Oxygen isotope evidence regarding the formation of spinel-bearing chondrules

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 O-16-rich (delta(18)O similar to delta(17)O similar to -35 parts per thousand) 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 O-16 (delta(18)O similar to delta(17)O similar to -10 parts per thousand). The degree of O-16-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 O-16-rich chondrule melt and O-16-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. (C) 1999 Elsevier Science B.V. All rights reserved.