Experimental constraints on degassing of magma: isothermal bubble growth during continuous decompression from high pressure

Numerical models predict that rapid ascent of hydrous magma can lead to sup ersaturation of dissolved volatile constituents, possibly leading to explos ive eruption. We have performed controlled decompression experiments to inv estigate the ascent rates required to maintain bubble-melt equilibrium. Hig h-silica rhyolitic melts were saturated with water at 200 MPa and 825 degre es C, decompressed to lower pressures at constant rates of 0.025, 0.25, 0.5 , and 1.0 MPa s(-1) and then rapidly quenched isobarically. Other samples w ere saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency duri ng decompression. At a decompression rate of 0.025 MPa s(-1), melt-vapor eq uilibrium was maintained over the entire pressure range examined: 200 to 17 .5 MPa. A single bubble nucleation event occurred in response to decompress ion, and quenched bubble sizes can be modeled by a equilibrium bubble growt h model that takes into account the number density of bubbles. At decompres sion rates of 0.25, 0.5, and 1.0 MPa s(-1), rhyolitic melts could not degas in equilibrium when pressure decreased from 200 MPa to 140 MPa, and water supersaturation (Delta P) in the melt reached up to 60 MPa, with higher val ues at faster decompression rates. Further pressure release resulted in nea r equilibrium degassing and Delta P dropped significantly. in each case, De lta P decreased when bubbles exceeded 10 vol.%. A single, heterogeneous bub ble nucleation event occurred in each experiment when Delta P < 20 MPa; no other bubbles nucleated despite Delta P reaching 60 MPa, which is probably too low to trigger homogeneous nucleation. Compared to estimates for magma decompression rates during lava dome eruptions, our results indicate that m agmas can degas efficiently throughout their ascent to the surface. In expl osive eruptions, decompression rates may exceed those of this study and hen ce melts may become supersaturated with water. Such fast decompressions are expected, however, only when magma is highly vesicular, which would aid ap proach to equilibrium degassing. (C) 1999 Elsevier Science B.V. All rights reserved.