Partitioning of sulfur and selenium between sulfide and silicate liquids and the thermochemistry of sulfide-selenide liquids

Abstract:

Selenium is a member of group 14 of the periodic table, directly below sulfur, and is generally acknowledged to exhibit similar geochemical behaviour to sulfur, as evidenced by complete solid solution between many sulfur and selenium minerals. However, in certain situations Se shows markedly different geochemical properties to S. In the context of igneous processes, for example, differences in the relative stabilities of Se^2-/SeO₄^2- and S^2-/SO₄^2- may be useful for unravelling the redox history of convergent margin magmas. Also, Se seems to be more resistant to late-stage degassing from magmas, allowing original S contents to be estimated from observed Se contents and assumed Se/S ratios (e.g. Jenner et al., 2010, Journal of Petrology, 51, 2445-2464). This raises the question of whether the Se/S ratios of these magmas can be assumed to reflect the Se/S ratios in the mantle source (usually assumed to be chondritic), or whether this ratio is fractionated during partial melting.

To address this issue, experiments were conducted in the piston-cylinder appratus to investigate the partitioning of S and Se between haplobasaltic silicate liquid and Fe-S-Se liquids near the FeS-FeSe join at 1400-1500 °C, 1.0-3.5 GPa and different Se/S ratios. Experimental results show that there is a slightly more compatible in Fe-S-Se liquid with increasing pressure, with K_D (XS^2- × XFeSe)/(XSe^2- × XFeS) increasing from 2.2 at 0.75 GPa to 2.74 at 3.5 GPa for compositions at molar S/(S+Se) of 0.82. This modest pressure dependence is consistent with earlier experimental results (Rose-Weston et al., 2009, Geochimica et Cosmochimica Acta, 73, 4598-4615). There is no significant temperature dependence over the relatively small temperature range investigated. Activity-composition relations derived from partitioning experiments suggest that FeS-FeSe liquids can be described by a symmetric regular solution model with a small positive deviation from ideality of 7.54 ± 1.16 kJ·mol^-1.