Nature and Scale of Mantle Heterogeneities in Upper Mantle Reservoirs: Evidence from trace element abundances and Os isotopic compositions in the Oman ophiolite

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Geochemical and isotopic evidence indicates that the upper mantle, the source of mid-ocean ridge basalts (MORB), has been depleted by melt extraction processes over geological periods. It has been suggested that the residual mantle, after extraction of MORB, represents a geochemically homogeneous reservoir. Recently, Re-Os systematics of MORB have been used to postulate global chemical heterogeneities of the depleted oceanic upper mantle that is related to the recycling process of ancient oceanic crust. The nature and scale of these mantle heterogeneities, introduced by the recycling processes, is unknown. The study of mantle heterogeneities by the direct analyses of mantle peridotites is difficult because of the extreme depletion of incompatible, lithophile elements. The Re-Os method is ideally suited for studying ultramafic (mantle) rocks and detecting recycled ancient oceanic crust in the upper mantle, as Os an element compatible with mantle minerals, while Re is moderately incompatible. Small degrees of partial melting of a fertile mantle will remove some Re from the source reducing the growth of the radiogenic 187Os in the residue. Our preliminary analyses on mantle rocks of the Oman ophiolite reveal a large spread in Os isotopic compositions suggesting an along-strike isotope variability of the paleo-ridge which suggest the presence of a highly heterogenous upper mantle. The purpose of the proposed study is, to determine the trace element signature and the Os isotopic composition of the different lithologies of the oceanic lithosphere and, in addition, to monitor the nature and scale of the geochemical and isotopic herteogeneities.