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The solubility of sulfur as S2– has been experimentally determined for 19 silicate melt compositions in the system CaO–MgO–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 S solubility is confirmed to be proportional to (fS2/fO2) 1/2, allowing the definition of the sulfide capacity (CS) of a silicate melt as CS = S (fO2/fS2) 1/2. Additional experiments covering over 150 melt compositions, including some with Na and K, were then used to determine CS as a function of melt composition at 1400°C. The results were fitted to the equation \ (Cₒ = A₀+₌A₌X₌+B₅₄-ₓ₈X₅₄Xₓ₈\) , where AFe ≫ ACa > AMg, ANa/K, ATi. The FeO content of natural basalts is the dominant control on CS. The equation for CS 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.
Hugh O’Neill (Sat,) studied this question.