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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.

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.

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.

– 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.

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.

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.

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

— In addition to the Mg-, Al-, 16O-rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr-rich, 16O-poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr-rich spinels, found in 13 chondrules and 4 “olivine-rich objects”. The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr2O3 contents like those of the Cr-spinels from Murchison. Some chondrules contain patchy-zoned spinel (Simon et al., 1994), which suggests that such grains did not form by sintering but perhaps by formation of overgrowths on relic grains. Unlike the olivine-rich objects, phases in all three chondrules that were analyzed by ion microprobe have uniform, near-normal O-isotopic compositions. One olivine-rich object, ALSP1, has a huge (1 mm) fragment of chevron-zoned spinel. This spinel has near-normal O-isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively 16O-rich and plot on the calcium-aluminum-inclusion (CAI) line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another olivine-rich object, ALSP11A, contains a rectangular, 150 ×s 100 μm, homogeneous spinel grain with 50 wt% Cr2O3 and 23 wt% FeO in a vuggy aggregate of finer-grained (5–90 μm), FeO-rich (Fo47–55) olivine. The magnesian core of one olivine grain has a somewhat 16O-rich isotopic composition like that of the large spinel, whereas the FeO-rich olivine is relatively 16O-poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr-rich spinel found to do so. Chevron-zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780 K at a Ptot of 10−3 atm and a dust/gas ratio of 100 relative to solar. The Cr-rich spinel in ALSP11A could condense at ∼1420 K, but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse Cr-rich spinels in Allende are relatively 16O-depleted and are isotopically distinct from the 16O-enriched MgAl2O4 from CAIs. Sample ALSP11A may represent a third population, one that is Cr-rich and plots on the CAI line. That the O-isotopic composition of ALSP1 is like those of Cr-rich spinels from chondrules indicates that O-isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.

— 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% TiOtot2) 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 TiOtot2, 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.

— We report the results of a study of TS2, an unusual compact Type A inclusion from Allende. A distinctive, major feature of this inclusion is that many of its melilite crystals have no dominant core-rim zoning but instead consist of 50–200 μm patches of Mg-rich melilite (Ak32–62, median Ak51) set in or partially enclosed by, and optically continuous with, relatively Al-rich melilite (Ak25–53, median Ak38). The Al-rich regions have jagged, dendritic shapes but occur within crystals having straight grain boundaries. Another unusual feature of this inclusion is the size and spatial distribution of spinel. In many places, especially in the interior of the inclusion, the aluminous melilite encloses numerous, fine (0.5–5 μm) inclusions of spinel and minor perovskite and fassaite. The latter phases also occur as isolated grains throughout the inclusion. Coarse-grained spinel, ∼50–150 μm across, occurs in clumps and chains enclosed in relatively Mg-rich melilite, whereas none of the fine spinel grains are clumped together. The sample also contains a spinel-free palisade body, 1.7 × 0.85 mm, that consists almost entirely of Ak-rich (45–65 mol%) melilite. Within the palisade body are two grains of perovskite with extremely Nb-rich (∼4–8 wt% Nb2O5) cores and rims of typical composition. All phases in this inclusion have chondrite-normalized REE patterns that are consistent with crystal/melt partitioning superimposed upon a bulk modified Group II pattern. We suggest that TS2 had an anomalous cooling history and favor the following model for the formation of TS2. Precursors having a bulk modified Group II pattern melted. Rapid growth of large, dendritic, nonstoichiometric melilite crystals occurred. The melilite trapped pockets of melt and incorporated excess spinel components and TiO2. Bubbles formed in the residual melt. As crystallization slowed, coarse spinel grew. Some spinel grains collected against bubbles, forming spherical shells, and others formed clumps and chains. Relatively Ak-rich melilite crystallized from the residual melt between dendritic melilite crystals and from melt trapped in pockets and between arms of dendrites, and incorporated the clumps and chains of coarse spinel. Bubbles broke and filled with late-stage melt, their shapes preserved by their spinel shells. Slow cooling, or perhaps an episode of reheating, allowed the early melilite to become stoichiometric by exsolving fine grains of spinel, perovskite and fassaite, and allowed the melilite to form smooth grain boundaries. Dendritic crystals are indicative of rapid growth and the melilite crystals in TS2 appear to be dendritic. Coarse, dendritic melilite crystals have been grown from Type B inclusion melts cooled at ∼50–100 °C/h. If those results are applicable to Type A inclusions, we can make the first estimate of the cooling rate of a Type A inclusion, and it is outside the range (2–50 °C/h) generally inferred for Type B inclusions. The rapid cooling inferred here may be part of an anomalous thermal history for TS2, or it may be representative of part of a normal thermal history common to Types A and B that involved rapid cooling early (at high temperatures) as inferred for TS2, and slower cooling later (at lower temperatures), as inferred for Type B inclusions. We prefer the former explanation; otherwise, the unusual features of TS2 that are reported here would be common in Type A inclusions (which they are not).

— 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

— It has been suggested that palisade bodies—shells of spinel found within some calcium, aluminum-rich inclusions (CAIs) and the phases the shells enclose—are intact mini-CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies formed instead in situ while their host inclusions were crystallizing. The evidence includes observations of spinel-lined cavities and glass-filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: (1) formation of vesicles in a largely molten inclusion; (2) nucleation of spinel upon and/or adherence to vapor-melt interfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to one proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (i.e., dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near-polar sections through palisade bodies and therefore do not make a genetic distinction between the two features.

— Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered a refractory inclusion, HIB-11, that is unique in terms of its texture, mineral compositions, and bulk composition. It consists of anhedral, Y-rich (1.6 wt% Y2O3) perovskite and lathlike spinel grains enclosed in a matrix of fine-grained, Sc-rich (10.5 wt% SC2O3 avg.), Ti-rich (12.6 wt% TiO2 avg., reporting all Ti as TiO2) clinopyroxene. The chondrite-normalized rare earth element (REE) pattern is complex, with light REE (LREE) at ∼10× C1, abundances increasing from Gd through Ho (the latter at ∼104× C1), decreasing through Yb at 200× C1, and Lu at ∼400× C1. The pattern reflects several stages of high-temperature volatility fractionation. Removal of Lu and Er from the source gas in the first condensation event was followed by partial to complete removal of the somewhat less refractory heavy REE, Gd through Ho, in the HIB-11 precursors by condensation from the fractionated residual gas in a second event. Both of these events probably reflect condensation of REE into ZrO2 or a mixed Zr-, Sc-, Ti-, Y-oxide at temperatures too high for hibonite stability. A second, lower-temperature component, which was subsequently added, had fractionated (Nd-poor, Ce-rich) LREE abundances that resulted from condensation from a gas that had undergone prior removal of the more refractory LREE, resulting in enrichment in Ce and the most volatile REE, Eu and Yb. The aggregate was then melted and quickly cooled, forming a fine-grained spherule. This is the first reported inclusion in which the two most refractory REE, Lu and Er, are strongly fractionated from the other REE. An absence of mass fractionation among the Ti isotopes indicates that HIB-11 is not an evaporative residue, implying that volatility fractionation of trace elements took place during condensation. The fact that the two most refractory heavy REE could be separated from the other, only slightly less refractory heavy REE suggests that a wide variety of REE patterns is possible, and that ultrarefractory inclusions with other unusual REE patterns, important recorders of nebular condensation, may yet be discovered.

We describe a previously unreported meteorite found in Axtell, Texas, in 1943. Based on the mineralogical composition and texture of its matrix and the sizes and abundance of chondrules, we classify it as a CV3 carbonaceous chondrite. The dominant opaque phase in the chondrules is magnetite, and that in refractory inclusions is Ni-rich metal (awaruite). Axtell, therefore, belongs to the oxidized subgroup of CV3 chondrites, although unlike Allende it escaped strong sulfidation. The meteorite bears a strong textural resemblance to Allende, and its chondrule population and matrix appear to be quite similar to those of Allende, but its refractory inclusions, thermoluminescence properties, and cosmogenic Co-60 abundances are not. Our data are consistent with a terrestrial age for Axtell of approximately 100 years and a metamorphic grade slightly lower than that of Allende.

A newly found polymict eucrite, EETA79006, is described. Lithic clasts are similar to those found in howardites and fall into four groups: fine-grained (aphanitic), coarse-grained, basaltic, and cataclastic. All have eucritic compositions and differ mainly in cooling and deformation histories. Some basaltic clasts cooled faster than others and may be impact melts. Analysis of pyroxene and feldspar in the matrix and in 20 lithic clasts indicates that the matrix was not derived from the observed lithic clast population. This meteorite and similar polymict eucrites may have formed by addition of younger more fractionated lithic clasts to the regolith of the parent body.

A total of seven lithic clasts from the polymict eucrites ALHA76005 and ALHA77302 have been studied petrographically and analyzed with the electron microprobe. All clasts are composed predominantly of pyroxene and plagioclase, + or - ilmenite, troilite, Fe-Ni metal, mesostasis, and silica. Pyroxene compositions in unequilibrated clasts and clast bulk compositions, calculated by modal recombination, indicate that the clasts originally crystallized under similar conditions and that they may be genetically related to each other by fractionation of pigeonite and plagioclase.