MICRODISTRIBUTION OF OXYGEN ISOTOPES IN A REFRACTORY INCLUSION FROM THE ALLENDE METEORITE

Isotope ratios of oxygen have been measured on a fassaite crystal and the surrounding area of a coarse-grained CAI from the Allende meteorit e using secondary ion mass spectrometry. Values of deltaO-18(SMOW) for fassaite, spinel, anorthite grains and alteration products are -27 +/ - 5, -38 +/- 2, +5 +/- 6, and +4% respectively, which are consistent w ith the results obtained on mineral separates. The O isotopic distribu tions are homogeneous either within individual single crystals of fass aite and anorthite or among the spinel grains within the analytical er ror. No gradual variation in O isotopic composition was detected even at the boundary between different phases or at cracks inside a phase w ith a spatial resolution of 10 mum. These observations strongly sugges t that the partial exchange of O isotopes in the minerals in the CAI w ith the surrounding gas through diffusion is an unlikely origin for th e observed inter-mineral heterogeneity of O isotopes in the Allende CA Is.