Mineralogical controls on the oxygen isotopic compositions of UOCs

We report the results of oxygen isotopic analyses made on mineral separates prepared from chondrules and whole meteorites (UOCs), Feldspar/glass (main ly chondrule mesostasis) is more O-16-poor than the olivine/low Ca pyroxene from which it was separated. This relationship is true both for chondrules and whole meteorites from the three UOC groups. Feldspar and glass have ex changed to a higher degree with O-16-poor gases than ferromagnesian mineral s because of the susceptibility of framework silicates and glass to gas-sol id diffusion. Olivine and low Ca pyroxene have preserved primitive isotopic compositions. In this way, mineralogy is the dominant control on the oxyge n isotopic compositions of UOCs. The similarity of chondrule and whole mete orite isotope compositions suggests that for many UOCs, matrix and chondrul es have undergone exchange with the same isotope reservoirs. This exchange postdates chondrule formation and is believed to have occurred after consol idation with the surrounding chondrite matrix in a parent body regolith. Af ter this gas-solid exchange. some isotopic equilibration took place between feldspar and the other relatively O-16-rich solids, as a result of metamor phism. A progressive decrease in the values of delta(18)O(feldspar/glass)- delta(18)O(olivine/pyroxene) for bulk meteorites occurs with increasing pet rologic subtype, from 8 parts per thousand for a type 3.2 to 2.5%, for a ty pe 3.9. A chemical origin for the initial O-16-poor gas reservoirs in the H , L, and LL parent bodies by processes of mass independent fractionation is considered more likely than nucleosynthesis. Feldspar/glass separated from Adrar 003 (L/LL3.2) may be in, or close to having, isotopic equilibrium wi th the O-16-poor gases for L or LL chondrites, On this basis and assuming t hat the gas was dominated by H2O and its isotopic composition lay along a l ine of slope 1 (corresponding to what has been called the equilibrated chon drite line), a tentative upper limit for gas-solid isotopic exchange in UOC s can be calculated, which gives 370 degrees C. Copyright (C) 1999 Elsevier Science Ltd.