Search

We performed high-precision SIMS (secondary ion mass spectrometry) 26Al-26Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” and “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 and Yamato-81020, respectively. Mesquite is unusually large (∼5 × 3 mm) for a CAI from any CO chondrite and exhibits a layered texture comprising a melilite-rich core surrounded by hibonite- and spinel-rich mantle layers and a semi-continuous spinel-dominated rim. The CAI Y24 stands out because of its distinct mineralogy: grossite, hibonite, and spinel are accompanied by abundant ultra-refractory-element-rich phases such as warkite, kangite, and perovskite. Silicates are absent in Y24. Negatively fractionated δ25Mg values of phases in the core and mantle layers of Mesquite suggest that the inclusion as a whole was never molten and, hence, represents an aggregate of condensates. The relatively large grain sizes of melilite in the core (up to ∼300 µm) most likely are the result of solid-state recrystallization and coarsening of melilite in the course of a heating event occurring in the solar nebula. This heating event, however, did not disturb the Al-Mg systematics of Mesquite. Regardless of their position within Mesquite and the phases analyzed, spots analyzed for Al-Mg plot on a single isochron characterized by an initial 26Al/27Al of (4.95 ± 0.08) × 10–5 and a δ26Mg*0 of –0.14 ± 0.05‰. We suggest that this initial 26Al/27Al ratio corresponds to the formation of Mesquite in the solar nebula that was slightly heterogeneous with respect to Mg isotopes. Spinel in the rim is uniform in Δ17O (∼–25‰); in contrast, hibonite in the core and mantle layers, albeit also 16O-rich, show variable oxygen isotope ratios (Δ17O ∼ –15‰ to –23‰), which would be consistent with hibonite condensation in a gas with quickly-changing oxygen isotope compositions. The 16O-poor composition of melilite (Δ17O ∼ –1‰ to 0‰) in the core could be the result of isotope exchange with an 16O-poor gas, perhaps during the heating event that caused the solid-state recrystallization and coarsening of melilite or the result of oxygen isotope exchange with a fluid on the parent body. Abundant calcite, phyllosilicates, and sodalite are witnesses to late-stage and low-temperature alteration of the Mesquite CAI; calcite and phyllosilicates most likely are of terrestrial origin, but sodalite could have formed in the parent body. Inclusion Y24 is irregularly-shaped, indicating a condensation origin. Completely enclosing other phases, warkite forms the matrix of Y24, which could be the result of simultaneous condensation and growth of warkite, grossite, and hibonite. Possibly, spinel formed by replacing grossite or hibonite or both minerals in a gradually cooling gas before any silicates condensed. SIMS analyses indicate that condensation occurred in an 16O-rich gas when 26Al/27Al was (5.4 ± 1.0) × 10–5. Oxygen isotope exchange with an 16O-poor fluid in the parent body or with an 16O-poor gas in a nebular setting caused the 16O-poor compositions in grossite and kangite.

Warkite (IMA 2013-129) is a new Sc-rich ultrarefractory mineral in the rhonite group of the sapphirine supergroup. It has a P 1 ¯ aenigmatite-type structure with a = 10.367 A, b = 10.756 A, c = 8.895 A, α = 106°, β = 96°, γ = 125°, and Z = 2, and general formula of Ca2(Sc,Ti,Al,Mg,Zr)6Al6O20. Warkite occurs as micrometer-sized crystals in eleven ultrarefractory Ca,Al-rich inclusions (UR CAIs) from the CM, CV, CO, and CH chondrites. In the CM, CO, and CV CAIs, warkite in the cores coexists with a Ti-rich oxide, either perovskite or kangite; the cores are generally mantled and rimmed by davisite and/or Sc-diopside. In the CH CAIs and one CO CAI, warkite in the cores coexists with perovskite and grossite; the cores are mantled by grossite ± gehlenite, and rimmed by low-Sc, Al-diopside. Therefore, there are two basic families of warkite-bearing inclusions, those containing Sc-rich clinopyroxene but no grossite and those containing grossite but no Sc-rich clinopyroxene. Scandian clinopyroxene in warkite-bearing CAIs generally formed by the reaction of warkite, which supplied most or all of the Sc, Ti, and Al, and a nebular gas that supplied much of the Ca and O and virtually all of the Mg and Si. The presence of Sc-rich clinopyroxenes may reflect exposure of some warkite-bearing CAIs to a dust-rich environment, which would enhance partial pressures of Si-, Mg-, and Ca-bearing species in the vapor and make it oxidizing relative to a dust-poor gas. Warkite in grossite-bearing inclusions is generally Ti3+-enriched relative to those in davisite-bearing inclusions, consistent with their formation in relatively dust-poor, more-reducing environments. Warkite compositions are sensitive to the presence or absence of spinel, melilite, Sc-rich clinopyroxenes, and grossite. Compositional variations of perovskite and warkite grains indicate a connection but, except for late-stage Fe exchange, they did not equilibrate with each other. The presence of at least two trends in Y-Sc among perovskites without corresponding trends in warkite suggests that at least some perovskite formed separately. Warkite-bearing CAIs from CM2s and CO3.0s are uniformly 16O-rich (Δ17O ∼ −23‰), whereas those from metamorphosed COs and CVs are isotopically heterogeneous: warkite, kangite, perovskite, melilite, and davisite are 16O-depleted to various degrees (Δ17O range from −22 to −2‰) relative to hibonite, spinel, and forsterite, all having 16O-rich compositions (Δ17O ∼ − 25 to −20‰). We infer that warkite-bearing CAIs originated in an 16O-rich nebular gas. Subsequently, CAIs from metamorphosed CVs and COs experienced O-isotope exchange with an 16O-depleted external reservoir, most likely aqueous fluids on the CV and CO chondrite parent asteroids; however, O-isotope exchange in the solar nebula cannot be excluded.

We have found two refractory inclusions in the CO3.00 carbonaceous chondrite Dominion Range (DOM) 08006 that appear to be primary condensates from the early solar nebula. One, inclusion 56–1, contains the first four phases predicted to form by equilibrium gas-solid condensation: corundum; hibonite; grossite; and perovskite. The other, 31–2, contains nine predicted condensate phases: hibonite; grossite; perovskite; melilite; spinel; FeNi metal; diopside; forsterite; and enstatite. Except for melilite/spinel, the phases occur in the predicted sequence from core to rim of the inclusion, which has an irregular shape inconsistent with a molten stage. This inclusion preserves the most complete record of condensation in the early solar nebula that has yet been found. The physical evidence reported here supports equilibrium condensation calculations that predict the observed sequence as well as the assumptions upon which they are based, such as total pressure (~10(−3) atm), bulk system composition (solar), and C—O—H proportions. All phases in both inclusions and the associated ferromagnesian silicates are 16O-rich, with ∆(17)O between −25 and −20‰, implying that this is the original composition of the vast majority of primary condensates and that (16)O-poor compositions observed in many isotopically heterogeneous inclusions are largely due to subsequent isotopic exchange. While the nebula was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed early and was preserved during condensation. Inclusion 31–2 did not incorporate live (26)Al and has nucleosynthetic anomalies in the heavy Ca and Ti isotopes (i.e., δ(48)Ca = 4.3 ± 1.9‰; δ(50)Ti = 8.8 ± 2.0‰). In contrast, inclusion 56–1 has radiogenic (26)Mg excesses yielding a ((26)Al/(27)Al)(0) ratio of (1.0 ± 0.1) × 10(−5) and negative nucleosynthetic isotopic anomalies in Ca (δ(48)Ca = −10.3 ± 4.2‰) and Ti (δ(50)Ti = −4.3 ± 2.9‰). Thus, it represents a deviation from the mutual exclusivity relationship between (26)Al incorporation and large nucleosynthetic anomalies. The reservoirs in which these inclusions formed had similar O-isotopic and different Al-, Ca– and Ti-isotopic compositions, suggesting that while the CAI-forming region was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed and was preserved during condensation.

Elemental abundance and isotopic fractionation profiles across zoned minerals from a martian meteorite (Shergotty) and from a lunar olivine-normative mare basalt (Apollo 15555) were used to place constraints on the thermal evolution of their host rocks. The isotopic measurements were used to determine the extent to which diffusion was responsible for, or modified, the zoning. The key concept is that mineral zoning that is the result of diffusion, or that was significantly affected by diffusion, will have an associated diagnostic isotopic fractionation that can quantify the extent of mass transfer by diffusion. Once the extent of diffusion was determined, the mineral zoning was used to constrain the thermal history. An isotopic and chemical profile measured across a large zoned pigeonite grain from Shergotty showed no significant isotopic fractionation of either magnesium or lithium, which is evidence that the chemical zoning was dominantly the result of crystallization from an evolving melt and that the crystallization must have taken place at a sufficiently fast rate that there was not time for any significant mass transfer by diffusion. Model calculations for the evolution of the fast-diffusing lithium showed that this would have required a cooling at a rate of about ∼150 °C/h or more. Measurable isotopic fractionation across a zoned olivine grain from lunar mare basalt 15555 indicated that the chemical zoning was mainly due to crystallization that was modified by a small but quantifiable amount of diffusion. The results of a diffusion calculation that was able to account for the amplitude and spatial scale of the isotopic fractionation across the olivine grain yielded an estimate of 0.2 °C/h for the cooling rate of 15555. The results of an earlier study of zoned augite and olivine grains from martian nakhlite meteorite NWA 817 were reviewed for comparison with the results from Shergotty. The isotopic fractionations near the edges of grains from NWA 817 showed that, in contrast to Shergotty, the lithium zoning in augite and of magnesium in olivine was due entirely to diffusion. The isotopic fractionation data across zoned minerals from the martian meteorites and from the lunar basalt were key for documenting and quantifying the extent of mass transfer by diffusion, which was a crucial step for validating the use of diffusion modeling to estimate their cooling rates.

Previous studies have shown that CV and CM chondrites incorporated Ca, Al-rich inclusions (CAIs) with different isotopic characteristics, which may represent different snapshots in the isotopic evolution of the early Solar System. To better understand how the isotopic characteristics of CAIs vary between different chondrite groups, we have studied calcium and titanium isotopes in CAIs from CM, CO, and CR chondrites. We show that all three chondrite groups contain CAIs with large anomalies in 48Ca and/or 50Ti (10s of ‰ or 100s of e-units) as well as CAIs with no anomalies resolved beyond measurement uncertainties. Isotopically, the anomalous CO and CR chondrite CAIs resemble the platy hibonite crystals (PLACs) from CM chondrites, but they are more mineralogically complex. The new data are consistent with the well-established mutual exclusivity relationship between incorporation of 26Al and the presence of large anomalies in 48Ca and 50Ti. The two highly anomalous CO chondrite CAIs have correlated anomalies in 46Ti and 50Ti, while most other highly anomalous CAIs do not. This result could indicate that the reservoir with coupled 46Ti and 50Ti that was sampled by bulk meteorites and CV chondrite CAIs already existed before arrival and/or homogeneous distribution of 26Al in the protoplanetary disk. Among the studied CM chondrite CAIs are ten spinel-hibonite inclusions (SHIBs) with known oxygen isotopic compositions. Our results show that these objects sampled a reservoir that was well-mixed in oxygen, calcium, and titanium isotopes. We further show that SHIBs tend to be slightly enriched in the heavy calcium isotopes, suggesting that their formation history was different from CV chondrite CAIs.

Accepted: 2017-05-26, 資料番号: SA1170244000

The behavior of boron during the early evolution of the Solar System provides the foundation for how boron reservoirs become established in terrestrial planets. The abundance of boron in the Sun is depleted relative to adjacent light elements, a result of thermal nuclear reactions that destroy boron atoms. Extant boron was primarily generated by spallation reactions. In the initial materials condensing from the solar nebula, boron was predominantly incorporated into plagioclase. Boron abundances in the terrestrial planets exhibit variability, as illustrated by B/Be. During planetary formation and differentiation, boron is redistributed by fluids at low temperature and during crystallization of magma oceans at high temperature.

The valences of Ti, V, and Cr in olivine and pyroxene, important indicators of the fO2 of the source region of their host rocks, can be readily measured nondestructively by XANES (X-ray absorption near edge structure) spectroscopy, but little such work has been done on lunar rocks, and there is some uncertainty regarding the presence of Ti3+ in lunar silicates and the redox state of the lunar mantle. This is the first study involving direct XANES measurement of valences of multivalent cations in lunar rocks. Because high alumina activity facilitates substitution of Ti cations into octahedral rather than tetrahedral sites in pyroxene and Ti3+ only enters octahedral sites, two aluminous basalts from Apollo 14, 14053 and 14072, were studied. Most pyroxene contains little or no detectable Ti3+, but in both samples relatively early, magnesian pyroxene was found that has Ti valences that are not within error of 4; in 14053, this component has an average Ti valence of 3.81 ± 0.06 (i.e., Ti3+/[Ti3+ + Ti4+ = 0.19]). This pyroxene has relatively low atomic Ti/Al ratios ( 0.5 contains Ti3+ and pyroxene with lower ratios does not. Later pyroxene, with lower Mg/Fe and higher Ti/Al ratios, has higher proportions of Ti (all Ti4+) in tetrahedral sites. All pyroxene analyzed contains divalent Cr, ranging from 15 to 30% of the Cr present, and all but one analysis spot contains divalent V, accounting for 0 to 40% (typically 20–30%) of the V present. Three analyses of olivine in 14053 do not show any Ti3+, but Ti valences in 14072 olivine range from 4 down to 3.70 ± 0.10. In 14053 olivine, ~50% of the Cr and 60% of the V are divalent. In 14072 olivine, the divalent percentages are ~20% for Cr and 20–60% for V. These results indicate significant proportions of divalent Cr and V and limited amounts of trivalent Ti in the parental melts, especially when crystal/liquid partitioning preferences are taken into account. These features are consistent with an fO2 closer to IW − 2 than to IW − 1. Apollo 15 basalt 15555, analyzed for comparison with A-14 materials, has olivine with strongly reduced Cr (Cr2+/(Cr2+ + Cr3+) ~0.9). Basalts from different sites may record redox differences between source regions.

One way to better understand processes related to chondrite metamorphism is to evaluate changes in chondrite features as a function of petrologic type. Toward this end the valence and coordination of Ti in olivine and pyroxene in suites of ordinary (H, L, and LL) and enstatite (EH and EL) chondrites of types 3 through 6 have been determined with XANES spectroscopy. Trivalent Ti, typically 10–40% of the Ti in the analytical volumes, was found in ordinary chondrites of all types, despite the stability of oxidized iron in the samples. Average valences and the proportions of Ti that are in tetrahedral coordination generally decrease with increasing grade between types 3.0 and 3.5, increase from 3.5 to 4, and then level off. These trends are consistent with previous studies of chondrite oxidation states using other methods, except here the onset of oxidation is observed at a lower type, ∼3.5, than previously indicated (4). These results are also consistent with previous suggestions that oxidation of higher-grade ordinary chondrite samples involved exposure to aqueous fluids from melting of accreted ice. In the enstatite chondrites, typically 20–90% of the Ti is trivalent Ti, so it is reduced compared to Ti in the ordinary chondrites. Valence decreases slightly from petrologic type 3 to 4 and increases from 4 to 6, but no increases in tetrahedral coordination with petrologic type are observed, indicating a redox environment or process distinct from that of ordinary chondrite metamorphism. The presence of Ti4+ in the E chondrites supports previous suggestions that they formed from oxidized precursors that underwent reduction. Unlike ordinary chondrites, enstatite chondrites are thought to have been derived from a body or bodies that did not accrete ice, which could account for their different valence-coordination-petrologic type relationships. The hypothesis, based on observations of unmetamorphosed chondrules and supported by laboratory experiments, that equilibration of Ti valence is sluggish compared to that of Fe could account for the coexistence of reduced Ti and oxidized Fe seen in chondrites of all petrologic types.

Pyroxene is arguably the most powerful, single-phase geochemical and petrologic recorder of Solar System processes, from nebular condensation through planetary evolution, over a wide range of temperatures, pressures, and f o 2 . It is an important mineral phase in the crusts and mantles of evolved planets, in undifferentiated and differentiated asteroids, and in refractory inclusions—the earliest Solar System materials. Here, we review the valence state partitioning behavior of Cr (Cr 2+ , Cr 3+ ), Ti (Ti 3+ , Ti 4+ ), and V (V 2+ , V 3+ , V 4+ , V 5+ ) among crystallographic sites in pyroxene over a range of f o 2 from approximately fayalite-magnetite-quartz (FMQ) to ~7 log units below iron-wustite (IW-7), and decipher how pyroxene can be used as a recorder of conditions of planetary and nebular environments and planetary parentage. The most important crystallographic site in pyroxene with respect to its influence on mineral/melt partitioning is M2; its Ca content has a huge effect on partitioning behavior, because the large Ca cation expands the structure. As a result, distribution coefficients ( D s) for Cr and V increase with increasing Ca content from orthopyroxene to pigeonite to augite. In addition, it is noted that V 3+ is favored over V 4+ in olivine and pyroxene. In pyroxene in refractory inclusions, Ti 3+ is favored over Ti 4+ and incorporation of Ti is facilitated by the high availability of Al for coupled substitution. The most important results from analysis of pyroxene in martian meteorites (e.g., QUE 94201) are the oxygen fugacity estimates of IW+0.2 and IW+0.9 derived from partitioning and valence data for Cr and V, respectively, obtained from experiments using appropriate temperatures and melt compositions. In angrites, changes in V valence state may translate to changes in f o 2 , from IW-0.7 during early pyroxene crystallization, to IW+0.5 during later episodes of pyroxene crystallization. In addition to f o 2 , the partitioning behavior of Cr, V, and Ti between pyroxene and melt is also dependent upon availability of other cations, especially Al, for charge-balancing coupled substitutions.

A coordinated, electron-backscatter-diffraction (EBSD) and transmission electron microscope (TEM) study was undertaken to obtain information on the origin of rims on refractory inclusions in the Allende and Axtell CV3 chondrites. These measurements were supported by theoretical modeling using density functional theory. Crystal-orientation analysis of Wark-Lovering rims via EBSD revealed pyroxene grains with similar crystallographic orientations to one another in both inclusions. An epitaxial relationship between grains within the diopside and anorthite rim layers was observed in Allende. TEM examination of the rims of both samples also revealed oriented crystals at depth. The microstructural data on the rims suggest that grain clusters grew in the form of three-dimensional islands. Density functional theory calculations confirm that formation of oriented grain islands is the result of energy minimization at high temperature. The results point toward condensation as the mode of origin for the rims studied here.

We report on the mineralogy, petrography, and in situ oxygen isotopic composition of twenty-five ultrarefractory calcium-aluminum-rich inclusions (UR CAIs) in CM2, CR2, CH3.0, CV3.1–3.6, CO3.0–3.6, MAC 88107 (CO3.1-like), and Acfer 094 (C3.0 ungrouped) carbonaceous chondrites. The UR CAIs studied are typically small

Grossite (CaAl4O7) is one of the one of the first minerals predicted to condense from a gas of solar composition, and therefore could have recorded isotopic compositions of reservoirs during the earliest stages of the Solar System evolution. Grossite-bearing Ca,Al-rich inclusions (CAIs) are a relatively rare type of refractory inclusions in most carbonaceous chondrite groups, except CHs, where they are dominant. We report new and summarize the existing data on the mineralogy, petrography, oxygen and aluminum-magnesium isotope systematics of grossite-bearing CAIs from the CR, CH, CB, CM, CO, and CV carbonaceous chondrites. Grossite-bearing CAIs from unmetamorphosed (petrologic type 2―3.0) carbonaceous chondrites preserved evidence for heterogeneous distribution of 26Al in the protoplanetary disk. The inferred initial 26Al/27Al ratio [(26Al/27Al)0] in grossite-bearing CAIs is generally bimodal, ˜0 and ˜5×10−5; the intermediate values are rare. CH and CB chondrites are the only groups where vast majority of grossite-bearing CAIs lacks resolvable excess of radiogenic 26Mg. Grossite-bearing CAIs with approximately the canonical (26Al/27Al)0 of ˜5×10−5 are dominant in other chondrite groups. Most grossite-bearing CAIs in type 2–3.0 carbonaceous chondrites have uniform solar-like O-isotope compositions (Δ17O ˜ ‒24±2‰). Grossite-bearing CAIs surrounded by Wark-Lovering rims in CH chondrites are also isotopically uniform, but show a large range of Δ17O, from ˜ ‒40‰ to ˜ ‒5‰, suggesting an early generation of gaseous reservoirs with different oxygen-isotope compositions in the protoplanetary disk. Igneous grossite-bearing CAIs surrounded by igneous rims of ±melilite, Al-diopside, and Ca-rich forsterite, found only in CB and CH chondrites, have uniform 16O-depleted compositions (Δ17O ˜ ‒14‰ to ‒5‰). These CAIs appear to have experienced complete melting and incomplete O-isotope exchange with a 16O-poor (Δ17O ˜ ‒2‰) gas in the CB impact plume generated about 5 Ma after CV CAIs. Grossite-bearing CAIs in metamorphosed (petrologic type >3.0) CO and CV chondrites have heterogeneous Δ17O resulted from mineralogically-controlled isotope exchange with a 16O-poor (Δ17O ˜ ‒2 to 0‰) aqueous fluid on the CO and CV parent asteroids 3–5 Ma after CV CAIs. This exchange affected grossite, krotite, melilite, and perovskite; corundum, hibonite, spinel, diopside, forsterite, and enstatite preserved their initial O-isotope compositions. The internal 26Al-26Mg isochrons in grossite-bearing CAIs from weakly-metamorphosed CO and CV chondrites were not disturbed during this oxygen-isotope exchange. HCCJr is grateful to Klaus Keil for all his sound profession counsel and collegial friendship over the years. He has always been willing to talk and has the generous nature of listening and sharing his thoughts freely and constructively. Professor Klaus Keil has been a mentor to and played a key role in the careers of three of the authors of this paper (ANK, KN, and GRH). He has also influenced the careers of the other authors and most of the people who have worked on meteorites over the past 50+ years. We therefore dedicate this paper to Professor Keil and present it in this Special Issue of Geochemistry.

The impact hypothesis for the origin of CB chondrites was tested by performing equilibrium condensation calculations in systems composed of vaporized mixtures of projectile and target materials. When one of the impacting bodies is composed of the metal from CR chondrites and the other is an H chondrite, good agreement can be found between calculated and observed compositions of unzoned metal grains in CB chondrites but the path of composition variation of the silicate condensate computed for the same conditions that reproduce the metal grain compositions does not pass through the measured compositions of barred olivine (BO) or cryptocrystalline (CC) chondrules in the CBs. The discrepancy between measured chondrule compositions and those of calculated silicates is not reduced when diogenite, eucrite or howardite compositions are substituted for H chondrite as the silicate-rich impacting body. If, however, a CR chondrite body is differentiated into core, a relatively CaO-, Al 2 O 3 -poor mantle and a CaO-, Al 2 O 3 -rich crust, and later accretes significant amounts of water, a collision between it and an identical body can produce the necessary chemical conditions for condensation of CB chondrules. If the resulting impact plume is spatially heterogeneous in its proportions of crust and mantle components, the composition paths calculated for silicate condensates at the same P tot , Ni/H and Si/H ratios and water abundance that produce good matches to the unzoned metal grain compositions pass through the fields of BO and CC chondrules, especially if high-temperature condensates are fractionated in the case of the CCs. While equilibrium evaporation of an alloy containing solar proportions of siderophiles into a dense impact plume is an equally plausible hypothesis for explaining the compositions of the unzoned metal grains, equilibrium evaporation can explain CB chondrule compositions only if an implausibly large number of starting compositions is postulated. Kinetic models applied to co-condensing metal grains and silicate droplets in a region of the plume with very similar composition, but with high cooling rate and sharply declining P tot during condensation, produce very good matches to the zoning profiles of Ir, Ni, Co and Cr concentrations and Fe and Ni isotopic compositions observed in the zoned metal grains in CB chondrites but produce very large positive δ 56 Fe in the cogenetic silicate, which are not found in the chondrules.

The Antarctic carbonaceous chondrites DOM 08004 and DOM 08006 have been paired and classified as CO3.0s. There is some uncertainty as to whether they should be paired and whether they are best classified as CO chondrites, but they provide an opportunity for the study of refractory inclusions that have not been modified by parent body processes. In this work, refractory inclusions in thin sections of DOM 08004 and 08006 are studied and compared with inclusions in ALHA77307 (CO3.0) and Acfer 094 (C3.0, ungrouped). Results show that the DOM samples have refractory inclusion populations that are similar to each other but not typical of CO3 chondrites; main differences are that the DOM samples are slightly richer in inclusions in general and, more specifically, in the proportions of grossite-bearing inclusions. In DOM 08004 and DOM 08006, 12.4% and 6.6%, respectively, of the inclusions are grossite-bearing. This is higher than the proportion found in Acfer 094 (5.2%), whereas none were found in ALHA77307. Like those in Acfer 094, DOM inclusions are small (mostly

Ti valence measurements in MgAl_2O_4 spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti^(+4). Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti^(+3)-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti^(+4)-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ^(25)Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ^(25)Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ^(25)Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti^(+4) concentrations in spinel.

We present high-precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine- and pyroxene-rich and have low 27 Al/ 24 Mg ratios (0.012-0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (d 26 Mg ranging from 0.95& to 0.15& relative to the DSM-3 standard) than those of Murchison (d 26 Mg ranging from 1.27& to +0.77&). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least-altered CM2 chondrules are on average lighter (average d 26 Mg = 0.39 0.30&, 2SE) than the moderately to heavily altered CM2 chondrules (average d 26 Mg = 0.11 0.21&, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low-Al CM2 chondrules analyzed here do not exhibit any mass-independent variations in 26 Mg from the decay of 26 Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable 26 Mg excess suggests that it either formed >1 Ma after calcium-aluminum-rich inclusions, or that its Al-Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of 26 Al.

– For fayalite formation times of several thousand years, and systems enriched in water by a factor of ten relative to solar composition, 1 μm radius olivine grains could reach 2 mole% fayalite and 0.1 μm grains 5 mole% by nebular condensation, well short of the values appropriate for precursors of most chondrules and the values found in the matrices of unequilibrated ordinary chondrites. Even 10 μm olivine crystals could reach 30 mole% fayalite above 1100 K in solar gas if condensation of metallic nickel-iron were delayed sufficiently by supersaturation. Consideration of the surface tensions of several phases with equilibrium condensation temperatures above that of metallic iron shows that, even if they were supersaturated, they would still nucleate homogeneously above the equilibrium condensation temperature of metallic iron. This phenomenon would have provided nuclei for heterogeneous nucleation of metallic nickel-iron, thus preventing the latter from supersaturating significantly and preventing olivine from becoming fayalitic. Unless a way is found to make nebular regions far more oxidizing than in existing models, it is unlikely that chondrule precursors or the matrix olivine grains of unequilibrated ordinary chondrites obtained their fayalite contents by condensation processes. Perhaps stabilization of FeO occurred after condensation of water ice and accretion of icy planetesimals, during heating of the planetesimals and/or in hot, dense, water-rich vapor plumes generated by impacts on them. This would imply that FeO is a relatively young feature of nebular materials.

When a shock wave passes through a nebular gas, increasing water enrichment leads to higher temperatures and postshock P tot , but lower cooling rates. A kinetic evaporation model is developed for tracking the chemical and isotopic changes that would occur in a clump of chondrule precursor dust surrounded by nebular gas in a closed system traversed by a nebular shock wave, taking into account effects of non-equilibrium melting and fractional crystallization on the liquid composition and the temperature difference between the gas and the droplet. A range of shock wave temperature–pressure histories computed for systems enriched relative to solar composition by factors of 550 in water, to achieve the redox state of chondrules, and 600 in dust, to retard evaporation, are employed, and redox changes are assumed to occur on the time–scale of heating and cooling in each. Two different system compositions are assumed, with the mean Fe/Si ratios of Types I and II chondrules. Two different textural outcomes are modeled, PO, in which nuclei are preserved and olivine crystallization begins immediately upon reaching saturation, and BO, in which no nuclei are preserved and olivine crystallization begins only after 300–400 K of supersaturation. In all cases, all iron evaporates, regardless of its oxidation state, as well as alkalis and smaller fractions of Mg and Si. In most cases, recondensation occurs on the time-scale of cooling, resulting in droplets whose bulk compositions have small isotopic anomalies in Mg, Si and Fe, comparable to those seen in bulk chondrules. Because fractional crystallization of olivine occurs before recondensation is complete, however, large isotopic variations, especially for iron, would have been recorded both within olivine crystals and between olivine and glass within these objects. Even after diffusive relaxation during crystal growth and cooling, variations in d 25 Mg of several tenths of a & to several & ,i nd 29 Si of 0.1& to several & and in d 56 Fe of several & would be measurable within large grains that grew throughout the olivine crystallization interval in many cases, and olivine-glass differences of Pseveral tenths of a & in d 29 Si, and of several & in d 56 Fe would be preserved. Such internal isotopic heterogeneities have not yet been observed in chondrules, suggesting that the latter did not form in these shock wave thermal histories. Suppression of production of internal isotopic variations requires heating times that are shorter by a factor of 100, combined with dust enrichments P 6 � 10 4 and/or P tot P10 � 2 bar. Together with relatively high fO2 , these

Dyl et al. (2011) state that their results confirm the conclusion of J. Simon et al. (2005) that the pyroxene in Wark–Lovering rims ( Wark and Lovering, 1977 ) found on Ca-, Al-rich refractory inclusions has lower Ti 3+ /Ti tot ratios than the primary pyroxene in the interiors of inclusions. While true, the claim is misleading because J. Simon et al. (2005) concluded that there was no Ti 3+ in the rims, whereas Dyl et al. (2011) found Ti 3+ in 41 of 42 new rim analyses. In addition, J. Simon et al. (2005) concluded that rims formed under much more oxidizing conditions, log f O 2 ⩾ IW-1, or ⩾6–7 log units higher, than inclusion interiors. The conclusions of J. Simon et al. (2005) were disputed by S. Simon et al. (2007) and are not supported by the new data of Dyl et al. (2011) . The present work is intended for clarification of this and other issues.

– 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 akermanite 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.

Micrometer-scale analyses of a calcium-, aluminum-rich inclusion (CAI) and the characteristic mineral bands mantling the CAI reveal that the outer parts of this primitive object have a large range of oxygen isotope compositions. The variations are systematic; the relative abundance of (16)O first decreases toward the CAI margin, approaching a planetary-like isotopic composition, then shifts to extremely (16)O-rich compositions through the surrounding rim. The variability implies that CAIs probably formed from several oxygen reservoirs. The observations support early and short-lived fluctuations of the environment in which CAIs formed, either because of transport of the CAIs themselves to distinct regions of the solar nebula or because of varying gas composition near the proto-Sun.

Refractory Ti-bearing minerals in the calcium-, aluminum-rich inclusion (CAI) Inti, recovered from the comet 81P/Wild 2 sample, were examined using analytical (scanning) transmission electron microscopy (STEM) methods including imaging, nanodiffraction, energy-dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inti fassaite (Ca(Mg,Ti,Al)(Si,Al)2O6) was found to have a Ti3+/Ti4+ ratio of 2.0 ± 0.2, consistent with fassaite in other solar system CAIs. The oxygen fugacity ( log f O 2 ) of formation estimated from this ratio, assuming equilibration among phases at 1509 K, is −19.4 ± 1.3. This value is near the canonical solar nebula value (−18.1 ± 0.3) and in close agreement with that reported for fassaite-bearing Allende CAIs (−19.8 ± 0.9) by other researchers using the same assumptions. Nanocrystals of osbornite (Ti(V)N), 2–40 nm in diameter, are embedded as inclusions within gehlenite, spinel and diopside in Inti. Vanadium is heterogeneously distributed within some osbornite crystals. Compositions range from pure TiN to Ti0.36V0.64N. The possible presence of oxide and carbide in solid solution with the osbornite was evaluated. The osbornite may contain O, but C is not present at detectable levels. The presence of osbornite, likely a refractory early condensate, together with the other refractory minerals in Inti, indicates that the parent comet contains solids that condensed closer to the proto-sun than the distance at which the parent comet itself accreted. The estimated oxygen fugacity and the reported isotopic and chemical compositions are consistent with Inti originating in the inner solar system like other meteoritic CAIs. These results provide insight for evaluating the validity of models of radial mass transport dynamics in the early solar system. The oxidation environments inferred for the Inti mineral assemblage are inconsistent with an X-wind formation scenario. In contrast, radial mixing models that allow accretion of components from different heliocentric distances can satisfy the observations from the cometary CAI Inti.

Calcium Tschermak’s pyroxene (CaTs), CaAlAlSiO_6, is well known as an important component in pyroxene. It is a member of the Ca clinopyroxene group in which Al dominates in the M1 site. Pyroxenes with more than 80 mol% CaTs were observed previously in Ca-,Al-rich refractory inclusions (CAI) from five carbonaceous chondrites. This study re-investigated the near end-member CaTs in the Allende and Murray chondrites. Electron backscatter diffraction (EBSD) is used to establish that its crystal structure is monoclinic, C2/c; a = 9.609 A, b = 8.652 A, c = 5.274 A, β =106.06°, V = 421.35 A^3, and Z = 4. Its EBSD pattern is an excellent match to that of synthetic CaAlAlSiO_6 with the C2/c structure. MicroRaman is also carried out to confirm the crystal structure. The Allende CaTs, with 46.00 wt% Al_(2)O_3 and 97 mol% Al in the M1 site, has the formula Ca_(1.02)(Al_(0.97)Fe_(0.01)Mg_(0.01))0.99(Si_(1.00)Al_(1.00))_(Σ2.00)O_6. It occurs as micrometer-sized crystals along with melilite, hibonite, perovskite, spinel, corundum, Ti^(3+)-rich pyroxene, and grossular in a fluffy Type A CAI. It is probably a secondary phase resulting from the alteration of gehlenitic melilite. The CaTs in Murray, with a formula Ca_(0.98)(Al_(0.81)Mg_(0.16)Ti_(0.04)^(4+))_(Σ1.01)(Si_(1.11)Al_(0.89))_(Σ2.00)O_6, occurs with hibonite and Al-rich diopside in a glass-free refractory spherule. This sample formed by solidification of a once-molten droplet early in the history of the solar system.

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10−6 to 10−1 bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al2O3 relative to more volatile MgO + SiO2 is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.

We compare the observed composition ranges of olivine, pyroxene, and Fe-Ni sulfides in Wild 2 grains with those from chondritic interplanetary dust particles (IDPs) and chondrite classes to explore whether these data suggest affinities to known hydrous materials in particular. Wild 2 olivine has an extremely wide composition range, from Fa096, with a pronounced frequency peak at Fa1. The composition range displayed by the low-calcium pyroxene is also very extensive, from Fs48 to Fs0, with a significant frequency peak centered at Fs5. These ranges are as broad or broader than those reported for any other extraterrestrial material. Wild 2 Fe-Ni sulfides mainly have compositions close to that of FeS, with less than 2 atom% Ni; to date, only two pentlandite grains have been found among the Wild grains, suggesting that this mineral is not abundant. The complete lack of compositions between FeS and pentlandite (with intermediate solid solution compositions) suggests (but does not require) that FeS and pentlandite condensed as crystalline species, i.e., did not form as amorphous phases, which later became annealed. While we have not yet observed any direct evidence of water-bearing minerals, the presence of Ni-bearing sulfides, and magnesium-dominated olivine and low-Ca pyroxene does not rule out their presence at low abundance. We do conclude that new investigations of major- and minorelement compositions of chondrite matrix and IDPs are required.

Crystallization experiments on liquids with compositions similar to those of compact Type A, Type B1 and Type B2 refractory inclusions were conducted under controlled temperature and fO2 conditions. Application of the results to the compositions of coexisting Ti 3+ -bearing fassaitic clinopyroxene + melilite pairs in natural inclusions shows that, if they crystallized at ~1509 K, they did so at log fO2 = −19.8 ± 0.9, only slightly below the equilibrium log fO2 of a partially condensed system of solar composition at the same temperature, − −

Recent 182Hf–182W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial e182W (−3.47 ± 0.20, 2σ) identical to that of magmatic iron meteorites after correction of cosmogenic 182W burn-out (−3.47 ± 0.35, 2σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred 103, which is lowered by the ubiquitous presence of metal inclusions to 180Hf/184W > 10 in bulk fassaite. Metal alloy (Ni ∼ 50%) is the sole host of W (∼500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has 180Hf/184W 100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the 182Hf–182W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new 182Hf–182W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.

The clinopyroxene in coarse-grained refractory inclusions contains significant amounts of Ti and V, two elements that are multivalent over the range of temperatures and oxygen fugacities under which the inclusions formed. The Ti3+/Ti4+ ratios and the valence of V of these pyroxenes are valuable recorders of nebular conditions. The former can be calculated stoichiometrically from electron probe analyses, but only for relatively Ti-rich grains (i.e., >∼4 wt% TiO 2 tot ). For Ti-poor pyroxene, and for measurement of V valence, another technique is needed. We have, for the first time, applied K-edge X-ray absorption near edge structure (XANES) spectroscopy to the measurement of Ti and V valence in meteoritic clinopyroxene in refractory inclusions. Use of MicroXANES, a microbeam technique with high (μm-scale) spatial resolution, allowed measurement of Ti and V valence along traverses across (a) Ti-, V-rich “spikes” in pyroxene in Type B1 inclusions; (b) typical grains in a Type B2 inclusion; and (c) the pyroxene layer of the Wark–Lovering rim sequence on the outsides of two inclusions. Measurements of Ti3+/(Ti3+ + Ti4+), or Ti3+/Titot, by XANES agree with values calculated from electron probe analyses to within ∼0.1, or ∼2σ. The results show that Ti3+/Titot increases sharply at the spikes, from 0.46 ± 0.03 to 0.86 ± 0.06, but the V valence, or V2+/(V2+ + V3+), does not change, with V2+ ≈ V3+. We found that pyroxene in both Types B1 and B2 inclusions has Ti3+/Titot and V2+/Vtot ratios between 0.4 and 0.7, except for the spikes. These values indicate, to first order, formation at similar, highly reducing oxygen fugacities that are consistent with a solar gas. The pyroxene in the rim on an Allende fluffy Type A coarse-grained refractory inclusion, TS24, has an average Ti3+/Titot of 0.51 ± 0.08 and an average V2+/Vtot of 0.61 ± 0.06, determined by XANES. These values are within the range of those of pyroxene in the interiors of inclusions, indicating that the rims also formed under highly reducing conditions. Measurements of Ti3+/Titot of pyroxene in the rim of a Leoville compact Type A inclusion, 144A, by both XANES and electron probe give a wide range of results. Of our 72 XANES analyses of this rim, 66% have Ti3+/Titot of 0.40–0.71, and the remaining analyses range from 0 to 0.38. In data from Simon et al. [Simon J. I., Young E. D., Russell S. S., Tonui E. K., Dyl K. A., and Manning C. E. (2005) A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al–26Mg dating of CAI rims. Earth Planet. Sci. Lett. 238, 272–283.] for this sample, 7 electron probe analyses yield calculated Ti3+/Titot values that are positive and 15 do not. In the probe analyses that have no calculated Ti3+, Ca contents are anticorrelated and Al contents directly correlated with the total cations per 6 oxygens, and the data fall along trends calculated for addition of 1–7% spinel to pyroxene. It appears likely that electron probe analyses of pure pyroxene spots have Ti3+/Titot values that are typical of refractory inclusions, in agreement with the majority of the XANES results. The average of the XANES data for 144A, 0.41 ± 0.14, is within error of that for TS24. The rim of 144A probably formed under reducing conditions like those expected for a solar gas, and was later heterogeneously altered, resulting in an uneven distribution of secondary, FeO-, Ti-bearing alteration products in the rim, and accounting for the measurements with low Ti3+/Titot values.

We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-hibonite-perovskite; spinel-perovskite-pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO 2 contents (7.8 wt%). The bulk CaO/Al 2 O 3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the “missing melilite” is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with F Mg = 4.3 ± 2.6‰/amu and another with isotopically light Ca ( F Ca = −3.4 ± 2.0‰/amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel than for melilite to form from hibonite.

Most of the petrologic data available for Type B inclusions comes from Type B1s. Relatively little comes from the B2s, and there has not been a systematic comparison of the properties of their two most abundant minerals. In this work, we document the compositions and zoning patterns of melilite and fassaite in Type B2 inclusions, and compare and contrast them with the features of their counterparts in Type B1 inclusions. We find that melilite compositions in Type B2 inclusions are similar to those of Type B1s, with maximum Ak contents of ∼75 mol % and a positive correlation between Ak and Na 2 O contents. Asymmetrically zoned melilite is common in Type B2s as are melilite grains with reversely zoned regions, and the reversely zoned portions of crystals are thicker than in B1s. In B2s, like B1s, fassaite is zoned with decreasing Ti, Sc, and V oxide contents from cores to rims of grains. Approximately half of the Ti is trivalent, but unlike that in B1s, within fassaite grains in B2s the Ti 3+ /(Ti 3+ + Ti 4+ ) ratio does not decrease from core to rim, and sharp enrichments (“spikes”) in Ti 3+ and V are not observed. Sector-zoned fassaite is much more common in B2s than in B1s. The differences we observed can be accounted for by the differences in bulk compositions between B1s and B2s. Type B2 inclusions tend to have higher SiO 2 contents, hence higher An/Ge component ratios, than Type B1s. Phase equilibria show that, compared to B1s, in B2s less melilite should crystallize prior to the appearance of fassaite, so that in B2s a higher proportion of melilite cocrystallizes with fassaite, causing more of the crystals to be reversely zoned; more melilite crystallizes while adjacent to other crystals, leading to asymmetrical zoning; and with more liquid available, transport of components to growing fassaite occurs more readily than in B1s, facilitating crystal growth and giving rise to sector zoning. The lack of zoning with respect to Ti 3+ /Ti tot and the absence of Ti 3+ -, V-rich spikes suggest that Type B2 melts maintained equilibrium with the nebular gas throughout crystallization, while the interiors of B1s were probably isolated from the gas, perhaps by their melilite mantles. This makes the similarity of Na-Ak relationships in B1 and B2 melilite difficult to understand, but apparently enclosure by melilite mantles was not necessary for the retention of Na 2 O during crystallization of Type B refractory inclusions.

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 m 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 (Ak 30-70) inward from the rim of the inclusion with reverse zoning over the last ~200 m to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Ak 5-12) occur. These grains are enclosed in strongly zoned (Ak 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: 29 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 (Ak 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 k40 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 kermanite 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.

Analyses of coarse-grained refractory inclusions typically do not have the solar CaO/Al2O3 ratio, probably reflecting nonrepresentative sampling of them in the laboratory. Many previous studies, especially those done by instrumental neutron activation analysis (INAA), were based on very small amounts of material removed from those restricted portions of inclusions that happened to be exposed on surfaces of bulk meteorite samples. Here, we address the sampling problem by studying thin sections of large inclusions, and by analyzing much larger aliquots of powders of these inclusions by INAA than has typically been done in the past. These results do show convergence toward the solar CaO/Al2O3 ratio of 0.792. The bulk compositions of 15 coarse-grained inclusions determined by INAA of samples >2 mg have an average CaO/Al2O3 ratio of 0.80 ± 0.18. When bulk compositions are obtained by modal recombination based on analysis of thin sections with cross-sections of entire, large, unbroken inclusions, the average of 11 samples (0.79 ± 0.15) also matches the solar value. Among those analyzed by INAA and by modal recombination, there were no inclusions for which both techniques agreed on a CaO/Al2O3 ratio deviating by >∼15% from the solar value. These results suggest that: individual inclusions may have the solar CaO/Al2O3 ratio; departures from this value are due to sample heterogeneity and nonrepresentative sampling in the laboratory; and it is therefore valid to correct compositions to this value. We present a method for doing so by mathematical addition or subtraction of melilite, spinel, or pyroxene. This yields a set of multiple, usually slightly different, corrected compositions for each inclusion. The best estimate of the bulk composition of an inclusion is the average of these corrected compositions, which simultaneously accounts for errors in sampling of all major phases. Results show that Type B2 inclusions tend to be more SiO2-rich and have higher normative Anorthite/Gehlenite component ratios than Type B1s. The inclusion bulk compositions lie in a field that can result from evaporation at 1700–2000K of CMAS liquids with solar CaO/Al2O3, but with a wide range of initial MgO (30–60 wt%) and SiO2 (15–50 wt%) contents.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 m. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, 1480 to 1468 K at Ptot 10 3 bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 CI that likely represent mixing of grains that condensed at various temperatures. Copyright © 2003 Elsevier Ltd

Most studies of Tagish Lake have considered features that were either strongly affected by or formed during the extensive hydrous alteration experienced by this meteorite. This has led to some ambiguity as to whether Tagish Lake should be classified a CI, a CM, or something else. Unlike previous workers, we have focused upon the primary, anhydrous component of Tagish Lake, recovered through freeze-thaw disaggregation and density separation and located by thin section mapping. We found many features in common with CMs that are not observed in CIs. In addition to the presence of chondrules and refractory forsterite (which distinguish Tagish Lake from the CIs), we found hibonite-bearing refractory inclusions, spinel-rich inclusions, forsterite aggregates, Cr-, Al-rich spinel, and accretionary mantles on many clasts, which clearly establishes a strong link between Tagish Lake and the CM chondrites. The compositions of isolated olivine crystals in Tagish Lake are also like those found in CMs. We conclude that the anhydrous inclusion population of Tagish Lake was, originally, very much like that of the known CM chondrites and that the inclusions in Tagish Lake are heavily altered, more so than even those in Mighei, which are more heavily altered than those in Murchison.

Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 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 Ptot ≤ 5 x 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 x CI chondrites, light REE abundances ~20 x 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

Berman’s (1983) activity-composition model for CaO-MgO-Al2O3-SiO2 liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO2, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of PH2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at PH2 = 10−6 bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO2 evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO2 evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO2-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO2 at cooling rates of 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.

Literature data for major element oxide compositions of most coarse-grained Types A and B inclusions in CV3 chondrites may be in error due to non-representative sampling of spinel relative to other phases because of small sample sizes. When reported compositions are corrected to the solar CaO/Al2O3 ratio by addition or subtraction of spinel, distinct trends result on oxide–oxide plots. These trends lie close to trajectories of bulk compositions of equilibrium condensates calculated for solar or dust-enriched gases under various conditions, except on a plot of MgO vs. SiO2 contents, where there is considerable scatter of the data points to the MgO-poor side of the condensation trajectory. The irreversible process of evaporative mass loss from a liquid droplet into an unsaturated H2 gas is modeled as a series of small equilibrium steps. This model is used to show that evolutionary paths of CMAS liquid compositions are identical for evaporation at all PH2 from 1 × 10−15 to 1 bar, with the ratio of the fraction of the SiO2 evaporated to that for MgO increasing both with increasing temperature from 1700 to 2000 K and with increasing SiO2 content of the starting composition. Such calculations show that compositions of most Type B inclusions can be explained by non-equilibrium evaporation of 10 to 30% of the MgO and 0 to 15% of the SiO2 into an H2 gas at 1700 K from liquid droplets whose compositions originated on any one of many possible equilibrium condensation trajectories. Some Type As may have suffered similar evaporative losses of MgO and SiO2 but at higher temperature. This degree of evaporation is consistent with the amount of Mg and Si isotopic mass fractionation observed in Types A and B inclusions. Evaporation probably happened after most Mg and Si were removed from the nebular gas into lower-temperature condensates.

The zoning of Na in the melilite of Type B1 Ca-, Al-rich inclusions from carbonaceous chondrites reflects a combination of factors including the maximum temperature, T_(max), for melting, bulk composition, crystal dimensions and the degree of diffusive relaxation. We used experimentally determined partition coefficients for Na between melilite and liquid together with modes and compositions of phases in meteoritic inclusions to calculate Na zoning produced in melilite by crystallization from Na-bearing liquids. We then used a simple model for the diffusive relaxation of an originally igneous zoning profile to constrain the thermal history. For Allende Type B1 inclusion TS-34, the melilite compositions are consistent with an initial melting event (T_(max) ~1450°C) during which essentially Na-free melilite crystallized, followed by the introduction of Na during a hiatus, a second melting event with T_(max) ~1290°C in which Na-bearing melilite with X^(Mel)_Al ≥ 0.47 grew on relict cores of melilite from the first melting event, and partial diffusive relaxation. For the Type B1 inclusion Vigarano 1623-8, previously published bulk and melilite compositions lead to the conclusion that T_(max) did not exceed ~1280°C during the last major melting event. A survey of literature data for other inclusions suggests that Na zoning in melilite may be a generally useful indicator of thermal history and that Type B1 inclusions were commonly subjected to multiple melting events with intervening periods of alteration. At any given value of X^(Mel)_(Ak) in TS-34, calculated melilite/liquid partition coefficients for Na are lower than values determined experimentally for melilite of similar composition because of liquid composition effects. The difference is more than a factor of two for X^(Mel)_(Ak) > 0.5, and it is likely, based on the activities of major oxides in the melt, that liquid composition effects are also important for other elements. On the other hand, modeling of diffusive relaxation for Na in melilite suggests that the diffusion coefficients are not strongly dependent on either X^(Mel)_(Ak) or Na concentration.