THE OXYGEN ISOTOPIC COMPOSITION OF OLIVINE AND PYROXENE FROM CI CHONDRITES

The CI chondrites are taken to represent average solar system material based on the similarity of their elemental compositions to that of th e solar photosphere. However, their oxygen isotope geochemistry is dom inated by secondary minerals that formed during aqueous alteration on the CI parent body. Precursors to this alteration, namely olivine and pyroxene, are extremely rare in CI chondrites, precluding previous mea surements of their oxygen isotopic composition. We report ion micropro be analyses of oxygen isotopes in single olivine and pyroxene grains s eparated from CI chondrites Orgueil and Ivuna. The CI chondrite olivin e and pyroxene grains most likely represent liberated chondrule phenoc rysts, based on petrographic, chemical, and isotopic evidence consiste nt with crystallization from a melt. The oxygen isotope data form an a rray that falls nearly along the carbonaceous chondrite O-16 mixing li ne with delta(18)O values ranging from -9.3 parts per thousand to + 12 .3 parts per thousand and delta(17)O from -11.3 parts per thousand to +7.89 parts per thousand, consistent with nebular processes being the source of the oxygen isotopic compositions. The degree of O-16-enrichm ent in Orgueil olivines is negatively correlated with FeO content, but the exact nature and timing of the process that introduced this varia tion remains unknown. The pyroxene oxygen isotopic compositions are si milar to those of olivines with >5 mol% fayalite. The oxygen isotopic analyses of the olivine and pyroxene in CI chondrites have been used t o revise previous models for the isotopic evolution of CI materials. O ur data require more complete gas-solid equilibration in the nebula an d constrain the initial aqueous fluids on the CI parent body to have l ower Delta(17)O values than previously postulated. The refined model i ndicates that the temperature of aqueous activity on the CI parent bod y was no higher than similar to 50 degrees C, and the fluid:rock ratio was significantly less than previously estimated. Even prior to alter ation and formation of secondary minerals, the CI chondrites were the most O-16-depleted carbonaceous chondrites and thus the solids origina lly contained in the CI chondrites are the most equilibrated nebular m aterials represented in the carbonaceous chondrites. The data suggest the oxygen isotopic composition of average solar system to be approxim ately equivalent to average terrestrial oxygen as recorded in the comp ositions of terrestrial and lunar basalts. Copyright (C) 1997 Elsevier Science Ltd.