GENETIC TAXONOMY OF METEORITE MINERALS BASED ON OXYGEN ISOTOPES

Abstract

Clayton and others have made systematic determinations of the isotopic composition of meteorite minerals, opening the way for a genetic interpretation. The predominant minerals in meteorites are pyroxenes and olivines; we present a plot of the data on their oxygen isotopes versus data on the chondrules of chondrites. We distinguish three different trends (I, II and III) in the isotopic compositions of both pyroxene and olivine, which are also followed by the chondrules. We attribute the increasingly oxidizing conditions to the migration of hydrogen from the parent planets and an increase in the H2O:H2 ratio in the fluids. This variation in the isotopic composition of the chondrules is correlated with similar changes in the oxygen isotopes of pyroxenes and olivines. The patterns of variation in the oxygen isotopic composition in the pyroxenes and olivine of meteorites enable us to arrange their parent planets in order of increasing abundance of oxygen. We distinguish the following types: Undifferentiated planets LL, L and H, which did not undergo internal stratification; HH planets, which stratified into pallasites and achondrites; and C3-Ure planets, which stratified under very highly reducing conditions involving anomalous fractionation of oxygen isotopes. We further distinguish the composite HH-E type of chondrite planets, in which stratification associated with normal mass fractionation of oxygen isotopes gave rise to most types of achondrites. They are analogs of the protoplanet from which the Earth and Moon originated, as indicated by the similarity in this respect of the minerals of the lunar and terrestrial crust to the minerals of E chondrites and to aubrites, which are directly derived from them by differentiation.