Crystals in rhyolitic melts can nucleate multiple bubbles: an experimentally constrained model

Abstract

Heterogeneous nucleation of bubbles on crystals in magma is thought to be an important driver of volcanic eruptions. Decompression experiments were carried out to elucidate the kinetics of H2O bubble nucleation on crystals of different sizes. Samples used were mixtures of hydrated rhyolitic melt and populations of augite or hornblende crystals of < 20 µm or 20–40 µm. We found that bubbles nucleated on surfaces of both types of crystals at supersaturation pressures ≤ 10 MPa, which is a small fraction of the supersaturation needed to homogeneously nucleate H2O bubbles in the melt. Very small crystals (< 5–10 µm) will nucleate only ~ 1 bubble, whereas on larger crystals, the number of bubbles that nucleate at a given supersaturation increases with an increased crystal surface area. In addition, the number of bubbles that nucleate on crystals of a given surface area increases with increasing supersaturation, indicating that the distance between nucleation sites decreases with increased supersaturation. We propose that the closer spacing of nucleation sites at higher supersaturation arises from the competition between bubble nucleation rate and the rate of H2O diffusion. Based on these results, we develop a modified heterogeneous nucleation rate equation that accounts for the control of crystal surface area on heterogeneous bubble nucleation and the resulting bubble number density generated. We provide a spreadsheet that allows users to calculate both total bubble number density and number densities per crystal size for a specified crystal-size population and different conditions (H2O content, temperature, and final pressure).