MANTLE SULFIDE GEOBAROMETRY

A method of oxygen and sulfur barometry is based on the equilibrium, 2 Fe2SiO4 + S2 = 2 FeS + Fe2Si2O6 + O2, as contained in the peridotite mineral assemblage olivine-orthopyroxene-MSS (monosulfide solid soluti on {Fe,Ni,Co,Cu}S-S2). The method was applied to spinel peridotite xen oliths in basalts from the Massif Central and Montferrier and to the A riege (Pyrenees) orogenic peridotite massifs. Monosulfide solid soluti on compositions were taken exclusively from sulfide inclusions within silicates and are interpreted to represent subsolidus MSS formed durin g high P- T recrystallization and deformation of the peridotites. Calc ulated f(O2)'s are consistent with spinel oxybarometry of shallow subc ontinental mantle lithosphere but are more scattered. Scatter is due, at least in part, to nonrandom sectioning of multiphase sulfide grains . The f(S2)'s, which are coupled to f(O2) through the equilibrium abov e, average 1-2 log units above the quartz-fayalite-magnetite-pyrrhotit e reference buffer. At such conditions, in primitive mantle containing 200-250 ppm S, at least 95% of that S would be present in MSS, and 5% or less (as minor H2S) in CO2-H2O fluids.