Evolution of melt-vapor surface tension in silicic volcanic systems: Experiments with hydrous melts

[1] We evaluate the melt- vapor surface tension ( sigma) of natural, water- saturated dacite melt at 200 MPa, 950 - 1055 degrees C, and 4.8 - 5.7 wt % H2O. We experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for s at those conditions using classical nucleation theory. The solutions obtained give dacite melt- vapor surface tensions that vary inversely with dissolved water from 0.042 ( +/- 0.003) J m(-2) at 5.7 wt % H2O to 0.060 ( +/- 0.007) J m(-2) at 5.2 wt % H2O to 0.073 ( +/- 0.003) J m(-2) at 4.8 wt % H2O. Combining our dacite results with data from published hydrous haplogranite and high- silica rhyolite experiments reveals that melt-vapor surface tension also varies inversely with the concentration of mafic melt components ( e. g., CaO, FeOtotal, MgO). We develop a thermodynamic context for these observations in which melt- vapor surface tension is represented by a balance of work terms controlled by melt structure. Overall, our results suggest that cooling, crystallization, and vapor exsolution cause systematic changes in s that should be considered in dynamic modeling of magmatic processes.