Heterogeneous bubble nucleation on pyroxene and plagioclase in andesite magmas

Understanding bubble nucleation and growth has long been considered a key to improving our knowledge of magmatic evolution, and aiding our goal of predicting violent volcanic eruptions. The role crystals play as heterogeneous nucleation sites for bubbles has become an active area of research because they have the potential to reduce the supersaturation necessary for bubble growth. The nucleation sites of bubbles in magmas are elusive because they cannot be directly observed in natural volcanic systems. Studies are generally conducted on natural, post-eruption samples or quenched experimental charges, but both provide only a view of the final state and provide little information on how this state was achieved. We directly observed bubble nucleation and growth by 4D in-situ synchrotron X-ray tomography of bubble nucleation and growth at the Swiss Light Source. Experiments were conducted on previously prepared, hydrous, crystal-bearing andesitic melts to observe bubble nucleation and track bubble growth and movement. We collected 3D images every 0.5 s while heating hydrated melts at 1 atm. We observed that bubbles first nucleated heterogeneously at clinopyroxene/melt and near plagioclase/melt interfaces, rather than homogeneously within the melt. Heterogeneous nucleation on one oxide crystal and homogeneous nucleation within the melt occurred significantly after nucleation on the silicates. The measured bubble-crystal contact angle was not constant and decreased with time. Bubbles grew much larger than the crystals in the experiments, producing textures similar to those seen in some natural volcanic samples. Our results show that the presence of silicate phases in magmas must be taken into account when discussing bubble nucleation in magmatic systems.