Computer Science
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000gecoa..64.1473g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 64, Issue 8, pp.1473-1483
Computer Science
18
Scientific paper
High melt viscosity is thought to hinder bubble growth in water-bearing silicate melts, and viscosities above ~10 9 Pa s may prevent growth and viscously quench a bubbly liquid. To investigate the influence of melt viscosity ( ) 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 ( = 2.5 × 10 6 -6.3 × 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 = 2.5 × 10 6 Pa s, whereas when > 10 8 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 = ~10 7-8 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 m bubbles expanded when = 1.6 × 10 8 Pa s; 0-20 m bubbles expanded when = 6.3 × 10 8 Pa s). The shift from efficient to inefficient degassing probably resulted from the decrease in water diffusivity ( D H 2 O ) 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 ~10 9 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.
Carroll Michael R.
Gardner James E.
Hilton Matt
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