OXYGEN ISOTOPIC COMPOSITION OF INDIVIDUAL OLIVINE GRAINS FROM THE ALLENDE METEORITE

The oxygen isotopic compositions of individual olivine grains from the Allende meteorite were determined by ion microprobe techniques. A variety of olivine grains were analysed: refractory forsterite grains (<1 wt% FeO), cores of two isolated olivine grains (2-20 wt% FeO), their FeO-rich (>26 wt%) rims, and twenty-eight individual matrix olivine grains (21-41 wt% FeO). In an oxygen three-isotope plot refractory forsterite grains (deltaO-17 almost-equal-to -11 parts per thousand; deltaO-18 almost-equal-to -8 parts per thousand) fall on the Allende mixing line but are significantly less enriched in O-16 than spinel and pyroxene from Ca,Al-rich inclusions (CAIs), possibly indicating a later formation of forsteritic olivine. Cores of isolated olivine grains with somewhat higher FeO contents (IOGs) have oxygen isotopic compositions similar to those of the refractory forsterites (deltaO-17 almost-equal-to -12 parts per thousand and deltaO-18 almost-equal-to -9 parts per thousand). Both types of olivine grains have oxygen isotopic compositions significantly different from those of chondrules. This clearly demonstrates that these olivine grains are not the result of crystallization of chondrule melts. Fayalite-rich rims of the IOGs are less enriched in O-16(deltaO-17 almost-equal-to -3 parts per thousand; deltaO-18 almost-equal-to 0 parts per thousand) than the forsteritic cores. Arguments are presented that this supports a condensation origin of the rims from a gas phase less enriched in O-16 than the gas phase from which the forsteritic cores had formed. Matrix olivines have oxygen isotopic compositions similar to fayalite-rich rims. However, there are variations in the oxygen isotopic composition of matrix olivines that exceed analytical uncertainties, suggesting a more complex origin.