Bubble growth in rhyolitic melts: Experimental and numerical investigation

Bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically. Rhyolitic melts were hydrated at 150 MPa. 780-850 degrees C to uniform water content of 5.5-5.3 wt%. The pressure was then dropped and held constant at 15-145 MPa. Upon the drop bubbles nucleated and were allowed to grow for various periods of time before final, rapid quenching of the samples. The size and number density of bubbles in the quenched glasses were recorded. Where number densities were low and run duration short, bubble sizes were in accord with the growth model of Scriven (1959) for solitary bubbles. However, most results did not fit this simple model because of interaction between neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between bubbles, was further developed and used to simulate bubble growth. The good agreement between experimental data, numerical simulation, and analytical solutions enables accurate and reliable examination of bubble growth from a limited volume of supersaturated melt. At modest supersaturations bubble growth in hydrated silicic melts (3-6 wt% water, viscosity 10(4)-10(6) Pa . s) is diffusion controlled. Water diffusion is fast enough to maintain steady-state concentration gradient in the melt. Viscous resistance is important only at the very early stage of growth (t < 1 s). Under the above conditions growth is nearly parabolic, R(2)=2Dt rho(m)(C-0-C-f)/rho(g) until the bubble approaches its final size. In melts with low water content: viscosity is higher and maintains pressure gradients in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascentduring eruptions. At high levels of supersaturation, advection of hydrated melt towards the growing bubble becomes significant. Our results indicate that equilibrium degassing is a good approximation for modeling vesiculation in melts with high water concentrations (C-0 > 3 wt%) in the region above the nucleation level. When the melt accelerates and water content decreases, equilibrium can no longer be maintained between bubbles and melt. Supersaturation develops in melt pockets away from bubbles and new bubbles may nucleate. Further acceleration and increase in viscosity cause buildup of internal pressure in the bubbles and may eventually lead to fragmentation of the melt.