OXYGEN ISOTOPE EVIDENCE REGARDING THE FORMATION OF SPINEL-BEARING CHONDRULES

Abstract

Oxygen isotope compositions of individual mineral grains in two spinel-bearing chondrules have been determined by secondary ion mass spectrometry. O isotope ratios of spinel grains in a plagioclase-rich chondrule were extremely 16O-rich (δ18O ~ δ17O ~ -35%%) indicating close relationship to spinels in Ca-Al-rich inclusions (CAIs), whereas O isotope ratios of a spinel grain in barred-olivine chondrule were less enriched in 16O (δ18O ~ δ17O ~ -10%%). The degree of 16O-excesses among minerals within each chondrule was parallel to the crystallization sequences. These measurements suggest that some CAI-spinel grains were incorporated into the chondrule precursors, and that the O isotopic variation among minerals may result in incomplete exchange of O isotopes between 16O-rich chondrule melt and 16O-poor ambient nebular gas. The mesostasis having an O-isotopic composition near the terrestrial fractionation line may have been somewhat affected by the aqueous alteration in the parent body.