Dispersion of Magmatic Volatiles through the Groundwater Flow System in and around Iwate Volcano, NE Japan

Groundwater flow system in and around volcano plays an important role as a dispersion pathway of magmatic volatiles from volcanic body. We investigated groundwater flow system based on chemical and isotopic compositions of groundwater and thermal water from variety of depth in and around Iwate volcano, NE Japan. Then we discuss spatial distribution and the amounts of dispersion of magmatic volatiles through the groundwater flow system. In the area of north and east flanks, big shallow groundwater flow system is existed. The groundwater have magmatic signature, such as high HCO3 and 3He/4He ratios (up to 4.0Ra). As the recharge area of this groundwater flow system contain the fumarolic area of summit, the magmatic volatiles in these springs would be supplied from gas conduits. In contrast, the deep thermal waters in this region have lower 3He/4He ratios (0.1-0.6Ra). This suggests that deep groundwater aquifers are isolated from the shallow groundwater flow system. In the southwest flank area, both deep thermal water and shallow groundwater along the active fault have magmatic signature with high HCO3 and 3He/4He ratios (up to 5.3Ra). This suggests that the magmatic volatiles are likely supplied via fault in this region. The residence time of groundwater estimated from helium isotopic compositions varied but closely related to geological structure; which are several hundred years at shallow depth inside the Iwate volcanic body, several thousand years at Tertiary volcanics, and several tens of thousand years at pre-Tertiary sediments, respectively. Fluxes of magmatic volatiles from Iwate volcano via groundwater evaluated from the residence time and magmatic 3He concentration of groundwater are large at the big springs in the area of north and east flanks and the groundwater along the active fault, whereas small at deeper groundwater aquifer in Tertiary volcanics and pre-Tertiary sediments. These results suggest that the major dispersion pathway of magmatic volatiles are shallow groundwater flow system and fault, and are restricted to the subsurface geological structure.