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We describe the mineralogy, petrology and oxygen isotopic compositions of high-temperature rims around mineralogically pristine calcium–aluminum-rich inclusions (CAIs) from the CR, CB and CH carbonaceous chondrites. In CR chondrites, nearly all CAIs are surrounded by single- or multi-layered rims composed of CAI-like minerals; relict CAIs inside chondrules in which the rims were resorbed by the host chondrule melt (Aléon et al., 2002; Makide et al., 2009) are the only exception. A complete multi-layered rim sequence (from inside outward: spinel + hibonite + perovskite → melilite → anorthite replacing melilite → Al-diopside → forsterite) is rarely observed; Al-diopside ± forsterite rims are more common. The CR CAIs and all rim layers are uniformly 16 O-rich (Δ 17 O ∼−24‰), indicating formation in a 16 O-rich gaseous reservoir. The mineralogy, petrology and 16 O-rich compositions of these rims suggest formation by evaporation/condensation, melting (?), and thermal annealing in the formation region of the host CAIs. We define such rims as the primordial Wark–Lovering (WL) rims. In CH chondrites, most CAIs are uniformly 16 O-rich and surrounded by the primordial WL rims. One of the 16 O-rich CAIs is surrounded by an anorthite–Al-diopside WL rim showing a range of Δ 17 O values, from ∼−24‰ to ∼−6‰; Δ 17 O decreases towards the CAI core. We infer that this rim experienced incomplete melting and O-isotope exchange in an 16 O-poor nebular gas, most likely during chondrule formation. Most CAIs in CB chondrites and about 10% of CAIs in CH chondrites are uniformly 16 O-depleted igneous inclusions; Δ 17 O values between individual CAIs vary from ∼−12‰ to ∼−5‰. These CAIs have diverse mineralogies (grossite-rich, hibonite-rich, melilite-rich, spinel-rich, and Al,Ti-diopside ± forsterite-rich), but are surrounded by the mineralogically similar igneous rims composed of ±melilite, Al-diopside and Ca-rich forsterite (0.5–1.4 wt% CaO). The igneous rims and the host igneous CAIs have identical (within uncertainties of our SIMS measurements) O-isotope compositions, suggesting that they crystallized from isotopically similar, but chemically distinct melts. We suggest that the uniformly 16 O-depleted igneous rims around the uniformly 16 O-depleted igneous CAIs in CB and CH chondrites formed during melting of pre-existing CAIs in an impact-generated plume invoked for the origin of CB chondrites (Krot et al., 2005), followed by O-isotope exchange with an 16 O-poor plume gas (Δ 17 O ∼−2‰), condensation of gaseous SiO and Mg into CAI melt, and its subsequent crystallization. We conclude that high-temperature rims around CAIs from CR, CH and CB chondrites recorded thermal processing in gaseous reservoirs with different oxygen isotopic compositions. In contrast to the isotopically heterogeneous WL rims around CV CAIs, our data provide no evidence that CAIs were transported between 16 O-rich and 16 O-poor gaseous reservoirs multiple times. We suggest instead that oxygen-isotope heterogeneity in the CV WL rims resulted from a fluid-rock interaction on the CV parent asteroid. [ABSTRACT FROM AUTHOR]

Oxygen isotopic variations across the outer margins and Wark–Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16 O-rich protosolar gas reservoir and were later exposed to both relatively 17,18 O-rich and 16 O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time–temperature combinations for their exposure in the nebula. At mean temperatures of 1400 K, models that match the isotope gradients in the inclusions yield timescales ranging from 5 × 10 3 to 3 × 10 5 years. Assuming CAIs originated with a relatively 16 O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16 O-rich but subtly distinct among different CAIs. Melilite is often relatively 16 O-poor, but rare relatively 16 O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16 O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body. [ABSTRACT FROM AUTHOR]

Abstract: Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (f O2) near that of the Iron–Wüstite (IW) f O2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f O2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for f O2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) f O2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f O2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3×105 yr of their formation in the solar nebula. [Copyright &y& Elsevier]