Refractory Siderophile Elements Research Articles

Siderophile element and Hf-W isotope characteristics of the metal-rich chondrite NWA 12273 – Implication for its origin and chondrite metal formation

Metal phases in primitive chondrites contain important information for understanding early Solar System processes. Northwest Africa (NWA) 12273 is an ungrouped chondrite and, due to its preternatural high modal abundance of metal grains (>60%), provides a unique perspective on the formation and evolution of primitive planetesimals. To investigate the formation of metal phases in early solar system materials we report detailed petrography, siderophile geochemistry and Hf-W isotope geochronology for NWA 12273. The primitive textures and heterogeneous mineral chemistry of chondrules, as well as siderophile element compositions of Ni-rich metal, all indicate an affinity of NWA 12273 to unequilibrated (type 3.8) ordinary chondrites that experienced limited thermal metamorphism. Modeling shows that crucial inter-element fractionations among refractory siderophile elements for kamacite in NWA 12273, were most likely caused by solid metal-liquid metal partitioning, for example during partial melting of Fe-Ni metal containing ∼3.9 wt.% of S. Analysis of the metal fractions within NWA 12273 gives a Hf-W model age of ∼2.4 Ma after CAI formation (ε182W6/4=−3.29±0.11), older than metal formation in most of H chondrites and IIE irons. In comparison with published data for chondrites and ‘non-magmatic’ iron meteorites, siderophile element data suggest that the metal phases in NWA 12273 may have originated from similar precursor materials and/or accreted in adjacent nebular locations to IIE irons and H chondrites. These primary planetesimals experienced impact-related evaporation and mixing followed by rebuilding of the secondary body. This scenario is consistent with that proposed for IIE and other silicate-bearing iron meteorites, indicating that the building blocks of some chondrite parent bodies were not entirely primitive (undifferentiated) materials.

Impact‐induced devolatilization of four ungrouped ataxites and the formation of a silica glass spherule in ALHA77255

AbstractAllan Hills A77255, Babb's Mill (Blake's Iron), Nordheim, and Chinga are ungrouped ataxitic iron meteorites that are similar to the IAB group of noncarbonaceous‐type irons in their concentrations of common and refractory siderophile elements. Mo‐isotopic data show that ALHA77255, Nordheim, and Chinga are carbonaceous‐type (CC) irons. (The Mo‐isotopic composition of Babb's Mill [Blake's Iron] has not yet been measured, but it also seems likely to be a CC iron.) Relative to mean IAB irons, these four ataxites are severely depleted in moderately volatile elements: Ga, >99%; Ge, >99%; Cu, 79%–97%; As, 70%–96%; P, 76%–90%. These samples were probably devolatilized by major collisions on separate parent asteroids (consistent with fractional crystallization modeling showing they are unlikely to be derived from the same metallic core). Collisionally induced devolatilization of ALHA77255 likely facilitated the formation of a 5‐mm diameter silica–glass spheroid in this meteorite. The spheroid may have formed by a complex process involving impact‐induced vaporization of mantle material in its parent asteroid, followed by fractional condensation.

Nickel-rich, volatile depleted iron meteorites: Relationships and formation processes

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10–100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected 182W/184W consistent with metal-silicate segregation on their parent bodies within 1–3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4–5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from 26Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8–9 GPa, is consistent with the latter.Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and 183W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and 183W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System.Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive.For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼ IW + 1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant.

A revised trapped melt model for iron meteorites applied to the IIIAB group.

As the largest magmatic iron meteorite group, the IIIAB group is often used to investigate the process of core crystallization in asteroid‐sized bodies. However, previous IIIAB crystallization models have not succeeded in both explaining the scatter among IIIAB irons around the main crystallization trends and using elemental partitioning behavior consistent with experimental determinations. This study outlines a revised approach for modeling the crystallization of irons that uses experimentally determined partition coefficients and can reproduce the IIIAB trends and their associated scatter for 12 siderophile elements simultaneously. A key advancement of this revised trapped melt model is the inclusion of an effect on the resulting solid metal composition due to the formation of troilite. The revised trapped melt model supports the previous conclusion that trapped melt played an important role in the genesis of IIIAB irons and matches the trace element fractionation trends observed in the Cape York suite as due to different amounts of trapped melt. Applying the revised trapped melt model to 16 elements as well as S and Fe, the bulk composition of the IIIAB core is found to have a composition consistent with that expected from a chondritic precursor for refractory siderophile elements but with evidence for depletions of more volatile elements. The bulk S composition of the IIIAB core is estimated as 9 ± 1 wt%, implying that a substantial amount of S‐rich material from the IIIAB core is underrepresented in our meteorite collections. Future applications of the revised trapped melt model to other magmatic iron meteorite groups can enable comparisons between the core compositions and crystallization processes across the early solar system.

Origin of metal from CB chondrites in an impact plume – A combined study of Fe and Ni isotope composition and trace element abundances

The formation processes of the unusually metal-rich CB chondrites are a matter of debate. It is widely accepted that metal grains have formed by condensation. However, it is still debated whether they condensed directly from the solar nebula or from an impact-induced vapor plume. In this study, we present high precision Fe and Ni isotope and trace element composition of zoned and unzoned metal grains from the CBb chondrites Hammadah al Hamra 237, QUE 94411, and MAC 02675, and the CH/CBb breccia Isheyevo and unzoned metal from the CBa chondrites Bencubbin, Gujba, and NWA 4025. Data were obtained using femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry (fs-LA-(MC)-ICP-MS). Zoned metal grains from CBb meteorites generally display parallel profiles of Ni and Fe isotope compositions with very low δ56Fe and δ60Ni, and elevated concentrations of refractory siderophile elements in their cores. These findings are consistent with dominantly kinetic isotope- and trace element fractionation during condensation from a confined and fast cooling gas reservoir. Tungsten and Mo are frequently depleted relative to other refractory elements, particularly in zoned metal grains, which is suggestive for elevated oxygen fugacities in the gas reservoir. Such conditions are indicative of the formation of these metal grains during an impact event.Compared to zoned metal, unzoned metal grains are isotopically more homogeneous and more similar to the heavier rims of the zoned metal grains. This indicates that they formed under different conditions than the zoned metals, i.e., in a more slowly cooling environment. However, several unzoned grains still display significantly variable and correlated δ56Fe and δ60Ni, suggesting that their formation was related to that of the zoned metal grains. The kinetic fractionation-dominated isotopic signatures of the zoned metal grains strongly point to their formation during fast cooling, as may be expected for the exterior envelope of an impact plume. In contrast, the more homogenous isotopic signatures of the unzoned metal grains are more consistent with dominantly equilibrium-like isotope fractionation during condensation, as may be expected for the interior of an impact plume. In this scenario, the isotopically heavier rims of the zoned grains are best explained by a depletion of the outer plume gas reservoir in refractory elements and light isotopes. Accordingly, these findings indicate that zoned and unzoned metal grains likely formed during the same event. The compositional differences among individual unzoned metal grains, but also within some of the zoned grains, indicate turbulent gas mixing, also including movement of metals during their formation, between inner and outer regions of the impact plume.

Trace element geochemistry of CR chondrite metal

AbstractWe report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior (“interior grains”), chondrule surficial shells (“margin grains”), and the matrix (“isolated grains”). Save for a few anomalous grains, Ni‐normalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least‐melted, fine‐grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high‐temperature isolation of refractory platinum‐group‐element (PGE)‐rich condensates before mixing with lower temperature PGE‐depleted condensates. The rounded shape of the Ni‐rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1–100 K h−1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni‐poorer, amoeboid margin grains, often included in the pyroxene‐rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high‐temperature event, of which the chondrule margin records a late “quenching phase,” in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule‐forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.

Fast accretion of the Earth with a late Moon-forming giant impact

Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.

Group IVA irons: New constraints on the crystallization and cooling history of an asteroidal core with a complex history

We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal–liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities. Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body. Phosphate inclusions in EET show Δ 17O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal–liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.

The elemental composition of Almahata Sitta

Abstract– We quantified up to 60 elements in four individual fragments (#4, #7, #15, and #47) of the Almahata Sitta meteorite, which entered Earth’s atmosphere on October 7, 2008. Almahata Sitta is indisputably the product of fragments of asteroid 2008 TC3 surviving passage through Earth’s atmosphere. Each of the four analyzed fragments has a ureilitic composition based on comparisons with literature data. The analyzed fragments represent each of the major lithologies of Almahata Sitta. Highly refractory siderophile elements are similarly enriched in all fragments of Almahata Sitta, while volatile elements show more compositional variability. The abundances of rare earth elements (REE) only reach CI values in the case of a single Almahata Sitta fragment (#15). More typically, REE abundances in Almahata Sitta are between 0.01 and 0.3 × CI. None of the fragments displays the classic V‐shaped REE pattern that are often found in ureilites. Given that Almahata Sitta was collected shortly after its fall, these REE compositional results indicate that the V‐shaped REE pattern found in many ureilites may be a result of extended terrestrial residence times.

Bulk compositions of metallic Fe-Ni of chondrites: Constraints on fractionation of siderophile and chalcophile elements

Bulk compositions of metallic Fe-Ni from two equilibrated ordinary chondrites, Jilin (H5) and Anlong (H5), and two unequilibrated ones, GRV 9919 (L3) and GRV 021603 (H3), were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The CI-, Co-normalized abundances of siderophile and chalcophile elements of metallic Fe-Ni from the unequilibrated ordinary chondrites correlate with 50% condensation temperatures (i.e., volatility) of the elements. The refractory siderophile elements (i.e., platinum group elements, Re), Au, Ni and Co show a flat pattern (1.01×CI Co-normalized), while moderate elements (As, Cu, Ag, Ga, Ge, Zn) decrease with volatility from 0.63×CI (Co-normalized, As) to 0.05×CI (Co-normalized, Zn). Cr and Mn show deficit relative to the trend, probably due to their main partition in silicates and sulfides (nonmagnetic). Metallic Fe-Ni from the equilibrated ordinary chondrites shows similar patterns, except for strong deficit of Cr, Mn, Ag and Zn. It is indicated that these elements were almost all partitioned into silicates and/or sulfides during thermal metamorphism. The similar deficit of Cr, Mn, Ag and Zn was also found in iron meteorites. Our analyses demonstrate similar behaviors of W and Mo as refractory siderophile elements during condensation of the solar nebula, except for slight depletion of Mo in the L3 and H5 chondrites. The Mo-depletion of metallic Fe-Ni from GRV 9919 (L3) relative to GRV 021603 (H3) could be due to a more oxidizing condition of the former than the latter in the solar nebula. In contrast, the Mo-depletion of the metallic Fe-Ni from the H5 chondrites may reflect partition of Mo from metal to silicates and/or sulfides during thermal metamorphism in the asteroidal body.

Platinum-Group-Element Systematics of Peridotites from Ophiolite Complexes of Northwest Anatolia, Turkey: Implications for Mantle Metasomatism by Melt Percolation in a Supra-subduction Zone Environment

Platinum-group-element (PGE) studies of peridotites from the supra-subduction zone (SSZ) ophiolites of northwest Anatolia provide evidence for the nature of melt extraction within the uppermost mantle, and interactions between subduction-related magma and oceanic lithosphere. The peridotite samples from the mantle section of the ophiolites are mainly spinel-harzburgites and dunites, accompanied by subordinate amounts of spinel-lherzolite. Whole-rock major-trace element and mineral chemical characteristics indicate that the peridotites originated as the solid residues of varying degrees of partial melting (~5 to ~20%), and were subsequently modified by interaction with metasomatizing melts. The samples have non-chondritic, fractionated chondrite-normalized PGE patterns. Melt-depleted (e.g., low Al2O3 and CaO contents) mantle harzburgites and dunites show moderate to strong enrichments in the palladium group relative to the iridium group PGEs (PdN/IrN = 1.81±0.23; N = CI-chondrite normalized), and in most samples, pronounced Rh and Pd enhancements relative to Pt (RhN/PtN = 2.31 ± 0.66; PdN/PtN = 1.93 ± 0.20). These signatures cannot be reconciled with a simple in situ melt extraction and removal of sulfide phases, but most likely reflect a multi-stage petrogenetic process that selectively enriched the local mantle environment in incompatible and less refractory siderophile elements that are mobilized during continuous melt percolation, while relatively depleting the mantle wedge in Pt, which was not as effectively mobilized by silicate melts (or fluids). The results of quantitative model calculations indicate that the addition of sulfides that originated from interaction between solid mantle and percolating hydrous basaltic melts may account for the strongly supra-chondritic ratios of both Pd/Ir and Ir/Os, as well as for the formation of abundant chromite deposits within the ophiolite complex. The peridotites show no systematic variation of Ir-group PGE (Ir, Ru, Os; I-PGE) abundances relative to melt depletion indices such as Mg#, Al2O3, CaO, or spinel Cr#, despite their remarkable inter-element PGE variations. These along with elevated values of strongly incompatible lithophile elements (e.g., Ba, U, and LREE) in the reactive harzburgites and dunites suggest a post-melting metasomatism and melt impregnation in a suprasubduction zone environment. Enrichment in various incompatible elements (Hf, U, Ta, Sr) relative to the expected values for melt-depleted mantle residues and pronounced negative anomalies in fluid-insoluble high-field-strength elements (Ta, Nb, Zr, Hf, Ti) further suggest that both siliceous melts and slab-derived hydrous fluids were involved in mantle metasomatism.

Siderophile geochemistry of ureilites: A record of early stages of planetesimal core formation

New bulk-compositional data, including trace siderophile elements such as Ir, Os, Au, and Ni, are presented for 25 ureilites. Without exception, ureilites have siderophile abundances too high to plausibly have formed as cumulates. Ureilites undoubtedly underwent a variety of “smelting,” by which C was oxidized to CO gas while olivine FeO was reduced to Fe-metal. However, pressure-buffered equilibrium smelting is not a plausible model for engendering the wide range (75–96 mol%) of mafic-silicate core mg among ureilites. The smelting reaction produces too much CO gas. Even supposing a disequilibrium process with the smelt-gas leaking out of the mantle, none of the ureilites, least of all the ureilite with the most “reduced” (highest) olivine-core mg (ALH84136), has the high Fe-metal abundance predicted by the smelted-cores model. In principle, the Fe-metal generated by smelting could have been subsequently lost, but siderophile data show that ureilites never underwent efficient depletion of Fe-metal. Ureilites display strong correlations among siderophile ratios such as Au/Ir, Ni/Ir, Co/Ir, As/Ir, Se/Ir, and Sb/Ir. Ureilite siderophile depletion patterns loosely resemble siderophile fractionations, presumably nebular in origin, among carbonaceous chondrites. However, Zn, for an element of moderate volatility, is anomalously high in ureilites. A tight correlation between Au and Ni extrapolates to the low-Ni/Au side of the compositional range of carbonaceous chondrites. From this mismatch, mild but nonetheless significant depletions of refractory siderophile elements such as Ir and Os, and moderate depletions of strongly siderophile, weakly chalcophile elements such as Ni and Au, we infer that the ureilite siderophile fractionations are largely the result of a non-nebular process, i.e., removal of S-rich metallic melt, possibly with minor entrainment of Fe-metal. Several lines of trace-element evidence indicate that melt porosity during ureilite anatexis was at least moderate. The ureilite pattern of very mild depletions of extremely siderophile elements, but much deeper depletions of moderately siderophile, chalcophile elements, suggests that asteroidal core formation probably occurs in two discrete stages. In general, separation of a considerable proportion (several wt%) of S-rich metallic melt probably occurs long before, and at a far lower temperature than, separation of the remaining S-poor Fe-metal. Apart from the Fe-metal itself, only extremely siderophile elements wait until the second stage to sequester mainly into the core.

Formation of metal in the CH chondrites ALH 85085 and PCA 91467

Siderophile element distributions within individual metal grains in two CH chondrites, Allan Hills 85085 and Pecora Escarpment 91467, were measured by laser ablation inductively coupled plasma mass spectrometry. Those metal grains that are zoned in Ni were also found to be zoned in other refractory siderophile elements, such as Ru, but not in Pd, which is not refractory but is highly siderophile. This pattern is consistent with an origin by condensation from a gas of approximately solar composition, but not with an origin by redox processes or fractional crystallization. The unzoned metal grains in CH chondrites were found to be frequently depleted in Ru but not in Pd, consistent with later stage condensation from a solar gas after removal of the zoned metal. Gold is inversely correlated with Ni in the unzoned metal grains, and mean Au abundances in zoned metal are always low. Both zoned and unzoned metal in CH chondrites could plausibly be produced from a thermostatically regulated nebula, followed by rapid removal of the zoned metal, and slower removal of the unzoned metal, both at temperatures near or above the condensation temperature of Au (∼1250 K). This is also consistent with the isolation temperatures inferred from silicate grains in CH chondrites by previous workers based on their volatile element inventories. The volatile siderophile Cu is enriched in the rims relative to the interiors of both zoned and unzoned grains, and is interpreted as the product of diffusion during low-grade thermal processing. The similarity of Cu distributions, and degree of kamacite/taenite exsolution, between zoned and unzoned metal in CH chondrites suggests that the two populations of metal experienced modest thermal metamorphism after they were brought together in the same environment, probably on the CH parent body. Fragmentation and size-sorting of the metal must have post-dated the Cu zoning, and may have occurred in a regolith on the CH parent body. The compositions of CH metal, like that of metal from QUE 94411 and HH 237, are consistent with a nebular origin, and may be the most primitive nebular materials (as distinct from presolar grains) sampled by chondrites.

Reevaluation of formation of metal nodules in ordinary chondrites

Abstract— Compositions of four metal nodules from two ordinary chondrites, WIS91627 (H3.7) and Juin (H5), were determined by instrumental and radiochemical neutron activation analyses. Compared with bulk metal fractions, the metal nodules are characterized by strong and variable depletion of the refractory siderophile elements Re, Os, Ir, Ru, Pt and Rh but normal W and Mo, some fractionation of Co from Ni, and low Cu concentrations. These characteristics are difficult to explain by shock‐induced vaporization followed by fractional condensation, a mechanism suggested by Widom et al. (1986). We propose formation of metal nodules during metamorphism in the parent body. Refractory siderophile elements, such as Ir, Os, Rh, etc., are partly locked up in noble metal nuggets and cannot participate in kamacite formation. The occurrence of metal nodules in both equilibrated and unequilibrated ordinary chondrites suggests that diffusion along grain boundaries was important in the development of kamacite and taenite in ordinary chondrites.

Rhodium and other platinum-group elements in carbonaceous chondrites

Five carbonaceous chondrites (including the CI chondrites Orgueil and Ivuna) were analyzed by spark source mass spectrometry (SSMS) for the platinum-group elements Ru, Rh, Os, Ir, Pt, as well as W, Re, An, Th, and U. Conventional photoplate detection and a recently developed multi-ion counting system were used for ion detection.Results obtained for CI chondrites agree with compiled values within 6%. This study contains the first Rh analyses for this chondrite group. Rhodium concentrations for Orgueil and Ivuna agree well, implying a Solar System abundance of 0.140 ± 0.004 ppm and corresponding to 0.359 ± 0.010 atoms relative to 1 × 106 Si atoms.Concentrations in CM2, CV3, and CK4 chondrites are enriched compared to those of CI chondrites. However, the abundances of the refractory siderophile elements Re, Os, Ir with condensation temperatures above 1600 K (at 10−4 atm) are higher by about 15–20% compared to the less refractory elements Ru, Pt, and Rh. Elements with similar condensation temperatures correlate very well resulting in uniform concentration ratios of RhPt = 0.136 ± 0.006, ReOs = 0.0821 ± 0.0019, and IrOs = 0.949 ± 0.039.

Fractionation of refractory siderophile elements in metal from the Rose City meteorite

Abstract— An ∼4 × 9 × 12‐mm concentration of metal (dubbed RC1) situated between silicate melt and a relict chondritic clast in the Rose City H5 impact‐melt breccia is compositionally heterogeneous. Approximately 65 wt% of RC1 is enriched in the refractory siderophile elements, Os and Ir, by 30–40% relative to bulk H chondrite metal; ∼20 wt% is depleted in these elements by 31–35%; and 15 wt% is depleted by a considerably greater amount (75%). Common and volatile siderophile elements are essentially unfractionated in all three regions; W is fractionated to only a moderate degree. The compositions of the different regions of RC1 are similar to those of previously analyzed metal nodules and veins in shocked but unmelted ordinary chondrites. All of these objects probably formed by a complex process involving vaporization of chondritic material, rapidly followed by oxidation of W to form volatile oxides, fractional condensation of refractory siderophile elements, transport of the residual vapor (containing common and volatile siderophile elements as well as W oxide) and condensation of this vapor in fractures and voids or on metallic liquid substrates. The common occurrence of vugs in shock‐heated chondrites and the pervasiveness of vaporization effects recorded in metal masses and veins underscores the important role of superheating in the formation of impact breccias.

Noble gas studies in vapor-growth diamonds: Comparison with shock-produced diamonds and the origin of diamonds in ureilites

We synthesized vapor-growth diamonds by two kinds of Chemical Vapor Deposition (CVD) using microwave (MWCVD) and hot filament (HFCVD) ionization of gases, and examined elemental abundances and isotopic compositions of the noble gases trapped in the diamonds. It is remarkable that strong differences existed in the noble gas concentrations in the two kinds of CVD diamonds: large amounts of noble gases were trapped in the MWCVD diamonds, but not in the HFCVD diamonds. The heavy noble gases (Ar to Xe) in the MWCVD diamonds were highly fractionated compared with those in the ambient atmosphere, and are in good agreement with the calculated fractionation patterns for plasma at an electron temperature of 7000–9000 K. These results strongly suggest that the trapping mechanism of noble gases in CVD diamonds is ion implantation during diamond growth. The degrees of fractionation of heavy noble gases were also in good agreement with those in ureilites. The vapor-growth hypothesis is discussed in comparison with the impact-shock hypothesis as a better model for the origin of diamonds in ureilites. The diamond (and graphite, amorphous carbon, too) may have been deposited on early condensates such as Re, Ir, W, etc. This model explains the chemical features of vein material in ureilites; the refractory siderophile elements are enriched in carbon and noble gases and low in normal siderophiles. The vapor-growth model is also compatible with the oxygen isotopic data of ureilites which suggests that nebular processes are primarily responsible for the composition of ureilites.

High Coherence between the Most Refractory Siderophile Elements in Metallic Materials of Meteorites.

Re and Os abundances in twelve iron meteorites of various groups and five stony iron meteorites have been determined by isotope dilution employing an inductively coupled plasma mass spectrometry (ICP-MS). Re and Os abundances were found to have a wide variation covering nearly four orders of magnitude, with a very high correlation coefficient (0.996), showing a slope very slightly gentler than unity. The regression line observed here covers various groups of iron meteorites and stony iron meteorites. This feature can be interpreted to reflect a single“universal”effect such as direct mixing of particles of most refractory metallic elements with gaseous cloud of less refractory matrix elements, predating the formation of meteorites, since Re and Os are predicted theoretically to be the first elements that condense as a solid phase from high temperature solar nebula. The aim of this paper is to study the relationship between the siderophile refractory elements, Re, Os and Ir, together with their bearings on Ni, and to draw some implications for the early sequences of the solar system.

'Domestic' origin of opaque assemblages in refractory inclusions in meteorites

Experimental studies indicate that opaque assemblages rich in refractory siderophile elements were formed within host calcium- and aluminium-rich inclusions (CAIs) by exsolution, oxidation and sulphidization of homogeneous alloys, rather than by aggregation of materials in the solar nebula before the formation of CAIs. These opaque assemblages are thus not the oldest known solid materials, as was once thought, and they do not constrain processes in the early solar nebula before CAI formation. Instead, the assemblages record the changing oxygen fugacity experienced by CAIs during slow cooling in nebular and/or planetary environments.

Isotopic studies of Mg, Fe, Mo, Ru and W in Fremdlinge from Allende refractory inclusions

The isotopic abundances of Mg, Fe, Mo, Ru and W in Fremdlinge and a refractory metal nugget in coarse-grained, Ca-Al-rich inclusions from the Allende meteorite have been determined using the PANURGE ion microprobe. These data represent the first measurement of the isotopic composition of Fremdlinge. No evidence for large, non-mass-dependent deviations (≳1%) from normal solar isotopic abundances was found for any of the elements measured. The only hint, of a nonlinear isotopic anomaly is for Mo in Fremdling 1 of Allende inclusion 5171; the measured 100 Mo 98 Mo ratio lies slightly more than two standard deviations above the value measured in standards. Isotopic variations due to mass-dependent fractionation are suggested for Mo in two Fremdlinge, Zelda of Egg 6 and Fremdling 1 of 5171, and for Ru in a refractory metal nugget in the FUN inclusion Cl. Mo is enriched in the lighter isotopes by ∼4‰/ amu and ∼9‰/ amu , respectively, while Ru is enriched in the heavier isotopes by ∼4‰/ amu . Despite their unusual mineralogy and extreme enrichment in refractory siderophile elements, Fremdlinge do not contain an enhanced component from a single nucleosynthetic source, but appear to contain the normal solar system mixture of e-, s-, and r-process material. While the possibility that some Fremdlinge may have formed under presolar conditions cannot be excluded, we believe that the chemical, petrologic and isotopic data are most plausibly interpreted in terms of the origin of Fremdlinge within the solar nebula