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

不同构造背景的镁铁质火山岩具有比原始地幔(PM)更低的铂/钯比值，这不能用已知的钯和铂在硫化物熔体和硅酸盐熔体之间的分配系数来解释。各种过程被用来解释这一观察结果，包括热液岩石/水的相互作用或蛇纹岩作用。然而，到目前为止，还没有系统地研究热液蚀变对PGE预算的影响，例如对整个被蚀变的上洋壳的影响。因此，对蚀变过程中铂、钯的分馏作用知之甚少。我们建议通过研究大约15 Ma前形成于东太平洋隆起的蚀变上洋壳和最上部辉长岩部分来填补这一空白，并在多核的ODP1256D钻孔钻探(Wilson等人，2006年)。与船上其他成员共享的关键矿藏样品中铂族元素(PGE)和铜-硫同位素的联合测量，将有助于评估蚀变过程中PGE的活动性，并有助于定量研究海洋地壳中铂和钯沿热液流体路径的分馏行为。由于大型PGE矿床与镁铁质火山作用(即Norils‘k)有关，这些见解对于理解洋壳中的成矿过程具有很高的相关性。

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).