The Influence of Oxygen Fugacity and Water on Silicate Crystallization in the Emeishan Large Igneous Province: Constraints from Thermodynamic Modeling Using MELTS

The Emeishan Large Igneous Province (ELIP) contains high-Mg volcanic rocks, evolved basalts, and Fe-Ti-V-oxide-bearing layered intrusions, providing a natural system for investigating the coupled effects of H2O and the redox state on silicate crystallization. We modeled three representative starting melts, namely Melt-6 (basaltic), Melt-8 (meimechitic), and Melt-10 (komatiitic), at 5 kbar under prescribed fO2 conditions spanning ΔFMQ = −3 to + 3 using rhyolite-MELTS. The simulations show that the starting melt composition sets the liquidus temperature and controls whether olivine saturates first. H2O depresses silicate saturation temperatures, with the strongest effect on plagioclase, whereas fO2 modifies phase relations mainly through Fe2+/Fe3+, effective FeO activity, and competition with Fe-Ti oxide saturation. The orthopyroxene saturation temperature and enstatite-ferrosilite (En-Fs) composition show the strongest redox response, whereas clinopyroxene is buffered by Ca occupancy in the M2 site and plagioclase records mainly the H2O content and residual melt evolution. These results support a coupled model in which the starting melt, H2O, and fO2 jointly control crystallization paths and Fe-Ti-V oxide enrichment in ELIP layered intrusions.