Water in rhyolitic magmas: Getting a grip on a slippery problem

Molecular water and hydroxyl groups are stable species in hydrous rhyolitic melts. To quantify the species concentrations we have used in situ IR spectroscopic measurements at 773-1073 K and 100-300 MPa in combination with an internally consistent calibration. The derived reaction enthalpy and entropy values for the homogeneous species reaction (H₂O_melt +O_melt = 2 OH_melt) are 35.0 ± 1.2 kJ mol^-1 and 27.7 ± 1.3 J mol^-1 K^-1, respectively and are independent of water content in the range 1.27-5.15 wt% water. Both values are significantly higher than extrapolated data based on species measurements of hydrous glasses at room temperature. The reaction entropy and enthalpy of hydroxyl formation are fundamental quantities for understanding and modeling hydrous magma properties.