Solubility of H2O in rhyolitic melts at low pressures and a new empirical model for mixed H2O–CO2 solubility in rhyolitic melts

Experiments of H2O solubility in synthetic haplogranitic and natural rhyolitic melts were conducted at 700–1200 °C and 0.098–25 MPa. Attainment of equilibrium was proven by reversals using starting materials with different initial H2O contents. Dissolved H2O contents were determined using Fourier transform infrared spectroscopy. When temperature decreases from 1000 to 700 °C, the solubility of H2O increases from 0.100 to 0.124 wt.% at 0.098 MPa, from 0.99 to 1.36 wt.% at 11 MPa, and from 1.46 to 2.17 wt.% at 25 MPa. At 6 MPa, the solubility of H2O increases from 0.63 to 0.77 wt.% from 1200 to 850 °C. Combining our data with those from the literature on pure H2O and pure CO2 solubility, mixed H2O–CO2 solubility, and H2O solubility for H2O–H2 gas mixture (240 measurements for H2O and 59 points for CO2), we develop a single empirical model for solubility of pure H2O vapor, that of pure CO2 vapor, as well as that of mixed H2O–CO2 vapor in metaluminous high-silica rhyolitic melt. This model applies to the conditions of 700–1200 °C and 0–500 MPa, and consists of two equations: H 2 O t = ( 354.94 P w 0.5 + 9.623 P w − 1.5223 P w 1.5 ) / T + 0.0012439 P w 1.5 + P CO 2 ( − 1.084 × 10 − 4 P w 0.5 − 1.362 × 10 − 5 P w ) CO 2 = P CO 2 ( 5668 − 55.99 P w ) / T + P CO 2 ( 0.4133 P w 0.5 + 2.041 × 10 − 3 P w 1.5 ) where H2Ot is total dissolved H2O content in wt.%, CO2 content is in ppm by mass, and T is temperature in Kelvin, Pw=XwfP and PCO2=XCO2fP (in MPa), where Xwf and XCO2f are the mole fraction of water and CO2 in the fluid. The 2σ relative uncertainty is 15% for the H2O equation, and 20% for the CO2 equation. The above H2O equation also applies to H2O solubility in mixed H2O–H2 fluid for XH2f<0.54 at 100 MPa, for XH2f<0.49 at 200 MPa, and XH2f≤0.44 at 300 MPa (that is, the presence of H2 in the fluid insignificantly affects H2O solubility). This empirical model is recommended for the modeling of explosive volcanic eruptions and magma chamber dynamics.