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

Abstract

Spinel and garnet fades 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 ratios by conventional and laser-assisted fluorination techniques yield a very restricted range of whole-rock δ¹⁸O values for both spinel peridotites (+ 5.4 to + 5.8‰) and garnet lherzolites (+ 5.5 to + 5.8‰) equivalent to that observed for lunar rocks and midocean ridge basalts (MORB). Mineral δ18O values for the xenoliths are: olivine = + 5.1 to + 5.8%., orthopyroxene = + 5.7 to + 6.0%., clinopyroxene = + 5.5 to + 6.2%., garnet = + 5.5 to +6.0%., and spinel = + 4.9 to + 5.5%. Similarly, δ18O ranges for silicate mineral pairs vary from only 0.5 to 0.7%. 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 18O distribution between olivine and pyroxenes in spinel-bearing peridotites.