Experimental demonstration of continuous bubble nucleation in rhyolite

Bubble nucleation is the first step in magma degassing. The kinetics of bubble nucleation during eruptive magma ascent determines the rate of magma degassing, which in turn, has a significant control on the explosivity of eruptions. Here, we examine if bubble nucleation in rhyolite can be continuous over the inferred timescale of magma travel time from reservoir to the surface in explosive silicic eruptions. We performed homogeneous bubble nucleation experiments using cylinders cored from natural rhyolite. Samples were hydrated at an initial pressure and temperature, then decompressed to a final pressure, and held at that pressure for various amounts of time before being quenched. Within a given suite samples had the same temperature as well as initial and final pressures, but were held for different annealing times. The resulting bubble number density of samples within each suite increases by up to 2 orders of magnitude as annealing time increases, indicating that nucleation continued during annealing. We estimate the nucleation rate and duration from measured bubble number densities and show that nucleation in the majority of samples was still ongoing at the time of quenching. Our findings confirm theoretical predictions that bubble nucleation can be a continuous process during eruptive magma ascent in explosive eruptions.