Siderophile element abundances in the upper mantle: evidence for a sulfide signature and equilibrium with the core

Abstract

Measurements of the intrinsic oxygen fugacities of samples from the Earth's upper mantle have shown that the characteristic oxidation state ranges from values close to Fe-Ni metal saturation to more oxidized values. Tectonic processes can account for the progressive and variable oxidation of a mantle that was originally in equilibrium with metal, so that the oxygen fugacity data give strong support to the possibility of protocore -protomantle equilibrium. The data for apparently overabundant siderophile elements in the mantle are reviewed and 2 groups (Group A = Ge, Au, Pd, Pt, Ru, Rh, Ir, Re, Os; Group B = Sn, Sb, Ag, Ga, Cu, As, Co, Ni, W, Mo) are recognized. Group A abundances result from an overprinting, by a late-accreting chondritic component, of a mantle that had suffered major extraction of the indigenous complement by core-forming metal ( Chou, 1978). Group B, however, are ≈- 10 × more abundant than Group A and may be distinguished as a group by their greater chalcophile tendencies. This is highlighted, for example, by the large Cl-normalized Sn/ Ge ratio of the mantle, which is unexpected as a result of metal-silicate fractionation alone, but can be accounted for if some Sn. together with sulfur was retained in the mantle during core separation. Industrial experience in the equilibrium of molten Fe and silicate slags shows that sulfur is not overwhelmingly partitioned into metal so that the concept of partial (≈-20% of original accreting component) retention of sulfur and Group B elements in the mantle has experimental and theoretical support. Following core separation, progressive loss of sulfide from the mantle is indicated by the anomalously high Ni (at a given Mg-number) in Archaean basalts and siliceous, Mg-poor sediments. It is also a feature of oceanic and continental basalts that some U/Pb fractionation must have occurred in the mantle source regions following core separation, and this is most satisfactorily achieved if Pb is partially removed from U in a sulfide phase.