The Effect of FeO on the Sulfur Content at Sulfide Saturation (SCSS) and the Selenium Content at Selenide Saturation of Silicate Melts

Abstract

The concentration of sulfur in basalt-like silicate melts as S^2– is limited to the amount at which the melt becomes saturated with a sulfide phase, such as an immiscible sulfide melt. The limiting solubility is called the ‘sulfur content at sulfide saturation’ (SCSS). Thermodynamic modelling shows that the SCSS depends on the FeO content of the silicate melt from two terms, one with a negative dependence that comes from the activity of FeO in the silicate melt, and the other with a positive dependence that comes from the strong dependence of the sulfide capacity of the melt (C_S) on FeO content. The interaction between these two terms should yield a net SCSS that has an asymmetric U-shaped dependence on the FeO content of the melt, if other variables are kept constant. We have tested this thermodynamic model in a series of experiments at 1400°C and 1·5 GPa to determine the sulfur contents at saturation with liquid FeS in melt compositions along the binary join between a haplobasaltic composition and FeO. The SCSS is confirmed to have the asymmetric U-shaped dependence, with a minimum at ∼5 wt % FeO. The effect of FeO on the selenide content at selenide saturation (SeCSeS) was investigated in an analogous fashion. SeCSeS shows a similar, though not identical, U-shaped dependence, implying that the solubility mechanism of selenide in basalt-like silicate melts is similar to that of sulfide. The observation of increasing SCSS with decreasing FeO in hydrous silicic melts was explored by inverse modelling of datasets from pyrrhotite-saturated hydrous silicic liquids, revealing that high SCSS at low FeO can be explained in terms of the low-FeO limb of the ‘U’, rather than dissolution of sulfur as hydrous species such as H₂S or HS^–. Recent measurements of the composition of the surface of Mercury prompted examination of the high-SCSS, low-FeO limb of the ‘U’ as a potential explanation for the sulfur-rich but Fe-poor surface of Mercury.