Super-hydrous arc magmas delay magnetite saturation and sulfide precipitation

Abstract Magnetite crystallization strongly influences iron (Fe) behavior, sulfide saturation, and redox evolution in arc magmas. However, the combined effects of H2O and oxygen fugacity (fO2) on magnetite solubility in hydrous silicate melts remain unclear. We conducted piston-cylinder experiments on hydrous dacitic melts at 1–2 GPa, 850–1000 °C, and fO2 from FMQ–1.5 to FMQ+5.0 (FMQ = Fayalite-Magnetite-Quartz buffer). Magnetite solubility (melt FeOtot at magnetite saturation) increases with temperature and melt H2O content but decreases with rising fO2. Increasing melt H2O from 5 to 15 wt% at ∼FMQ+2 doubles melt FeOtot (3 to 7 wt%), whereas increasing fO2 from FMQ+1 to FMQ+3 at fixed H2O (5 wt%) decreases FeOtot (4 to 1 wt%). Highest FeOtot occurs at intermediate fO2 (FMQ+2) when melt H2O exceeds 10 wt%. Additionally, increasing FeOtot content in the melt enhances the sulfur solubility. Thereby, elevated H2O delays magnetite and sulfide saturation, significantly affecting Fe and chalcophile element evolution in hydrous arc magmas.