The mobilisation of Platinum-Group Elements in altered oceanic crust from ODP-borehole 1256D for tracing the noble metal flux in the crust

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Mafic volcanic rocks of different tectonic settings display a wide range of Pt/Pd-ratios lower than that of the primitive mantle (PM) which cannot be accounted for by known partition coefficients of Pd and Pt between sulphide melt and silicate melt. Various processes have been invoked to explain this observation, including hydrothermal rock/water interactions or serpentinisation. However, so far no study has systematically investigated the effects of hydrothermal alteration on the PGE budget for example on a complete section of altered upper oceanic crust. Hence, little is known on Pt-Pd fractionation during alteration. We propose to fill this gap by studying a complete profile of altered upper oceanic crust and the uppermost gabbroic section, formed at the East Pacific Rise some 15 Ma ago and drilled at the multicruise ODP-borehole 1256D (Wilson et al., 2006). Combined measurements of platinum-group elements (PGE) and Cu-Sisotopes in key pool samples, shared with other shipboard party members, will help to evaluate PGE mobility during alteration and to study quantitatively the fractionation behaviour of Pt and Pd along the hydrothermal fluid path in the oceanic crust. These insights are highly relevant for understanding ore forming processes in the oceanic crust, since large PGE deposits are related to mafic volcanism (i.e. Norils’k).