Volcanic ash surface chemistry modified by elemental diffusion during late-stage microlite crystallization

Experiments were conducted to fragment cores from a glass-bearing andesitic lava bomb at Tungurahua. The resulting ash was sieved and the 63-90 μm-sized fraction was characterized by x-ray photoelectron spectroscopy (XPS) and QEMSCAN, an automated mineralogy technique using energy dispersive x-ray spectroscopy (EDX). Bulk and surface componentry were calculated from the QEMSCAN data with a resolution of 2 μm, and showed that the fraction of diopside microlites increased at the surface of the ash particles by a factor of 1.6. The average fraction of all phases at the ash-particle boundaries was used to calculate the bulk surface chemistry of the ash samples. XPS data were collected from the same sample mounts, however at a much finer depth resolution of <12 nm. Comparison of these two datasets showed large, unexpected element depletions in the XPS data, most significantly in Mg (decreased by 1.1-1.2 orders of magnitude), but also in Fe and Ca.

Textural inspection of the diopside microlites shows hypidiomorphic textures suggesting rapid growth rates. We observe a 1-2 μm compositional boundary layer in the surrounding glass, which is depleted in Mg and Fe. Elemental diffusion into the microlites is inferred to have depleted the compositional boundary layer of elements present at higher fractions in the crystal structure. We explain the element depletions found in the XPS data by fracture propagation focused within the compositional boundary layers surrounding diopside microlites during fragmentation. In this scenario, EDX measurements would detect elemental ratios matching diopside, however the XPS data would measure glass in the compositional boundary layer at the surface.

We show that late-stage microlite growth can generate substantial deviations in ash surface chemistry from the bulk magma. Ash surface chemistry has a fundamental role in ash-plume and ash-atmosphere interactions, including precipitation and fixing of volatile species, ice nucleation, charge balances and volcanic lightning, and may modify the fusion hazard during jet engine ingestion.