BUBBLE-GROWTH IN RHYOLITIC MELTS - EXPERIMENTAL AND NUMERICAL INVESTIGATION

Bubble growth controlled by mass transfer of water from hydrated rhyol itic melts at high pressures and temperatures was studied experimental ly 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 press ure was then dropped and held constant at 15-145 MPa. Upon the drop bu bbles nucleated and were allowed to grow for various periods of time b efore final, rapid quenching of the samples. The size and number densi ty of bubbles in the quenched glasses were recorded. Where number dens ities were low and run duration short, bubble sizes were in accord wit h the growth model of Scriven (1959) for solitary bubbles. However, mo st results did not fit this simple model because of interaction betwee n neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between bubbles, was furt her developed and used to simulate bubble growth. The good agreement b etween experimental data, numerical simulation, and analytical solutio ns enables accurate and reliable examination of bubble growth from a l imited volume of supersaturated melt. At modest supersaturations bubbl e 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 resi stance 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 gra dients in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascentduring eruptions. At high levels of supe rsaturation, advection of hydrated melt towards the growing bubble bec omes 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 c an no longer be maintained between bubbles and melt. Supersaturation d evelops in melt pockets away from bubbles and new bubbles may nucleate . Further acceleration and increase in viscosity cause buildup of inte rnal pressure in the bubbles and may eventually lead to fragmentation of the melt.