Possibility of effective magma degassing into groundwater flow systems beneath Unzen volcanic area, SW Japan, inferred from the evaluation of volcanic gas fluxes using electrical conductivity structures

摘要

The mass and heat budget of volcanic gases released from magma is critical to understanding a number of volcanic activities, including the ease with which magma can ascend. Due to its elevated temperature and salinity, the crustal electrical conductivity in a groundwater flow system increases through the addition of hydrother-mal fluids which are produced by mixing of volcanic gases with meteoric-origin water. Therefore, the spatial extent of high electrical conductivity regions within groundwater flow systems may be used to evaluate the mass flux of volcanic gases to the systems. The present study attempts to estimate the mass flux of volcanic gases beneath the Unzen volcanic area in Southwest Japan, by developing a simple flow model of hydrothermal fluids and applying this model to the electrical conductivity structure of the area. The estimated mass flux of volcanic gases (10~(4.8 ± a3) t/yr) yields results for CO_2 flux (10~(3.1 ± 0.3) t/yr) and magma input rate (10~(0.1 ± 0.3) million m~3/yr) that are consistent with those estimated by geochemical and geodetic observations. This suggests that volcanic gases are steadily released from magma into the overlying groundwater flow system beneath the area, and that effective degassing may be one of the factors controlling the relatively effusive style of recent volcanism at Unzen volcano.