Sulfur partitioning between aqueous fluids and felsic melts at high pressures: Implications for sulfur migration in subduction zones

Sulfur (S), an essential volatile in subduction zone magmatism, exhibits higher solubility in aqueous fluids compared to silicate melts. Despite its importance, the partitioning of S between aqueous fluids and silicate melts under the conditions of subduction zone, critical for magma generation and evolution, remains poorly understood. To address this knowledge gap, we performed piston-cylinder experiments at a temperature of 950 ℃ and pressures of 1 and 2 GPa, investigating the effects of various parameters including oxygen fugacity, melt composition, fluid composition (salinity) and pressure on S partitioning between aqueous fluid and silicate melt (D_S^fluid/melt). Our results indicate that the D_S^fluid/melt is always large (> > 1), and S prefers to enter the aqueous fluid at high pressures. However, the D_S^fluid/melt decreases with increasing pressure from 1 to 2 GPa. Specifically, under reducing conditions (Ni-NiO buffer), D_S^fluid/melt decreased from 147 ± 40 to 20 ± 2, whereas under moderately oxidizing conditions (Re-ReO₂ buffer), it decreased from 27 ± 1 to 20 ± 2. These results stress the strong affinity of S for aqueous fluids at high pressures. Together with the great capacity for S dissolution in the H₂O-rich magma within the deep Earth, fluid-saturated felsic magma efficiently transports substantial amounts of S from deep to shallow regions in subduction zone settings. This process plays a crucial role in the formation of giant porphyry deposits and provides a potential source of excess S released during explosive volcanic eruptions.