Hydrothermal chemistry of seawater from 25 degrees to 350 degrees C

Progressive heating of seawater to 350 degrees C causes it to become increasingly acid and depleted in Ca, Mg, and SO4, because anhydrite and a previously undescribed magnesium oxysulfate are precipitated. Observed composition changes and calculation of activities of dissolved species reveal a progressive increase of HSO4-, HCl, and MgOH+ and decrease of HCO3-, C03-, and CaHCO3+ at temperatures above 150 degrees C. The pH at experimental conditions drops to 3.3, while the buffer capacity of seawater increases from 0.24 meq/pH at 25 degrees C to 54 meq/pH at 350 degrees C. Theoretical solubilities of brucite, anhydrite, magnesite, dolomite, and calcite were compared to ion-activity products. Comparison of solution composition with theoretical solubilities indicates that carbonate minerals were supersaturated with increasing temperature. The solution composition indicates equilibrium saturation with anhydrite at temperatures of 150 degrees C.