element. External reproducibility on PGE analyses using the Element 2 was better than 0.5% for 0.1 ppb solutions and 0.3% for 1 ppb solutions. Total procedural blanks run with the samples had an average 187Os/188Os isotope composition of 0.1448 ± 0.0024, with average concentrations of 1.5pg [Re], 37pg [Pd], 20pg [Pt], 5pg [Ru], 2pg [Ir] and <1pg [Os]. This resulted in negligible blank corrections and a total analytical blank in line with typical blank contributions for Carius tube analyses at the University of Maryland.

Oxygen-isotope measurements Oxygen isotope analyses were performed at the Geophysical Laboratory, Carnegie

Institution of Washington and are reported in δ18O, δ17O (δXOn is the per mille [‰] deviation of XO/16O in n from the international standard [std] V-SMOW given by the relationship: δXOn = 1000 × ((XO/16On)/(

XO/16Ostd) -1), where X is 17O or 18O and n

denotes the unknown), and ∆17O notation, which represents deviations from the terrestrial fractionation line (λ = 0.526 (Ref. S18); ∆17O = 1000ln ((δ17O/1000) + 1) – 0.526 × 1000ln ((δ18O/1000) + 1)) (after Ref. S19). The value of 0.526 was obtained by

linear regression of linearised values for δ17O and δ18O of terrestrial silicate minerals (Refs. S18, S20). Small sub-chips of GRA 06128, 22 and GRA 06129, 9 were crushed



under ethanol in a boron nitride pestle and mortar, ultrasonicated in dilute hydrochloric acid, and magnetic material was removed with a hand magnet. The resulting sample for analysis, as observed with a low-power binocular microscope, was predominantly a colourless mineral with some coloured mineral grains. Samples were loaded in a Sharp reaction chamber (Ref. S21). Successive, repeated blanks with BrF5 and vacuum pumping were carried out for 12 hours until there was less than 150 microns non-condensable gas pressure remaining after a blank run. Quantitative release of oxygen by fluorination reaction was performed by heating samples individually with a CO2 laser in the presence of BrF5. Standardisation of delta values was achieved by comparison with the Gore Mountain garnet standard, USNM 107144, analysed during every analytical session. O-isotope analyses, performed on pre-leached powder whole-rock aliquots stripped of magnetic fractions and minerals, differ from the initial reported O-isotope values for the meteorites (Ref. S22), which were not leached and may have been subject to terrestrial contamination.

SUPPLEMENTARY DISCUSSION

Petrology and mineral chemistry of GRA 06128 and GRA 06129

A brief petrological description of the Graves Nunatak (GRA) 06128 (Mass = 447.6g) and GRA 06129 (196.5g) paired achondrite meteorite finds is presented here. Pairing of these samples is confirmed by identical petrological and geochemical characteristics (Refs S2-9). The stones were found with a ~50% covering of patchy, vesiculated fusion crust (Fig. S3). GRA 06128/9 are characterized by a blocky or slab-like appearance, with a stratified fabric that is unusual in meteorites and has been attributed to intense parallel fracturing of the samples (Ref. S7). Both meteorites are partially brecciated but represent a single lithology (Refs S2-9). In unbrecciated regions, GRA 06128/9 have granoblastic textures but variable ranges in grain-size for the major silicate phases (<0.1 to >0.5 mm, diameter), and are partially to totally chemically re-equilibrated. GRA 06128/9 consist of oligoclase (Modal abundance - 71-90%), ortho- and clinopyroxene (<1-11%), olivine (4-14.2%), chlorapatite and merrillite (0.5-5%), troilite and pentlandite (0.2-2.5%) in addition to minor spinel and ilmenite (<0.2%) (Table S1). FeNi metal is a minor phase and is located in conjunction with pentlandite inclusions within silicates. In detail, we noted that GRA 06128, 42 and 51 contain more Na-merrillite and less sulphide than GRA 06129, 22, consistent with some degree of mineralogical heterogeneity within the meteorites. Alteration is pervasive and occurs as rusty regions on grain boundaries and within cracks; gypsum as well as Fe-sulphates and Fe-oxides have been recognized in GRA 06128/9 (Refs. 2-9).

Major mineral phases display a wide range in grain size. Large subhedral plagioclase grains possess granoblastic textures with polysynthetic twinning. Transformation twinning (cf., microcline) was observed in some plagioclase grains. Large anhedral olivine grains (>1 mm long) have rims of clinopyroxene, orthopyroxene, and phosphates. Compositions of silicate minerals are extremely uniform (Table S2), with over 100 electron microprobe analyses of the main silicate phases yielding nearly identical values. These grains display inter- and intra-grain homogeneity at all scales and in all sections studied. Plagioclase is oligoclase in composition



(An14.6±0.4Ab83.6±0.4Or1.8±0.1). Olivines have constant forsterite content (Fo39.9±0.4) for all

morphologies and sizes of grains measured. Orthopyroxenes are En52.9±0.3Wo2.3±0.1 with

Mg# of 54.1±0.4; and augites are En37.7±0.4Wo42.4±0.8 and Mg# of 65.5±1.1. Co-existing

orthopyroxene and augite yield a temperature of 810°C using the two pyroxene geothermometer and the QUILF program (Refs. S23, S24). It is notable that olivine is chemically in disequilibrium with both pyroxenes, being more Fe-rich than expected from the Mg-Fe exchange between these phases (Ref. 25), possibly reflecting slow

cooling, as derived from thin exsolution lamellae of 5µm in the augite (Ref. S26).

Large phosphate grains typically show intergrowths of chlorapatite and Na-merrillite. Two subhedral apatite grains in GRA 06128, 42 and 51 were found with grain sizes up to 2 mm. Chlorapatite contains variable Cl (3.3-5.5 wt.%) and the F

concentrations vary from 0.5-1.9 wt.%, slightly above the 1σ standard deviation of the measurements (0.5 wt.%). Intergrown Na-merrillite contains 2.2-2.5 wt% Na2O, which is higher than those in lunar rocks and some martian meteorites but similar to those in ALH 84001 and in Pallasites (Ref. S27). The Mg# of Na-merrillite varies from 68 to 82 - higher than for the Mg# of coexisting silicates.

Almost all large sulphide grains (10-60 µm) are intergrowths of pentlandite and

troilite. Smaller pentlandite and FeS droplets (~5 µm) are ubiquitously enclosed in all

silicate phases. Small FeNi metal grains (1-3µm) were only found associated with pentlandite included in silicate minerals. The composition of large sulphide grains is essentially FeS (troilite). Pentlandite lamellae, (FeNi)9S8, contain up to 26 wt.% Ni and 2.3-3.3 wt.% Co. The coexisting pentlandite and troilite reflect exsolution of pentlandite from a monosulphide solid solution at a temperature of ~610ºC under a slow-cooling condition at a relatively high S fugacity (Ref. S28), indicative of possible sulphide immiscible melt during most of the crystallization process. The small pentlandite inclusions in silicate minerals contain low Co (0.5-0.6 wt.%), but otherwise are similar to the pentlandite lamellae. FeNi metals associated with these pentlandite inclusions are of a composition near awaruite (FeNi3), with about 0.9 wt.% Co (Table S2). The association of pentlandite and FeNi metal suggests a melt with low S content (Ref. S28) and implies inconsistency between the two sulphide (±metal) associations.

Anhedral ilmenite and titaniferous chromite grains occur as individual grains or as intergrowths. Titaniferous chromite contains up to 39.8 wt% Cr2O3 with 1.8-2.7 wt% V2O3 and Mg# of 4.5-6.7. Ilmenite contains 1 to 1.9 wt.% MgO.

Weathering effects in the GRA 06128/9 meteorites The GRA 06128/9 achondrite meteorites are weathered samples with pervasive

‘rust’ regions within cracks and at grain boundaries. Indeed, Treiman et al. (Ref. S7) noted that as much as 20% of iron in GRA 06129 is now present as nano-phase ferric iron. Of concern is whether whole-rock compositions, most especially the highly siderophile elements, have been re-distributed or ‘disturbed’ by terrestrial Antarctic weathering.



Studies of chondrites and low abundance lunar mare basalt meteorites have noted disturbance of the Re-Os isotope system resulting in deviations from isochronous relations on plots of 187Re/188Os-187Os/188Os (Refs S29-30). These deviations are consistent with minor, relatively recent, re-distribution of Re and/or Os on millimetre to centimetre scales within the samples. However, in the case of chondrites (which have slightly higher, to similar abundances, of the HSE as the GRA 06128/9 meteorites) some of this redistribution has been attributed to aqueous alteration or shock on their parent body in the past 2 Gyr (Ref S29). We do not consider this to have been significant in the case of the GRA 06128/9 meteorites, or Brachina. In Figure 3 of the main manuscript the Re-Os isotope data for Brachina and GRA 06128/9 plot within error of a 4.56 Gyr reference isochron, indicating limited isotopic disturbance over that period of time. This results in calculated initial 187Os/188Os for GRA 06128/9 and Brachina close to the solar system initial (Table 2); therefore Re-Os systematics are not disturbed.

Other studies have suggested that some CK carbonaceous chondrites have suffered alteration and terrestrial weathering of sulphides, causing disturbance to the siderophile elements (Ref S31). Again, the GRA 06128/9 do not appear to have suffered this type of alteration. Our study of primary magmatic sulphides in the meteorites show no evidence of alteration rims or for extensive leaching. Mass-balance calculations using HSE compositions of sulphides in the meteorites are in close agreement with measured whole-rock HSE compositions (Fig. 3 of the main manuscript). This is important because fresh primary magmatic sulphides were measured that retain the original compositions of HSE in the meteorite. Thus, there is no evidence for modification of HSE through alteration of the meteorites. Alteration of other typically fluid-mobile elements (e.g., Rb, Sr, Ba) require further study and cannot be discounted, but do not impact the outcomes of this study.

Pb-Pb age determination for GRA 06128/9 The average Pb-Pb age for GRA 06128/9 was derived from analyses of different

phosphates in the meteorites. The mean chlorapatite age was derived from grains (n = 8) with the highest total counts of Pb. For merrillite, only 3 grains in the meteorites provided >10,000 total counts of 207Pb and these were selected for the mean age of merrillite phases. The basis for this filter is that samples with lower total Pb counts are more susceptible to terrestrial contamination which can generate both older and younger ages.

Mass balance calculations for major elements, the REE and highly

siderophile elements in GRA 06128/9 Given that individual mineral grains are compositionally homogeneous and the bulk

compositions of both GRA 06128/9 stones are similar, it is possible to compare bulk measured compositions of the samples with those calculated using the modal abundances of minerals, mineral compositions, and their respective densities. This exercise is useful for understanding distribution of elements within the GRA 06128/9 meteorites. These calculations also serve to demonstrate that the trachy-andesite to andesite composition measured on whole-rock powders of the meteorite are representative of the overall composition of the GRA 06128/9 stones. Plotted in Figure



1 of the main manuscript are the measured compositions of GRA 06128 and GRA 06129 made on aliquots of fused powder from 2g fragments of the meteorite (GRA 06128, 22 and GRA 06129, 9). Also shown are the calculated compositions of the meteorites from modal recombination based on modal data presented in table S1 and major element compositions of mineral phases in GRA 06128/9 from table S2. We have also utilised the Na2O content of GRA 06128 measured via INAA on small unground splits of the meteorites (~25 mg; Ref. S8). Because of the consistency of major element compositions in mineral phases for the meteorites, the only major variable to change is the modal proportions of those minerals. INAA gave ~6.3 wt.% Na2O, only marginally greater (but beyond analytical uncertainty) than bulk rock compositional results reported here (Table S4). Variation in major element compositions anticipated for the meteorites is mainly due to the variable abundances of mineral phases and weathering mobilization of more fluid-mobile elements.

Trace element data are dependant upon the amount and proportions of feldspar for Eu, phosphate minerals for the rest of the REE, and sulphide for the HSE. Resultant compositions allow isolation of mineral controls on the major-, minor- and trace-elements. This is exemplified by the discrepancy between calculated REE abundances in the meteorites from their individual mineral trace element compositions versus measured ‘whole-rock’ REE abundances from fused beads. Such inhomogeneity has been recognised in previous studies of extra-terrestrial materials, where large sample allocations are not feasible (e.g., Ref. S32).

Modelling the GRA 06128/9 bulk composition: Possible relations to the

brachinite meteorite group

A number of primitive achondrite groups have minor mineral phases similar to those seen in GRA 06128/9, but of those the brachinite meteorite group appears to be the closest for any petrogenetic relationship. O-isotope variability between different brachinite meteorites have been attributed to both incomplete melting of their parent body, or multiple parent bodies with different oxygen isotope compositions (e.g., Ref.

S33). The deviation in δ18O for GRA 06128/9 from the brachinite mixing line may be a reflection of their high plagioclase contents, consistent with measured fractionation

factors between feldspar and olivine in terrestrial rocks (δ18O = ~0.6 ±0.3 (2σ); Ref. 34). The cumulate, proto-granular and re-equilibrated nature of the brachinites indicates metamorphic reprocessing as well as an igneous origin for this suite of meteorites (Ref. S35). The brachinites also possess similar concentrations of HSE to GRA 06128/9 and incompatible element systematics and an oxidation state consistent with a possible petrogenetic link to GRA 06128/9 (Ref. S35). The age of GRA 06128/9 is within error of reported ages for brachinites (Brachina 4563.7 ± 0.9 Myr (Ref. S36)).

It has been argued that brachinites represent orthocumulates (Refs. S35, 37), consistent with an igneous origin, or that they represent anatectic residues after melt extraction (Ref. S38). Brachinites are composed predominantly of olivine, so a direct petrogenetic link with GRA 06128/9 can only be achieved by crystal fractionation or partial melting. Possible, albeit non-definitive, links between GRA 06128/9 and the brachinites make them potential analogues for complementary crust-mantle/cumulate residue materials in the GRA 06128/9 parent body.



In Figure S4, we show whole-rock REE patterns for GRA 06128/9 versus brachinites. Brachinites have somewhat light REE-enriched, sub-chondritic to chondritic REE profiles with small positive Eu-anomalies. Brachinites have around 80 to 93% modal olivine, 3.6 to 9.4% clinopyroxene, typically no orthopyroxene, 0-10% plagioclase (Na composition) and trace amounts to 0.5% phosphate. Mittlefehldt et al. (Ref. S35) performed a calculation to estimate the composition of parental melts in equilibrium with brachinites EET 99402 and EET 99407. They found the parental melt composition to be roughly 3 × CI chondrites for the REE, indicating ~30% melting of their source region.

Assuming that GRA 06128/9 formed from a chondritic precursor via lower degrees of partial melting (≤30%), then experiments reveal that andesitic composition melts can be achieved at ~1200ºC and fO2 IW -1 (Ref. S39). These experiments also indicate that olivines formed at these temperatures have Fo66-70 – i.e., close to the brachinite olivines (Fo64-71). These results are consistent with brachinites being complementary residue or orthocumulate analogues to GRA 06128/9. Calcic pyroxene and plagioclase in some brachinites is not predicted from these experiments, but could be explained by inefficient melt extraction during partial melting.

The HSE abundances provide further insights. In order to generate the fractionated HSE pattern of GRA 06128/9 from a broadly chondritic initial composition, melt-solid fractionation of the HSE must have taken place. We discount the possibility of initial nebular heterogeneity for the HSE profile seen in GRA 06128/9, since this is inconsistent with the broadly chondritic 187Os/188Os as well as the oxygen-isotope, trace- and major-element composition of GRA 06128/9 that imply a broadly chondritic source. There are three primary ways in which HSE fractionation can occur within the GRA 06128/9 parent body: 1) in the source, 2) during ascent, or 3) via metamorphism and preferential mobilization of HSE. Given the lack of evidence for core formation, there is no reason to invoke solid metal/liquid metal equilibration. Instead, the high fO2 and presence of sulphides in this meteorite indicate that sulphide fractionation was an important control on resultant HSE compositions in GRA 06128/9.

It has been demonstrated that base metal sulphide melts crystallise to a monosulphide solid solution (MSS) in the (Cu)-Fe-Ni-S system. With a decrease in temperature, pentlandite (and minor chalcopyrite) will exsolve from the MSS. With continued equilibration to low temperatures, the final assemblage will be a Fe-S phase of troilite and hexagonal pyrrhotite or hexagonal + monoclinic pyrrhotite and pentlandite, depending on the activity of sulphur (Ref. S40). The assemblage of troilite and pyrrhotite is essentially what is observed in GRA 06128/9. Our analyses of sulphides in the GRA 06128/9 meteorites, via LA-ICP-MS, demonstrate that the complement of HSE in the whole-rock is present in these phases. Assuming an initially chondritic composition, there is ~70% depletion of Pd, Pt and Ir, relative to Re, Os and Ru. A number of terrestrial platinum-rich ore deposits have been studied via LA-ICP-MS for their inventory of sulphide minerals, which are hosts to almost the entire inventory of HSE in these systems (Ref. S41). In figure S5 we compare sulphides and whole-rock data from GRA 06128/9 to selected sulphide data from some of the studied terrestrial magmatic intrusions. Importantly, these intrusions have pentlandite and hexagonal and monoclinic pyrrhottite (Fe1-XS) with elevated Ru as well as depletions in Pt and Ir, like those observed for GRA 06128/9. Notably, despite ideal stoichiometry of Fe-S, these attributes are not systematic, suggesting the initial parental melt composition and sulphide inventory plays a major role in the resultant HSE pattern of these deposits.



The most likely cause of Pd, Pt and Ir depletion relative to Re, Os and Ru in GRA 06128/9 is through sulphide-sulphide liquid immiscibility and segregation during ascent. Previously in brachinites, fractionation of siderophile elements has been attributed to metal remaining after loss of a metal-sulphide partial melt (Ref. S35). This process could account for the high abundance of HSEs in both GRA 06128/9 and Brachina.

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Figure S1: Photo-mosaics of GRA06128 and GRA06129 sections illustrating the distribution of minerals in the meteorites and the difference between polished sections studied here. Scale bars are 1 mm.



Figure S2: Mineral and whole-rock rare earth element data for GRA 06128/9 obtained by LA-ICP-MS. GRA 06128 shown as unfilled symbols and GRA 06129 as filled symbols, except for orthopyroxene and olivine data, which are from GRA 06128 and plagioclase data which derives from GRA 06129. Also shown are REE data for the terrestrial continental crust (Ref. S42), the composition of which is more REE-rich than GRA 06128/9. Whole-rock compositions (as dashed lines) measured via laser ablation of glass beads. Normalization after Ref. S43.

Figure S3: Field-collection images of (a) GRA 06128 and (b) GRA 06129 taken at the Graves Nunatak Icefield during the 2006/2007 Antarctic Search of Meteorites collection campaign. Note the blocky texture of the meteorites and the patchy fusion crust (black outer material) with orange-coloured, rusty interior. Images courtesy of ANSMET.



Figure S4: Rare earth element plot of GRA 06128/9 (this study) and brachinite data (La, Sm, Eu and Yb obtained via INAA; Refs. S35, S37). Also shown is the calculated (grey) field of possible complementary residuum compositions for GRA 06128/9 that assumes mineral chemistry corresponding to the GRA 06128/9 meteorites and a modal mineralogy similar to that of the brachinites (from Ref. S35). Normalisation after Ref. S43.



Figure S5: Highly siderophile element profiles of GRA 06128/9 whole rocks (solid black lines and filled squares), pentlandite (red lines) and Fe1-XS (blue lines) versus terrestrial platinum-group element ore bearing intrusions, including the Great Dyke (Zimbabwe), Penikat (AP-Reef and PV-Reef, Finland), Merensky Reef, Bushveld Igneous Complex (South Africa) and the Medvezky Reef, Noril’sk (Russia). Data from this study and Ref. S41. Normalisation values from Ref. S44.


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SUPPLEMENTARY METHODS Petrology and mineral chemistry … · SUPPLEMENTARY METHODS Petrology and mineral chemistry ... 190Os, 191Ir, 194Pt), and 6 mL of inverse ... analysis, as observed