OXYGEN ISOTOPE COMPOSITION OF MANTLE PERIDOTITE

Abstract

The oxygen isotopic composition of 76 samples of olivine in spinel-, garnet- and diamond-facies peridotites, and as syngenetic inclusions within diamond has been determined by laser fluorination (LF). δ18OV-SMOW values of olivine are almost invariant, averaging 5.18 ± 0.28‰ (2 S.D.) with an overall range of 4.8–5.5‰. Coexisting clinopyroxenes display positive Δ18Ocpx-olivine fractionations averaging 0.4‰ that are consistent with isotopic equilibrium at normal mantle temperatures. The bulk compositions of spinel-, garnet- and diamond-facies mantle are similar, with a calculated bulk mantle δ18O of +5.5‰. Olivine inclusions in diamonds of Archaean age yield δ18O values that are indistinguishable from olivine in post-Archaean mantle and the LF data do not provide evidence for gross spatial or temporal δ18O variation in peridotitic mantle. There is no evidence that mantle hydration processes impart distinctive oxygen isotope characteristics, other than a subtle isotopic disequilibrium disturbing pyroxenes relative to olivine.