Uranium, thorium and REE partitioning into sulfide liquids: Implications for reduced S-rich bodies

We have performed experiments at 1.5 GPa over the temperature range 1400- 2100 degrees C to determine the partitioning of lithophile elements (U, Th, Eu, Sm, Nd, Zr, La, Ce, Yb) between sulfide liquid, low-S metals and silicate melt. The data demonstrate pronounced increases in partitioning of all the lithophile elements into sulfide at very low FeO contents (< 1 wt%) of the silicate melt such that D-U = ([U](sulfide)/ [U](silicate) exceeds 1 and may be > 10 in some cases. Similarly D-Sm may be > 2 under the same conditions of low silicate FeO. This strong partitioning behaviour is found only be important in S- rich metals, however because the observed effect of low FeO on partitioning is uniquely confined to metallic melts close to stoichiometric FeS in composition. The results and the effects of FeS content of the metal and FeO content (or activity) of the silicate may be understood in terms of exchange reactions such as: UO2 + 2FeS = 2FeO + US2 Silicate sulfide silicate sulfide High concentrations of FeS (in metal) and low FeO contents of the silicate melts drive the reaction to the right, favouring high US2 in the sulfide and hence high D-U. The effect is, we find, enhanced by the high solubility of S in the silicate (up to 11 wt%) at low FeO contents. This S content greatly reduces the activity coefficient of FeO in the silicate melt, enhancing the displacement of the reaction to the right. For sulfide-silicate partitioning at 1.5 GPa and 1400 degrees C we obtain D-Nd/ D-Sm of about 1.4 and D-Th similar to 0.1DU. With increasing temperature the differences between these geochemically similar element pairs decreases such that, at 2100 degrees C D-Nd/D-Sm is 1.0 and D-Th/D-U is about 0.3. We used these results, together with D-U and DSm to model addition of a putative Mercury-like component (with FeS core) to early Earth. We find that the 1400 degrees results could lead to a significant (similar to 11 ppm) 142 Nd anomaly in silicate Earth and add > 8 ppb U to the core, but lead to an unreasonably high Th/U of silicate Earth (4.54). Based on the 2100 degrees C results the Nd-142 anomaly would be 0 but addition of the sulfur- rich body could add up to 10 ppb of U to the core, generating, when the accompanying 21 ppb Th is also considered, similar to 3 TW of the energy required for the geodynamo. In this case, the Th/U ratio of silicate Earth would approximate 4.3, within the range of some estimates. (C) 2017 The Author(s).