THE INFLUENCE OF SULFUR ON PARTITIONING OF SIDEROPHILE ELEMENTS

Sulfur is a potential light element in the liquid outer core of the Ea rth. Its presence in segregating metal may have had an influence in di stribution of metal-loving (siderophile) elements during early accreti on and core formation events in the Earth. The observed ''excess'' abu ndance of siderophile elements in the terrestrial mantle, relative to an abundance expected from simple core-mantle equilibrium at low tempe rature and pressure, may indicate a reduction in the iron-loving tende ncy of siderophile elements in the presence of sulfur in the metallic phase. The present experimental partitioning study between iron-carbon -sulfur-siderophile element bearing liquid metal and liquid silicate s hows that for some siderophile elements this sulfur effect may be sign ificant enough to even change their character to lithophile. Large and intricate variations in metal-silicate partition coefficients (D-met/ sil) have been observed for many elements, e.g., Ni, Co, Ge, W, P, Au, and Re as a function of sulfur content. Moderately siderophile elemen ts Ge, P, and W show the most significant response (sulfur-avoidance) by an enhanced segregation into the associated sulfur-deficient phases . Highly siderophile elements Ir, Pt, and Re show a different style of sulfur-avoidance (alloy-preference) by segregating as sulfur-poor, si derophile element-rich alloys. Both groups are chalcophobic. D-met/sil for Ni, CO, and Au moderately decreases with increasing sulfur-conten t in the liquid metal. D-met/sil for chalcophile element, Cr, in contr ast, increases with sulfur. Irrespective of the sulfur-content, in the presence of a carbon-saturated liquid metal, P is always lithophile. The general nonmetal-avoidance tendency of siderophile elements (and a cceptance of chalcophile elements) in the liquid metal, postulated by Jones and Malvin (1990) in the Fe-Ni-S(sulfur)-M (siderophile) system is found to be present in the metal-silicate system as well. A sulfur- bearing liquid metal segregation can potentially reduce the metal-lovi ng nature of many elements to explain the excess paradox. Sulfur-beari ng core segregation, however, might require an efficient draining of e xsolved immiscible sulfide liquids from the molten silicate, or an inc reasing siderophility of sulfur at high pressure to reduce the mantle sulfur content to the observed (<300 ppm) value. Moreover, the chondri tic relative abundance pattern of many moderately or highly siderophil e elements in the upper mantle is not explained by the presence of sul fur in the segregating metals. Core formation is more complex and intr icate than equilibrium segregation. Copyright (C) 1997 Elsevier Scienc e Ltd.