The sulfide capacity and the sulfur content at sulfide saturation of silicate melts at 1400°C and 1 bar

The solubility of sulfur as S² has been experimentally determined for 19 silicate melt compositions in the system CaOMgO-Al₂O₃-SiO₂ (CMAS) ± TiO₂ ± FeO, at 1400°C and 1 bar, using CO-CO₂-SO₂ gas mixtures to vary oxygen fugacity (fO₂) and sulfur fugacity (fS₂). For all compositions, the solubility is confirmed to be proportional to (fS₂/fO₂)^1/2, allowing the definition of the sulfide capacity (C_s) of a silicate melt as C_s = [S](fO₂/fS₂)^1/2. Additional experiments covering over 150 melt compositions, including some with Na and K, were then used to determine C_s as a function of melt composition at 1400°C. The results were fitted to the equation C_S = A_O + ∑_MA_MX_M + B_Fe-_TiX_FeX_Ti, where A_Fe ≫ A_Ca > A_Mg,A_Na/K,A_Ti. The FeO content of natural basalts is the dominant control on C_S. The equation for C_S was then combined with the equation for the thermodynamic equilibrium between silicate melt and immiscible FeS-rich sulfide melt, to develop an expression for the sulfur content at sulfide saturation (SCSS) of the silicate melt. The value of SCSS is independent of fO₂ and fS₂, but shows an asymmetric U-shaped dependence on the FeO content of the silicate melt. Many of the experiments on Fe-containing melt compositions were saturated with immiscible FeS melt, and these experiments were used to calibrate quantitatively SCSS at 1400°C as a function of melt composition.