OXYGEN ISOTOPES IN SEPARATED COMPONENTS OF CI AND CM METEORITES

Oxygen isotopic compositions have been determined for whole-rock and separated components from the CI chondrites Alais, Ivuna, and Orgueil, and from the atypical CM chondrites Bells and Essebi. The composition of CI matrix is in agreement with previous measurements and is systematically different from that of CM matrix, probably due to warmer and wetter conditions of aqueous alteration for CI matrix. Although Bells matrix is like that of other CM chondrites, the matrix of Essebi falls in the CI field. Essebi also contains abundant magnetite, like CIs and unlike other CMs. The isotopic compositions of magnetite in Alais, Ivuna, Orgueil, and Essebi define a trend with DELTA O-17 of +1.3 to + 1.8, distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite. The magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation, and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more O-16-depleted than the earth.