Variations in water saturation states and their impact on eruption size and frequency at the Aso supervolcano, Japan

Abstract

The exsolution of a magmatic volatile phase in the plumbing systems of volcanoes plays a key role in controlling growth dynamics of subvolcanic reservoirs and eruptive styles. By using common petrological proxies found in volcanic deposits, such as melt inclusions and apatite crystals, the presence of such an exsolved magmatic volatile phase can be traced, specifically the exsolution of water from silicate melt. To monitor variations in the water saturation state of magmas from the Aso volcanic complex (Kyushu, Japan) prior to and during the catastrophic Aso-4 caldera-forming event (at ∼86 ka BP), we investigate a set of pre-Aso-4 and Aso-4 deposits, combining volatile budgets (F, Cl, OH, S) of melt inclusions, matrix glasses, and apatite crystals, with sulfur isotope signatures in apatite. F-Cl-OH partitioning in apatite along with melt inclusion data from pre-Aso-4 units indicate water-undersaturated conditions during magma evolution until around 10 ka prior to the Aso-4 event. In contrast, eruptions occurring within the last ∼10 ka prior to the Aso-4 event indicate the presence of a water-rich exsolved magmatic volatile phase in the eruptible portions of the subvolcanic reservoir. Likewise, sulfur systematics in apatite and melt inclusions from the Aso-4 event suggest the exsolution of a water-rich magmatic volatile phase at some point prior to the caldera-forming event. Recharge of mafic magmas shortly before the Aso-4 eruption induced chemical hybridization in the resident upper crustal mush, bringing the reservoir back to less evolved compositions and water-undersaturated conditions. This hybridization event is recorded by volatile contents of both apatite and matrix glasses from late-erupted, crystal-rich products of the Aso-4 event, all yielding water-undersaturated signatures. During this hybridization event, chemical dilution and partial redissolution of the exsolved volatile phase reduced the magma compressibility significantly, so that additional magma influx from depth might have allowed a sharply increasing overpressurization in the subvolcanic reservoir and served as a potential trigger for the cataclysmic Aso-4 eruption. Drawing from our observations made on Aso, we propose that recharging of large silicic upper-crustal reservoirs with increased volumes of drier and more mafic melts can influence their water saturation states and associated physical properties. Such changes could contribute to the triggering of large-scale caldera-forming events.