Oxygen isotope composition of garnet and spinel peridotites in the continental mantle: Evidence from the Vitim xenolith suite, southern Siberia

Details Oxygen isotope composition of garnet and spinel peridotites in the continental mantle: Evidence from the Vitim xenolith suite, southern Siberia Oxygen isotope composition of garnet and spinel peridotites in the continental mantle: Evidence from the Vitim xenolith suite, southern Siberia

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1463i&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 5, pp.1463-1470

Computer Science

6

Scientific paper

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 18 O / 16 O ratios by conventional and laser-assisted fluorination techniques yield a very restricted range of whole-rock 18 O 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 18 O 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, 18 O 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 18 O 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 O-isotope character. With increasing temperature and degree of partial melting, such a mantle source could generate basaltic liquids with 18 O values varying from C. + 6.1 to + 5.7%. Thus, the broad 18 O range of c. + 4.5 to + 7.5%. 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.

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