Accretion and core formation: The effects of sulfur on metal–silicate partition coefficients.

The accretion of the Earth was marked by the high-pressure segregation of the core, accompanied by dissolution of about 10% of one or more “light” elements into the metal. Cosmochemical data suggest that, of these 10% “light” elements, the core contains ∼1.7% S (Dreibus and Palme, 1996) and there is evidence that volatile elements such as S accreted to the Earth late in planetary growth, plausibly as a sulfide “matte” (O’Neill, 1991). Given that metallurgical data indicate that dissolution of even small amounts of sulfur in liquid Fe can have profound effects on the activities of some trace components, we have undertaken a study of the affect of S on the metal–silicate partitioning of a number of the most important chalcophile and siderophile elements. We performed experiments at 1.5 GPa and 1460–1650 °C on metal–silicate partitioning of W, Mo, Ni, Co, Cu, Ag, Mn, Cd, Tl, Cr, Sb, In, Pb, Ga, Ge, V and Zn under conditions where the S content of the metal was varied from 0 to 37 wt%. Mn and Ag were found to exhibit the highest ratio of chalcophile to siderophile behaviour while W, Ga and Sb are the most “chalcophobic” of the elements studied. In terms of the 1-parameter epsilon model (Wagner, 1962) we derived values for each element at 1600 °C as follows (negative values indicate chalcophile behaviour): ε Cu S = - 2.57 ; ε Mn S = - 6.41 ; ε Ag S = - 4.15 ; ε Sb S = 4.36 ; ε Cd S = - 3.78 ; ε In S = - 0.24 ; ε Tl S = - 6.21 ; ε Ga S = 6.54 ; ε Pb S = - 3.73 ; ε Cr S = - 3.70 ; ε Ge S = 7.03 ; ε V S = - 3.14 ; ε Zn S = - 1.68 ; ε Mo S = 2.27 ; ε W S = 6.47 ; ε Ni S = 2.17 ; ε Co S = 2.40 . We use these new data in conjunction with published pressure–temperature dependences of metal–silicate partitioning to test the effects of accreted S on the calculated trace element concentrations in bulk silicate Earth. The approach employs a continuous accretion model in which the oxidation state of the Earth and pressure of core segregation both increase during accretion. We find that, without addition of S in the latter stages of accretion, the Mo/W ratio of silicate Earth would be several times larger than that observed. Addition of ∼2% S accompanied by small amounts of carbon in the last 15% of accretion, however, enables us to match the observed concentrations of these elements in silicate Earth. This confirms an earlier conclusion that the Mo/W ratio of silicate Earth requires late sulfide addition to the core (Wade et al., 2012). Further support for late sulfide addition to the core comes from the depletion factors of volatile chalcophile elements Cu, Ga, Sb, Ag, Zn, Pb, Cd, In and Tl in silicate Earth relative to lithophile elements of similar volatility. We find that depletions of these elements are well correlated with their partition coefficients into sulfide (FeS) liquids at 1.5 GPa and temperatures of 1460–1650 °C. In contrast there is essentially no correlation between their depletion factors and sulfur-free liquid Fe–silicate partition coefficients. [ABSTRACT FROM AUTHOR]