Magmatic and fragmentation controls on ash surface chemistry

Abstract:

The chemical effects of silicate ash particles ejected by explosive volcanic eruptions on biotic and abiotic systems are fundamentally mediated by ash particle surfaces. Ash surface properties can be presumed to be functions of magmatic state and fragmentation processes, as well as in-plume and atmospheric alteration by volcanic and/or environmental gases and liquid aerosols. Recently, attention has been focussed on the capacity of alteration processes to shape ash surfaces, with the chemistry and mineralogy of the pre-existing ash surface presumed to be equivalent to those of the bulk particle, or even of the ash deposit. Here we present findings which highlight the influence of magma composition and fragmentation mechanisms on ash surfaces. We conducted rapid decompression experiments at varying temperature and pressure conditions on porous andesitic rocks to produce fragmented ash materials, untouched by secondary alteration. These materials were compared to samples produced by crushing of clasts from the same experiments. The bulk chemistry and surface mineralogy of ash particles from a selected size fraction (63-90 µm) was determined via XRF, SEM-BSE, and EPMA, while the surface chemistry (<10 nm) was investigated by X-ray photoelectron spectroscopy (XPS). We identify similar disparities between whole-rock and surface chemistry as identified in previous ash studies, demonstrating ash surface chemistry to be a product of surface generation mechanisms, in addition to alteration. We observe dependences on both fragmentation pressure and temperature of ash surface chemistry. The mechanisms, pressure and temperature of magma fragmentation may thus influence ash surface chemistry and mineralogy, and subsequently, the post-eruptive alteration of ash particles and their reactivity within biotic and abiotic systems.