Bubbles and atom clusters in rock melts: A chicken and egg problem

Abstract

Bubble and crystal nucleation and growth dynamics play a vital function in determining the eruptive behaviour of a magma. Investigating the initial stages of bubble nucleation and crystallization processes, including their rate and relative timing, can thus give us insight into the eruption parameters of magmas including their rheology. Although many experiments and numerical simulations of decompression-induced vesicle nucleation in magmas have been carried out, the mechanisms of bubble nucleation remain poorly understood. It is widely accepted that there are two ways bubbles can form: either by homogeneous nucleation within a melt, or by heterogeneous nucleation on a surface, interface, or particle. It has been tentatively suggested that what had been interpreted as homogeneous nucleation in magmas was instead heterogeneous nucleation on the surface of elusive nano- or micro-scale crystals present throughout the melt. Herein, we have conducted an experiment whereby bubble nucleation is induced in a synthetic andesitic glass sample using radiation damage from a focused ion beam (FIB) to investigate the initial stages of bubble nucleation. Scanning transmission electron microscopy, electron energy-loss spectroscopy and atom probe tomography (APT) data reveal sub-nanometer scale chemical heterogeneities in the form of Fe-, Na-, and/or Ca-rich clusters at the bubble-glass interface of the newly created bubbles. Comparatively, specimens prepared by cryogenic FIB, in which bubble nucleation is suppressed, are without bubbles; however, APT data shows similar chemical heterogeneities suggesting these clusters existed within the synthetic glass prior to bubble formation. Based on these observations, our data demonstrate the existence of nanoscale chemical heterogeneities within a synthetic silicate glass and at bubble-glass interfaces. Although our experiments do not replicate the conditions of natural processes, our results contribute to the hypothesis that previously described homogeneous nucleation may be re-interpreted as a variety of heterogeneous nucleation. These findings highlight the need to revise the commonly used definition of homogeneous nucleation among magma scientists.