Experimental Hydrothermal Alteration of Rhyolite and Andesite at 325 C and 300 Bar: Implications for a Potential Role of Volcanic Glass in the Fluid Composition in the Okinawa Trough

The experimental study of water-rock reactions under high-temperature and -pressure conditions is a useful approach to constrain controlling factors of the fluid composition in a natural hydrothermal system. Previous studies have focused mainly on the mid-ocean ridge fields, and the hydrothermal alteration of intermediate-to-felsic rocks has been less emphasized despite its potential importance in the fluid chemistry in an arc/back-arc basin setting. We examined the alteration processes of fresh rhyolite and andesite rocks collected from the middle and southern Okinawa Trough, respectively, at 325 degrees C and 300 bar (the estimated condition at the reaction zone in the fields), especially focusing on the behavior of silica between the solid and liquid phases. The experimental fluids are characterized by the high Si concentration up to 30 mM, indicating the substantial dissolution of volcanic glass in the analyzed rocks. The high Si concentration in the fluids was presumably buffered by amorphous silica, precipitated from the fluids as a precursor of hydrothermal quartz, during the experiments. Our results emphasize a previously overlooked role of volcanic glass/amorphous silica in the fluid composition in the Okinawa Trough and are consistent with the previous model of pumice replacement mineralization for the SMS deposit formation in the trough.