Sulfide and sulfate saturation of dacitic melts as a function of oxygen fugacity

Abstract

Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2) across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here, we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2O-saturated natural dacitic melt at 1000 °C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the sulfide-sulfate transition (log fO2 = ΔFMQ−0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.48, ΔFMQ+1.54, ΔFMQ+1.75, ΔFMQ+2.08 and ΔFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfide-sulfate transition occurs at ΔFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate to evolved silicate melts as a function of fO2:SCSSdacitic = [S2−] (1 + 10(2.00ΔFMQ – 3.05))SCASdacitic = [S6+] (1 + e(1.26 – 2.00ΔFMQ))The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that predicted by thermodynamics.