Magma fragmentation processes during a large-scale explosive phreatomagmatic eruption constrained from volatile contents of volcanic glass

Phreatomagmatic eruptions are one of the most common and hazardous eruption styles on Earth. To understand the explosive phreatomagmatic eruptions, it is essential to constrain the mechanisms of magma–water interaction and the environments in which they occur. We investigated the quenching depths of ascending magma during the large-scale and explosive phreatoplinian eruption that occurred at Kutcharo volcano, Japan, based on the water content measurements for volcanic glass shards using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy (FT-IR) analysis. Results for the two analytical methods show approximately similar total water content ranges despite using different glass shards. This indicates that the water content ranges can be regarded as representative of those of this phreatoplinian eruption. However, for FT-IR results, OH and H2Om species contents against H2Ot contents for individual glass shards do not plot within the equilibrium curves calculated using the H2O speciation model of rhyolite with temperatures of 500–800 °C. This suggests that glass shards have experienced secondary hydration after deposition, resulting in disequilibrium H2O speciation by adding only H2Om. Using measured OH contents and applying appropriate models for H2O solubility, speciation, and a glass transition temperature of 800 °C, we estimated quenching pressures of up to ~24.3 MPa. These correspond to quenching depths ranging from near-surface levels to approximately 1,200 m, depending on the assumed pressure gradient. When the presence of a paleo-caldera lake is considered, quenching depths concentrate at 100–300 m below ground level, suggesting that magma-water interaction occurred primarily in the upper conduit directly beneath the lake bottom. Bubble-wall/micro-pumice glass shards and dense/blocky glass shards show overlapping OH contents and quenching depths, implying that volatile-driven fragmentation and magma-water interaction occurred concurrently within the conduit. Together with the high mass discharge rate, our results indicate that this phreatoplinian eruption was fundamentally highly explosive, with external water mainly modifying fragmentation, quenching, and depositional characteristics rather than driving the overall eruption intensity. Phreatoplinian eruptions may therefore develop during highly explosive silicic eruptions when abundant external water interacts with ascending magma, conditions that may be particularly favorable in mature caldera systems.