Formation of orange hibonite,as inferred from some Allende inclusions

Abstract— We studied three fluffy Type A refractory inclusions from Allende that contain orange hibonite. The melilite in the present samples is very Al‐rich, averaging Åk₆, Åk₁₄, and Åk₁₂ in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of <10 × CI; in the other inclusion, the hibonite, melilite and perovskite have Group II‐like patterns. The hibonite and melilite in all three inclusions studied have excess ²⁶Mg consistent with (²⁶Al/²⁷Al)_I = 5 × 10^?5. Much of the hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, hibonite should not be an early‐crystallizing phase in these inclusions. We conclude that the hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene. Studies have shown that blue hibonite contains Ti³⁺ while orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti³⁺, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable, we find that (a) the hibonite is V‐rich (～1 wt% V₂O₃) and (b) there are no negative Ce anomalies in Allende hibonite. This indicates that the hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti³⁺. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ～2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.     

The origin of the ‘FUN’ anomalies and the high temperature inclusions in the allende meteorite

The discovery of isotopic anomalies in white inclusions of the meteorite Allende has led to fundamental questions concerning the origin of these anomalies and of the white inclusions themselves. An analysis of the FUN anomalies in the inclusions C1 and EK1-4-1 demonstrates that these isotopic anomalies may be decomposed into individual nucleosynthetic components, which have been subjected to separate mass and component fractionations. There is no evidence that any freshlysynthesized material injected into the primitive solar nebula was of abnormal isotopic composition, or that the FUN anomalies were due to an injection of unusual material. Rather, they show the effects of form of interstellar grains whose size or chemistry served as a memory for the nucleosynthetic origins of their constituent atoms. Giant gaseous protoplanets, as described for the early solar nebula by Cameron (1978), are a potential site for achieving both mass and component fractionations, and for producing white inclusions in general.     

Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite

Two hibonite‐spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite‐spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine‐grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite‐spinel inclusions represent high‐temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca‐containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite‐spinel inclusions in ALHA77307 formed by high‐temperature condensation under disequilibrium conditions.     

Re‐examining the role of chondrules in producing the elemental fractionations in chondrites

Abstract— The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI‐like component. The matrix‐normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI‐like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe‐metal is added to the chondrule composition. Measured matrix compositions are not CI‐like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ?10–30% of refractory‐rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI‐dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg‐Si‐Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.     

Nebular thermal evolution and the properties of primitive planetary materials

Abstract— Models of the solar nebula are constructed to investigate the hypothesis that surviving planetary objects began to form as the nebula cooled from an early, hot epoch. The imprint of such an epoch might be retained in the spatial distribution of planetary material, the systematic deviations of its elemental composition from that of the Sun, chemical indicators of primordial oxidation state, and variations in oxygen and other isotopic compositions. Our method of investigation is to calculate the time‐dependent, two‐dimensional temperature distributions within model nebulas of prescribed dynamical evolution, and to deduce the consequences of the calculated thermal histories for coagulated solid material. The models are defined by parameters which characterize nebular initial states (mass and angular momentum), mass accretion histories, and coagulation rates and efficiencies. It is demonstrated that coagulation during the cooling of the nebula from a hot state is expected to produce systematic heterogeneities which affect the chemical and isotopic compositions of planetary material. The radial thermal gradient at the midplane results in delayed coagulation of the more volatile elements. Vertical thermal gradients isolate the most refractory material and concentrate evaporated heavy elements in the gas phase. It is concluded that these effects could be responsible for the distribution of terrestrial planetary masses, the systematic depletion patterns of the moderately volatile elements in chondritic meteorites and the Earth, the range of oxygen isotopic compositions exhibited by calcium‐aluminum‐rich inclusions (CAIs) and other refractory inclusions, and some geochemical evidence for a moderately enhanced oxidation state. However, nebular fractionations on a global scale are unlikely to account for the more oxidizing conditions inferred for some CAIs and chondritic silicates, which require dust enhancements greater than a few hundred. This conclusion, along with the well‐established evidence from studies of chondrules and CAIs for thermal excursions of short duration, make it likely that local environments, unrelated to nebular thermal evolution, were also important.     

Refractory inclusions in the unique carbonaceous chondrite Acfer 094

Abstract– Acfer 094 is an unshocked, nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions. The refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work, and to report new or unusual inclusion types. A total of 289 Ca‐, Al‐rich inclusions in the thin section and 67 among the disaggregated material, having a total of 31 different mineral assemblages, were found. Inclusions are largely free of secondary alteration products, and are typically ≤200 μm across. The most common are gehlenitic melilite+spinel±perovskite, spinel+perovskite, and spinel with a thin, silicate rim, typically melilite±diopside. Such rims and (thicker) mantles are very common among Acfer 094 inclusions, and they exhibit a variety of zoning patterns with respect to åkermanite and FeO contents. In the thin section, about 13% of the inclusions contain hibonite and approximately 5% are grossite‐bearing; in the disaggregated material, the percentages are 14 and 9, respectively, comparable to previous work. Among the unusual inclusions are a fine‐grained, porous, Ti‐rich hibonite+spinel+perovskite+melilite inclusion with a compact, coarse, Ti‐poor hibonite+spinel+melilite clast; two inclusions in which hibonite has reacted to form grossite; two inclusions with FeO‐rich spinel; and a small object consisting of fassaite enclosing euhedral spinel, the first fragment of a Type B inclusion reported from Acfer 094. Inclusions similar to those found in CM or CV chondrites are rare; Acfer 094 contains a distinctive population of inclusions. The population, dominated by small, melilite‐bearing inclusions, is most similar to that of CO chondrites. A distinguishing feature is that in Acfer 094, almost every phase in almost every refractory inclusion contains 0.5–1.5 wt% FeO. A lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to FeO‐bearing fluid prior to accretion into the Acfer 094 parent body. There are no known nebular conditions under which the refractory phases found in the present samples could acquire FeO enrichments to the observed levels. The most likely setting is therefore in an earlier, FeO‐rich parent body. The inclusions were ejected from this parent body, mixed with typical CAIs, chondrules, amoeboid olivine aggregates, and amorphous material, and incorporated into the Acfer 094 parent body.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.     

A hibonite‐corundum inclusion from Murchison: A first‐generation condensate from the solar nebula

Abstract— Through freeze‐thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ?90 times 75 μm refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (?10 μm), rounded grains enclosed in hibonite laths (?10 μm wide and 30–40 μm long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically‐corundum followed by hibonite followed by perovskite‐is that predicted for the first phases to form by equilibrium condensation from a solar gas for P^tot ≤5 times 10^?3 atm. In addition, the texture of the inclusion, with angular voids between subhedral hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ?40 × CI chondrites, light REE abundances ?20 × CI chondrites, and negative Eu anomalies. The chondrite‐normalized abundance patterns, especially one for a hibonite‐perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for hibonite and perovskite at 10^?3 atm and are not consistent with formation of the inclusion by closed‐system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 times 10^?3 atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10^?4 atm). Both hibonite and corundum have δ17O ? δ18O ? ?50%, indicating formation from an 16O‐rich source. The inclusion does not contain radiogenic ²⁶Mg and apparently did not contain live ²⁶Al when it formed. If the short‐lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that ²⁶Al‐free refractory inclusions such as this one formed within the first ?6 times 10⁵ years of nebular collapse.     

The stability of hibonite,melilite and other aluminous phases in silicate melts: Implications for the origin of hibonite-bearing inclusions from carbonaceous chondrites

Abstract— Phase fields in which hibonite and silicate melt coexist with spinel, CaAl₄O₇, gehlenitic melilite, anorthite or corundum at 1 bar in the system CaO-MgO-Al₂O₃-SiO₂-TiO₂ were determined. The hibonites contain up to 1.7 wt% SiO₂. For TiO₂, the experimentally determined partition coefficients between hibonite and coexisting melt, D^Hib/L_i, vary from 0.8 to 2.1 and generally decrease with increasing TiO₂ in the liquid. Based on Ti partitioning between hibonite and melt, bulk inclusion compositions and hibonite-saturated liquidus phase diagrams, the hibonite in hibonite-poor fluffy Type A inclusions from Allende and at least some hibonite from hibonite-rich inclusions is relict, although much of the hibonite from hibonite-glass spherules probably crystallized metastably from a melt Bulk compositions for all of these CAIs are consistent with an origin as melilite + hibonite + spinel + perovskite phase assemblages that were partially altered and in some cases partially or completely melted The duration of the melting event was sufficient to remove any Na introduced by the alteration process but frequently insufficient to dissolve all of the original hibonite. Simple thermochemical models developed for meteoritic melilite and hibonite solid solutions were used to obtain equilibration temperatures of hibonite-bearing phase assemblages with vapor. Referenced to 10^?3 atm, hibonite + corundum + vapor equilibrated at ～1260 °C and hibonite + spinel ± melilite + vapor at 1215 ± 10 °C. If these temperatures reflect condensation in a cooling gas of solar composition, then hibonite ± corundum condensed first, followed by spinel and then melilite. The position of perovskite within this sequence is uncertain, but it probably began to condense before spinel. This sequence of phase appearances and relative temperatures is generally consistent with observed textures but differs from expectations based on classical condensation calculations in that equilibration temperatures are generally lower than predicted and melilite initially condenses with or even after spinel. Simple thermochemical models for the substitution of trace elements into the Ca site of meteoritic hibonites suggest that virtually all Eu is divalent in early condensate hibonites but that Eu²⁺/Eu³⁺ decreases by a factor of 20 or more during the course of condensation primarily because the ratio is proportional to the partial pressure of Al, which decreases dramatically as aluminous phases condense. The relative sizes of Eu and Yb anomalies in meteoritic hibonites and inclusions may be partly due to this effect     

Fine‐grained,spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy,petrology, and bulk chemistry

Abstract— Fine‐grained, spinel‐rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al‐diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite‐rich accretionary rims. In contrast to heavily altered fine‐grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine‐grained CAIs are ¹⁶O‐enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine‐grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al‐diopside. Other fine‐grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite‐free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al‐diopside. The mantles are calcium‐enriched, magnesium‐depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine‐grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel‐perovskite‐melilite ± hibonite gas‐solid condensates from a reservoir that was ¹⁶O‐enriched but depleted in the most refractory REEs. These aggregates later experienced low‐temperature gas‐solid nebular reactions with gaseous SiO and Mg to form Al‐diopside and ±anorthite. The zoned structures of many of the fine‐grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi‐stage formation history of fine‐grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse‐grained CAIs in CV chondrites.     

Al‐Mg systematics of hibonite‐bearing Ca,Al‐rich inclusions from Ningqiang

Abstract– Hibonite‐bearing Ca,Al‐rich inclusions (CAIs) usually occur in CM and CH chondrites and possess petrographic and isotopic characteristics distinctive from other typical CAIs. Despite their highly refractory nature, most hibonite‐bearing CAIs have little or no ²⁶Mg excess (the decay product of ²⁶Al), but do show wide variations of Ca and Ti isotopic anomalies. A few spinel‐hibonite spherules preserve evidence of live ²⁶Al with an inferred ²⁶Al/²⁷Al close to the canonical value. The bimodal distribution of ²⁶Al abundances in hibonite‐bearing CAIs has inspired several interpretations regarding the origin of short‐lived nuclides and the evolution of the solar nebula. Herein we show that hibonite‐bearing CAIs from Ningqiang, an ungrouped carbonaceous chondrite, also provide evidence for a bimodal distribution of ²⁶Al. Two hibonite aggregates and two hibonite‐pyroxene spherules show no ²⁶Mg excesses, corresponding to inferred ²⁶Al/²⁷Al < 8 × 10^?6. Two hibonite‐melilite spherules are indistinguishable from each other in terms of chemistry and mineralogy but have different Mg isotopic compositions. Hibonite and melilite in one of them display positive ²⁶Mg excesses (up to 25‰) that are correlated with Al/Mg with an inferred ²⁶Al/²⁷Al of (5.5 ± 0.6) × 10^?5. The other one contains normal Mg isotopes with an inferred ²⁶Al/²⁷Al < 3.4 × 10^?6. Hibonite in a hibonite‐spinel fragment displays large ²⁶Mg excesses (up to 38‰) that correlate with Al/Mg, with an inferred ²⁶Al/²⁷Al of (4.5 ± 0.8) × 10^?5. Prolonged formation duration and thermal alteration of hibonite‐bearing CAIs seem to be inconsistent with petrological and isotopic observations of Ningqiang. Our results support the theory of formation of ²⁶Al‐free/poor hibonite‐bearing CAIs prior to the injection of ²⁶Al into the solar nebula from a nearby stellar source.     

The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Abstract— In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K‐isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had δ⁴¹K compositions that were normal within error (typically ±3%, 2s?). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large ⁴¹K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ⁴¹K is produced by ～12% loss of K. The range of L‐chondrite‐normalized K/Al ratios (a measure of the K‐elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L‐chondrite‐like K abundances and the K loss occurred via Rayleigh fractionation, the most K‐depleted chondrules would have had compositions of up to δ⁴¹K ? 200%. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K‐isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K‐isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim‐matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K‐isotopic fractionation in them requires that the K‐isotopic exchange took place during chondrule formation, which would probably require gas‐chondrule exchange. Potassium evaporated from fine‐grained dust and chondrules during chondrule formation may have produced sufficient K‐vapor pressure for gas‐chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.     

Early solar system aqueous activity: K isotope evidence from Allende

The alkali element K is moderately volatile and fluid mobile; thus, it can be influenced by both primary processes (evaporation and recondensation) in the solar nebula and secondary processes (thermal and aqueous alteration) in the parent body. Since these primary and secondary processes would induce different isotopic fractionations, K isotopes could become a potential tracer to distinguish them. Using recently developed methods with improved precision (0.05‰, 95% confidence interval), we systematically measured the K isotopic compositions and major/trace elemental compositions of chondritic components (18 chondrules, 3 CAIs, 2 matrices, and 5 bulks) in the carbonaceous chondrite fall Allende. Among all the components analyzed in this study, CAIs, which formed initially under high‐temperature conditions in the solar nebula and were dominated by nominally K‐free refractory minerals, have the highest K₂O content (average 0.53 wt%) and have K isotope compositions most enriched in heavy isotopes (δ⁴¹K: ?0.30 to ?0.25‰). Such an observation is consistent with previous petrologic studies that show CAIs in Allende have undergone alkali enrichment during metasomatism. In contrast, chondrules contain lower K₂O content (0.003–0.17 wt%) and generally lighter K isotope compositions (δ⁴¹K: ?0.87‰ to ?0.24‰). The matrix and bulks are nearly identical in K₂O content and K isotope compositions (0.02–0.05 wt%; δ⁴¹K: ?0.62 to ? 0.46‰), which are, as expected, right in the middle of CAIs and chondrules. This strongly indicates that most of the chondritic components of Allende suffered aqueous alteration and their K isotopic compositions are the ramification of Allende parent‐body processing instead of primary nebular signatures. Nevertheless, we propose the small K isotope fractionations observed (< 1‰) among Allende components are likely similar to the overall range of K isotopic fractionation that occurred in nebular environment. Furthermore, the K isotope compositions seen in the components of Allende in this study are consistent with MC‐ICP‐MS analyses of the components in ordinary chondrites, which also show an absence of large (10‰) isotope fractionations. This is not expected as evaporation experiments in nebular conditions suggest there should be large K isotopic fractionations. Nevertheless, possible nebular processes such as chondrules back exchanging with ambient gas when they formed could explain this lack of large K isotopic variation.     

Multiple generations of hibonite in spinel-hibonite inclusions from Murchison

Abstract— Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered, in addition to many spinel-hibonite spherules, several hibonite-rich inclusion fragments in which the hibonite has wider ranges in TiO₂ contents (e.g., 0.07–8.6 wt% in one inclusion and 2–10 wt% in another) than previously observed within single inclusions. In these inclusions, there are sharp contacts between texturally early, Ti-poor hibonite and relatively late, Ti-rich hibonite, and the two types occur in complex intergrowth textures that are not consistent with crystallization from a melt in a single-stage cooling event. One inclusion has, in addition to relatively TiO₂-rich hibonite, some that is virtually TiO₂-free but contains ～1 wt% MgO and ～1.5 wt% SiO₂. Instead of the common substitution of Mg + Ti for 2Al, Mg coupled with Si in this case, probably reflecting crystallization from an unusual, Ti-free silicate liquid. Ion microprobe analyses of Ti-rich and Ti-poor hibonite from this inclusion yield quite similar trace-element patterns and Mg-isotopic compositions. The results are most consistent with formation of Ti-rich hibonite from Ti-poor hibonite by addition of Mg and Ti to the latter by exchange with a hot, Ti-rich liquid. That this occurred without a resolvable change in the Mg-isotopic composition requires that the Ti-rich, second generation of hibonite formed <2.5 times 10⁵ yr later than the Ti-poor hibonite. Observations of spinel-hibonite spherules and spherule fragments in our sample suite provide additional evidence that, despite the claims by Greenwood et al. (1994), spinel-hibonite spherules crystallized from individual, molten droplets, as earlier suggested by Macdougall (1981) and MacPherson et al. (1983).     

Origin of hibonite-pyroxene spherules found in carbonaceous chondrites

Abstract— We have studied both of the known glass-free, hibonite-pyroxene spherules: MYSM3, from Murray (CM2), and Y17–6, from Yamato 791717 (CO3). They consist of hibonite plates (～2 wt% TiO^tot₂) enclosed in Al-rich pyroxene that has such high amounts of CaTs (CaAl₂SiO₆) component, up to ～80 mol%, that it must have crystallized metastably. Within the pyroxene, abundances of MgO and SiO₂ are strongly correlated with each other and are anticorrelated with those of Al₂O₃, reflecting an anticorrelation between the diopside and CaTs components of the pyroxene. In contrast with previous results for Type B fassaite, however, we do not observe an anticorrelation between MgO and TiO^tot₂, possibly reflecting different relative distribution coefficients for Ti³⁺ and Ti⁴⁺ in the aluminous pyroxene of the spherules from those found for fassaite in Type B inclusions. Previously described hibonite-silicate spherules have ²⁶Mg deficits but the present samples do not. Furthermore, the pyroxene in Y17-6 has excess ²⁶Mg, while the hibonite it encloses does not, indicating that the two phases either had different initial ²⁶Al/²⁷Al ratios or different initial ²⁶Mg/²⁴Mg ratios. The Ti isotopic compositions of the present samples are highly unusual: δ⁵⁰Ti = 103.4 ± 5.2%o in MYSM3 and -61.4 ± 4.1%0 in Y17-6, which are among the largest ⁵⁰Ti anomalies reported for any refractory inclusion. The textures suggest that hibonite crystallized first; but based on the calculated bulk compositions of both spherules, it is not the liquidus phase in either sample, which suggests that the hibonite in both samples is relict. The presence of ragged hibonite grains in MYSM3 and rounded hibonite grains in Y17-6 and a lack of isotopic equilibrium between pyroxene and hibonite support this conclusion. The spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included hibonite. The heating events were too short and/or not hot enough to melt all the hibonite. The droplets cooled quickly enough that CaTs-rich pyroxene crystallized instead of anorthite. Based on the observed differences in isotopic composition, it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described hibonite-silicate spherules, providing further evidence of the isotopic heterogeneity of the early solar nebula.     

Mineralogy,chemistry, and oxygen isotopes of refractory inclusions from stratospheric interplanetary dust particles and micrometeorites

Abstract— Calcium, aluminum-rich inclusions (CAIs) are characteristic components in carbonaceous chondrites. Their mineralogy is dominated by refractory oxides and silicates like corundum, perovskite, spinel, hibonite, melilite, and Ca-pyroxene, which are predicted to be the first phases to have condensed from the cooling solar nebula. Allowing insights into processes occurring in the early solar system, CAIs in carbonaceous and ordinary chondrites were studied in great detail, whereas only a few refractory inclusions were found and studied in stratospheric interplanetary dust particles (IDPs) and micrometeorites. This study gives a summary of all previous studies on refractory inclusions in stratospheric IDPs and micrometeorites and will present new data on two Antarctic micrometeorites. The main results are summarized as follows: (a) Eight stratospheric IDPs and six micrometeorites contain Ca, Al-rich inclusions or refractory minerals. The constituent minerals include spinel, perovskite, fassaite, hibonite, melilite, corundum, diopside and anorthite. (b) Four of the seven obtained rare-earth-element (REE) patterns from refractory objects in stratospheric IDPs and micrometeorites are related to Group III patterns known from refractory inclusions from carbonaceous chondrites. A Group II related pattern was found for spinel and perovskite in two micrometeorites. The seventh REE pattern for an orthopyroxene is unique and can be explained by fractionation of Gd, Lu, and Tb at highly reducing conditions. (c) The O-isotopic compositions of most refractory objects in stratospheric IDPs and micrometeorites are similar to those of constituents from carbonaceous chondrites and fall on the carbonaceous chondrites anhydrous minerals mixing line. In fact, in most cases, in terms of mineralogy, REE pattern and O-isotopic composition of refractory inclusions in stratospheric IDPs and micrometeorites are in good agreement with a suggested genetic relation of dust particles and carbonaceous chondrites. Only in the case of one Antarctic micrometeorite does the REE pattern obtained for an orthopyroxene point to a link of this particle to enstatite chondrites.     

Condensation and aggregation of solar corundum and corundum‐hibonite grains

Abstract— Forty‐three corundum grains (1–11 μm in size) and 5 corundum‐hibonite grains with corundum overgrown by hibonite (4–7 μm in size), were found in the matrix of the mineralogically pristine, ungrouped carbonaceous chondrite Acfer 094 by using cathodoluminescence imaging. Some of the corundum and corundum‐hibonite grains occur as aggregates of 2 to 6 grains having similar sizes. The oxygen isotopic compositions of some of the corundum‐bearing grains suggest their solar nebula origin. ²⁶Al‐²⁶Mg systematics of one corundum grain showed the canonical initial ²⁶Al/²⁷Al ratio, also suggesting a solar nebula origin. Quantitative evaluation of condensation and accretion processes made based on the homogeneous nucleation of corundum, diffusion‐controlled hibonite formation, collisions of grains in the nebula, and critical velocity for sticking, indicates that, in contrast to the hibonite‐bearing aggregates of corundum grains, the hibonite‐free corundum aggregates could not have formed in the slowly cooling nebular region with solar composition. We suggest instead that such aggregates formed near the protosun, either in a region that stayed above the condensation temperature of hibonite for a long time or in a chemically fractionated, Ca‐depleted region, and were subsequently physically removed from this hot region, e.g., by disk wind.     

Defining the mechanisms that disturb the Sm‐Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

Abstract— Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm‐Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high‐Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non‐indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non‐igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm‐Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.     

The formation of rims on calcium‐aluminum‐rich inclusions: Step I—Flash heating

Abstract— Wark‐Lovering rims of six calcium‐aluminum‐rich inclusions (CAIs) representing the main CAI types and groups in Allende, Efremovka and Vigarano were microsurgically separated and analysed by neutron activation analysis (NAA). All the rims have similar ～4x enrichments, relative to the interiors, of highly refractory lithophile and siderophile elements. The NAA results are confirmed by ion microprobe and scanning electron microscope (SEM) analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb, V, Sr, Ca and Ni are less enriched in the rims. The refractory element patterns in the rims parallel the patterns in the outer parts of the CAIs. In particular, the rims on type B1 CAIs have the igneously fractionated rare earth element (REE) pattern of the melilite mantle below the rim and not the REE pattern of the bulk CAI, proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the CAIs. The refractory elements were enriched in an Al₂O₃‐rich residue <50 μm thick after the most volatile ～80% of the outermost 200 μm of each CAI had been volatilized, including much Mg, Si and Ca. Some volatilization occurred below the rim, and created refractory partial melts that crystallized hibonite and gehlenitic melilite. The required “flash heating” probably exceeded 2000 °C, but for only a few seconds, in order to melt only the outer CAI and to unselectively volatilize slow‐diffusing O isotopes which show no mass fractionation in the rim. The volatilization did, however, produce “heavy” mass‐fractionated Mg in rims. In some CAIs this was later obscured when “normal” Mg diffused in from accreted olivine grains at relatively high temperature (not the lower temperature meteorite metamorphism) and created the ～50 μm set of monomineralic rim layers of pyroxene, melilite and spinel.     

The pyroxene pallasites,Vermillion and Yamato 8451: Not quite a couple

Abstract— Two pallasites, Vermillion and Yamato (Y)‐8451, have been studied to obtain petrologic, trace element, and O‐isotopic data. Both meteorites contain low‐Ca and high‐Ca pyroxenes (<2% by volume) and have been dubbed “pyroxene pallasites.” Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Olivine compositions are Fa_10–12, similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. The O‐isotopic compositions of Vermillion and Y‐8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Although Vermillion and Y‐8451 have similar mineralogy, pyroxene compositions, REE patterns, and O‐isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.