High-temperature rims around calcium–aluminum-rich inclusions from the CR, CB and CH carbonaceous chondrites

Abstract

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 16O-rich (Δ17O ∼−24‰), indicating formation in a 16O-rich gaseous reservoir. The mineralogy, petrology and 16O-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 16O-rich and surrounded by the primordial WL rims. One of the 16O-rich CAIs is surrounded by an anorthite–Al-diopside WL rim showing a range of Δ17O values, from ∼−24‰ to ∼−6‰; Δ17O decreases towards the CAI core. We infer that this rim experienced incomplete melting and O-isotope exchange in an 16O-poor nebular gas, most likely during chondrule formation.Most CAIs in CB chondrites and about 10% of CAIs in CH chondrites are uniformly 16O-depleted igneous inclusions; Δ17O 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 (0.5–1.4wt% 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 16O-depleted igneous rims around the uniformly 16O-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 16O-poor plume gas (Δ17O ∼−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 16O-rich and 16O-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.