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 var iety 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 gr ains (21-41 wt% FeO). In an oxygen three-isotope plot refractory forst erite 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 pyrox ene from Ca,Al-rich inclusions (CAIs), possibly indicating a later for mation of forsteritic olivine. Cores of isolated olivine grains with s omewhat higher FeO contents (IOGs) have oxygen isotopic compositions s imilar to those of the refractory forsterites (deltaO-17 almost-equal- to -12 parts per thousand and deltaO-18 almost-equal-to -9 parts per t housand). Both types of olivine grains have oxygen isotopic compositio ns significantly different from those of chondrules. This clearly demo nstrates that these olivine grains are not the result of crystallizati on of chondrule melts. Fayalite-rich rims of the IOGs are less enriche d in O-16(deltaO-17 almost-equal-to -3 parts per thousand; deltaO-18 a lmost-equal-to 0 parts per thousand) than the forsteritic cores. Argum ents are presented that this supports a condensation origin of the rim s from a gas phase less enriched in O-16 than the gas phase from which the forsteritic cores had formed. Matrix olivines have oxygen isotopi c compositions similar to fayalite-rich rims. However, there are varia tions in the oxygen isotopic composition of matrix olivines that excee d analytical uncertainties, suggesting a more complex origin.