The effects of composition and temperature on chalcophile and lithophile element partitioning into magmatic sulphides

We develop a comprehensive model to describe trace and minor element partitioning between sulphide liquids and anhydrous silicate liquids of approximately basaltic composition. We are able thereby to account completely for the effects of temperature and sulphide composition on the partitioning of Ag, Cd, Co, Cr, Cu, Ga, Ge, In, Mn, Ni, Pb, Sb, Ti, Tl, V and Zn. The model was developed from partitioning experiments performed in a piston-cylinder apparatus at 1.5 GPa and 1300 to 1700 °C with sulphide compositions covering the quaternary FeSNiSCuS0.5FeO.Partitioning of most elements is a strong function of the oxygen (or FeO) content of the sulphide. This increases linearly with the FeO content of the silicate melt and decreases with Ni content of the sulphide. As expected, lithophile elements partition more strongly into sulphide as its oxygen content increases, while chalcophile elements enter sulphide less readily with increasing oxygen. We parameterised the effects by using the ε-model of non-ideal interactions in metallic liquids. The resulting equation for partition coefficient of an element M between sulphide and silicate liquids can be expressed aslog⁡DM sulph /sil=A+BT−n2log⁡[FeO]corr+1673T[εMSn/2FeOlog⁡(1−xFeO)+εMSn/2NiSlog⁡(1−xNiS)+εMSn/2CuS0.5log⁡(1−xCuS0.5)] where A is a constant related to the entropy change of the partitioning reaction, B is a constant related to its enthalpy and n is the valency of the element of interest. Interaction parameters εFeO, εNiS and εCuS0.5 refer to non-ideal interactions between trace element and matrix, xFeO, xNiS, xCuS0.5 and xFeS are mole fractions of FeO, NiS, and CuS0.5 in sulphide and FeOcorr is FeO content of silicate liquid (wt%) corrected for the ideal activity of FeS in the sulphide as follows:[FeO]corrected=[FeO]silicate[Fe/(Fe+Ni+Cu)]sulph We find, for most elements, that the effect of Ni and Cu on partitioning is significantly smaller than the effect of oxygen. The effects of temperature are greatest for Ni, Cu and Ag.We used our model to calculate the amount of sulphide liquid precipitated along the liquid line of descent of MORB melts and find that 70% of silicate crystallisation is accompanied by ∼0.23% of sulphide precipitation. The latter is sufficient to control the melt concentrations of chalcophile elements such as Cu, Ag and Pb. Our partition coefficients and observed chalcophile element concentrations in MORB glasses were used to estimate sulphur solubility in MORB liquids. We obtained between ∼800 ppm (for primitive MORB) and ∼2000 ppm (for evolved MORB), values in reasonable agreement with experimentally-derived models. The experimental data also enable us to reconsider Ce/Pb and Nd/Pb ratios in MORB. We find that constant Ce/Pb and Nd/Pb ratios of 25 and 20, respectively, can be achieved during fractional crystallisation of magmas generated by 10% melting of depleted mantle provided the latter contains >100 ppm S and about 650 ppm Ce, 550 ppm Nd and 27.5 ppb Pb.Finally, we investigated the hypothesis that the pattern of chalcophile element abundances in the mantle was established by segregation of a late sulphide matte. Taking the elements Cu, Ag, Pb and Zn as examples we find that the Pb/Zn and Cu/Ag ratios of the mantle can, in principle, be explained by segregation of ∼0.4% sulphide matte to the core.