Chondritic Metal Research Articles

Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites

We have developed a technique to measure abundances of Os, Ir, Pt, and Au in situ in Fe-Ni metal on a microscale using secondary-ion mass spectrometry. A Cs + beam is used to generate negative secondary ions, and interferences from molecular ions are eliminated by a combination of 40 eV of energy filtering and a mass resolution of ∼1900. Ion yields for Fe, Co, Ni, and the PGEs vary considerably from spot to spot in meteoritic metal samples. Ion yield variations for Os and Ir correlate strongly with the Fe ion yield, that for Pt is weakly correlated, and that for Au is essentially uncorrelated. From ion yields for meteorite standards and the correlations with Fe ion yield, it is possible to obtain concentrations of Os, Ir, Pt, and Au in meteorite samples. Using this technique, a Cs + beam current of 10 nA gives a spatial resolution of 10–20 μm and detection limits of less than 1 ppm for Os, less than 0.1 ppm for Ir, and 10–20 ppb for Pt and Au. With refinement, it should be possible to measure Rh, Pd, and Ag and to improve the spatial resolution. However, Re, Ru, W, and Hf can only be measured as positive secondary ions. We have measured the abundances of Os, Ir, Pt, and Au in kamacite, taenite, and/or plessite in five iron meteorites: Canyon Diablo (IA), Cape York (Agpalilik) (IIIA), Colomera (IIE), Cruz del Aire (Anom), and Wallapai (IID). Our measurements show that Os, Ir, Pt, and Au partition preferentially into taenite relative to kamacite during slow cooling. Measured abundance ratios (taenite/kamacite) range from ∼1.3 to ∼2.1 for Os, Ir, and Pt, and from ∼2 to ∼6 for Au. These ratios are consistent with those determined recently by laser-ablation ICPMS, but differ significantly from those determined by earlier workers. Low-temperature taenite/kamacite distribution coefficients inferred from our data are ∼2.1 for Os, ∼1.6 for Ir, ∼1.9 for Pt, and ∼6 for Au. PGEs are not enriched in taenite as much as Ni. Partitioning is controlled by the interplay of the size and electron configuration of Os, Ir, Pt, and Au and the crystal structures of kamacite and taenite. The ion probe can be used to further investigate the distribution and subsolidus redistribution of PGEs and Au in iron meteorites and in the metal in chondrites and stony irons.

The origin and nebular history of the metal phase of ordinary chondrites

We present new INAA results for bulk metal from H and LL chondrites. Both have characteristic elemental patterns similar to L chondrite metal, in particular increasing abundances of W and Ga from unequilibrated to equilibrated chondrites but a reverse variation of V and Cr abundances. These characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material. The similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor, similar to CI or CM material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites. Co Ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups. This suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions. The chondrites that accreted earliest have preserved their melting characteristics, while those that accreted later reequilibrated with the ambient gas at different temperatures. The differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies. The initial metal composition established during the melting stage (13 wt% Ni; Co Ni = 0.031 ) is inferred from the metal in highly unequilibrated chondrites. The accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction. H chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups. LL chondrites were accreted at the lowest temperature and are most oxidized. Compositionally, the metal component that would account for the Fe Si fractionation among H, L, and LL chondrites is different from the metal formed by melting, indicating that the different Fe S ratios of H, L, and LL chondrites were established before their accretion.

Metal phases of L chondrites: Their formation and evolution in the nebula and in the parent body

Metal phases of six (three equilibrated and three unequilibrated) L chondrites were studied by INAA, SEM, and Mössbauer spectroscopy. Characteristics retained in the bulk metals of unequilibrated chondrites (abundant carbon, high contents of Cr, V, Mn, and low contents of W, Mo, and Ga compared to metals of equilibrated chondrites, less enrichment of W than Mo, and fractionation of Co from Ni) demonstrate that chondrite metals are not nebular condensates. All those characteristics can be well explained by melting. Chondrite metals are not melting remnants of previously condensed metals, rather, they were produced by reduction of CI- or CM-like material during the melting process. The complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event. Distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the low-temperature diffusion products and must have been developed in the chondrite parent body. The difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body. Thus, while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity, the crystallization of EOC silicates resulted from an external heating. The internal metamorphism was mild (400–600°C) and long whereas the external heating was intense (with a maximum temperature in range of 800–950°C) and short. Tetrataenite is present not only in UOCs but also in EOCs, suggesting that the external heating occurred during the internal metamorphism, i.e., within 100 myr of chondrite formation.

Distribution of W and Mo in ordinary chondrites and implications for nebular and parent body thermal processes

W and Mo abundances in the bulk metals of 10 H, 5 L and 6 LL chondrites were determined by instrumental neutron activation analysis (INAA). Partitioning of W and Mo between metal and non-metal phases was evaluated by comparing Ni-normalized W and Mo abundances in the metal phases with those in the bulk samples. It is observed that W distributions differ significantly between equilibrated ordinary chondrites (EOCs) and unequilibrated ordinary chondrites (UOCs). However, no correlation is confirmed between the W distribution and the petrographic type for EOCs. This implies that the W partitioning among mineral phases can provide a clue for estimating the metamorphic temperature intervening between EOCs and UOCs. The difference in W equilibrium temperatures was observed among H, L and LL chondrite groups and may correspond to the difference in cooling rates and subsequently in sizes of the chondrite parent bodies; LL chondrites have the lowest equilibrium temperature and, hence, have the largest parent body, whereas H chondrites have the highest equilibrium temperature and the smallest parent body. Mo/W abundance ratios remain constant in EOC metals, but are variable in UOC metals, suggesting that the W solid equilibrium has not been achieved in UOCs. The W and Mo distributions in UOCs still preserve the characteristics of W and Mo in the nebula, which demonstrates that the chondritic metal was formed by melting highly oxidized precursors before or during theaccretion of chondrite parent bodies.

Experimental determination of the solubility of Au in silicate melts

We report here new data on the solubility of Au in silicate melts of anorthite-diopside eutectic composition at a wide range of oxygen fugacities, from pure oxygen to 10−8 atm, and at a temperature range of 1300 °C to 1480 °C. Because experiments were done with metal loops at temperatures above the Au-melting temperature, PdAu-metal-alloys had to be used. Pd-solubility data derived from the same set of experiments agree with earlier data obtained from experiments with pure Pd-metal (Borisov et al., 1994a). The results of the present experiments show that Pd-solubilities are by a factor of 2 to 6 higher than Au-solubilities. Both, Au and Pd solubilities decrease with decreasing oxygen fugacity. At oxygen fugacities below the iron-wiistite buffer (IW) Au solubility increases with decreasing fO2 probably reflecting formation of Au-silicides at such reducing conditions. Compared to Pd, Au has higher activity coefficients in Fe-metal and lower solubility in silicate melts. This leads to similar metal-silicate partition coefficients for both elements. At a temperature of 1350 °C and an oxygen fugacity corresponding to IW-2 DAu (met/sil) is about 2.5 · 107 and DPd (met/sil) about 1.6 · 107. Thus similar behavior is expected during metal separation in planetary bodies including core formation in the Earth. The metal/silicate partition coefficient of Ir is, however, by several orders of magnitudes higher (Borisov and Palme, 1995a). Equilibration with chondritic metal will therefore lead to grossly non-chondritic Pd/Ir or Au/Ir ratios in coexisting silicate phases. Chondritic ratios are thus indicative of the presence of unfractionated meteoritic components. Samples from the upper mantle of the Earth, for example, reflect the admixture of a late unfractionated (chondritic) veneer (e.g.,Kimura et al., 1974;Jagoutz et al., 1979).

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.

A comparison of metallographic cooling rate methods used in meteorites

The primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites. Both types of metal occur in some Group IAB meteorites, which are mixtures of massive metal with well-developed Widmanstätten structures and chondritic inclusions with dispersed metal grains. The grains have textures and compositions similar to chondritic metal, including negligible P. The meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before Widmanstätten patterns formed during cooling. Cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central Ni contents. Modeling entails solving diffusion equations using experimental diffusion coefficients, phase relations, and bulk or local Ni and P contents, taking into account geometry, undercooling, and impingement. There is one set of parameters for grains and another, quite different set for Widmanstätten lamellae, including a factor of 30 difference in diffusion coefficients. Yet cooling rates obtained from Widmanstätten structures and metal grains in chondritic inclusions of the same meteorite are consistent; uncertainties in the best data are ±10° /Ma, equivalent to a factor of 1 ± 0.25. This agreement implies that the data and models are correct or contain fortuitously offsetting errors, which is quite unlikely. Cooling rates range from 40°/Ma to 70° /Ma in IAB meteorites that contain both grains and Widmanstätten structures. Rates based on grains in Ni-poor and Ni-rich meteorites lacking Widmanstätten patterns expand the range from 30°/Ma to perhaps 200°/Ma. Cooling rates correlate with Ni content; Ni-poor meteorites have slower rates than Ni-rich ones. Evidently, IAB meteorites were radially distributed over >30km in a body with a radius >50km. A comparison of the available Ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established.

Reflectance spectra of mafic silicate-opaque assemblages with applications to meteorite spectra

The reflectance spectra of wüstite and mixtures of mafic silicates plus carbon or magnetite can be used to interpret meteorite and asteroid spectra. Mafic silicate + magnetite spectra show many features characteristics of magnetite-bearing meteorites— an overall decline or constant reflectance and lown overall reflectance. Mafic silicate + amorphous carbon spectra show low overall reflectance and a red slope unlike that seen in CV and CO carbonaceous chondrite spectra, probably because the meteoritic carbon is in a more ordered form. The reflectance spectra of ureilites are largely consistent with an assemblage of mafic silicates and abundant carbon. Ordinary chondrite reflectance spectra cannot be reproduced by any of the laboratory mixture spectra. The reflectance spectrum of wüstite is a reasonable match to the spectrum of ordinary chondrite metal, suggesting that most ordinary chondrite metal grains are probably coated with an optically thick layer of an oxide. Ordinary chondrite and mafic silicate reflectance spectra are consistently less red-sloped than S-class asteroid spectra. The various spectral criteria use to deco0nvolve mafic silicate spectra are also applicable to CV and CO carbonaceous chondrites, ureilites, and ordinary chondrites, because the opaque phases present in these meteorites are spectrally neutral.

The Trace Element Chemistry of the Metal in IAB Iron Meteorites

Abstract Metal grains from a silicate inclusion in Landes, a group IAB iron-meteorite, were separated and analyzed via IN A A for 14 siderophile elements. The same 14 elements were determined in matrix metal from Landes, 5 other IAB meteorites and three anomalous iron meteorites. Compared to the other samples and to chondritic metal, the inclusion metal is enriched in siderophile elements. In chondritic metal, the siderophile element contents are higher when Fe0 is oxidized to FeO. Silicates in IAB inclusions contain little FeO, yet the metal is as rich in siderophile elements as the most oxidized chondrites. Evidently the IAB body contained its full cosmic complement of S resulting in an FeS content ~5 x that of ordinary chondrites. This explains the composition of the metal since sulfurization, like oxidation, reduces the available Fe0. IAB meteorites contain their full cosmic complement of other moderately volatile elements, which implies an equally high S content. A high S content significantly affects the melting and freezing of FeNi-metal. About 85% of the metal melts at the Fe-FeS eutectic (~960 °C), and all of the metal is molten at ~1050°C. During cooling, ~85% of the metal crystallizes with a Ni content of 6±1% between 1050°c and 960°C. Besides a small depletion in elements with low melting points or chalcophile tendencies this metal is little fractionated. Bellow 960 °C, the Ni content of the metal increases until the melt is exhausted. This Ni-rich metal also becomes progressively depleted in refractory elements and enriched in volatile elements. The crystallization process thus yields products that mimic the abundance and composition of group IAB meteorites.

Xenon in Chondritic Metal

Abstract We report xenon isotopic abundances observed in the stepwise release of noble gases in a highpurity metal separate of the Forest Vale (H4) chondrite. We identify a 244Pu-derived fission component, due to recoils into the metal, a cosmic-ray-produced spallation component and a new trapped component (FVM; 132Xe: 134Xe: 136Xe = 3.16:1.235:1.000) which is isotopically distinct from known solar system reservoirs. We discuss several processes which might account for observed isotopic shifts and conclude that the signature of FVM xenon may provide clues regarding the origin of chondritic metal.

Petrology of ALH85085: a chondrite with unique characteristics

ALH85085 has several characteristics which set it apart from any known chondritic group. Its chondrules are smaller (25–75 μm), the majority are cryptocrystalline, and all are volatile (Na, K, S) depleted. FeNi metal is more abundant (40.9 wt.%) than in any other chondrite and sulfide abundance is low (1.1%). Matrix material does not surround the chondrules and fragments, but occurs as lithic clasts (lumps) up to 300 μm across. The bulk composition of ALH85085 is lower in Na/Si and S/Si and higher in Ni/Si, Fe/Si and Mg/Si than in other chondritic groups. The Ca/Si, Al/Si and Mg/Si ratios are closest to those of the Renazzo chondrite grouplet (CR). Matrix lumps are most similar to CR and Murchison group (CM) chondrite matrices. Survival of this fragile matrix material as lithic clasts indicates that they were added just prior to the agglomeration of the chondrite. All components (chondrules, fragments, matrix lumps) fall within a narrow size range, which may be the result of aerodynamic sorting. If so, the sorting must have occurred after chondrule solidification and following a period of chondrule breakage since fragments from larger chondrules are present. The Ni vs. Co trend in the FeNi metal overlaps that of metal in CR chondrites and Bencubbin. These trends are similar to those of calculated nebular condensation paths indicating a nebular origin. The cryptocrystalline texture and volatile depletion of the chondrules indicate that most chondrules were heated to temperatures above the liquidus. Although ALH85085 cannot now be assigned to any known chondrite group, it shares many similarities with CR chondrites and the Bencubbin metal-silicate chondritic assemblage and may be part of a chondritic group closely related to these meteorites.

Boron in chondritic meteorites

Alpha-track images (ATI), prepared by etching neutron-irradiated cellulose nitrate films attached to polished thin sections of meteorites, provide useful qualitative maps of boron and lithium distribution. However, extreme care must be taken to avoid contamination during preparation, particularly by boron in diamond polishing paste. Boron and lithium are virtually absent from chondrules, metal and sulphide grains in carbonaceous and ordinary chondrites. Matrix material, by contrast, contains sufficient boron to account for the 0.2–1.0 ppm boron usually present. Many Antarctic meteorites contain secondary Fe-Si-rich weathering products: ion microprobe analysis showed significant enrichment of boron and chlorine in one meteorite, probably attributable to effects of sea-spray aerosols. Fifteen new analyses of 11 Antarctic chondrites, gave boron contents in the range 0.24–1.0 ppm, with a mean value of 0.55 ppm. The set comprised three carbonaceous chondrites and eight ordinary chondrites, with no obvious difference in boron content between the two groups. Types 3 and 4 ordinary chondrites show a wider range in values of B 10 6Si than previously found. Using literature abundances for other elements, the present data do not confirm that boron resembles sulphur more closely than sodium in depletion patterns from Cl chondrite abundances.

The nature of the meteoritic components of Apollo 16 soil, as inferred from correlations of iron, cobalt, iridium, and gold with nickel

Concentrations of Co, Ni, Ir, and Au in small samples of submature and mature soil from Apollo 16 are highly variable. Correlations of Fe, Co, Ir, and Au with Ni indicate that the variation in siderophile element concentrations results from variation in the concentration of Fe‐Ni metal. From these correlations, the mean composition of the metal is inferred to be 5.6% Ni and 0.36% Co for a Ni/Co ratio of 15.5 ± 0.7, i.e., dissimilar to the chondritic ratio of 20–21. The Fe‐Ni metal is also characterized by low Ir/Ni and Ir/Au ratios compared to metal in ordinary chondrites. Metal with a similar composition is also found in metal‐rich, noritic impact melt breccias that occur as discrete rocks as well as small clasts in ancient regolith breccias at Apollo 16. Mass‐balance models using lithophile element concentrations indicate that about 35% of the soil is noritic impact melt. The Fe‐Ni metal associated with this component is sufficient to supply virtually all the ancient Fe‐Ni metal found in the typical Apollo 16 soil (0.4–0.5%), i.e., no other source is required. The soil also contains 1–2% carbonaceous chondrite. The concentration of this component appears to be relatively constant from sample to sample and does not contribute significantly to the variation in siderophile element abundances. These two meteoritic components contribute nearly equal concentrations of Ni to the typical Apollo 16 soil.

Enstatite chondrites and enstatite achondrites (aubrites) were not derived from the same parent body

Enstatite achondrites (aubrites) were not derived from known enstatite chondrites by melting and fractionation on one and the same parent body, for these and other reasons: (1) There is no satisfactory mechanism for fractionating metal plus troilite in enstatite chondrites to form these phases in different proportions and with different Ti contents in aubrites. (2) Many enstatite chondrites and aubrites are regolith or fragmental breccias, but clasts of one within the other have not been found. (3) Cosmic ray exposure ages of the two groups are difficult to explain if they are from the same parent body, but are easy to explain if they are from different parent bodies. Siderophile element abundances in metal from the Mt. Egerton meteorite, which consists of enstatite and metallic Fe,Ni, preclude it from being a complementary differentiate of the aubrites. Rather, it appears that Mt. Egerton was formed from the same source material as enstatite chondrites, but the components were mixed in different proportions.

The spectral and physical properties of metal in meteorite assemblages: Implications for asteroid surface materials

The metal grains in chondritic meteorites from terrestrial collections are coated with an optically thick surface layer, probably composed of iron oxide and/or iron sulfide. This coat on the metal grains suppresses the spectral contribution of NiFe metal in the reflectance curves of these meteorites. Only if this surface layer is disrupted will the strongly reddened signature of metallic NiFe be seen in chrondritic spectra. While origin of this surface layer is not yet established, it is probable that it is either pre-terrestrial or formed by the weathering of an unstable mineral species, such as lawrencite (FeCl 2), which was present as a thin, pre-terrestrial veneer on the chondritic metal grains. In either case, the surfaces of intact metal grains in asteroidal chondritic assemblages most probably will not resemble NiFe metal. Low-nickel metal grains, such as those in H-type chondrites, will be brittle at asteroid surface temperatures. High-nickel metal grains, such as those in LL-type chondrites, remain ductile down to at least 50°K, below even asteroid night side temperatures. The metal phase, even when brittle, will be at least as strong as the silicate phase in asteroid regoliths. Therefore, preferential fragmentation of brittle metal is not a viable mechanism to increase the spectral contribution of the NiFe phase in an asteroid regolith. Under plausible proposed regolith processes, only the metal-rich H-type subset of the ordinary chondrites can be expected to produce an S-type asteroid spectrum from an undifferentiated assemblage, and then only if optically thick metal grain coats are absent. Known regolith processes cannot reasonably produce an S-type spectrum from metal-poor L-, LL-, or C3-type assemblages. The strong NiFe signatures and the mafic silicate features in the reflectance spectra of the S-type asteroids appear to require that the most of them represent metal-rich, differentiated assemblages. The spectral properties of M-type asteroids do not require metal-rich or differentiated surface materials, although it is plausible that this is the case.

HIGH POSSIL AND STRATHMORE—A STUDY OF TWO L6 CHONDRITES

High Possil and Strathmore are the first (1804) and last (1917) meteorite falls, respectively, of the three recorded in Scotland. Olivine compositions and total Fe contents in High Possil (Fa25.2; 21.35 wt %) and Strathmore (Fa25.3; 20.6 wt %) confirm their classification as L‐group chondrites (Mason, 1963), and the presence of abundant plagioclase feldspar shows that both chondrites belong to petrologic type 6. Both chondrites display thermal and mechanical alteration attributable to moderate shock‐loading appropriate to facies c (High Possil) and c‐d (Strathmore) (Dodd and Jarosewich, 1979). Incipient shock‐melting of metal and troilite in both chondrites is the first described from Lc chondrites, and differences in the responses of metallic and silicate minerals to shock‐loading are discussed.

Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites

The new interpretive calibrations (olivine-pyroxene abundance, spectral signature of chondritic metal grains) have been applied to visible and near-infrared spectral reflectance data for the typical S-type asteroid, (8) Flora. Chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel-iron (NiFe) metal phase. The conditions required to produce a S-type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed. Five mineralogic and patrologic parameters have been derived for the surface material of Flora from the analysis of the spectral reflectance and rotational spectral variations of this object. These include bulk mineralogy (NiFe metal, olivine, pyroxene), mafic mineral abundance (ol/py ≈ 2.8), metal nature [abundant (≈50%) and coarse-grained in the substrate], pyroxene composition (orthopyroxene > F s 50 and/or abundant clinopyroxene), and spatial mineralogic variations (olivine/pyroxene ratio and pyroxene composition do not covary in a chondritic manner). All of the diagnostic mineralogic properties of the Flora surface material indicate that this is a differentiated body. None of the derived surface properties unambiguously support a chondritic-type material and several contradict any type of undifferentiated assemblage. Flora is most probably the residual core of an intensely heated, thermally evolved, and magmatically differentiated planetesimal which was subsequently disrupted. The present surface samples layers formed at and near the core-mantle boundary in the parent body. Several lines of evidence suggest that the S-type asteroids as a class are predominantly the core fragments of disrupted, differentiated planetesimals. A main belt source of the ordinary chondrites remains elusive.

A search for evidence of superheavy-element fission in chondritic metal

There is evidence for the early melting of the Moon1,2, and Runcorn2–4 has concluded from palaeomagnetic data that a fluid core dynamo must have been present for ∼109 yr and that the decay of a siderophile superheavy element (SHE) with a half life of ∼108 yr may have been responsible for these phenomena3–5 Libby and co-workers6 have suggested that SHE-decay could have produced certain trace-element abundance patterns in iron meteorites. Several searches7–9 have failed to detect evidence for SHE decay in iron meteorites. We describe here a search for SHE in metal fragments within ordinary chondrites. The absence of excesses of tracks along the boundaries between olivines and metal grains allows upper limits to the SHE content of the metal of ∼2.5×10−13 to ∼2.5×10−15 kg per kg to be set at a time ∼4×109 yr ago, determined from the 244Pu fission track densities in chondritic phosphates.

Si-rich Fe–Ni grains in highly unequilibrated chondrites

Some Fe–Ni grains in the Bishunpur L3 chondrite have Si contents in the range 1.3–4.5 mg per g (mole fractions of 0.003–0.009); the only previous report of Si in ordinary chondrite metal is a lower limit of 0.7 mg per g for metal in the Leighton H4 chondrite1. The Bishunpur Si contents are 1.1–4 tunes that found in a large metal grain in the Murchison CM2 chondrite2. As whole-rock systems both meteorites have high degrees of oxidation and consist of unequilibrated, essentially unmetamorphosed mineral assemblages. Thus the Si was not introduced into the metal during metamorphic reheating, but rather during nebular condensation2. Grossman et al.2 calculated that the Si mole fraction of 0.0024 in the Murchison grain required a total nebular pressure ≥10−5atm, which is higher than that generally inferred for the CM formation location. We show here that improvements in the Si activity coefficients and plausible constraints on the nucleation of mafic silicate condensates allow Si mole fractions of 0.003 at nebular pressures of <10−6atm. Thus such chondrites could have formed over a wide range of distances from the Sun.

Trace element distribution between magnetic and non-magnetic portions of ordinary chondrites

The contents of As, Au, Ni, Co, Ir, Os, Re and Pt were determined in the metal and silicates-plus-troilite portions of 10 H-, 7 L-, and 5 LL-group chondrites. All elements are strongly siderophile and concentrated in the metal phase. The different values of the As/Ni, Au/Ni and Co/Ni ratios in the magnetic and non-magnetic portions of the three chemical groups indicate that the contents of these elements in the latter fractions cannot be entirely attributed to metal contamination. Analyses of minerals separated from L-group chondrites reveal relatively high concentrations of Ni and Co in troilite and of Au in chromite, feldspar and phosphate. Iridium, Os, Re and Pt are enriched with respect to Ni in the non-magnetic fractions of ordinary chondrites, and the proportion of refractory metals, in these fractions, increases from H- to LL-group chondrites, and, within each group, in the order Ir ≳ Os ≳ Pt ≫ Re. The Ir/Re ratio in ordinary chondritic metal is about 35% below C1 ratios, but higher than C1 in the non-magnetic fractions. The Ir-Re fractionation appears to have taken place in the nebula before condensation of Fe-Ni. The Ir-rich inclusions within chondritic silicates may represent a sample of the material that was lost during the agglomeration of ordinary chondrites.