Fractionation of highly siderophile elements in the lower oceanic crust

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The processes that lead to the strong fractionation of highly siderophile elements (HSE, the platinum group elements, rhenium and gold) in the crust are not well understood. Compared to the mantle, abundances of some HSE in ocean ridge basalts are lower by a factor of 1000 or more, with Re, Au and Pd being more abundant than other HSE. Because most erupting basalts are sulphide saturated it has been suggested that the highly variable abundances of the HSE in basalts may reflect crystal fractionation processes in the deeper oceanic crust. The relevance of magmatic HSE fractionation in the crust vs. the mantle is still an unresolved major question, particularly because of the limited availability of HSE data and systematic work on lower crustal rocks. We propose to study oceanic middle to lower crustal rocks from ODP site 735B on the SW Indian ridge. The main goals will be to constrain HSE, Se and Te abundances in typical lithologies, to identify HSE carrier phases and their composition, and to assess their impact on the HSE budget of oceanic crust. This will produce constraints on possible magmatic enrichment of HSE in the lower crust and will improve our knowledge of the fractionation of the HSE in the crust-mantle system.