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 spec troscopic measurements at 773-1073 K and 100-300 MPa in combination with an internally consistent calibration. The derived reaction enthalpy and entro py values for the homogeneous species reaction (H2Omelt+O-melt=2 OHmelt) ar e 35.0+/-1.2 kJ mol(-1) and 27.7+1.3 J mol(-1) K-1, respectively and are in dependent of water content in the range 1.27-5.15 wt% water. Both values ar e 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 mode ling hydrous magma properties. (C) 2001 Elsevier Science B.V, All rights re served.