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.     

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.     

Conditions in the protoplanetary disk as seen by the type B CAIs

Abstract— Type B coarse‐grained calcium‐aluminum‐rich inclusions (CAIs) are the oldest known materials to have formed in the solar system and are a unique source of information regarding conditions and processes in the protoplanetary disk around the young sun. Recent experimental results on the crystallization and evaporation of type B‐like silicate melts allow us to place the following constraints on the conditions in the protoplanetary disk during the formation of type B CAIs. 1) Once type B CAIs precursors have been condensed from a solar composition gas, they were reheated at 1250–1450 °C, as is indicated by their igneous texture. 2) The melilite mantles characteristic of type B1 CAIs could be formed by crystallization of magnesium‐ and silicon‐depleted melt in the outer part of the partially molten droplets. Such depletion can arise when evaporation is fast compared to chemical diffusion in the melt. This requires the pressure of the surrounding solar composition gas to be at least 10⁻⁴ bars during the initial crystallization of melilite mantle. Type B2 CAIs with uniform distribution of melilite are expected to form at pressures less than 10⁻⁵ bars. 3) Evaporation calculations are used to place bounds on the thermal history of the type B CAIs. Observed compositional zoning in melilite suggests that the temperatures in the protoplanetary disk where the type B CAIs resided after crystallization could not have exceeded ˜1000 °C for more than a few tens of thousands of years. A recent calculation of the physical conditions associated with nebular shocks produced transient temperatures and gas pressures very much like what we find is required to melt reasonable CAI precursors and evaporate these sufficiently quickly to make a type B1 CAI.     

Metamorphosed calcium‐aluminum‐rich inclusions in CK carbonaceous chondrites

CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium‐aluminum‐rich inclusion (CAI)‐poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine‐grained CAIs recrystallized as irregular assemblages of plagioclase + Ca‐rich pyroxene ± olivine ± Ca‐poor pyroxene ± magnetite. Coarse‐grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca‐rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al‐Mg isotopic systematics measured in fine‐ and coarse‐grained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al‐Mg isotopic signatures obtained in the core of a coarse‐grained CAI (CG1‐CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.     

Calcium‐aluminum‐rich inclusions in enstatite chondrites (II): Oxygen isotopes

Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield ¹⁶O‐rich compositions (δ ¹⁸O and δ ¹⁷O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ ¹⁸O and δ ¹⁷O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with ¹⁶O‐rich and ¹⁶O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.     

Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of ²⁶Mg, whereas the primary CAI phases mostly yield correlated excesses of ²⁶Mg with increasing Al/Mg corresponding to “canonical” initial ²⁶Al/²⁷Al ～ 4.5–5 × 10^?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in ²⁶Mg/²⁴Mg with increasing Al/Mg, yielding (²⁶Al/²⁷Al)0 = (4.9 ± 2.8) × 10^?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of ²⁶Mg consistent with initial ²⁶Al/²⁷Al ～ 4.5 × 10^?5. Especially in the compact type A CAI, where ²⁶Mg/²⁴Mg in grossular correlates with increasing Al/Mg, these ²⁶Mg excesses are almost certainly due to in situ decay of ²⁶Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.     

Calcium‐aluminum‐rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

Abstract— Calcium‐aluminum‐rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 μm) and irregularly‐shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite ± hibonite‐rich, melilite‐rich, and pyroxene‐anorthite‐rich CAIs. Other types of refractory objects include fine‐grained spinel‐melilite‐pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene‐anorthite‐rich CAIs have igneous textures, and most melilite‐rich CAIs share similarities to both the fluffy and compact type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare. In situ ion microprobe analyses of oxygen‐isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are ¹⁶O‐rich (δ¹⁷O of hibonite, melilite, spinel, pyroxene, and anorthite < ?22‰) and isotopically homogeneous within 3–4‰. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, ¹⁶O‐rich compositions (?24‰ < δ¹⁷O < ?20‰). In contrast, objects which show petrographic evidence for extensive melting are not as ¹⁶O‐rich (δ¹⁷O less than ?18‰). Secondary alteration minerals replacing ¹⁶O‐rich melilite in melilite‐rich CAIs plot along the terrestrial fractionation line. Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous ¹⁶O‐rich compositions. It is likely that these objects (or their precursors) condensed in an ¹⁶O‐rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in ¹⁶O, probably as a result of their having melted in the presence of a relatively ¹⁶O‐poor nebular gas. If the precursors of these CAIs were as ¹⁶O‐rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI‐forming regions and/or radial transport of some CAI precursors into an ¹⁶O‐poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite‐rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen‐isotopic compositions of their CAIs.     

26Al‐26Mg isotope systematics of the first solids in the early solar system

High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk ²⁶Al‐²⁶Mg isochron with an inferred initial ²⁶Al/²⁷Al ratio of approximately 5.25 × 10^?5, named the canonical ratio. Nonigneous CV CAIs yield well‐defined internal ²⁶Al‐²⁶Mg isochrons consistent with the canonical value. These observations indicate that the canonical ²⁶Al/²⁷Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI‐forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial ²⁶Al/²⁷Al ratios, (4.2–5.2) × 10^?5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial ²⁶Al/²⁷Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in ²⁶Al. At least some of these refractory objects may have formed prior to injection of ²⁶Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of ²⁶Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial ²⁶Al/²⁷Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical ²⁶Al/²⁷Al ratio.     

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.     

A petrographic,chemical, and isotopic study of calcium‐aluminum‐rich inclusions and aluminum‐rich chondrules from the Axtell (CV3) chondrite

Abstract— Petrographic, compositional, and isotopic characteristics were studied for three calcium‐aluminum‐rich inclusions (CAIs) and four plagioclase‐bearing chondrules (three of them Al‐rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial ²⁶Al/²⁷Al ratios ((²⁶Al/²⁷Al)₀) ranging from ～5 × 10^?5 to <1.1 × 10^?5, and plagioclase‐bearing chondrules have (²⁶Al/²⁷Al)₀ ratios of ～3 × 10^?6 and lower. One type‐A CAI has the characteristics of a FUN inclusion. The Al‐Mg data imply that the plagioclase‐bearing chondrules began to form >2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al‐Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase‐bearing chondrules (two with large anorthite laths and two barred‐olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH‐1471, but neither process explains the other compositions. Our study indicates that plagioclase‐bearing objects originated through a variety of processes.     

Refractory materials in comet samples

Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium‐aluminum‐rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al‐rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine‐grained, spinel‐rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.     

26Al‐26Mg systematics of Ca‐Al‐rich inclusions,amoeboid olivine aggregates,and chondrules from the ungrouped carbonaceous chondrite Acfer 094

Abstract— We report in situ magnesium isotope measurements of 7 porphyritic magnesium‐rich (type I) chondrules, 1 aluminum‐rich chondrule, and 16 refractory inclusions (14 Ca‐Al‐rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic ²⁶Mg excesses corresponding to initial ²⁶Al/²⁷Al ratios between ～5 × 10^?5 ～7 × 10^?5 suggesting that formation of the Acfer 094 CAIs may have lasted for ～300,000 years. Four CAIs show no evidence for radiogenic ²⁶Mg; three of these inclusions (a corundum‐rich, a grossite‐rich, and a pyroxene‐hibonite spherule CAI) are very refractory objects and show deficits in ²⁶Mg, suggesting that they probably never contained ²⁶Al. The fourth object without evidence for radiogenic ²⁶Mg is an anorthite‐rich, igneous (type C) CAI that could have experienced late‐stage melting that reset its Al‐Mg systematics. Significant excesses in ²⁶Mg were observed in two chondrules. The inferred ²⁶Al/²⁷Al ratios in these two chondrules are (10.3 ± 7.4) × 10^?6 (6.0 ± 3.8) × 10^?6 (errors are 2σ), suggesting formation 1.6^+1.2_‐0.6 and 2.2^+0.4_‐0.3 Myr after CAIs with the canonical ²⁶Al/²⁷Al ratio of 5 × 10^?5. These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0–LL3.1) and carbonaceous (CO3.0) chondrites.     

Fassaites in compact type A Ca‐Al‐rich inclusions in the Ningqiang carbonaceous chondrite: Evidence for partial melting in the nebula

Abstract— Fassaite is a major component of Ca‐Al‐rich inclusions (CAIs) of Types B and C that crystallized from liquids. In contrast, this mineral is rarely reported in Type A inclusions and has been much less studied. In this paper, we report highly Ti‐, Al‐enriched fassaite that occurs as rims on perovskite in two compact Type A inclusions from the Ningqiang meteorite. In addition, one of the inclusions contains an euhedral grain of Sc‐fassaite (16.4 wt% Sc₂O₃) isolated in melilite. The occurrence and mineral chemistry of the fassaite rims can be explained by a reaction of pre‐existing perovskite with CAI melts. Hence, such rims may serve as an indicator for partial melting of Type A inclusions. The Sc‐fassaite is probably a relict grain. A third spherical CAI contains several euhedral grains of V‐fassaite (4.8–5.4 wt% V₂O₃) enclosed in a melilite fragment. The high V content of fassaite cannot be related to any Fremdlinge, magnetite, or metallic Fe‐Ni, because these phases are absent in the inclusion. In the same CAI, other fassaites intergrow with spinel and minor perovskite, filling voids inside of the melilite and space adjacent to the Wark‐Lovering rim. The fassaite intergrown with spinel is almost V‐free. The coexistence of two types of fassaite suggests that this CAI has not been completely melted.     

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.     

Aluminum‐26 in H4 chondrites: Implications for its production and its usefulness as a fine‐scale chronometer for early solar system events

Abstract— In order to investigate whether or not ²⁶Al can be used as a fine‐scale chronometer for early solar system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste Marguerite (SM), Forest Vale (FV), Beaver Creek and Quenggouk and compared the results with the canonical ²⁶Al/²⁷Al ratio for calcium‐aluminum‐rich inclusions (CAIs). For SM and FV, Pb/Pb and Mn‐Cr ages have previously been determined (Göpel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of ²⁶Mg. The ²⁶Al/²⁷Al ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 ± 0.64) × 10^?7 for SM and (1.52 ± 0.52) × 10^?7 for FV. The differences between these ratios and the ratio of 5 times 10^?5 in CAIs indicate time differences of 5.4 ± 0.1 Ma and 6.1 ± 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates but there are large discrepancies between the U‐Pb and Mn‐Cr system for the relative ages for CAIs, SM and FV. For example, Mn‐Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this “adjusted” CAI age and the Mn‐Cr ages of SM and FV require that ²⁶ Al was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X‐wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough ²⁶Al to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that ²⁶Al can serve as a fine‐scale chronometer for early solar system events.     

Synthesis of refractory metal nuggets and constraints on the thermal histories of nugget‐bearing Ca,Al‐rich inclusions

Tiny refractory metal nuggets are mainly observed inside Ca, Al‐rich inclusions (CAIs) from chondritic meteorites and are commonly assumed to be condensates from a solar composition gas. However, recent detailed studies of metal nugget compositions and their comparison with predictions from condensation show that the observed abundance patterns are extremely difficult to achieve in this way. As a test for the proposed alternative, precipitation from a silicate liquid, we conducted melting experiments, in which nine different refractory metals (nugget components) were equilibrated with each other along with a CAI‐like liquid at reducing conditions. When quenched, minerals similar to those in CAIs formed from such liquids including refractory metal nuggets exhibiting compositions and appearances similar to those of the meteoritic nuggets. The run products and their comparison with a meteoritic nugget‐bearing CAI is evidence for formation of refractory metal nuggets during cooling of Ca, Al‐rich liquids at rates about 1000°/40 s (in the interval from 1900 to 900 °C). To achieve the formation of refractory metal nuggets and the textures observed in the host inclusions, during cooling the rate probably changed. Refractory metal nuggets apparently formed during quenching before spinel crystallized.     

A new model for the origin of Type‐B and Fluffy Type‐A CAIs: Analogies to remelted compound chondrules

Abstract– In the scenario developed here, most types of calcium‐aluminum‐rich inclusions (CAIs) formed near the Sun where they developed Wark‐Lovering rims before being transported by aerodynamic forces throughout the nebula. The amount of ambient dust in the nebula varied with heliocentric distance, peaking in the CV–CK formation location. Literature data show that accretionary rims (which occur outside the Wark‐Lovering rims) around CAIs contain substantial ¹⁶O‐rich forsterite, suggesting that, at this time, the ambient dust in the nebula consisted largely of ¹⁶O‐rich forsterite. Individual sub‐millimeter‐size Compact Type‐A CAIs (each surrounded by a Wark‐Lovering rim) collided in the CV–CK region and stuck together (in a manner similar to that of sibling compound chondrules); the CTAs were mixed with small amounts of ¹⁶O‐rich mafic dust and formed centimeter‐size compound objects (large Fluffy Type‐A CAIs) after experiencing minor melting. In contrast to other types of CAIs, centimeter‐size Type‐B CAIs formed directly in the CV–CK region after gehlenite‐rich Compact Type‐A CAIs collided and stuck together, incorporated significant amounts of ¹⁶O‐rich forsteritic dust (on the order of 10–15%) and probably some anorthite, and experienced extensive melting and partial evaporation. (Enveloping compound chondrules formed in an analogous manner.) In those cases where appreciably higher amounts of ¹⁶O‐rich forsterite (on the order of 25%) (and perhaps minor anorthite and pyroxene) were incorporated into compound Type‐A objects prior to melting, centimeter‐size forsterite‐bearing Type‐B CAIs (B3 inclusions) were produced. Type‐B1 inclusions formed from B2 inclusions that collided with and stuck to melilite‐rich Compact Type‐A CAIs and experienced high‐temperature processing.     

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.     

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The ²⁰Ne/²²Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The ²¹Ne/²²Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure ²²Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The ³⁶Ar/³⁸Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic ³⁶Ar produced by neutron capture on ³⁵Cl with subsequent β^?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.     

Calcium‐aluminum‐rich inclusions in enstatite chondrites (I): Mineralogy and textures

Abstract— Like calcium‐aluminum‐rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite CAIs are composed of refractory minerals such as spinel, perovskite, Al, Ti‐diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na‐(±Cl)‐rich minerals. Porous, aggregate, and compact textures of the refractory cores in enstatite chondrite CAIs and rare Wark—Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100μm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite‐rich types, and presence of primary Ti‐(±V)‐oxides, and secondary geikelite and Ti, Fe‐sulfides distinguish the assemblage of enstatite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of indicators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high‐temperature processing of the CAIs did not occur under the reducing conditions characteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite‐chondrite‐forming regions or (2) O fugacities fluctuated within the enstatite‐chondrite‐forming region. In contrast, secondary geikelite and Ti‐Fe‐sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alteration of enstatite chondrite CAIs occurred in a nebular or parent‐body setting. We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production or dispersal of CAIs.