Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa_38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa_38–41); (h) spinels have a high TiO₂ of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ¹⁷O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ¹⁷O = 5.36 ± 0.43, δ¹⁸O = 5.07 ± 0.86, Δ¹⁷O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.     

The bulk composition of Mars

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.     

Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: The record of their source material and atmospheric entry heating

New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al₂O₃, and 0.14 wt.% to 0.31 wt.% for TiO₂) and refractory trace elements (from 6.2 μg/g to 19.3 μg/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (δ¹⁸O = 13.2-17.3‰ and δ¹⁷O = 7.6-9.2‰). They are systematically enriched in Ni and Co (Ni = 24-500 μg/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 μg/g, δ¹⁸O = 13.1-28.0‰ and δ¹⁷O = 5.1-14.0‰). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale.     

Origin of SiO2-rich components in ordinary chondrites

Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO₂ (with bulk SiO₂ >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655-1666]. Several different explanations of how SRCs formed have been published. The main question is how silica-enrichment was achieved, because CI-chondritic atomic Mg/Si-ratio is 1.07 and as a consequence only olivine and pyroxene, but no free silica should be stable. There are two basic possibilities for the SiO₂-enrichment: (1) a RedOx-mechanism or magmatic fractionation on the parent body and (2) fractional condensation or recycling of chondrule mesostasis in the solar nebula. To better constrain the origin of these objects, we measured major and rare earth elements in SRCs of various types of ordinary chondrites, and in addition, we studied silica polymorphism in these objects using an in situ micro-Raman technique. Bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene. The SRCs extend these lines to an SiO₂ end member. In contrast, magmatic trends grossly deviate from these mixing lines. Concentrations of CaO, Al₂O₃, and REE in the pyroxenes of the SRCs are low (0.01 to 1× CI) and the CI-normalized REE-patterns are virtually flat, typical of bulk chondrules, but untypical of magmatic trends. We therefore conclude that SiO₂-rich objects are not of magmatic origin. They are the result of fractional condensation in the solar nebula. The silica in SRCs occurs mainly as tridymite and sometimes as cristobalite or—in very rare cases—as quartz. Some SiO₂-phases yielded a yet unknown micro-Raman spectrum, which we were unable to identify. The often chondrule-like shape of SRCs as well as the presence of high-temperature SiO₂-polymorphs lead to the following model for the origin of SRCs: formation of SiO₂-rich precursors in the solar nebula by fractional condensation, reheating to temperatures between 1140 and >1968 K, thereby forming the SRCs,—probably during the chondrule-forming process—followed by rapid cooling.     

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

Of the six chondrites that were listed as EH6 or EH6-an during the course of this study, we confirm the EH classification of Y-8404, Y-980211 and Y-980223 and the EH-an classification of Y-793225; two chondrites (A-882039 and Y-980524) are reclassified as EL (the former contains ferroan alabandite and both contain kamacite with ∼1 wt% Si). All of the meteorites contain euhedral enstatite grains surrounded by metal ± sulfide (although this texture is rare in Y-793225), consistent with enstatite crystallizing from a mixed melt. All contain enstatite with <0.04 wt% MnO; the three EH chondrites average 0.25 wt% Mn in troilite. (Literature data show that typical EH3-EH5 chondrites contain enstatite with 0.13-0.20 wt% MnO and troilite with 0.05-0.11 wt% Mn.) The three EH chondrites contain keilite [(Fe_>0.5,Mg_<0.5)S], which has been interpreted in the literature as a product of impact melting. Y-8404 and Y-980223 contain abundant silica (∼13 and ∼10 wt%, respectively), a rare phase in most enstatite chondrites. We suggest that all six meteorites have experienced impact melting; Mn was preferentially partitioned into sulfide during subsequent crystallization. The silica-rich samples may have become enriched in the aftermath of the impact by a redox reaction involving FeO and reduced Si. A-882039, Y-8404, Y-980211, Y-980223 and Y-980524 were incompletely melted; they contain rare relict chondrules and are classified as impact-melt breccias; Y-793225 is a chondrule-free impact-melt rock. If these EH and EH-an chondrites (which were previously listed as petrologic type 6) have, in fact, been impact melted, it seems plausible that collisional heating is generally responsible for EH-chondrite metamorphism. This is consistent with literature data showing that a large fraction (?0.7) of those chondrites classified EH5-7 and a significant fraction (?0.3) of those chondrites classified EH4 and EH4/5 possess textural and mineralogical properties suggestive of impact melting. In addition, ∼60% of classified EL6-7 chondrites (now including A-882039 and Y-980524) appear to have formed by impact melting. It thus seems likely that collisional heating is mainly responsible for EL- and EH-chondrite metamorphism.     

Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in δD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of ∼0‰ (least altered, most volatile rich) and −200‰ (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-δD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a δD value of ∼−158‰, consistent with initial accretion of CM parent bodies at ∼4 AU.     

The Yamato-type (CY) carbonaceous chondrite group: Analogues for the surface of asteroid Ryugu?

We report new mineralogical, petrographic and noble gas analyses of the carbonaceous chondrite meteorites Y-82162 (C1/2_ung), Y-980115 (CI1), Y-86029 (CI1), Y-86720 (C2_ung), Y-86789 (C2_ung), and B-7904 (C2_ung). Combining our results with literature data we show that these meteorites experienced varying degrees of aqueous alteration followed by short-lived thermal metamorphism at temperatures of >500 °C. These meteorites have similar mineralogy, textures and chemical characteristics suggesting that they are genetically related, and we strongly support the conclusion of Ikeda (1992) that they form a distinct group, the CYs (“Yamato-type”). The CY chondrites have the heaviest oxygen isotopic compositions (δ¹⁷O ˜12‰, δ¹⁸O ˜22‰) of any meteorite group, high abundances of Fe-sulphides (˜10 ‒ 30 vol%) and phosphates, and contain large grains of periclase and unusual objects of secondary minerals not reported in other carbonaceous chondrites. These features cannot be attributed to parent body processes alone, and indicate that the CYs had a different starting mineralogy and/or alteration history to other chondrite groups, perhaps because they formed in a different region of the protoplanetary disk. The short cosmic-ray exposure ages (≤1.3 Ma) of the CY chondrites suggest that they are derived from a near-Earth source, with recent observations by the Hayabusa2 spacecraft highlighting a possible link to the rubble-pile asteroid Ryugu.     

Glasses in the D'Orbigny angrite

The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO₂ and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.     

Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids

IR spectroscopy is one of the few techniques that can directly probe water molecules in rocks. This method has been used to characterize the mineralogy of hydrated/hydrous carbonaceous chondrites, and to link known meteorite families with spectroscopic observations of low albedo asteroids. In this paper, we present measurements of the infrared transmission spectra of matrix chunks from 3 CI and 9 CM chondrites. Spectra were measured at ambient conditions and then at different temperatures along a dehydration path toward high-T (∼300 °C) under primary vacuum. At ambient conditions, the 3-μm spectral range is always dominated by adsorbed atmospheric water molecules. Upon moderate (∼100 °C) and high (∼300 °C) heating under low pressure (P < 10⁻⁴ mbar), adsorbed water and then phyllosilicates interlayer water are removed, revealing a residual absorption band around 3 μm. This band is a characteristic IR feature of the phyllosilicate phases which dominate the mineralogical assemblage of hydrated carbonaceous chondrites. Among the CM chondrites, the high-T spectra reveal a strong variability that appears correlated with the alteration classification scheme of Rubin et al. (2007) and Howard et al. (2009a). The 3-μm band continuously evolves from a broad feature peaking at 3550-3600 cm⁻¹ for the weakly altered CMs (Murchison-type) to a sharp asymmetric peak at ∼3675 cm⁻¹ for the more extensively altered samples (Cold Bokkeveld-type). We attribute this spectral evolution to variations in the chemistry of the phyllosilicate phases from Fe-rich to Mg-rich. On the other hand, the 10-μm spectral region shows a single broad peak which does not compare with known terrestrial serpentine spectra, probably due to high structural disorder of the chondrite phyllosilicate phases. The present work clearly shows that previously published reflectance spectra of chondrites are biased by the presence of adsorbed terrestrial water molecules. Laboratory data collected under dry conditions are needed to reinterpret the chondrite-asteroid connection from the comparison of their 3-μm absorption features.     

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland

New mapping, geochemistry and zircon U-Pb ion microprobe geochronology of pre-3750 Ma rocks from West Greenland was used to identify sedimentary protoliths in a problematic high-grade metamorphic terrane. Samples were collected from southernmost part of the Itsaq Gneiss Complex where Akilia association supracrustal rocks have previously been noted. Supracrustal lithologies include laterally continuous and variably deformed units of amphibolite, ultramafics and ferruginous quartz-pyroxene rocks. Oxygen isotope and mass-independently fractionated sulfur isotopes, immobile trace elements and rare earth element patterns are consistent with origin of quartz-pyroxene rocks as chemical sediments deposited in a marine hydrothermal setting. We describe a further supracrustal lithology: Garnet-bearing quartz-biotite schists with elevated oxygen isotope values (δ¹⁸O_SMOW ? +16‰) and mass-independently fractionated S isotopes consistent with a low-temperature aqueous sedimentary origin. In several enclaves, granitoid gneisses within low-strain limbs transect lithologic contacts and contain inclusions of surrounding rocks. This supports the interpretation that some orthogneisses were originally emplaced as igneous veins that cut supracrustal lithologies. Zircon geochronology on orthogneisses that preserve intrusive relationships confirms minimum ages of ca. 3750 Ma for the supracrustals and pooled [Th/U]_zircon and δ¹⁸O_zircon values of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Low [Zr]_WR and high Zr saturation temperatures further minimize the possibility of zircon inheritance. A >3750 Ma age and chemical sedimentary origin for various Akilia association lithologies underscores the widespread occurrence of rocks of this kind beyond the type locality on Akilia (island) at the southern limit of the Itsaq Gneiss Complex.     

Graves Nunataks 95209: A snapshot of metal segregation and core formation

GRA 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies. This lodranite preserves a physically, chemically, and mineralogically complex set of metal-sulfide veins. High-resolution X-ray computed tomography revealed three distinct lithologies. The dominant mixed metal-silicate-sulfide matrix is cut by metal-rich, graphite-bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal-poor region. Silicate compositions and modal abundances are typical for lodranites, while the mineralogy of the metal-sulfide component is complex and differs among the three lithologies. Kamacite and troilite occur with chromite, tetrataenite, schreibersite, graphite, and a range of phosphates. An ³⁹Ar-⁴⁰Ar age of 4.521 ± 0.006 Ga measures the time of closure of the K-Ar system. Carbon rosettes within the metal-rich vein are nitrogen-poor, well crystallized, include kamacite sub-grains of composition comparable to the host metal, and are essentially isotopically homogeneous (δ¹³C ∼ −33‰). In contrast, carbon rosettes within metal of the metal-poor lithology are N-poor, poorly crystallized, include kamacite grains that are Ni-poor compared to their host metal, and are isotopically heterogeneous (δ¹³C ranging from −50 to +80‰) even within a single metal grain. The silicate portion of GRA 95209 is similar to the lodranite EET 84302, sharing a common texture, silicate mineral compositions, and Ar-Ar age. GRA 95209 and EET 84302 are intermediate between acapulcoites and lodranites. Both experienced Fe,Ni-FeS melting with extensive melt migration, but record only the onset of silicate partial melting with limited migration of silicate melt. The complex metal-sulfide veins in GRA 95209 resulted from low-degree partial melting and melt migration and intruded the matrix lithology. Reactions between solid minerals and melt, including oxidation-reduction reactions, produced the array of phosphates, schreibersite, and tetrataenite. Extensive reduction in the metal-rich vein resulted from its origin in a hotter portion of the asteroid. This difference in thermal history is supported by the graphite structures and isotopic compositions. The graphite rosettes in the metal-rich vein are consistent with high-temperature igneous processing. In contrast, the carbon in the metal-poor lithology appears to preserve a record of formation in the nebula prior to parent-body formation. Carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal-sulfide melts that form a core, but does not achieve isotopic homogeneity until extensive thermal processing occurs.     

Martian regolith in Elephant Moraine 79001 shock melts? Evidence from major element composition and sulfur speciation

Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S²⁻ versus S⁶⁺). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt - pyroxene (En_72-75Fs_20-21Wo_5-7) and olivine (Fo_75-80). Reconstruction of glass + crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite.Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.     

The LaPaz Icefield 04840 meteorite: Mineralogy, metamorphism, and origin of an amphibole- and biotite-bearing R chondrite

The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo₆₂), orthopyroxene (En₆₉Fs₃₀Wo₁), albite (An₈Ab₉₀Or₂), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe³⁺, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe³⁺/Fe_tot values are olivine ?5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (^ANa + ^IVAl ↔ ^A□ + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60 °C from a amphibole-plagioclase thermometer; P_H2O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P_CO2 unconstrained; f_O2 at QFM + 0.5 log units; . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660 ± 75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.     

Experimental aqueous alteration of the Allende meteorite under oxidizing conditions: Constraints on asteroidal alteration

We have performed an experimental study of the aqueous alteration of the Allende CV3 carbonaceous chondrite under highly oxidizing conditions, in order to examine the alteration behavior of Allende’s anhydrous mineralogy. The experiments were carried out at temperatures of 100, 150, and 200 °C, for time periods between 7 and 180 days, with water/rock ratios ranging from 1:1 to 9:1. Uncrushed cubes of Allende were used so that the spatial relationships between reactant and product phases could be examined in detail. Scanning electron microscope studies show that in all the experiments, even those of short duration (7 days), soluble salts of Ca and Mg (CaSO₄, CaCO₃, and MgSO₄) precipitated on the sample surface, indicating that these elements are rapidly mobilized during alteration. In addition, iron oxides and hydroxides formed on the sample surfaces. The sulfates, carbonates, and the majority of the iron-bearing secondary minerals are randomly distributed over the surface of samples. In some instances the iron oxides and hydroxides are constrained to the boundaries of altering mineral grains. Transmission electron microscope studies show that the FeO-rich olivine in the interior of the samples has altered to form interlayered serpentine/saponite and Fe-oxyhydroxides. The degree of alteration increases significantly with increasing water/rock ratio, and to a lesser extent with increasing duration of heating. The serpentine/saponite forms both by direct replacement of the olivine in crystallographically oriented intergrowths, and by recrystallization of an amorphous Si-rich phase that precipitates in pore space between the olivine grains. The alteration assemblage bears many similarities to those found in altered carbonaceous chondrites, although in detail there are important differences, which we attribute to (a) the relatively high temperatures of our experiments and (b) comparatively short reaction times compared with the natural examples. In terms of mineral assemblage, our experiments most closely resemble alteration in the CI chondrites, although the degree of alteration of our experiments is much lower. CI chondrites contain serpentine/saponite intergrowths and veins of Ca-sulfate and Ca-carbonate as well as the Fe-oxyhydroxide, ferrihydrite. However, the phyllosilicate phases formed in our experiments are somewhat coarser-grained than the finest phyllosilicate fraction present in CI chondrites, suggesting that alteration of the CI chondrites occurred at lower temperatures. In terms of mineral assemblage, our experiments also appear to come close to matching CR chondrites, although we infer that CR alteration probably occurred at temperatures <100 °C, based on the very fine-grained size of phyllosilicates in CR matrices.     

An exotic kind of cosmic material: graphite-containing xenoliths from the Krymka (LL3.1) chondrite

Seven graphite-containing xenoliths were found in the Krymka (LL3.1) chondrite. The xenoliths have the following chemical and mineralogical characteristics which distinguish them from the Krymka host: (1) low totals in bulk chemical analyses obtained by electron microprobe; (2) high bulk Fe abundances; (3) a uniform recrystallized, chondrule-free texture; (4) the presence of euhedral graphite and carbon-rich material; (5) higher quantities of troilite and metal; (6) a relatively homogeneous composition of silicates; (7) a distinctive composition of metal, chromite and phosphate; (8) isotopically heavy C in graphite compared to both bulk Krymka and graphite in other ordinary chondrites. The xenoliths are mineralogically similar, but not identical, to the Krymka carbonaceous clast K1, which bears graphite microcrystals, organic compounds and mysterite. They resemble carbonaceous chondrites, both chemically and isotopically. The mineralogical, chemical and isotopic data for the graphite-containing fragments suggest that this material represents metamorphosed varieties of a previously unknown type of unequilibrated carbonaceous matter. Most likely, the graphite has a metamorphic origin and was crystallized from C-containing precursor materials through the following transformation sequence: organic compounds → C-rich material → graphite.     

Highly siderophile elements in ureilites

The abundances of the highly siderophile elements (HSE) Ru, Pd, Re, Os, Ir, and Pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples, including monomict, augite-bearing, and polymict lithologies. This report adds significantly to the quantity of available Pt and Pd abundances in ureilites, as these elements were rarely determined in previous neutron activation studies. The CI-normalized HSE abundance patterns of all ureilites analyzed here except ALHA 81101 show marked depletions in the more volatile Pd, with CI chondrite-normalized Pd/Os ratios (excluding ALHA 81101) averaging 0.19 ± 0.23 (2σ). This value is too low to be directly derived from any known chondrite group. Instead, the HSE bulk rock abundances and HSE interelement ratios in ureilites can be understood as physical mixtures of two end member compositions. One component, best represented by sample ALHA 78019, is characterized by superchondritic abundances of refractory HSE (RHSE—Ru, Re, Os, Ir, and Pt), but subchondritic Pd/RHSE, and is consistent with residual metal after extraction of a S-bearing metallic partial melt from carbonaceous chondrite-like precursor materials. The other component, best represented by sample ALHA 81101, is RHSE-poor and has HSE abundances in chondritic proportions. The genesis of the second component is unclear. It could represent regions within the ureilite parent body (UPB), in which metallic phases were completely molten and partially drained, or it might represent chondritic contamination that was added during disruption and brecciation of the UPB. Removal of carbon-rich melts does not seem to play an important role in ureilite petrogenesis. Removal of such melts would quickly deplete the ureilite precursors in Re/Os and As/Au, which is inconsistent with measured osmium isotope abundances, and also with literature As/Au data for the ureilites. Removal of ²⁶Al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions.     

Carbonates in CI chondrites: clues to parent body evolution

All CI chondrites are regolith breccias consisting of various types of chemically and mineralogically distinct mineral and lithic fragments (or units). In the CI chondrite Ivuna, for example, four different lithological units were identified and are referred to as lithology I, II, III, and IV. So far, lithologies III and IV have been identified in Orgueil as well. It appears that at least Ivuna and Orgueil consist of the same basic lithologies in different proportions. Carbonates in CIs occur as individual grains within such lithic units or exist as large fragments between them. Carbonate fragments are remnants of former carbonate veins and, in contrast to individual carbonate grains, are not genetically linked to lithological units. Four different types of carbonates (dolomite, breunnerite, calcite, siderite) occur in CIs and they constitute, on average, 5 vol% of each studied section. In this study, carbonates in the CI chondrites Orgueil, Ivuna, Alais, and Tonk were studied petrographically, mineralologically, and chemically. The results clearly indicate that, in contrast to most previous studies, compositional differences exist between dolomites within and among CI chondrites. From these differences it can be derived that (1) several episodes of alteration occurred on the CI parent body, (2) physicochemical conditions during carbonate formation must have been different among CIs, and (3) CI carbonates obviously were formed at low temperatures in equilibrium with surrounding fluid(s). While local compositional changes in fluid(s) on a micrometer to millimeter scale (as reflected by dolomite compositions within CI chondrites) were most likely controlled by the availability of Ca2+ Mg2+, Fe2+, and especially Mn2+ ions in the aqueous solutions, more widespread compositional changes on a meter (or even larger) scale were controlled by variable pH, Eh, CO2 partial pressure, and, especially, temperature conditions (as reflected by the compositional variability of dolomites among CIs).     

New lithologies in the Zagami meteorite: evidence for fractional crystallization of a single magma unit on Mars

Zagami consists of a series of increasingly evolved magmatic lithologies. The bulk of the rock is a basaltic lithology dominated by pigeonite (Fs_28.7–54.3), augite (Fs_19.5–35.0) and maskelynite (Ab_42–53). Approximately 20 vol.% of Zagami is a basaltic lithology containing FeO-enriched pyroxene (pigeonite, Fs_27.0–80.8) and mm- to cm-sized late-stage melt pockets. The melt pockets are highly enriched in olivine-bearing intergrowths, mesostases, phosphates (both whitlockite and water-bearing apatite), Fe,Ti-oxides and sulfides. The systematic increases in abundances of late-stage phases, Fs and incompatible element (e.g., Y and the REEs) contents of pigeonite, Ab contents of maskelynite, and FeO concentrations of whitlockite all point to a fractional crystallization sequence.The crystallization order in Zagami and the formation of these various lithologies was controlled by the abundances of iron, phosphorus, and calcium. During fractional crystallization, iron and phosphorus enrichment occurred, ultimately forcing the crystallization of calcium phosphates and olivine-bearing intergrowths. The limited amount of calcium in the melt and its partitioning between phosphates and silicates controlled the crystallization of phosphates, plagioclase, pigeonite, and augite. The presence of these FeO-enriched, water-poor late-stage lithologies has important implications. Discrepancies between experimental and petrologic studies to infer the history of basaltic shergottites may be partially explained by the use of starting compositions which are too FeO-poor in the experimental studies. The water-poor nature of the late-stage melt pockets suggests crystallization from a very dry magma, although whether this magma was always dry or experienced significant near-surface degassing remains an open question. Finally, the presence of fractional crystallization products within Zagami suggests that this may be a relatively common process on Mars.     

Non-basaltic asteroidal magmatism during the earliest stages of solar system evolution: A view from Antarctic achondrites Graves Nunatak 06128 and 06129

The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 ± 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2-30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe-Ni-S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium- or alkali-rich chondritic parent body with additional incorporation of Fe-Ni-S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600 °C at an fO₂ of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years.The existence of assemblages similar to GRA on several other planetary bodies with different geochemical characteristics (ureilite, winonaites, IAB irons) implies that this type of early asteroidal melting was not rare. Whereas, eucrites and angrites represent extensive melting of a parent body with low concentrations of moderately-volatile elements, GRA represents low-degrees of melting of a parent body with chondritic abundances of moderately volatile elements. The interpretation of the low-temperature mineral assemblage is somewhat ambiguous. Textural features suggest multiple episodes of alteration. The earliest stage follows the interaction of magmatic assemblages with a Cl-rich fluid. The last episode of alteration appears to cross-cut the fusion crust and earlier stages of alteration. Stable isotopic measurements of the alteration can be interpreted as indicating that an extraterrestrial volatile component was preserved in GRA.