THREE THOUSAND YEARS OF BUILD-UP TO THE BIG ONE: MAGMATIC STORAGE AND TIMESCALES OF PRECURSORY AND CLIMACTIC ERUPTIONS OF AIRA CALDERA, JAPAN

Elucidating the magmatic processes, storage conditions, and timescales of caldera-forming eruptions (CFEs) is a critical endeavor for both the field of igneous petrology and for hazard mitigation. The Aira Caldera of southern Kyushu, Japan has a well-preserved record of precursory explosive volcanism that occurred within thousands of years of the culminating CFE, providing a rare opportunity to study snapshots of an evolving rhyolite magma system leading up to the catastrophic CFE. Aira had three pre-caldera pyroclastic eruptions prior to the 30 ka Ito Ignimbrite CFE (400 km³), each approximately 1,000 years apart. To investigate the evolution of this system, we compare the compositions, storage pressures, and crystallization timescales of these eruptions. Although all eruptions were high-silica rhyolites (75-78 wt. % SiO₂), we found several distinct magma compositions within each eruption using multivariate hierarchical clustering. A pumice population from the second pre-CFE is statistically indistinguishable from one in the final pre-CFE, suggesting persistence of magma storage. To investigate the persistence of the magma system further, we calculated Ti diffusion timescales using cathodoluminescence images of quartz phenocrysts. While most magma populations from the precursory eruptions have quartz core timescales that indicate magma crystallization over less than 500 years, one population from the second eruption was crystallizing for 1,500 years, again suggesting that rhyolite magma was not evacuated during the eruption that had occurred 1,000 years prior. We used the rhyolite-MELTS geobarometer to estimate storage pressures of these magmas. We find that all pre-caldera magmas were stored at similar pressures, between 75 and 125 MPa. The persistence of eruptible magma between these smaller precursory events, stored at similar depths, suggests a complex lateral magmatic storage system that allows some magma to remain stored during eruption. However, CFE magma was stored deeper, with one population stored at 100-150 MPa and a quartz-free population stored at 200-350 MPa. Thus, we find that although precursory eruptions may have incompletely tapped a persistent magma storage system for 3,000 years, a change in magma storage and processes allowed for the climactic eruption of the Aira Caldera.