Spatial and Temporal Variations in Helium Isotope Ratios of Fumaroles and Hot/Cold Spring Gases at the Kirishima Volcanic Group

Helium is present in very small amounts in volcanic gases, and its isotopic ratio (3He/4He ratio) sensitively reflects the state of magma underground. Spatial and temporal variations in the 3He/4He ratios of volcanic and hydrothermal fluids in and around the Kirishima Volcanic Group are discussed to clarify how magmatic fluids are incorporated into the shallow hydrothermal system and how their contributions reflect variations in magmatic activity. The spatial variation in the 3He/4He ratio of the fumaroles and hot/cold spring gases shows systematic 3He/4He decreases with increasing distance from the magma reservoir, the location of which is estimated as a pressure source of crustal deformation associated with the 2011 Shinmoe-dake eruption, to each sampling site. The spatial 3He/4He trend can be explained by a hydrodynamic dispersion model, in which crustal helium with a low 3He/4He ratio progressively leaches into an originally 3He-enriched magmatic fluid as it migrates through the local crust. However, the 3He/4He distribution is also controlled by the local permeable structure at shallow depths, resulting in a difference in the residence times of migrating fluids underground. The 3He/4He ratios of fumaroles in Iwo-yama slightly increase before Shinmoe-dake eruptions, and decrease after each eruption. This variation cannot be accounted for by the contribution of the radiogenic 4He relative to total helium in the fumaroles before and after the eruptions, because it is estimated that the amount of radiogenic 4He that volcanic gas can acquire during its migration from magma to the surface is negligible. Alternatively, the variation results from the change in mixing ratio of gases derived from two reservoirs having high and low 3He/4He ratios. Assuming that the magma chamber has a high 3He/4He ratio, the increase in the 3He/4He ratios of the fumaroles before the eruption would result from an increase in the supply of gas from the chamber to the Iwo-yama fumaroles. Once an eruption occurs at Shinmoe-dake, magmatic gas is effectively released through the volcanic vent, resulting in a decrease in its supply to the Iwo-yama fumaroles. Thus, the temporal variation of 3He/4He ratios in volcanic gases may reflect the pressure variation of the magma chamber.