Corundum, the thermodynamically stable phase of alumina (Al₂O₃), is one of the most refractory dust species to condense around evolved stars. Presolar alumina in primitive chondrites has survived various kinds of processing in circumstellar environments, the interstellar medium (ISM), the Sun’s parent molecular cloud, and the protosolar disk. The morphology and crystal structure of presolar alumina grains may reflect their formation and evolution processes, but the relative importance of these two types of processes is poorly understood. In this study, we performed detailed morphological observations of 185 alumina grains extracted from unequilibrated ordinary chondrites (Semarkona, Bishunpur, and RC075). We also performed electron back-scattered diffraction analyses of 122 grains and oxygen isotopic analyses of 107 grains. Dissolution experiments on corundum and transition alumina phases were carried out to examine the possibility of the alteration of surface structures of alumina grains by the chemical separation procedures of chondrites.
The average size of the alumina grains was 1 μm, and neither whiskers nor extremely flat grains were observed. About one-third of the grains had smooth surfaces, while ~60% of the grains had rough surfaces with 10 to 100 nm-sized fine structures. The rough-surface grains have varieties of morphology and crystallinity, suggesting that the rough surface structures are secondary in origin. Electron back-scattered diffraction patterns from 95% of alumina grains matched with α-Al₂O₃ (corundum), and more than 75% of the alumina grains are single crystals of corundum. Nine presolar alumina grains with anomalous oxygen isotopic compositions were found among 107 alumina grains, and most of them were characterized by rough surface structures. While most of the presolar alumina grains were corundum, the relative abundance of amorphous or low-crystallinity grains is higher in presolar alumina grains than in solar alumina grains. The dissolution experiments showed that all phases except for corundum dissolved during the acid treatments of chondrites. This suggests that smooth surface structures of corundum grains were originally formed in space, and that original surfaces of alumina that had been damaged by energetic particle irradiation in the ISM or the protosolar disk were lost during chemical separations to form the rough surface structures, and that amorphous or low-crystallinity alumina grains in chondrites have acid-resistant structures different from sol-gel-synthesized amorphous alumina. The present results also imply the possible presence of acid-soluble alumina phases, undiscovered by chemical separations, in chondrites.

The fluence and isotopic composition of solar wind xenon have been determined from silicon collector targets flown on the NASA Genesis mission. A protocol was developed to extract gas quantitatively from samples of ~9 – 25 mm², and xenon measured using the RELAX mass spectrometer. The fluence of implanted solar wind xenon is 1.202(87) 10⁶ atoms ¹³²Xe cm^-2, which equates to a flux of 5.14(21) 10⁶ atoms ¹³²Xe cm^-2 year^-1 at the L1 point. This value is in good agreement with those reported in other studies. The isotopic composition of the solar wind is consistent with that extracted from the young lunar regolith and other Genesis collector targets.
The more precise xenon isotopic data derived from the Genesis mission confirm models of relationships among planetary xenon signatures. The underlying composition of Xe-Q is mass fractionated solar wind; small, varying contributions of Xe-HL and ¹²⁹Xe from ¹²⁹I decay are present in reported meteorite analyses. In contrast, an s-process deficit is apparent in Xe-P3, which appears to have been mass fractionated to the same extent as Xe-Q from a precursor composition, suggesting similar trapping mechanisms. Solar wind xenon later evolved by the addition of ~1% (at ¹³²Xe) of s-process xenon to this precursor. As an alternative model to a single source reservoir for Xe-P3, we propose that trapping of xenon onto carbonaceous carriers has been an ongoing process across galactic history, and that preparation of the residues in which Xe-P3 has been identified preferentially preserves longer lived host phases; a higher proportion of these sample xenon isotopic compositions from earlier in galactic chemical evolution, allowing the s-process deficit to become apparent. The relationships among SW-Xe, Xe-Q and Xe-P3 predict that the ¹²⁴Xe/¹³²Xe ratio for the solar wind is 0.00481(6).

‘Quenched’ angrite meteorites are among the best time markers of igneous activity in early formed planetesimals of the Solar System. They can be precisely dated by the Mn-Cr extinct nuclide decay system because they contain olivine with high Mn/Cr. Nevertheless, there is disagreement between various determinations of the initial ⁵³Mn/⁵⁵Mn for this meteorite, hindering their use for cross-calibration between chronometric systems and between Secondary Ion Mass Spectrometry (SIMS) and bulk measurement techniques. Here we re-evaluate the Mn-Cr systematics of olivine from the quenched angrite D’Orbigny using Sensitive High-mass Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG) to search for heterogeneity in isotope systematics and check for inter-laboratory bias. We investigated possible bias arising due to different data reduction methods and have paid careful attention to the relative sensitivities of Mn and Cr by utilising a three-component mixing model to correct for matrix effects associated with Mg, Fe and Ca zoning in angrite olivine. We have determined an initial ⁵³Mn/⁵⁵Mn of 3.60 (±0.39) × 10^-6 and 3.44 (±0.29) × 10^-6 (2σ errors) for D’Orbigny olivine by the Mean of Ratios and Ratio of Total Counts data reduction methods. These values are in agreement with those found by some previous bulk and mineral-scale determinations, and with the generally accepted initial ⁵³Mn/⁵⁵Mn of this meteorite, but not with previous SIMS work on this material. The source of this discrepancy remains unclear. We can exclude heterogeneity in D’Orbigny as a source of discrepancy because we used the same sample and the meteorite appears to have consistent initial ⁵³Mn/⁵⁵Mn over both micro- and macro-scales. The discrepancy between this and the previous SIMS study probably reflects an unrecognised systematic analytical bias, possibly associated with relative sensitivities of Mn and Cr or with mass spectrometric backgrounds (isobaric interferences or scattered ions) which may become significant at very low Cr count rates.

The chemical compositions of Martian basalts are enriched in iron with respect to terrestrial basalts. Their rheology is poorly known and liquids of this chemical composition have not been experimentally investigated. Here, we determine the viscosity of five synthetic silicate liquids having compositions representative of the diversity of Martian volcanic rocks including primary Martian mantle melts and alkali basalts. The concentric cylinder method has been employed between 1500 °C and the respective liquidus temperatures of these liquids. The viscosity near the glass transition has been derived from calorimetric measurements of the glass transition. Although some glass heterogeneity limits the accuracy of the data near the glass transition, it was nevertheless possible to determine the parameters of the non-Arrhenian temperature-dependence of viscosity over a wide temperature range (1500°C to the glass transition temperature). At superliquidus conditions, the Martian basalt viscosities are as low as those of the Fe-Ti-rich lunar basalts, similar to the lowest viscosities recorded for terrestrial ferrobasalts, and 0.5 to 1 orders of magnitude lower than terrestrial tholeiitic basalts. Comparison with empirical models reveals that Giordano et al. (2008) offers the best approximation, whereas the model proposed by Hui and Zhang (2007) is inappropriate for the compositions considered.
The slightly lower viscosities exhibited by the melts produced by low degree of mantle partial melting versus melts produced at high degree of mantle partial melting (likely corresponding to the early history of Mars), is not deemed sufficient to lead to viscosity variations large enough to produce an overall shift of Martian lava flow morphologies over time. Rather, the details of the crystallization sequence (and in particular the ability of some of these magmas to form spinifex texture) is proposed to be a dominant effect on the viscosity during Martian lava flow emplacement and may explain the lower range of viscosities (10² – 10⁴ Pa·s) inferred from lava flow morphology. Further, the differences between the rheological behaviors of tholeiitic vs. trachy-basalts are significant enough to affect their emplacement as intrusive bodies or as effusive lava flows. The upper range of viscosities (10⁶ – 10⁸ Pa·s) suggested from lava flow morphology is found consistent with the occurrence of alkali basalt documented from in-situ analyses and does not necessarily imply the occurrence of basalt-andesite or andesitic rocks.

CR chondrites are considered as one of the most primitive classes of meteorites. Most of them experienced a mild aqueous alteration and show no evidence of significant effect of thermal metamorphism. We present here a search for low degree metamorphic effects in CR chondrites. We studied 15 CR chondrites using different metamorphic indicators: 1) structure and Ni content of metal grains; 2) hydration state of matrix; 3) structure and composition of organic matter. The different metamorphic indicators show that two of the analyzed CR chondrites, GRA 06100 and GRO 03116, experienced thermal metamorphism. Indeed, all of the metal grains in GRA 06100 and half of the metal grains in GRO 03116 show Ni-rich phases; the matrix of GRA 06100 is almost completely dehydrated, and the matrix of GRO 03116 is partially dehydrated; Raman spectra of organic matter in these two meteorites are clearly different from those obtained for organic matter in the other CR chondrites, which resemble Raman spectra of organic matter in unmetamorphosed, CM2 meteorites; IR spectra of insoluble organic matter extracted from GRA 06100 and GRO 03116 show lower carbonyl abundance and higher CH₂/CH₃ ratio with respect to organic matter of unmetamorphosed chondrites. The other CR chondrites analyzed here lack these characteristics and only show a few metal grains with Ni-rich inclusions. Our results also show that the metamorphic effects observed in GRA 06100 and GRO 03116 are different from those observed in type 3 chondrites, which experienced long-duration metamorphism of radiogenic origin. We infer that thermal processing in these two CRs extended over a short duration and was triggered by impacts.

Eighty-five D-rich carbonaceous particles were identified in the matrix of the NWA 801 CR2 chondrite using isotope microscopy. The occurrence of 67 D-rich carbonaceous particles was characterized using secondary electron microscopy combined with X-ray elemental mapping. The close association of H and C, and D-enrichment suggests that the D-rich carbonaceous particles correspond to organic matter. The D-rich organic particles were scattered ubiquitously throughout the matrix at a concentration of approximately 660 ppm. The morphology of the D-rich carbonaceous particles is globular up to about 1 μm in diameter and is classified into four types: ring globules, round globules, irregular-shaped globules, and globule aggregates. The ring globules are ring-shaped organic matter containing silicate and/or oxide, with or without a void in the center. This is the first report of silicate and oxide grains surrounded by D-rich organic matter. The globule aggregates are composed of several D-rich organic globules mixed with silicates. Morphology of ring globules is very similar to core-mantle grain produced in the molecular cloud or in the outer solar nebula inferring by astronomy, suggesting that the organic globules have formed by UV photolysis in the ice mantle. Silicates or oxides attached to D-rich organic globules are the first observation among chondrites so far and may be unique nature of CR2 chondrites. The hydrogen isotopic compositions of the ring globules, round globules, irregular-shaped globules, and globule aggregates are δD = 3,000-4,800, 2,900-8,100, 2,700-11,000, and 2,500-11,000‰, respectively. Variations of D/H ratio of these organic globules seemed to be attributed to variations of D/H ratio of the organic radicals or differences of content of the D-rich organic radicals. There are no significant differences in the hydrogen isotopic compositions among the four types of D-rich carbonaceous matter. The D-enrichments suggest that these organic globules have formed in a cold molecular cloud and/or the outer protoplanetary disk of the early solar system. The oxygen isotopic compositions of the silicates and oxides attached to the ring globules and globule aggregates range from δ¹⁷O = –49 to 50‰ and δ¹⁸O = –46 to 64‰. The oxygen isotopic compositions are not distinct from those of solar system materials, which suggests that the organic globules were formed in the outer solar system rather than in the presolar environment. Therefore, it is possible that the ring globules and globule aggregates in NWA 801 may have formed in the outer protoplanetary disk of the early solar system. Organic globules that exhibit clear presolar origin were not identified in this study. The lack of clear presolar signatures might suggest that modifications of isotopic compositions or morphologies of the presolar organic matter occurred in the early solar nebula.

¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (*Patrick.Peplowski@jhuapl.edu)
²Computer Science Corporation, Lanham-Seabrook, MD 20706, USA
³National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
⁵Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
⁶Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁷Physics Department, Catholic University of America, Washington, DC 20064, USA.

Ureilites are a group of ultramafic achondrites commonly thought to be residues of partial melting on a carbon-rich asteroid. They show a large variation in FeO content (olivine Fo values ranging from ~74 to 95) that cannot be due to igneous fractionation and suggests instead variation in oxidation state. The presence of chromite in only a few of the most ferroan (Fo 75-76) samples appears to support such a model. MicroXANES analyses were used in this study to determine the valence states of Cr (previously unknown) in olivine cores of eleven (11) main group ureilites. The goal of this work was to use a method that is independent of Fo to determine the oxidation conditions under which ureilites formed, in order to evaluate whether the ureilite FeO-variation is correlated with oxidation state, and whether it is nebular or planetary in origin. Two of the analyzed samples, LEW 88774 (Fo 74.2) and NWA 766 (Fo 76.7) contain primary chromite; two others, LAP 03587 (Fo 74.4) and CMS 04048 (Fo 76.2) contain sub-micrometer-sized exsolutions of chromite + Ca-rich pyroxene in olivine; and one, EET 96328 (Fo 85.2) contains an unusual chromite grain of uncertain origin. No chromite has been observed in the remaining six samples (Fo 77.4-92.3).
Chromium in olivine in all eleven samples was found to be dominated by the divalent species, with valences ranging from 2.10 ± 0.02 (1σ) to 2.46 ± 0.04. The non-chromite-bearing ureilites have the most reduced Cr, with a weighted mean valence of 2.12 ± 0.01, i.e., Cr²⁺/Cr³⁺ = 7.33. All low-Fo chromite-bearing ureilites have more oxidized Cr, with valences ranging from 2.22 ± 0.03 to 2.46 ± 0.04. EET 96328, whose chromite grain we interpret as a late-crystallizing phase, yielded a reduced Cr valence of 2.15 ± 0.07, similar to the non-chromite-bearing samples. Based on the measured Cr valences, magmatic (1200-1300°C) oxygen fugacities (fO₂) of the non-chromite-bearing samples were estimated to be in the range IW-1.9 to IW-2.8 (assuming basaltic melt composition), consistent with fO₂ values obtained by assuming olivine-silica-iron metal (OSI) equilibrium. For the primary chromite-bearing-ureilites, the corresponding fO₂ were estimated (again, assuming basaltic melt composition) to be ~IW to IW+1.0, i.e., several orders of magnitude more oxidizing than the conditions estimated for the chromite-free ureilites. In terms of Fo and Cr valence properties, ureilites appear to form two groups rather than a single “Cr-valence (or fO₂) vs. Fo” trend. The chromite-bearing ureilites show little variation in Fo (~74-76) but significant variation in Cr valence, while the non-chromite-bearing ureilites show significant variation in Fo (~77-95) and little variation in Cr valence. These groups are unrelated to petrologic type (i.e., olivine-pigeonite, olivine-orthopyroxene, or augite-bearing). The chromite-bearing ureilites also have lower contents of Cr in olivine than most non-chromite-bearing ureilites, consistent with predictions based on Cr olivine/melt partitioning in spinel saturated vs. non-spinel-saturated systems.
Under the assumption that at magmatic temperatures graphite-gas equilibria controlled fO₂ at all depths on the ureilite parent body, we conclude: 1) that ureilite precursor materials having the Fo and Cr valence properties now observed in ureilites are unlikely to have been preserved during planetary processing; and 2) that the Fo and Cr valence properties now observed in ureilites are consistent with having been established by high-temperature carbon redox control over a range of depths on a plausible-sized ureilite parent body. The apparent limit on ureilite Fo values around 74-76 suggests that the precursor material(s) had bulk mg# ≥ that of LL chondrites.

Sandra Pizzarello^a,*, Stephen K. Davidowski^a, Gregory P. Holland^a and Lynda B. Williams^b

^aDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604
^bSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404

Reference
Pizzarello S, Davidowski SK, Holland GP and Williams LB (2013) Processing of meteoritic organic materials as a possible analog of early molecular evolution in planetary environments. PNAS 110:15614-15619.
[doi:10.1073/pnas.1309113110]

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