The lead isotope composition of the Earth´s mantle from double-spike analysis of abyssal peridotites

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Oceanic basalts are widely used as probes of mantle composition, however this approach assumes that mantle melts inherit the isotopic composition of their source. Recent Pb isotope data for peridotites presumed to represent melting residues of the upper oceanic mantle extend to much less radiogenic compositions than observed in oceanic basalts. Incomplete extraction of Pb from an unradiogenic component in the mantle during melting could potentially explain the paradox that the bulk Earth apparently has a Pb isotope composition that is more radiogenic than is predicted if the Earth accreted from chondritic material. It could also explain the Pb isotope systematics of mid-ocean ridge basalts, which apparently reflect mixing between enriched and depleted mantle components. Reliable existing Pb isotope data for abyssal peridotites - residues of mantle melting at mid-ocean ridges - are too few (3 analyses) to test this hypothesis. Our proposed study will use a combination of new and existing methods to obtain some of the first reliable Pb isotope data for abyssal peridotites. We will analyse clinopyroxene and spinel separates, and use sequential dissolution methods to selectively dissolve different mineral phases within bulk-rock powders. Pb isotope compositions will be measured to high precision using a 202Pb-205Pb double-spike method. Our results will be used to test the hypothesis that a 'hidden' unradiogenic Pb component, which can balance the Earth's Pb budget, is present in the upper mantle.