Spinel and garnet facies peridotites from the Vitim Volcanic Field in Siberia comprise a suite of fertile mantle xenoliths, which are LREE depleted and have strongly depleted strontium and neodymium isotopic compositions. Determination of O-18/O-16 ratios by conventional and laser-assisted fluorination techniques yield a very restricted range of whole-rock deltaO-18 values for both spinel peridotites (+5.4 to +5.8 parts per thousand) and garnet lherzolites (+5.5 to +5.8 parts per thousand) equivalent to that observed for lunar rocks and mid-ocean ridge basalts (MORB). Mineral deltaO-18 values for the xenoliths are: olivine = +5.1 to +5.8 parts per thousand, orthopyroxene = +5.7 to +6.0 parts per thousand, clinopyroxene = +5.5 to +6.2 parts per thousand, garnet = +5.5 to +6.0 parts per thousand, and spinel = +4.9 to +5.5 parts per thousand. Similarly, deltaO-18 ranges for silicate mineral pairs vary from only 0.5 to 0.7 parts per thousand. The sixteen peridotite xenoliths studied exhibit equilibrium O-isotope fractionations between minerals of a magnitude expected from theoretical and experimental considerations, in particular a temperature-controlled O-18 distribution between olivine and pyroxenes in spinel-bearing peridotites. Because the Vitim garnet and spinel peridotites are rich in basaltic components and show long-term depletion in incompatible elements, they are derived from a MORB-like mantle source which makes up, according to current geochemical models, a predominant part of the Earth's upper mantle. The Vitim xenolith suite may, therefore, reflect its bulk 0-isotope character. With increasing temperature and degree of partial melting, such a mantle source could generate basaltic liquids with deltaO-18 values varying from c.+6.1 to +5.7 parts per thousand. Thus, the broad deltaO-18 range of c.44.5 to +7.5 parts per thousand reported in the literature for spinel peridotite xenoliths is probably a very restricted disequilibrium phenomenon that likely reflects recent metasomatic overprinting in highly localized zones of the uppermost continental mantle affected by basaltic magmatism and, therefore, should not be considered as representative of the upper mantle, in general.
