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 coarser-grained 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 degrees 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.