Dark inclusions in Allende, Leoville, and Vigarano: Evidence for nebular oxidation of CV3 constituents

Abstract

Dark lithic inclusions are known to occur in several CV3 chondrites; we report petrologic, chemical, and oxygen isotopic data on examples from Allende, Leoville, and Vigarano. The Allende inclusions comprise a petrographic sequence ranging from a type which contains chondrules and refractory inclusions and resembles CV3 or CO 3 chondrites to a type which contains porous aggregates of iron-rich olivine. Oxygen isotopic compositions correlate with the sequence; chondrule-rich inclusions have compositions similar to the host meteorite bulk values whereas olivine aggregate-rich inclusions have more 16O-depleted compositions. Chemical compositions, on the other hand, do not correlate with the petrographic sequence. The dark inclusions are all systematically enriched in iron, and depleted in potassium and sodium, relative to their host meteorites. The inclusions appear to be fragments of CV3 parent bodies which were processed to different degrees prior to their incorporation as clasts into Allende, Leoville, and Vigarano. The processing homogenized olivine compositions, presumably through heating, and also involved oxygen exchange with 16O-poorer surroundings. Either parent body metamorphism or nebular heating might have produced the petrographic and isotopic sequences. However, the most 16O-depleted inclusions also have layered silicate rims surrounding them which are better attributed to gas-solid reactions than parent body processes. In particular, andradite is abundant in the rims which implies that the rims grew under conditions significantly more oxidizing than those likely to have occurred on a CV3 parent body. At least part of the processing, therefore, appears to have taken place prior to final accumulation of the material when the fragments were dispersed in the nebula. The evidence for coupled oxidation and 16O-depletion in the nebula suggests that other features of the CV3 group such as the oxidized and reduced subgroups, ferrous rims on forsteritic olivines, and coarse-grained rims on chondrules may have been established by similar gas-solid reactions prior to the final assembly of the meteorites.