The efects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma

Geobarometers are commonly used to determine the pressure (and hence depth) of magmatic bodies. For instance, at equilibrium, the concentration of dissolved volatiles in a vapor-saturated melt can be used as a barometer: this is the pressure of vapor-saturation (Psat^v). Most determinations of Psat^v assume that melt and vapor contain only oxidized C-O-H species. However, sulfur is the third most abundant volatile element in magmas, and oxygen fugacity (fO2) exerts a strong influence on the speciation of the melt and vapor. To explore how S and fO2 affect calculations of Psat^v, we model a Hawaiian tholeiite that contains both reduced and oxidized C-O-H-S species in the melt and vapor. We find that excluding reduced C-O-H species in the system can result in significant underestimations of Psat^v under reducing conditions (Delta FMQ < 0). The effect of S on Psat^v is small except in the vicinity of the "sulfur solubility minimum" (SS^min; 0 < Delta FMQ < +2), where excluding S-bearing species can result in underestimates of Psat^v. The implications of these results depend on the volatile concentration of the system being investigated, its fO2, and the melt composition and temperature. Our results suggest there will be little impact on Psat^v calculated for mid-ocean ridge basalts because their fO2 is above where reduced C-O-H species become important in the melt and vapor and yet below the SS^min. However, the fO2 of ocean island and arc basalts are close enough to the SS^min and their S concentrations high enough to influence Psat^v. However, high-CO2 and high-H2O concentrations are predicted to reduce the effect of the SS^min. Hence, Psat^v calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SS^min than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate Psatv calculations for them require the inclusion of reduced C-O-H species.