VARIATIONS IN WATER SATURATION STATE WITHIN THE SUBVOLCANIC RESERVOIR OF THE ASO SUPERVOLCANO (JAPAN)

The exsolution of a magmatic volatile phase in volcanic plumbing systems plays a crucial role in the growth of subvolcanic reservoirs and in controlling eruptive frequency, and styles. This study focuses on the monitoring of the water saturation state in the subvolcanic system of the Aso volcanic complex (Japan) prior to and during the catastrophic Aso-4 caldera-forming event. We analyze pre-Aso-4 and Aso-4 deposits using petrological proxies, such as melt inclusions, matrix glasses and apatite crystals, to trace the presence of such an exsolved magmatic volatile phase, specifically the exsolution of water. In addition, the resulting volatile budgets (F, Cl, OH, S) obtained from these proxies are coupled to sulfur isotope signatures of apatite. F-Cl-OH partitioning in apatite combined with melt inclusion data indicate water-undersaturated conditions prevailed during magma evolution until around 10 ky prior to Aso-4 and during the Aso-4 event itself. Volatile budgets from young pre-Aso-4 units, however, provide evidence for the presence of a water-rich exsolved magmatic volatile phase in the eruptible portions of the reservoir within the last 10 ky preceding Aso-4. Moreover, sulfur systematics from apatite and melt inclusions of the Aso-4 event, support the prevalence of a water-rich magmatic volatile phase at some point prior to the caldera-forming event. We propose that recharge of mafic magmas shortly before the Aso-4 eruption induced chemical hybridization in the resident mush, which led to the return of the system to less evolved compositions and water-undersaturated conditions. Through thermo-mechanical modelling of the reservoir dynamics in the Aso-4 caldera cycle, we aim to establish the conditions under which such partial volatile re-dissolution can occur and explore its potential impact on triggering caldera-forming events. The observed transitions in water-saturation state in the Aso reservoir, resulting in water-undersaturated conditions shortly before the Aso-4 eruption, suggest that recharge-induced re-dissolution of a water-rich magmatic volatile phase (due to volatile dilution and changes in the water saturation limit of the melt due to overpressure) may constitute an additional factor with potential influence on the eruptibility of large-scale magmatic systems.