Shapes of volcanoes and fluid geochemistry

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Stratovolcanoes and caldera volcanoes have different shapes due to their formation processes. Stratovolcanoes are conical mountains formed by ejecta and lava around craters. On the other hand, after a large eruption releases a large amount of magma at stratovolcanoes, the magma chamber becomes a void and the central part of the volcano sinks, forming caldera volcanoes. There are a couple of questions such as the age of the magma chamber in stratovolcanoes and caldera volcanoes, the structural differences in the surrounding hydrothermal systems, and whether stratovolcanoes eventually evolve into caldera volcanoes. Geochemical data of fluids discharged at volcanoes give us crucial information about structures and conditions of volcanic-hydrothermal system. However few studies have investigated relationships between discharged fluid geochemistry and morphology difference between stratovolcanoes and caldera volcanoes.

Helium is a noble gas element which has two stable isotopes (3He and 4He), and helium isotope ratios (3He/4He) in the mantle are higher than those in the atmosphere and the crust. Therefore 3He/4He ratios can be useful geochemical tracers of mantle/magma-derived components. We measured 3He/4He ratios of geothermal fluids and groundwater inside and outside the caldera of Mt. Aso, southwest Japan, and compared these to 3He/4He rations in groundwater at Mt. Unzen, a stratovolcano in the same tectonic field.

3He/4He ratios decreased in proportion to distance from the central part of the volcano due to change of contributions from the magma and the crust to fluid components. And the computed 3He/4He ratio of magmas in Mt. Aso was lower than that in Mt. Unzen. We combined this result with seismic tomography data and suggest that Mt. Aso may not be receiving new mantle material to the magma reservoir. In addition, magma contributions to the sites distant from the central cone in Mt. Aso was higher than those in Mt. Unzen. Thus, we conclude that the caldera volcano Mt. Aso probably features older magma and more developed geothermal systems than the stratovolcano Mt. Unzen. Discharged fluid geochemistry reflect differences between stratovolcanoes and caldera volcanoes, which may be useful for assessing risks of caldera-forming large eruptions in the future.

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