Oxygen isotope cosmothermometer

Abstract

Variations in oxygen isotopic abundances of meteoritic minerals, chondrules, whole meteorites, and planets are discussed in terms of a model involving isotopic exchange between primordial dust and a cooling solar nebular gas. From the temperature-dependence of the isotopic fractionation factors temperatures have been assigned to the processes of initial condensation, chondrule formation and planetary accretion. Separated phases from carbonaceous chondrites fall into three isotopic groups representing widely differing conditions of formation: (a) low-iron olivine and pyroxene, and calcium-aluminum silicates condensed at temperatures >1000°K; (b) high-iron olivine and pyroxene melted to form chondrules after prior cooling and exchange to temperatures of 530–620°K; (c) hydrous silicates condensed at temperatures below 400°K. The ordinary chondrites, the Earth and the Moon have remarkably similar isotopic compositions, which are not readily accounted for in terms of mixtures of known materials of the primitive meteorites. Mean accretion temperatures of 450–470°K are estimated for members of this group. The model permits an estimate of the O 18 O 16 ratio of the solar nebular gas, which gives δO 18 = −1 ± 2%. relative to SMOW.