BUBBLE GROWTH IN HIGHLY VISCOUS SILICATE MELTS DURING CONTINUOUS DECOMPRESSION FROM HIGH PRESSURE

Abstract

High melt viscosity is thought to hinder bubble growth in water-bearing silicate melts, and viscosities above ~109 Pa s may prevent growth and viscously quench a bubbly liquid. To investigate the influence of melt viscosity (η) on magma degassing, rhyolitic melts were experimentally saturated with water at high pressures and then decompressed at a rate of either 0.125 or 0.25 MPa s-1; viscosity (η = 2.5 x 106-6.3 x 108 Pa s) was varied between experiments by changing the initial hydration pressures and temperatures. Dissolved water contents and bubble sizes and porosities indicate that melts degassed in equilibrium when η = 2.5 x 106 Pa s, whereas when η > 108 Pa s, the melts did not degas at all, despite pressure drops up to 50 MPa. The transition between efficient and inefficient degassing occurred when η = ~107-8 Pa s. In all experiments, bubbles expanded in size in response to pressure drops, but the extent of expansion and the size of bubbles that expanded both decreased as viscosity increased (e.g., 0-40 μm bubbles expanded when η = 1.6 x 108 Pa s; 0-20 μm bubbles expanded when η = 6.3 x 108 Pa s). The shift from efficient to inefficient degassing probably resulted from the decrease in water diffusivity (DH2O) as temperature decreased, whereas the decrease in degree of bubble expansion at higher viscosities resulted from increasing viscous resistance. Our results confirm model predictions that bubble expansion will be arrested when η ~109 Pa s, at decompression rates of 0.125 and 0.25 MPa s-1. Such rates are expected only in explosive volcanic eruptions, however, and so higher viscosities will be needed for the melt to resist bubble growth in effusive eruptions.