Investigation of stable Cr isotopic fractionation during Earths silicate differentiation and reassessment of the accurate Bulk Silicate Earth delta53Cr value

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It has been observed that Cr isotope compositions of magmatic rocks and reservoirs correlate with indicators for silicate differentiation (e.g. Mg#). It is also known for decades that mafic minerals such as spinel, olivine and the pyroxenes can contain significant amounts of Cr2+ besides the Cr3+ oxidation state. Moreover, recent studies show that melts formed during partial mantle melting have different Cr2+/Cr3+ ratios depending on the ambient oxygen fugacity. It is thus possible that Cr isotopes fractionate during silicate differentiation and that the size of this fractionation is depending on the prevailing oxygen fugacity. In this study we intend to investigate (i) the Cr isotopic compositions of basaltic reservoirs (MORB, OIB, continental flood basalts, archean komatiites) that had variable melting conditions (e.g. degree of melting, fO2), (ii) the cause for the large Cr isotopic variations found in rocks of orogenic peridotite bodies, (iii) whether olivine, pyroxene, spinel or garnet in peridotites have variable Cr isotopic compositions and if so (iv) how these may be dependent on the minerals composition and whether the minerals are in isotopic equilibrium. The ultimate goal of this study however is to test whether stable Cr isotopes can be used as a tool to trace prevailing oxygen fugacity during partial mantle melting.