Water in rhyolitic and dacitic melts

This dissertation contains experimental investigations of H₂O solubility and diffusivity in silicate melts. These results provide insights into an understanding of the chemical and physical properties of magma of different compositions and thereby the understanding of various magmatic processes. The method for determining H₂O contents in all chapters in this dissertation is Fourier transform infrared spectroscopy (FTIR). The solubility of H₂O in the pressure range of 0.0981–25 MPa, where few accurate data exist, was experimentally determined. Using data from this study for low pressures and those from the literature for higher pressures (mostly ≥50 MPa), an empirical model is constructed. This empirical model agrees with all but one experimental datum (202 out of 203) within 15% relative error, which is a good improvement compared with previous model. Therefore this empirical model is recommended for calculating H₂O solubility and for modeling volcanic eruptions. The speciation of H₂ O in dacitic glass and melt with total dissolved H₂O contents of 1.5–2.5 wt% at 480–590°C and 0.1 MPa was determined. Besides estimating the apparent equilibrium temperature, it has the potential for obtaining the viscosity of dacitic melt with future kinetic study. With the information on the speciation of H₂O in dacitic melt, the diffusivity of H₂O in dacitic melt was studied at 551–637°C for samples with 0.8–2.5 wt% total dissolved H₂O. In this temperature range, water diffuses slower in dacitic melt than in rhyolitic melt. The activation energy for H₂O diffusion in dacitic melt is greater than that in rhyolitic melt. Hence, extrapolation indicates that the H₂O diffusivity in the two melts would be similar at ∼900°C for 0.8 wt% and ∼740°C for 2.5 wt% of total dissolved H₂O.