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

Abstract

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^𝜈⁠). Most determinations of Pₛₐₜ^𝜈 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 Pₛₐₜ^𝜈⁠, 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^𝜈 under reducing conditions (ΔFMQ < 0). The effect of S on Pₛₐₜ^𝜈 is small except in the vicinity of the “sulfur solubility minimum” (SS^min; 0 < ΔFMQ < +2), where excluding S-bearing species can result in underestimates of Pₛₐₜ^𝜈⁠.

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 Pₛₐₜ^𝜈 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 SS^min. However, the f_O2 of ocean island and arc basalts are close enough to the SS^min and their S concentrations high enough to influence Pₛₐₜ^𝜈⁠. However, high-CO₂ and high-H₂O concentrations are predicted to reduce the effect of the SS^min. Hence, Pₛₐₜ^𝜈 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 Pₛₐₜ^𝜈 calculations for them require the inclusion of reduced C-O-H species.

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