Older magma at Aso caldera than at Unzen stratovolcano in south west Japan as recorded through helium isotopes

Stratovolcanoes form conical mountains by ejecta and lava, while caldera volcanoes have depressions caused by post caldera collapse. Few studies have investigated relationships between this morphology difference and discharged fluid geochemistry. Here we present helium isotope ratios of geothermal fluids and groundwater inside and outside the caldera of Mt. Aso, Japan, and compare these to helium ratios in groundwater at Mt. Unzen, a stratovolcano in the same tectonic field. We find that helium isotopes decrease in proportion to distance from the central part of the volcano. However, the helium isotopes of inferred magmas are lower in Mt. Aso relative to Mt. Unzen. We combine this result with seismic tomography data and suggest that Mt. Aso may not be receiving new mantle material to the magma reservoir. Thus, we conclude that the caldera volcano Mt Aso probably features older magma and more developed geothermal systems than the stratovolcano Mt. Unzen.