Heterogeneous condensation of presolar titanium carbide core‐graphite mantle spherules

Abstract— We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon‐rich asymptotic giant branch (AGB) stars employing a newly‐derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core‐graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core‐graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < n?_0.1 (M₅/ζ)^?1/2L₄^1/4 < 0.7, where v_0.1 is the gas outflow velocity at the formation site in units of 0.1 km s^?1, M₅ the mass loss rate in 10^?5 M⊙ year^?1, L₄ the stellar luminosity in 10⁴ L⊙, and M/ζ is the effective mass loss rate taking account of non‐spherical symmetry of the gas outflows. The total gas pressures P_c at the formation sites for the effective mass loss rates M/ζ = 10^?5‐10^?3 M⊙ year^?1 correspond to 0.01 < P_c < 0.9 dyn cm^?2, implying that the observed TiC core‐graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1 < ? ? 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon‐rich AGB stars. Brief discussion is given on other types of graphite spherules.     

Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption

Abstract— A popular model for ureilites assumes that during anatexis in an asteroidal mantle, pressure‐buffered equilibrium smelting (partial reduction coincident with partial melting) engendered their conspicuous mafic‐silicate‐core mg diversity (75–96 mol%). Several mass‐balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites’ entire 21 mol% range in olivine‐core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <～80 bar) should have reached ≥85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO‐leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le Chǎtelier's principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass‐leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites’ distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short‐lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive “smelting” episode.     

Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption

Abstract— A popular model for ureilites assumes that during anatexis in an asteroidal mantle, pressure‐buffered equilibrium smelting (partial reduction coincident with partial melting) engendered their conspicuous mafic‐silicate‐core mg diversity (75–96 mol%). Several mass‐balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites' entire 21 mol% range in olivine‐core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <˜80 bar) should have reached ≥85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO‐leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le Châtelier's principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass‐leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites' distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short‐lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive “smelting” episode.     

Primodial retention of carbon by the terrestrial planets

Chemical equilibrium calculations on the stability of pure and dissolved graphite and cohenite (Fe₃C), several other carbides, and several carbonates have been carried out for a system with solar elemental abundances over a very wide range of temperature and pressure. The calculated abundances of condensed carbon compounds are similar to the observed inventories on Earth and Venus, but fully 10 times smaller than the minimum carbon abundance found in ordinary chondrites. The total carbon content of most iron meteorites is compatible with their origin as a cooling FeNiCSP solution which was saturated with dissolved carbon at the solidus, such as would be produced by melting an ordinary chondrite, not by direct condensation from or equilibrium with the primitive solar nebula. It is argued that the carbon content of Mars need not be appreciably greater than that of the Earth. Material with even lower formation temperatures than Mars, such as the primitive material in the asteroid belt, may retain substantially more carbon as disequilibrium polymeric organic matter, possibly by the Fischer-Tropsch mechanism favored by Anders. Carbonates are not found as equilibrium products in a solar-composition system, and are probably secondary alteration products. CaCO₃ might, however, persist in a solar-composition gas at temperatures below 460°K and pressures below 10^?6.6 bar. The most stable condensed carbon compounds are found to be graphite, Fe₃C, and possibly TiC, all in solid solution in the metal phase.     

Petrogenesis of main group pallasite meteorites based on relationships among texture,mineralogy, and geochemistry

Main group pallasite meteorites are samples of a single early magmatic planetesimal, dominated by metal and olivine but containing accessory chromite, sulfide, phosphide, phosphates, and rare phosphoran olivine. They represent mixtures of core and mantle materials, but the environment of formation is poorly understood, with a quiescent core–mantle boundary, violent core–mantle mixture, or surface mixture all recently suggested. Here, we review main group pallasite data sets and petrologic characteristics, and present new observations on the low‐MnO pallasite Brahin that contains abundant fragmental olivine, but also rounded and angular olivine and potential evidence of sulfide–phosphide liquid immiscibility. A reassessment of the literature shows that low‐MnO and high‐FeO subgroups preferentially host rounded olivine and low‐temperature P₂O₅‐rich phases such as the Mg‐phosphate farringtonite and phosphoran olivine. These phases form after metal and silicate reservoirs back‐react during decreasing temperature after initial separation, resulting in oxidation of phosphorus and chromium. Farringtonite and phosphoran olivine have not been found in the common subgroup PMG, which are mechanical mixtures of olivine, chromite with moderate Al₂O₃ contents, primitive solid metal, and evolved liquid metal. Lower concentrations of Mn in olivine of the low‐MnO PMG subgroup, and high concentrations of Mn in low‐Al₂O₃ chromites, trace the development and escape of sulfide‐rich melt in pallasites and the partially chalcophile behavior for Mn in this environment. Pallasites with rounded olivine indicate that the core–mantle boundary of their planetesimal may not be a simple interface but rather a volume in which interactions between metal, silicate, and other components occur.     

Mineralogy of fine‐grained material in the Krymka (LL3.1) chondrite

Abstract— Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: (1) bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; (2) enrichment in FeO and depletion in S, MgO and SiO₂ due to a high abundance of Fe‐rich silicates and low sulfide abundance; (3) fine‐grained, almost chondrule‐free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; (4) occurrence of a small amount of once‐molten material (microchondrules) enclosed in fine‐grained materials; (5) occurrence of accretionary features, especially unique accretionary spherules; (6) high abundance of small calcium‐ aluminium‐rich inclusions (CAIs) in one of the fine‐grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite. Accretionary, fine‐grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast, its incorporation, and history within the bulk rock of Krymka. The differences in porosity, bulk composition, and mineralogy of cores and rims of the fine‐grained spherulitic objects allow us to speculate on the following processes: (1) Low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary, porous (fluffy) silicate spherules. (2) Within a dusty environment with decreasing silicate/(metal + sulfide) ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust, which formed an accretionary rim. Variations of the silicate/(sulfide + metal) ratio in the dusty nebular environment result in the formation of multi‐layered rims on the surface of the silicate‐rich spherules. (3) Soft accretion and lithification of rimmed, fluffy spherules, fine‐grained, silicate‐rich dust, metal‐sulfide particles, CAIs, silicate‐rich microchondrules, and coarse silicate grains and fragments followed. (4) After low‐temperature processing of the primary, accretionary rock collisional fragmentation occurred, the fragments were subsequently coated by fine‐grained material, which was highly oxidized and depleted in sulfides. (5) In a final stage this accretionary “dusty” rock was incorporated as a fragment within the Krymka host.     

Formation of Dust Grains in Circumstellar Envelopes of Oxygen-Rich AGB Stars

We discuss dust formation in steady state dust driven winds around oxygen-rich AGB stars, including not only homogeneous Al₂O₃ and silicate grains but also heterogeneous grains consisting of an Al₂O₃ core and a silicate mantle. In the inner subsonic region, Al₂O₃ grains with radii of ∼ 0.15 μm condense first, then condensation of silicate on Al₂O₃ starts slightly inside the sonic point, which accelerates the gas flow into the supersonic region. Also small silicate grains, whose radii are a few tens of ?ngstroms form beyond the sonic point. The carrier of 13 μm feature observed towards oxygen-rich AGB stars is considered to be the core-mantle grains consisting of an α-Al₂O₃ core and a silicate mantle from the radiation transfer calculations based on the results of dust formation calculations. This revised version was published online in September 2006 with corrections to the Cover Date.     

Survival of refractory presolar grain analogs during Stardust‐like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s^?1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si₃N₄ grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.     

Effect of silicon on activity coefficients of Bi,Cd, Sn,and Ag in liquid Fe‐Si,and implications for differentiation and core formation

The depletion of volatile siderophile elements (VSE) Sn, Ag, Bi, Cd, and P in mantles of differentiated planetary bodies can be attributed to volatile‐depleted precursor materials (building blocks), fractionation during core formation, fractionation into and retention in sulfide minerals, and/or volatile loss associated with magmatism. Quantitative models to constrain the fractionation due to core formation have not been possible due to the lack of activity and partitioning data. Interaction parameters in Fe‐Si liquids have been measured at 1 GPa, 1600 °C and increase in the order Cd (~6), Ag (~10), Sn (~28), Bi (~46), and P (~58). These large and positive values contrast with smaller and negative values in Fe‐S liquids indicating that any chalcophile behavior exhibited by these elements will be erased by dissolution of a small amount of Si in the metallic liquid. A newly updated activity model is applied to Earth, Mars, and Vesta. Five elements (P, Zn, Sn, Cd, and In) in Earth's primitive upper mantle can largely be explained by metal‐silicate equilibrium at high PT conditions where the core‐forming metal is a Fe‐Ni‐S‐Si‐C metallic liquid, but two other—Ag and Bi—become overabundant during core formation and require a removal mechanism such as late sulfide segregation. All of the VSE in the mantle of Mars are consistent with core formation in a volatile element depleted body, and do not require any additional processes. Only P and Ag in Vesta's mantle are consistent with combined core formation and volatile‐depleted precursors, whereas the rest require accretion of chondritic or volatile‐bearing material after core formation. The concentrations of Zn, Ag, and Cd modeled for Vesta's core are similar to the concentration range measured in magmatic iron meteorites indicating that these volatile elements were already depleted in Vesta's precursor materials.     

A unique type B inclusion from Allende with evidence for multiple stages of melting

Abstract— A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (?1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 μm long that occur at the inclusion rim. The core consists of blocky melilite grains 20–50 μm across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ?1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine‐grained (1–10 μm) spinel. The rim laths are normally zoned (Åk_30–70) inward from the rim of the inclusion with reverse zoning over the last ?200 μm to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk_5–12) occur. These grains are enclosed in strongly zoned (Åk_15–70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na₂O‐free, but the outer parts of the overgrowths are not. In the laths at the rim, Na₂O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 2–9 wt% TiO₂ + Ti₂O₃. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ?1400 °C and melts except for gehlenitic (Åk_5–12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na₂O‐free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ?1260 °C. Melilite more gehlenitic than Åk₄₀ does not melt. During cooling, Na₂O‐bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co‐crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na‐free and Na‐bearing melilite, plus a positive correlation between Na₂O and åkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass‐fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse‐grained inclusions also have features consistent with a sequence of early, high‐temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.     

Carbon in howardite,eucrite and diogenite basaltic achondrites

Abstract— The C contents and isotopic compositions of four eucrites, four diogenites and two howardites have been determined. Stepped heating in an O atmosphere was employed to convert selectively different carbonaceous materials to CO₂ gas at various temperatures. This technique successfully distinguishes between terrestrial contaminants and indigenous C. With the exception of the Kapoeta howardite, the howardite, eucrite, and diogenite (HED) meteorites contain ～10–30 ppm indigenous C with δ¹³C between ?29% and ?19%. Kapoeta (a regolith breccia) has an elevated C content and δ¹³C, due to the presence of ¹³C-enriched carbonate minerals (δ¹³C ～ +20%) in CM2- or CR2-like clasts. The range in δ¹³C displayed by HED samples is similar to that of other solar system basalts, such as lunar rocks and Martian meteorites but distinctly different from that of the terrestrial mantle. The diogenites have a slightly lower total C yield and higher δ¹³C than the eucrites, which is a result of degassing of trapped CO/CC₂/CO^2–₃ from the silicate lattice during metamorphism or annealing. However, three out of the four diogenites studied appear to contain a discrete component, possibly of graphitic C coating silicate grains, that is seemingly unaffected by the extended annealing period experienced by the diogenites. It is possible that this component might host the indigenous primitive Xe recently identified in diogenites.     

Formation of Al2O3 Grains and the 13µM Feature in Circumstellar Envelopes of Oxygen-Rich AGB Stars

The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13μm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13μm feature when the mass loss rate is less than 2 × 10-5M·/yr. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impact ejecta in upper Eocene deposits at Massignano,Italy

Abstract— Previous workers have shown that an impact ejecta layer at Massignano, Italy contains a positive Ir anomaly, flattened spheroids (pancake spherules), Ni‐rich spinel crystals, and shocked quartz with multiple sets of planar deformation features. Because of sample sizes and work by different investigators, it was not clear if the shocked quartz is associated with the Ir anomaly and pancake spherules or if it belongs to a separate impact event. To address this problem, we carried out a high‐resolution stratigraphic study of this ejecta layer. The ejecta layer was sampled continuously at 1 cm intervals in two adjacent columns. The carbonate was removed with dilute HCl, and the non‐carbonate fraction was gently sieved. Pancake spherules were recovered from the 250–500 μm size fraction and counted. At the peak abundance, the number of pancake spherules in the 250–500 μm size fraction is about 6–7/g of sample. The pancake spherules removed from the 250–500 μm size fraction are mostly translucent to opaque pale green, but some have a grey color or dark opaque patches due to a coating of Ni‐ and Cr‐rich spinel crystals. Energy‐dispersive X‐ray analysis and X‐ray diffraction data indicate that the green spherules are composed of iron‐rich smectite, probably nontronite. Black opaque spinel stringers (dark spinel‐rich pancake spherules), usually <200 μm across, can be seen in a polished section of a block that includes the ejecta layer. None of the dark spinel‐rich pancake spherules were recovered from the sieved non‐carbonate fraction due to their fragile nature, but we believe that they are from the same impact event as the green pancake spherules. The <250 μm size fractions from both columns were disaggregated using ultrasonics and re‐sieved. The 63–125 μm size fractions were then searched for shocked quartz using a petrographic microscope. At the peak‐abundance level, the number of shocked quartz grains in the 63–125 μm size fraction is about 7/g of sample. Some of the shocked quartz grains have a “toasted” appearance. These grains have a brownish color and contain a patchy distribution of faint, densely spaced planar deformation features (PDFs). Polymineralic fragments containing one or two shocked quartz grains with one or two sets of PDFs were observed. They appear to have an organic matrix and are probably fragments of agglutinated foraminiferal tests. We searched for, but did not find, coesite or shocked zircons. We found that the peak abundance of the shocked quartz is within a centimeter of the peak abundance of the green pancake spherules. We conclude that the pancake spherules are diagenetically altered clinopyroxene‐bearing spherules and that the shocked quartz, green (and presumably the dark spinel‐rich) pancake spherules, and Ir anomaly all belong to the same impact event. This conclusion is consistent with previous suggestions that the cpx spherule layer may be from the 100 km‐diameter Popigai impact crater in northern Siberia.     

Ferrous silicate spherules with euhedral iron‐nickel metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions

Abstract— The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) ? 0.1‐0.4) silicate spherules (chondrules), about 15–30 μm in apparent diameter, composed of cryptocrystalline olivinepyroxene normative material, ±SiO₂‐rich glass, and rounded‐to‐euhedral Fe, Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al₂O₃, and TiO₂ <0.04 wt%) and enriched in FeO, MnO, Cr₂O₃, and Na₂O relative to the dominant, volatile‐poor, magnesian chondrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, which suggests that this correlation is not due to oxidation, reduction, or both of iron (FeO_sil ? Fe_met) during melting of metal‐silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8–18 wt% Ni, <0.09 wt% Cr, and a sub‐solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded‐to‐euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe, Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400–500 K.     

Chemistry in low-mass star forming regions

We review molecular evolution in low-mass star-forming regions and discuss what we can observe with ALMA. Recent observations have revealed chemical fractionation, i.e. spatial variation of molecular abundances, in dense prestellar cores. In the central regions of cold prestellar cores, CO is heavily depleted, while the depletion of N-bearing species are rare. Models show that CO is frozen onto grains, while N-bearing species survive because of the CO depletion and slow formation of N₂ in the gas phase. CO depletion also enhances the molecular D/H ratio. Chemical fractionation and its variation among cores can be an indicator of evolutionary stage and/or accumulation process of cores. As the core contracts, central region of the core is eventually heated by compressional heating and a new-born protostar. CO is sublimated back to the gas phase, if the temperature reaches 20 K. Warm temperature enhances the endothermic reactions which were negligible in the prestellar core stage, and also enhances grain-surface reactions among heavy-element species to form large organic molecules, which sublimate when the temperature reaches ～100 K. Warm regions with high abundances of the gaseous organic species are called hot corinos or low-mass hot cores. Adopting a theoretical model of core contraction, we present the temporal variation of the radius inside which CO and large organic species are sublimated. We also investigate the molecular evolution in infalling shells to derive molecular distribution in a protostellar core.     

Nickel abundance in stony cosmic spherules: Constraining precursor material and formation mechanisms

Abstract– We report bulk and olivine compositions in 66 stony cosmic spherules (Na₂O < 0.76 wt%), 200–800 μm in size, from the Transantarctic Mountains, Antarctica. In porphyritic cosmic spherules, relict olivines that survived atmospheric entry heating are always Ni‐poor and similar in composition to the olivines in carbonaceous or unequilibrated ordinary chondrites (18 spherules), and equilibrated ordinary chondrites (one spherule). This is consistent with selective survival of high temperature, Mg‐rich olivines during atmospheric entry. Olivines that crystallized from the melts produced during atmospheric entry have NiO contents that increase with increasing NiO in the bulk spherule, and that range from values similar to those observed in chondritic olivines (NiO generally <0.5 wt%) to values characteristic of olivines in meteoritic ablation spheres (NiO > 2 wt%). Thus, NiO content in olivine cannot be used alone to distinguish meteoritic ablation spheres from cosmic spherules, and the volatile element contents have to be considered. We propose that the variation in NiO contents in cosmic spherules and their olivines is the result of variable content of Fe, Ni metal in the precursor. NiO contents in olivines and in cosmic spherules can thus be used to discuss their parent body. Ni‐poor spherules can be derived from C‐rich and/or metal‐poor precursors, either related to CM, CI, CR chondrites or to chondritic fragments dominated by silicates, regardless of the parent body. Ni‐rich spherules (NiO > 0.7 wt%) that represent 55% of the 47 barred‐olivine spherules we studied, were derived from the melting of C‐poor, metal‐rich precursors, compatible with ordinary chondrite or CO, CV, CK carbonaceous chondrite parentages.     

Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?¹⁷O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?¹⁷O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.     

Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346

Abstract— Pyroxene structural data, along with analyses of titanomagnetite, fayalite and mesostasis of the new nakhlite Miller Range (MIL) 03346, define equilibration near 1 bar, 1100 °C, and oxygen fugacity near the FMQ buffer. There is a clear progression of oxygen fugacity (fO₂) in Martian meteorites from reduced Allan Hills (ALH) 84001 to intermediate shergottites to oxidized nakhlites. This trend can be explained by polybaric graphite‐CO‐CO₂ equilibria in the Martian mantle. Shergottites would have formed at pressures between 1.2 and 3.0 GPa, and nakhlite parent liquids formed at pressures >3.0 GPa, consistent with geochemical and petrologic data for the shergottites and nahklites. Carbon buffering in the Martian mantle could be responsible for variation in fO₂ in Martian meteorites (rather than assimilation or crustal interaction), as well as C‐H‐O fluids that could be the source of ˜30 ppb CH₄ detected by recent spacecraft missions. The conundrum of an oxidized current mantle and basalts, but reduced early mantle during core‐mantle equilibrium exists for both the Earth and Mars. A polybaric buffering role for graphite can explain this discrepancy for Mars, and thus it may not be necessary to have an oxidation mechanism like the dissociation of MgFe‐perovskite to account for the oxidized terrestrial mantle.     

Grain alignment in dense interstellar environments: spectropolarimetry of the 4.67-μm CO-ice feature in the field star Elias 16 (Taurus dark cloud)

We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ∼20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ∼5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds.     

Vesicle dynamics during the atmospheric entry heating of cosmic spherules

Cosmic spherules are unique igneous objects that form by melting due to gas drag heating during atmospheric entry heating. Vesicles are an important component of many cosmic spherules since they suggest their precursors had finite volatile contents. Vesicle abundances in spherules decrease through the series porphyritic, glassy, barred, to cryptocrystalline spherules. Anomalous hollow spherules, with large off‐center vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behavior of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from nonporphyritic particles. Modest rotation rates of tens of radians s^?1 are, however, sufficient to impede loss of vesicles and may explain the presence of small solitary vesicles in barred, cryptocrystalline and glassy spherules. Rapid rotation at spin rates of several thousand radians s^?1 are required to concentrate vesicles at the rotational axis and leads to rapid growth by coalescence and either separation or retention depending on the orientation of the rotational axis. Complex rapid rotations that concentrate vesicles in the core of particles are proposed as a mechanism for the formation of hollow spherules. High vesicle contents in porphyritic spherules suggest volatile‐rich precursors; however, calculation of volatile retention indicates these have lost >99.9% of volatiles to degassing prior to melting. The formation of hollow spherules, by rapid spin, necessarily implies preatmospheric rotations of several thousand radians s^?1. These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping.