Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three-dimensional measurement of helium isotopes in groundwater

摘要

Magmatic volatile supply to the groundwater around lwate volcano, which is an active volcano with weak fumarolic activity, is investigated using chemical and isotopic compositions of 180 groundwater samples from various depths. Magmatic ~3He flux is estimated by an applied method using helium isotopes. The three-dimensional distribution of ~3He flux can be obtained as well as that of the concentrations. High ~3He flux with high concentration is found in shallow groundwaters at N and NE flanks, and in shallow and deep groundwater around the fault zone. Low ~3He flux with high concentration in deep groundwater suggests the slow and long-term accumulation of magmatic volatiles. High 3He fluxes and concentrations around the fault zone indicate that the fault acts as an ascending path of magmatic volatiles. Distribution of flux indicates the three-dimensional anisotropy of magmatic volatile supply, which is due to the groundwater flow variation restricted by the structure of the volcanic body. Supply rates of other magmatic volatiles can be also obtained using ~3He flux, and the total supply rates are estimated to be 2.3 x 10-3 (3He), 5.1 x 10~8 (magmatic C), 1.3 x 10~8 (C1), and 1.3 x 10~8 (S) mol/y. Comparison of supply rate to groundwater with volcanic gas emission rate reveals that the magmatic volatiles are largely supplied to the groundwater. However, the total emission combining the groundwater and volcanic gas is not prominently large compared with other active volcanoes. We infer that the method combining the magmatic volatile concentration and flux to groundwater is available for the evaluation of passive activity of volcano.