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 18O16O 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. 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 δ18O values varying from C. + 6.1 to + 5.7%. Thus, the broad δ18O 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.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
