Oxygen Isotopic Abundances in Calcium-Aluminum-Rich Inclusions from Ordinary Chondrites: Implications for Nebular Heterogeneity

Abstract

The oxygen isotopic compositions of two calcium-aluminum–rich inclusions (CAIs) from the unequilibrated ordinary chondrite meteorites Quinyambie and Semarkona are enriched in ¹⁶O by an amount similar to that in CAIs from carbonaceous chondrites. This may indicate that most CAIs formed in a restricted region of the solar nebula and were then unevenly distributed throughout the various chondrite accretion regions. The Semarkona CAI is isotopically homogeneous and contains highly ¹⁶O-enriched melilite, supporting the hypothesis that all CAI minerals were originally ¹⁶O-rich, but that in most carbonaceous chondrite inclusions some minerals exchanged oxygen isotopes with an external reservoir following crystallization.

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