Bubble nucleation in rhyolite and dacite melts: temperature dependence of surface tension

Surface tension (sigma) profoundly influences the ability of gas bubbles to nucleate in silicate melts. To determine how temperature impacts sigma, experiments were carried out in which high-silica rhyolite melts with 5 wt% dissolved water were decompressed at temperatures that ranged from 775 to 1,085A degrees C. Decompressions were also carried out using dacite melts with 4.3 wt% dissolved water at 1,150A degrees C. Water bubbles nucleated in rhyolite only when decompressions exceeded 95 MPa at all temperatures. Bubbles nucleated in number densities that increased as decompression increased and at hotter temperatures at a given amount of decompression. After correcting decompression amounts for temperature differences, values for sigma were estimated from nucleation rates and found to vary between 0.081 and 0.093 N m(-1). Surface tension decreases as temperature increases from 775 to 875A degrees C, but then increases as temperature increases to 1,085A degrees C. Those values overlap previous results, but only when melt viscosity is less than 10(4) Pa s. For low-viscosity rhyolite, there is a strong correlation of sigma with temperature, in which sigma increases by 6.9 x 10(-5) N m(-1) C-1. That variation is robust for 5-9 wt% dissolved water, as long as melt viscosity is a parts per thousand currency sign10(4) Pa s. More viscous rhyolite deviates from that correlation probably because nucleation is retarded in stiffer melts. Bubbles nucleated in dacite when decompressions exceeded 87 MPa, and occured in one or more events as decompression increased. Surface tension is estimated to be 0.083 (+/- 0.001) N m(-1) and when adjusted for temperature agrees well with previous results for colder and wetter dacite melts. At a given water content, dacite melts have lower surface tensions than rhyolite melts, when corrected to a fixed temperature.