Heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure

The nucleation of H2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 800 °C and either 50 or 125 MPa, and then decompressed by 20–91 MPa at temperatures that ranged between 550 and 700 °C, and held at the lower pressures for 10–720 s before being quenched. Bubbles nucleated in number densities (NB) that vary between 3 × 107 and 2 × 108 cm− 3. Blocky shaped magnetite or the ends of needle-shaped hematite acted as sites for nucleation, but only if a minimum super-saturation was exceeded, which increases with increasing melt viscosity. Bubbles did not nucleate along the lengths of hematite needles nor on plagioclase. Both the beginning and ending times of the nucleation event increases with increasing melt viscosity. Using nucleation theory predictions, neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature, water diffusivity, or viscosity. Instead, the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor, resulting from the increasing polymerization of the melt at lower temperatures and water contents. Because only ∼ 108 bubbles cm− 3 nucleate on magnetite in the rhyolite melt used, almost regardless of experimental conditions, results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions.