Partitioning of trace amounts of highly siderophile elements in the Fe-Ni-S system and their fractionation in nature

The partitioning of highly siderophile elements (HSEs: Re, Os, Ir, Pt, Pd, and Au) between solid metal and liquid metal sulfide in the Fe-Ni-S system at 1000 to 1200 degrees C and low pressure has been investigated at abundan ces close to those in nature by using neutron activation analysis. Partitio n coefficients (D) are high for the most refractory metals (Re, Os, and Ir) , intermediate for Ru and Pt, and near unity for the least refractory metal s (Pd, Ni, and Au), results broadly consistent with literature data. The pa rtitioning of HSE seems to be reasonably independent of the concentration o f these elements (trace to minor), temperature (1000 to 1900 degrees C), an d pressure (1 bar to 11 GPa), but varies markedly with variation in bulk co mposition. particularly for the refractory metals. We demonstrate fractiona tion of Re and Os in the Fe-S system [molar D(Os)/D(Re) = 1.15 +/- 0.11] co nsistent with fractionation patterns for Re and Os in magmatic iron meteori tes and qualitatively consistent with that hypothesised for Os-187-enrichme nt in some oceanic island basalt (OIB) sources. Laboratory partition coeffi cients for HSE in the Fe-Ni-S system correlate closely with log HSE vs. log Ni regression slopes of the magmatic iron meteorites. The selective partit ioning behavior of HSE is related to the extreme range in melting point of the pure elements, and is unlikely to be erased completely by pressures in the earth's core, even in the hypothetical absence of S. Also, the abundanc es of HSE in mantle peridotites and OIB could not have been influenced by p artitioning between large quantities of solid metal and liquid metal sulfid e. Copyright (C) 1999 Elsevier Science Ltd.