Oxygen isotopic composition of hydrous and anhydrous mantle peridotites

Oxygen isotope ratios, determined using the laser fluorination technique, are reported for minerals from anhydrous and hydrous (i.e., amphibole-bearing) spinel lherzolites from Yemen, as well as from hydrous spinel lherzolites and amphibole megacrysts from Nunivak Island, Alaska. Oxygen isotopic compositions of olivine vary from 5.1–5.4%c and of pyroxene from 5.5–6.0%c and no systematic difference exists between minerals in hydrous and anhydrous lherzolites. The oxygen isotopic composition of the amphibole in the peridotites and of the amphibole megacrysts is also very homogeneous and varies from δ18O = 5.3−5.6%o. These results indicate that the metasomatic minerals in the lherzolites are in oxygen isotopic equilibrium with the peridotitic minerals. The only isotopic disequilibria are observed in minerals which have grown in melt-pockets formed by partial melting of amphibole. The homogeneity of the oxygen isotopic 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.