Phase partitioning during fragmentation revealed by QEMSCAN Particle Mineralogical Analysis of volcanic ash

Volcanic ash particle properties depend upon their genetic fragmentation processes. Here, we introduce QEMSCAN Particle Mineralogical Analysis (PMA) to quantify the phase distribution in ash samples collected during activity at Santiaguito, Guatemala and assess the fragmentation mechanisms. Volcanic ash from a vulcanian explosion and from a pyroclastic density current resulting from a dome collapse were selected. The ash particles resulting from both fragmentation modes are dense and blocky, typical of open-vent dome volcanoes and have a componentry consistent with their andesitic composition. We use image analysis to compare the fraction of each phase at particle boundaries compared to the total particle fraction. Our results show that the explosion-derived ash has an even distribution of plagioclase and glass, but boundaries enriched in pyroxene and amphibole. In contrast, the ash generated during dome collapse has an increased fraction of glass and decreased fraction of plagioclase at particle boundaries, suggesting that fractures preferentially propagate through glass during abrasion and milling in pyroclastic flows. This study presents QEMSCAN PMA as a new resource to identify generation mechanisms of volcanic ash, which is pertinent to volcanology, aviation, respiratory health and environmental hazards, and highlights the need for further experimental constraints on the fragmentation mechanism fingerprint.