Oxygen Isotope Fractionation between Minerals and an Estimate of the Temperature of Formation

Abstract

Oxygen isotopic compositions of separated minerals from three type A and four type B rocks are very uniform. The δ¹⁸O values are: plagioclase, 6.20; clinopyroxene, 5.75; ilmenite, 4.45 (parts per thousand relative to Standard Mean Ocean Water). The isotopic distribution corresponds to equilibrium at 1120°C. The isotopic composition of lunar pyroxenes falls within the range for pyroxenes of terrestrial mafic and ultramafic rocks, ordinary chondrites, enstatite chondrites, and enstatite achondrites, but above the range for basaltic achondrites, hypersthene achondrites, and mesosiderites. Glass isolated from the lunar soil has a δ¹⁸O value of 6.2, significantly richer in ¹⁸O than the crystalline rock fragments in the soil.