Delayed, disequilibrium degassing in rhyolite magma: decompression experiments and implications for explosive volcanism

Recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface. In this study a series of decompression experiments using crystal-free, rhyolite melt were conducted to examine the development of large supersaturations due to delayed, homogenous (spontaneous) bubble nucleation. Melts were saturated at 900 degreesC and 200 MPa with either 5.2 wt% dissolved H2O, or with 4.2 wt% H2O and 640 ppm CO2, and isothermally decompressed at linear rates of either 0.003, 0.025, or 8.5 MPa/s to final pressures between 25 and 175 MPa. Additional isobaric saturation experiments (900 degreesC, 200-25 MPa) using pure H2O or mixed H2O-CO2 fluids establish reference equilibrium solubility curves/values. Homogenous nucleation is triggered in both H2O-only and H2O-CO2 experiments once the supersaturation pressure (DeltaP(ss)) reaches similar to 120-150 MPa and the melt contains similar to two times its equilibrium water contents. Bubble number density and nucleation rate depend on the supersaturation pressure, with values on the order of 10(2)/cm(3) and < 1/cm(3)/s for P-ss similar to 120 MPa; 10(6)/cm(3) and 10(3)-10(5)/Cm-3/s for DeltaP(ss) similar to 130-150 MPa; and 10(7)/cm(3) and 10(6)/cm(3)/s for DeltaP(ss) similar to 160-175 MPa. Nucleation rates are consistent with classical nucleation theory, and infer an activation energy for nucleation of 1.5 x 10(-18) J/nucleus, a critical bubble radius of 2 x 10(-9) m, and an effective surface tension for rhyolite at 5.2 wt% H2O and 900 degreesC of 0.10-0.11 N/m. The long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved H2O and CO2 contents persist to the end of our runs. The disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi-equilibrium paths found in other studies where bubble nucleation occurs heterogeneously on crystals or other discontinuities in the melt at low DeltaP(ss). Homogenous and heterogenous nucleation rates are comparable, however, as are bubble number densities, so that at a given decompression rate it appears that nucleation mechanism, rather than nucleation rate, determines degassing efficiency by fixing the pressure (depth) at which vesiculation commences and hence the time available for equilibration prior to eruption. Although real systems are probably never truly crystal-free, our results show that rhyolitic magmas containing up to 10(4) crystals/cm(3), and perhaps as high as 10(6) crystals/cm(3), are controlled by homogenous, rather than heterogenous, nucleation during ascent. (C) 2000 Elsevier Science B.V. All rights reserved.