Refractory inclusions in the unique carbonaceous chondrite Acfer 094

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

Formation of an unusual compact Type A refractory inclusion from Allende

Abstract— 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 (Åk_32–62, median Åk₅₁) set in or partially enclosed by, and optically continuous with, relatively Al-rich melilite (Åk_25–53, median Åk₃₈). 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 Åk-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 Åk-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).     

Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite‐free spinel inclusions in CM chondrites

Abstract— Queen Alexandra Range (QUE) 97990 (CM2.6) is among the least‐altered CM chondrites known. It contains 1.8 vol% refractory inclusions; 40 were studied from a single thin section. Inclusion varieties include simple, banded and nodular structures as well as simple and complex distended objects. The inclusions range in mean size from 30 to 530 μm and average 130 ± 90 μm. Many inclusions contain 25 ± 15 vol% phyllosilicate (predominantly Mg‐Fe serpentine); several contain small grains of perovskite. In addition to phyllosilicate, the most abundant inclusions in QUE 97990 consist mainly of spinel‐pyroxene (35%), followed by spinel (20%), spinel‐pyroxene‐olivine (18%), pyroxene (12%), pyroxene‐olivine (8%) and hibonite ± spinel (8%). Four pyroxene phases occur: diopside, Al‐rich diopside (with ≥ 8.0 wt% Al₂O₃), Al‐Ti diopside (i.e., fassaite), and (in two inclusions) enstatite. No inclusions contain melilite. Aqueous alteration of refractory inclusions transforms some phases (particularly melilite) into phyllosilicate; some inclusions broke apart during alteration. Melilite‐free, phyllosilicate‐bearing, spinel inclusions probably formed from pristine, phyllosilicate‐free inclusions containing both melilite and spinel. Sixty‐five percent of the refractory inclusions in QUE 97990 appear to be largely intact; the major exception is the group of spinel inclusions, all of which are fragments. Whereas QUE 97990 contains about 50 largely intact refractory inclusions/cm², estimates from literature data imply that more‐altered CM chondrites have lower modal abundances (and lower number densities) of refractory inclusions: Mighei (CM ? 2.3) contains roughly 0.3–0.6 vol% inclusions (?10 largely intact inclusions/cm²); Cold Bokkeveld (CM2.2) contains ?0.01 vol% inclusions (on the order of 6 largely intact inclusions/cm²).     

Na-bearing Ca-Al-rich inclusions in the Yamato-791717 CO carbonaceous chondrite

Abstract Ca-Al-rich inclusions (CAIs) in the Yamato-791717 CO carbonaceous chondrite contain 5 to 80 vol% of nepheline, along with minor sodalite, and thus are among the most nepheline-rich CAIs known. The primary phases in inclusions are mainly spinel, fassaite, aluminous diopside, perovskite, and hibonite. In contrast to many CO chondrites, melilite is rare. Spinel contains variable amounts of Fe (0 to 57 mol% FeAl₂O₄) and is commonly zoned. Texture suggests that nepheline is a secondary alteration product formed by replacing mainly melilite, fassaite, and spinel; melilite is the most susceptible to alteration of the primary phases, so most of it was probably already consumed to form nepheline. The majority of inclusions are single concentric objects or aggregates of concentric objects. Lightly altered inclusions have cores of spinel surrounded by bands of nepheline (replacing fassaite), fassaite, and diopside. In moderately altered inclusions, spinel cores are replaced by nepheline. In heavily altered inclusions, the major part of internal areas (50 to 80% in volume) are replaced by nepheline. In some moderately and heavily altered inclusions, only diopside rims remain unaltered. Textural relationships indicate that the resistance of primary phases to alteration increases in the order melilite, fassaite, spinel, diopside. The alteration probably proceeded with reaction of the primary phases with the low-temperature (≤ 1000 K) nebular gas rich in Na, Fe and CI. The degree of alteration in Y791717 CAIs appears to be much higher than those in CAIs in other reported meteorites.     

Oxygen isotopic alteration in Ca‐Al‐rich inclusions from Efremovka: Nebular or parent body setting?

Abstract— In situ SIMS oxygen isotope data were collected from a coarse‐grained type B1 Ca‐Al‐rich inclusion (CAI) and an adjacent fine‐grained CAI in the reduced CV3 Efremovka to evaluate the timing of isotopic alteration of these two objects. The coarse‐grained CAI (CGI‐10) is a sub‐spherical object composed of elongate, euhedral, normally‐zoned melilite crystals ranging up to several hundreds of Pm in length, coarse‐grained anorthite and Al, Ti‐diopside (fassaite), all with finegrained (～10 μm across) inclusions of spinel. Similar to many previously examined coarse‐grained CAIs from CV chondrites, spinel and fassaite are ¹⁶O‐rich and melilite is ¹⁶O‐poor, but in contrast to many previous results, anorthite is ¹⁶O‐rich. Isotopic composition does not vary with textural setting in the CAI: analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent ¹⁶O‐poor compositions. CGI‐10 originated in an ¹⁶O‐rich environment, and subsequent alteration resulted in complete isotopic exchange in melilite. The fine‐grained CAI (FGI‐12) also preserves evidence of a 1st‐generation origin in an ¹⁶O‐rich setting but underwent less severe isotopic alteration. FGI‐12 is composed of spinel ± melilite nodules linked by a mass of Al‐diopside and minor forsterite along the CAI rim. All minerals are very fine‐grained (<5 μm) with no apparent igneous textures or zoning. Spinel, Al‐diopside, and forsterite are ¹⁶O‐rich, while melilite is variably depleted in ¹⁶O (δ^17,18O from ～‐40‰ to ?5‰). The contrast in isotopic distributions in CGI‐10 and FGI‐12 is opposite to the pattern that would result from simultaneous alteration: the object with finer‐grained melilite and a greater surface area/ volume has undergone less isotopic exchange than the coarser‐grained object. Thus, the two CAIs were altered in different settings. As the CAIs are adjacent to each other in the meteorite, isotopic exchange in CGI‐10 must have preceded incorporation of this CAI in the Efremovka parent body. This supports a nebular setting for isotopic alteration of the commonly observed ¹⁶O‐poor melilite in coarse‐grained CAIs from CV chondrites.     

Forsterite‐rich accretionary rims around calcium‐aluminum‐rich inclusions from the reduced CV3 chondrite Efremovka

Abstract— It was suggested that multilayered accretionary rims composed of ferrous olivine, andradite, wollastonite, salite‐hedenbergitic pyroxenes, nepheline, and Ni‐rich sulfides around Allende calcium‐aluminum‐rich inclusions (CAIs) are aggregates of gas‐solid condensates which reflect significant fluctuations in physico‐chemical conditions in the slowly cooling solar nebula and grain/gas separation processes. In order to test this model, we studied the mineralogy of accretionary rims around one type A CAI (E104) and one type B CAI (E48) from the reduced CV3 chondrite Efremovka, which is less altered than Allende. In contrast to the Allende accretionary rims, those in Efremovka consist of coarse‐grained (20–40 μm), anhedral forsterite (Fa_1–8), Fe, Ni‐metal nodules, amoeboid olivine aggregates (AOAs) and fine‐grained CAIs composed of Al‐diopside, anorthite, and spinel, ± forsterite. Although the fine‐grained CAIs, AOAs and host CAIs are virtually unaltered, a hibonite‐spinel‐perovskite CAI in the E48 accretionary rim experienced extensive alteration, which resulted in the formation of Fe‐rich, Zn‐bearing spinel, and a Ca, Al, Si‐hydrous mineral. Forsterites in the accretionary rims typically show an aggregational nature and consist of small olivine grains with numerous pores and tiny inclusions of Al‐rich minerals. No evidence for the replacement of forsterite by enstatite was found; no chondrule fragments were identified in the accretionary rims. We infer that accretionary rims in Efremovka are more primitive than those in Allende and formed by aggregation of high‐temperature condensates around host CAIs in the CAI‐forming regions. The rimmed CAIs were removed from these regions prior to condensation of enstatite and alkalies. The absence of andradite, wollastonite, and hedenbergite from the Efremovka rims may indicate that these rims sampled different nebular regions than the Allende rims. Alternatively, the Ca, Fe‐rich silicates rimming Allende CAIs may have resulted from late‐stage metasomatic alteration, under oxidizing conditions, of original Efremovka‐like accretionary rims. The observed differences in O‐isotope composition between forsterite and Ca, Fe‐rich minerals in the Allende accretionary rims (Hiyagon, 1998) suggest that the oxidizing fluid had an ¹⁶O‐poor oxygen isotopic composition.     

Ca,Al‐rich inclusions in Rumuruti (R) chondrites

Abstract— Rumuruti chondrites (R chondrites) constitute a well‐characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al‐rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al‐rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al‐rich objects (101 CAIs, 19 Al‐rich chondrules, and 6 spinel‐rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel‐rich inclusions (42), (2) fassaite‐rich spherules, (3) complex spinel‐rich CAIs (53), (4) complex diopside‐rich inclusions, (5) Al‐rich chondrules, and (6) Al‐rich (spinel‐rich) fragments. The simple concentric and complex spinel‐rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al‐(Cl)‐rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R‐chondrite group. Some Ca,Al‐rich inclusions appear to be primitive (e.g., low FeO‐contents in spinel, low abundances of Na,Al‐(Cl)‐rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al‐(Cl)‐rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R‐chondrite lithologies have abundant secondary Ab‐rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.     

XANES and Mg isotopic analyses of spinels in Ca‐Al‐rich inclusions: Evidence for formation under oxidizing conditions

Ti valence measurements in MgAl₂O₄ 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 Δ²⁵Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ²⁵Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ²⁵Mg data are most simply explained by the low‐pressure evaporation model, but this model has difficulty explaining the high Ti⁺⁴ concentrations in spinel.     

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

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

Refractory calcium‐aluminum‐rich inclusions and aluminum‐diopside‐rich chondrules in the metal‐rich chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94411

Abstract— The metal‐rich chondrites Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627, contain relatively rare (<1 vol%) calcium‐aluminum‐rich inclusions (CAIs) and Al‐diopside‐rich chondrules. Forty CAIs and CAI fragments and seven Al‐diopside‐rich chondrules were identified in HH 237 and QUE 94411/94627. The CAIs, ～50–400 μm in apparent diameter, include (a) 22 (56%) pyroxene‐spinel ± melilite (+forsterite rim), (b) 11 (28%) forsterite‐bearing, pyroxene‐spinel ± melilite ± anorthite (+forsterite rim) (c) 2 (5%) grossite‐rich (+spinel‐melilite‐pyroxene rim), (d) 2 (5%) hibonite‐melilite (+spinel‐pyroxene ± forsterite rim), (e) 1 (2%) hibonite‐bearing, spinel‐perovskite (+melilite‐pyroxene rim), (f) 1 (2%) spinel‐melilite‐pyroxene‐anorthite, and (g) 1 (2%) amoeboid olivine aggregate. Each type of CAI is known to exist in other chondrite groups, but the high abundance of pyroxene‐spinel ± melilite CAIs with igneous textures and surrounded by a forsterite rim are unique features of HH 237 and QUE 94411/94627. Additionally, oxygen isotopes consistently show relatively heavy compositions with Δ¹⁷O ranging from ?6%0 to ?10%0 (1σ = 1.3%0) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This suggests that the CAIs formed in a reservoir isotopically distinct from the reservoir(s) where “normal”, ¹⁶O‐rich (Δ¹⁷O < ?20%0) CAIs in most other chondritic meteorites formed. The Al‐diopside‐rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain Al‐diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and Al‐rich, Ca‐poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the Al‐rich barred‐olivine chondrules in HH 237 and QUE 94411/94627, but have lower Cr concentrations than the latter, indicating that they may have formed during the same chondrule‐forming event, but at slightly different ambient nebular temperatures. Aluminum‐diopside grains from two Al‐diopside‐rich chondrules have O‐isotopic compositions (Δ¹⁷O ? ?7 ± 1.1 %0) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen‐isotopic composition of one Ca, Al‐poor cryptocrystalline chondrule in QUE 94411/94627 was analyzed and found to have Δ¹⁷O ? ?3 ± 1.4%0. The characteristics of the CAIs in HH 237 and QUE 94411/94627 are inconsistent with an impact origin of these metal‐rich meteorites. Instead they suggest that the components in CB chondrites are pristine products of large‐scale, high‐temperature processes in the solar nebula and should be considered bona fide chondrites.     

Transmission electron microscope study of compact Type A calcium-aluminum-rich inclusions from CV3 chondrites: Clues to their origin

Abstract— A transmission electron microscope (TEM) study of three coarse-grained Type A Ca, Al-rich inclusions (CAIs) from Allende, Acfer 082 and Acfer 086 (all CV3 chondrites) was performed in order to decipher their origin and effects of possible metamorphism. The constituent minerals of the CAIs are found to exhibit very similar microstructural characteristics in each of the inclusions studied. In general, the minerals show a well-developed equilibrium texture with typical 120° triple junctions. Melilites are clearly considerably strained and characterized by high dislocation densities up to 3 × 10¹¹ cm^?2. The dislocations have Burgers vectors of [001], [110] or [011] and often form subgrain boundaries subparallel {100}. Melilite in the Allende CAI additionally contains thin amorphous lamellae mostly oriented parallel to {001}. Fassaite (Al-Ti-diopside) is almost featureless even on the TEM scale. Only a few subplanar dislocation walls composed of dislocations with Burgers vectors [001] and 1/2 [110] were detected. Although enclosed within the highly strained melilites, the euhedral spinels contain only low dislocation densities (<2 × 10⁴ cm^?2). In the Allende CAI, spinels were found twinned on {111}. Perovskite is also characterized by a low number of linear lattice defects. All grains possess orthorhombic symmetry and are commonly twinned according to a 90° rotation around [101]. Many crystals exhibit typical domain structures as well as curved twin walls where two orthogonal sets intersect. In addition to the mineral phases described above, tiny inclusions of the simple oxides CaO and TiO₂ were found within melilite (CaO), spinel (CaO, TiO₂) and perovskite (CaO, TiO₂). Based on these observations, it is assumed that at the beginning of the formation of the CAIs a condensed solid precursor was present. Euhedral spinels poikilitically enclosed within melilites suggest that this solid aggregate was then molten. If the pure oxides represent relict condensates, their presence proves that this melting was incomplete. While still plastic, the CAIs were shocked by microimpacts causing the high dislocation densities in melilite as well as diaplectic melilite glass and twinned spinels in the Allende CAI. In Acfer 082 and 086, the deformation took place at elevated temperatures, preventing the solid phase transition and mechanical twinning. The absence of linear lattice defects in spinel, fassaite and perovskite most probably reflects inhomogeneous pressure distribution in the polycrystalline CAI as well as the different strengths of the minerals. According to cooling-rate experiments on perovskite by Keller and Buseck (1994), the dominating (101) twins in the CAI perovskites point to cooling rates ≤50 °C/min. Finally, after crystallization of the CAI was complete, mild thermal metamorphism caused the formation of subgrain boundaries, 120° triple junctions and chemical homogenization of the melilites.     

The petrogenesis of type B1 Ca‐Al‐rich inclusions: The spinel perspective

Abstract— Minor element variations in MgAl₂O₄ spinel from the type B1 calcium‐aluminum‐rich inclusion (CAI) Allende TS‐34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases. These correlations result from a combination of crystallization of a liquid produced by re‐melting event(s) and local re‐equilibration during subsolidus reheating. The correlation of the Ti and V in spinel inclusions with the Ti and V in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite, following a partial melting event. There are, however, difficulties in quantitative modeling of the observed trends, and it is easier to explain the Ti correlation in terms of complete re‐equilibration. The correlation of V in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re‐melting, but it can be explained by partial re‐equilibration. The distinct V and Ti concentrations in spinel inclusions in melilite from the edge regions of the CAI are best explained as being affected by only a minor degree of re‐equilibration. The center melilites and included spinels formed during crystallization of the liquid produced by re‐melting, while the edge melilites and included spinels are primary. The oxygen isotope compositions of TS‐34 spinels are uniformly ¹⁶O‐rich, regardless of the host silicate phase or its location within the inclusion. Similar to other type B1 CAIs, clinopyroxene is ¹⁶O‐rich, but melilite is relatively ¹⁶O‐poor. These data require that the oxygen isotope exchange in TS‐34 melilite occurred subsequent to the last re‐melting event.     

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

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

Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpine Hills 87300 and 88107

Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk_4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess ²⁶Mg, presumably from the in situ decay of ²⁶Al. Although well‐defined isochrons are not observed, the ²⁶Mg excesses are consistent with initial ²⁶Al/²⁷Al ratios of approximately 3–5 times 10^?5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of ²⁵Mg/²⁴Mg. The two crystals also show differences in their inferred initial ²⁶Al/²⁷Al ratios, 1 × 10^?5 vs. ≤3 × 10^?6.     

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

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

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

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

Fine‐grained,spinel‐rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions

Abstract— Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al‐Ti‐diopside, melilite, and anorthite) within four relatively unaltered, fine‐grained, spinel‐rich Ca‐Al‐rich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly ¹⁶O‐rich (Δ¹⁷O ≤ ?20‰) in all four CAIs; Al‐Ti‐diopside is similarly ¹⁶O‐rich in all but one CAI, where it has smaller ¹⁶O excesses (‐15‰ ≤ Δ¹⁷O ≤ ?10‰). Anorthite and melilite vary widely in composition from ¹⁶O‐rich to ¹⁶O‐poor (‐22‰ ≤ Δ¹⁷O ≤ ?5‰). Two of the CAIs are known to have group II volatility‐fractionated rare‐earth‐element patterns, which is typical of this variety of CAI and which suggests formation by condensation. The association of such trace element patterns with ¹⁶O‐enrichment in these CAIs suggests that they formed by gas‐solid condensation from an ¹⁶O‐rich gas. They subsequently experienced thermal processing in an ¹⁶O‐poor reservoir, resulting in partial oxygen isotope exchange. Within each inclusion, oxygen isotope variations from mineral to mineral are consistent with solid‐state oxygen self‐diffusion at the grain‐to‐grain scale, but such a model is not consistent with isotopic variations at a larger scale in two of the CAIs. The spatial association of ¹⁶O depletions with both elevated Fe contents in spinel and the presence of nepheline suggests that late‐stage iron‐alkali metasomatism played some role in modifying the isotopic patterns in some CAIs. One of the CAIs is a compound object consisting of a coarse‐grained, melilite‐rich (type A) lithology joined to a fine‐grained, spinel‐rich one. Melilite and anorthite in the fine‐grained portion are mainly ¹⁶O‐rich, whereas melilite in the type A portion ranges from ¹⁶O‐rich to ¹⁶O‐poor, suggesting that oxygen isotope exchange predated the joining together of the two parts and that both ¹⁶O‐rich and ¹⁶O‐poor gaseous reservoirs existed simultaneously in the early solar nebula.     

Compound ultrarefractory CAI‐bearing inclusions from CV3 carbonaceous chondrites

Two compound calcium‐aluminum‐rich inclusions (CAIs), 3N from the oxidized CV chondrite Northwest Africa (NWA) 3118 and 33E from the reduced CV chondrite Efremovka, contain ultrarefractory (UR) inclusions. 3N is a forsterite‐bearing type B (FoB) CAI that encloses UR inclusion 3N‐24 composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, and Zr,Sc‐rich Al,Ti‐diopside. 33E contains a fluffy type A (FTA) CAI and UR CAI 33E‐1, surrounded by Wark‐Lovering rim layers of spinel, Al‐diopside, and forsterite, and a common forsterite‐rich accretionary rim. 33E‐1 is composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, Zr,Sc,Y‐rich pyroxenes (Al,Ti‐diopside, Sc‐rich pyroxene), and gehlenite. 3N‐24’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are ¹⁶O‐poor (Δ¹⁷O approximately ?2‰ to ?5‰). Spinel in 3N‐24 and spinel and Al‐diopside in the FoB CAI are ¹⁶O‐rich (Δ¹⁷O approximately ?23 ± 2‰). 33E‐1’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are ¹⁶O‐depleted (Δ¹⁷O approximately ?2‰ to ?5‰) vs. Al,Ti‐diopside of the FTA CAI and spinel (Δ¹⁷O approximately ?23 ± 2‰), and Wark‐Lovering rim Al,Ti‐diopside (Δ¹⁷O approximately ?7‰ to ?19‰). We infer that the inclusions experienced multistage formation in nebular regions with different oxygen‐isotope compositions. 3N‐24 and 33E‐1’s precursors formed by evaporation/condensation above 1600 °C. 3N and 33E’s precursors formed by condensation and melting (3N only) at significantly lower temperatures. 3N‐24 and 3N’s precursors aggregated into a compound object and experienced partial melting and thermal annealing. 33E‐1 and 33E avoided melting prior to and after aggregation. They acquired Wark‐Lovering and common forsterite‐rich accretionary rims, probably by condensation, followed by thermal annealing. We suggest 3N‐24 and 33E‐1 originated in a ¹⁶O‐rich gaseous reservoir and subsequently experienced isotope exchange in a ¹⁶O‐poor gaseous reservoir. Mechanism and timing of oxygen‐isotope exchange remain unclear.     

Ca‐Al‐Fe‐rich inclusion in the Vigarano CV3 chondrite

Abstract– An anomalous Ca‐Al‐Fe‐rich spherical inclusion (CAFI) was found in the Vigarano CV3 chondrite. The CAFI has an igneous texture and contains large amounts of almost pure and coarse‐grained hercynite grains (approximately 56 vol%) as well as refractory phases such as grossite and perovskite. However, melilite and Mg‐spinel, which are common in ordinary Ca‐Al‐rich inclusions, are very rare (<1 vol%). Another unique characteristic of the CAFI is the presence in its core of dmitryivanovite (CaAl₂O₄), which was formed by shock metamorphism of a low‐pressure form of CaAl₂O₄ that was originally crystallized from a molten droplet. The fine‐grained hercynite and unidentified aluminous phase in the rim of the CAFI may have been produced from grossite during aqueous alteration in the Vigarano parent body.     

Hydrothermal origin of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV3 chondrite

We report an occurrence of hexagonal CaAl₂Si₂O₈ (dmisteinbergite) in a compact type A calcium‐aluminum‐rich inclusion (CAI) from the CV3 (Vigarano‐like) carbonaceous chondrite Northwest Africa 2086. Dmisteinbergite occurs as approximately 10 μm long and few micrometer‐thick lath‐shaped crystal aggregates in altered parts of the CAI, and is associated with secondary nepheline, sodalite, Ti‐poor Al‐diopside, grossular, and Fe‐rich spinel. Spinel is the only primary CAI mineral that retained its original O‐isotope composition (Δ¹⁷O ~ ?24‰); Δ¹⁷O values of melilite, perovskite, and Al,Ti‐diopside range from ?3 to ?11‰, suggesting postcrystallization isotope exchange. Dmisteinbergite, anorthite, Ti‐poor Al‐diopside, and ferroan olivine have ¹⁶O‐poor compositions (Δ¹⁷O ~ ?3‰). We infer that dmisteinbergite, together with the other secondary minerals, formed by replacement of melilite as a result of fluid‐assisted thermal metamorphism experienced by the CV chondrite parent asteroid. Based on the textural appearance of dmisteinbergite in NWA 2086 and petrographic observations of altered CAIs from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAIs from the oxidized Allende‐like CV3 chondrites that has been previously misidentified as a secondary anorthite.