OXYGEN-ISOTOPE COMPOSITION OF GARNET AND SPINEL PERIDOTITES IN THE CONTINENTAL MANTLE - EVIDENCE FROM THE VITIM XENOLITH SUITE, SOUTHERN SIBERIA

Spinel and garnet facies peridotites from the Vitim Volcanic Field in Siberia comprise a suite of fertile mantle xenoliths, which are LREE d epleted and have strongly depleted strontium and neodymium isotopic co mpositions. Determination of O-18/O-16 ratios by conventional and lase r-assisted fluorination techniques yield a very restricted range of wh ole-rock deltaO-18 values for both spinel peridotites (+5.4 to +5.8 pa rts per thousand) and garnet lherzolites (+5.5 to +5.8 parts per thous and) equivalent to that observed for lunar rocks and mid-ocean ridge b asalts (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 pe r thousand. Similarly, deltaO-18 ranges for silicate mineral pairs var y from only 0.5 to 0.7 parts per thousand. The sixteen peridotite xeno liths studied exhibit equilibrium O-isotope fractionations between min erals of a magnitude expected from theoretical and experimental consid erations, in particular a temperature-controlled O-18 distribution bet ween olivine and pyroxenes in spinel-bearing peridotites. Because the Vitim garnet and spinel peridotites are rich in basaltic components an d show long-term depletion in incompatible elements, they are derived from a MORB-like mantle source which makes up, according to current ge ochemical models, a predominant part of the Earth's upper mantle. The Vitim xenolith suite may, therefore, reflect its bulk 0-isotope charac ter. With increasing temperature and degree of partial melting, such a mantle source could generate basaltic liquids with deltaO-18 values v arying 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 literatu re for spinel peridotite xenoliths is probably a very restricted diseq uilibrium phenomenon that likely reflects recent metasomatic overprint ing in highly localized zones of the uppermost continental mantle affe cted by basaltic magmatism and, therefore, should not be considered as representative of the upper mantle, in general.