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

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