Effects of sulfide composition and melt H2O on sulfur content at sulfide saturation in basaltic melts

Abstract

Sulfur content at sulfide saturation (SCSS) is a critical parameter that controls the consumption of sulfide during partial melting as well as potential exsolution of sulfide during magma evolution. Natural magmatic systems involve H2O in silicate melt and NiS in FeS-dominated sulfide, but the combined effects of melt H2O and sulfide composition on SCSS have not been well constrained. We carried out 18 phase equilibrium experiments between anhydrous or hydrous basaltic melt and FeS-NiS sulfide at 1.0–2.5 GPa and 1523–1673 K in a piston cylinder apparatus. Experimental results demonstrate an increase of SCSS of approximately 104 ppm per wt% melt H2O and a proportional increase of SCSS with the mole fraction of FeS in sulfide (XFeS). In combination with literature data, we find that the effects of melt H2O and sulfide composition are independent and additive. A new model of SCSS for natural basaltic melt is developed:SCSS(ppm)=XFeS⋅exp13.88−9744T−328PT+104wH2O,where T is temperature in K, P is pressure in GPa, and wH2O is melt H2O content in wt%. This model not only fits our own data well but also reproduces literature data of SCSS in basaltic melt within 20% off. Application of this SCSS model to ridge magmatism shows that the observed S contents in MORBs corresponds to ~15% melting of mantle (after sulfide exhaustion). The higher S contents in some sulfide-saturated MORBs require a mantle source containing high-NiS sulfide (XNiS ≈ 0.4). Comparison of SCSS with S contents in melt inclusions from primary arc magmas suggests that sulfate is not required to be present for a majority of sub-arc mantle.