Bubble growth in highly viscous silicate melts during continuous decompression from high pressure

High melt viscosity is thought to hinder bubble growth in water-bearing sil icate melts, and viscosities above similar to 10(9) Pa s may prevent growth and viscously quench a bubbly liquid. To investigate the influence of melt viscosity (eta) On magma degassing, rhyolitic melts were experimentally sa turated with water at high pressures and then decompressed at a rate of eit her 0.125 or 0.25 MPa s(-1); viscosity (eta = 2.5 x 10(6)-6.3 x 10(8) 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 eta = 2.5 x 10(6) Pa a. w hereas when eta > 10(8) Pa s, the melts did not degas at all, despite press ure drops up to 50 MPa. The transition between efficient and inefficient de gassing occurred when eta = similar to 10(7-8) Pa s. In all experiments, bu bbles expanded in size in response to pressure drops, but the extent of exp ansion and the size of bubbles that expanded both decreased as viscosity in creased (e.g., 0-40 mu m bubbles expanded when eta = 1.6 x 10(8) Pa s; 0-20 mu m bubbles expanded when eta = 6.3 x 10(8) Pa s). The shift from efficie nt to inefficient degassing probably resulted from the decrease in water di ffusivity (D-H2O) 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 wil l be arrested when eta similar to 10(9) Pa s, at decompression rates of 0.1 25 and 0.25 MPa s(-1). Such rates are expected only in explosive volcanic e ruptions, however, and so higher viscosities will be needed for the melt to resist bubble growth in effusive eruptions. Copyright (C) 2000 Elsevier Sc ience Ltd.