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

1Anke Wohlers, 1Bernard J. Wood
Geochimica et Cosmochimica acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.050]
1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
Copyright Elsevier

We have performed experiments at 1.5 GPa over the temperature range 1400-2100°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 (10 in some cases. Similarly DSm 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+US2UO2+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 DU. 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 1.5GPa and 1400°C we obtain DNd/DSm of about 1.4 and DTh ∼ 0.1DU. With increasing temperature the differences between these geochemically similar element pairs decreases such that, at 2100°C DNd/DSm is 1.0 and DTh/DU is about 0.3. We used these results, together with DU and DSm to model addition of a putative Mercury-like component (with FeS core) to early Earth. We find that the 1400o results could lead to a significant (∼11ppm) 142Nd 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°C results the 142Nd 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, ∼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.

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