Nanoscale silicate melt textures determine volcanic ash surface chemistry

Explosive volcanic eruptions produce vast quantities of silicate ash, whose
surfaces are subsequently altered during atmospheric transit. These altered
surfaces mediate environmental interactions, including atmospheric ice
nucleation, and toxic effects in biota. A lack of knowledge of the initial, prealtered
ash surface has required previous studies to assume that the ash surface
composition created during magmatic fragmentation is equivalent to the
bulk particle assemblage. Here we examine ash particles generated by controlled
fragmentation of andesite and find that fragmentation generates ash
particles with substantial differences in surface chemistry. We attribute this
disparity to observations of nanoscale melt heterogeneities, in which Fe-rich
nanophases in themagmaticmelt deflect and blunt fractures, thereby focusing
fracture propagation within aureoles of single-phase melt formed during
diffusion-limited growth of crystals. In this manner, we argue that commonly
observed pre-eruptive microtextures caused by disequilibrium crystallisation
and/or melt unmixing can modify fracture propagation and generate primary
discrepancies in ash surface chemistry, an essential consideration for understanding
the cascading consequences of reactive ash surfaces in various
environments.