Experimental evidence for sulfur-rich martian magmas: Implications for volcanism and surficial sulfur sources

It has been proposed that sulfur outgassing by volcanism on Mars contributes to the sulfur budget and volatile cycle at and near the surface (Farquhar et al., 2000; Halevy et al., 2007). However, it remains unclear if shergottites and martian magmas are sulfide saturated or under-saturated, and therefore how much sulfur they can contribute via volcanism. Expressions derived from experimental studies allow prediction of S contents, though they are not calibrated for shergottitic liquids, which are FeO-rich and Al2O3-poor compared to the terrestrial experimental dataset upon which the models have been based. We have carried out new experiments designed to test current S saturation models, and then show that existing calibrations are not suitable for high FeO and low Al2O3 compositions characteristic of shergottitic liquids. The new results show that existing models either under-predict by a factor of 2, or over-predict by a factor of 2–3, the S contents of sulfide saturated shergottitic liquids. Primitive shergottite magmas are capable of dissolving no more than ~ 4000 ppm S at sulfide saturation. Melting of the martian mantle, and subsequent volcanism provides more than enough sulfur for the sulfate-rich soil and layered deposits recognized on the surface of Mars.