OXYGEN ISOTOPIC COMPOSITION OF HYDROUS AND ANHYDROUS MANTLE PERIDOTITES

Oxygen isotope ratios, determined using the laser fluorination techniq ue, are reported for minerals from anhydrous and hydrous (i.e., amphib ole-bearing) spinel lherzolites from Yemen, as well as from hydrous sp inel lherzolites and amphibole megacrysts from Nunivak island, Alaska. Oxygen isotopic compositions of olivine vary from 5.1-5.4 parts per t housand and of pyroxene from 5.5-6.0 parts per thousand and no systema tic difference exists between minerals in hydrous and anhydrous lherzo lites. The oxygen isotopic composition of the amphibole in the peridot ites and of the amphibole megacrysts is also very homogeneous and vari es from delta(18)O = 5.3-5.6 parts per thousand. These results indicat e that the metasomatic minerals in the lherzolites are in oxygen isoto pic equilibrium with the peridotitic minerals. The only isotopic diseq uilibria are observed in minerals which have grown in melt-pockets for med by partial melting of amphibole. The homogeneity of the oxygen iso topic ratios of mantle minerals in this study indicate that the fluids circulating in the mantle and precipitating amphibole or mica had the same oxygen isotopic compositions as the mantle protolith or that the fluids had been buffered by the isotopic composition of the olivine, the most abundant mineral, during percolation through the peridotites. Copyright (C) 1997 Elsevier Science Ltd