OXYGEN-ISOTOPE COMPOSITION OF MANTLE PERIDOTITE

The oxygen isotopic composition of 76 samples of olivine in spinel-, g arnet- and diamond-facies peridotites, and as syngenetic inclusions wi thin diamond has been determined by laser fluorination (LF). delta(18) O(V-SMOW) values of olivine are almost invariant, averaging 5.18 +/- 0 .28 parts per thousand (2 S.D.) with an overall range of 4.8-5.5 parts per thousand. Coexisting clinopyroxenes display positive Delta(18)O(c px-olivine) fractionations averaging 0.4 parts per thousand that are c onsistent with isotopic equilibrium at normal mantle temperatures. The bulk compositions of spinel-, garnet- and diamond-facies mantle are s imilar, with a calculated bulk mantle delta(18)O of +5.5 parts per tho usand. Olivine inclusions in diamonds of Archaean age yield delta(18)O values that are indistinguishable from olivine in post-Archaean mantl e and the LF data do not provide evidence for gross spatial or tempora l delta(18)O variation in peridotitic mantle. There is no evidence tha t mantle hydration processes impart distinctive oxygen isotope charact eristics, other than a subtle isotopic disequilibrium disturbing pyrox enes relative to olivine.