Alteration of volcanic ash glass chemistry due to lightning

Abstract

Volcanic lightning generates high temperatures (>1500 °C) that melt volcanic ash particles, which re-solidify into lightning-induced volcanic spherules (LIVS). These LIVS have been documented within several ashfall deposits from multiple explosive volcanic eruptions. While several studies have analyzed the morphology and formation processes of LIVS, there is limited research on the chemical composition of these glassy particles. High-current (7 and 25 kA) impulse experiments were performed on very fine volcanic ash (<32 μm) produced from andesitic pyroclasts to create lightning-induced volcanic particles. The pre- and post-experimental ash was analyzed using a combination of wavelength and energy dispersive spectroscopy. The unaltered post-experimental particles, in addition to the pre-experimental samples, are characterized by angular grain shapes and euhedral mineral microlites within a glass matrix. Conversely, the 29 analyzed lightning-induced particles have a rounded morphology and are composed of chemically distinct and highly variable glass with no preserved microlites. Observed within the post-experimental particles are chemically diverse flow structures that are compositionally different than the surrounding glass, indicating mingling between molten components derived from the initial mineral and glass. Transformation of volcanic ash particles into LIVS will provide evidence of lightning during explosive eruptions and may increase the leaching rate of elements into the environment following ash deposition.