OXYGEN ISOTOPIC HOMOGENEITY OF THE EARTH: NEW EVIDENCE

Abstract

The two oxygen isotope ratios18O/16O and17O/16O were analyzed in Precambrian (3.5 Gy) and modern cherts and in mantle-derived lavas. All samples exhibit oxygen isotopic compositions consistent with mass-dependent isotopic fractionation from a single reservoir and thus suggest that 3.5 Gy ago the Earth was already a well-mixed body. These results would therefore not support models of ocean accretion by cometary impact later than 3.7 ± 0.1 Gy. New measurements on lunar rocks confirm oxygen isotopic homogeneity of the Earth-Moon system and thus suggests that the internal mixing of the two bodies pre-dates their accretion. The recent giant impact model used to explain how the Moon was formed is also compatible with our results.