Trace element partitioning in the Fe–S–C system and its implications for planetary differentiation and the thermal history of ureilites

Element partitioning in metal–light element systems is important to our understanding of planetary differentiation processes. In this study, solid-metal/liquid-sulfide, liquid-metal/liquid-sulfide and solid-metal/troilite partition coefficients (D) were determined for 18 elements (Ag, As, Au, Co, Cr, Cu, Ge, Ir, Ni, Os, Pd, Pt, Mo, Mn, Re, Ru, Se and W) in the graphite-saturated Fe–S–C system at 1 atm. Compared at the same liquid S concentration, the solid/liquid partition coefficients are similar to those in the Fe–S system, but there are systematic differences that appear to be related to interactions with carbon dissolved in the solid metal. Elements previously shown to be “anthracophile” generally have larger solid/liquid partition coefficients in the Fe–S–C system, whereas those that are not have similar or smaller partition coefficients in the Fe–S–C system. The partitioning of trace elements between C-rich and S-rich liquids is, in most cases, broadly similar to the partitioning between solid metal and S-rich liquid. The highly siderophile elements Os, Re, Ir and W are partitioned strongly into the C-rich liquid, with D ≫ 100. The partition coefficients for Pt, Ge and W decrease significantly at the transition to liquid immiscibility, while the partition coefficient for Mo increases sharply. The bulk siderophile element patterns of ureilite meteorities appear to be better explained by separation of S-rich liquid from residual C-rich metallic liquid at temperatures above the silicate solidus, rather than by separation of S-rich liquid from residual solid metal at lower temperatures.