Solubilities of carbon dioxide and water in rhyolitic melt at 850°C and 750 bars

Concentrations of carbon dioxide and water dissolved in glasses quenched from rhyolitic melts equilibrated with H ₂O-CO ₂ fluids at 850°C and 750 bar were measured using infrared spectroscopy; concentrations of H ₂O and CO ₂ in the quenched fluids were measured manometrically. The mole fraction of CO ₂ in the quenched fluid ranged from 0.06 to 0.91. Concentrations of CO ₂ in the coexisting rhyolitic melt increased from 23(±6) ppm for the sample equilibrated with the most CO ₂-poor fluid to 515(±16) ppm for that equilibrated with the most CO ₂-rich fluid. The water content of the melt varied from 0.51(±0.06) to 3.34(±0.08) wt%. Our results show that concentrations of molecular CO ₂ and H ₂O in the glasses obey Henry's Law; i.e., the mole fractions of molecular CO ₂ and molecular H ₂O in the quenched melts are proportional to their fugacities in the coexisting vapor. CO ₂ contents of vapor-saturated melts are not enhanced by addition of water to CO ₂-rich vapor, contrary to previous reports for silicate melts at higher pressures. The Henrian behavior of CO ₂ and H ₂O at low pressure considerably simplifies modeling of the degassing of silicic magmas.