The effects 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 (P-sat(v)). Most determinations of P-sat(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 (f(O2)) exerts a strong influence on the speciation of the melt and vapor. To explore how S and f(O2) affect calculations of P-sat(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 P-sat(v) under reducing conditions (Delta FMQ < 0). The effect of S on P-sat(v) is small except in the vicinity of the "sulfur solubility minimum" (SSmin; 0 < Delta FMQ < +2), where excluding S-bearing species can result in underestimates of P-sat(v). The implications of these results depend on the volatile concentration of the system being investigated, its f(O2), and the melt composition and temperature. Our results suggest there will be little impact on P-sat(v) calculated for mid-ocean ridge basalts because their f(O2) is above where reduced C-O-H species become important in the melt and vapor and yet below the SSmin. However, the f(O2) of ocean island and arc basalts are close enough to the SSmin and their S concentrations high enough to influence P-sat(v). However, high-CO2 and high-H2O concentrations are predicted to reduce the effect of the SSmin. Hence, P-sat(v) calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SSmin than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate P-sat(v) calculations for them require the inclusion of reduced C-O-H species.