Multiple lines of evidence for crustal magma storage beneath the Mesozoic crystalline Iide Mountains, Northeast Japan

Umeda, Koji; Asamori, Koichi; Ninomiya, Atsushi; Kanazawa, Sunao; Oikawa, Teruki

It has been recognized that the Iide Mountains in Northeast Japan are unique because of their occurrence in a non-volcanic region yet hot springs have anomalously high heat discharge values similar to the heart discharge from hot springs in volcanic regions. In order to attempt to constrain the heat source of hydrothermal activity in the region, new helium isotopic data from hot springs surrounding the Iide Mountains was obtained. The He/He ratios of the hot spring gases sampled range from 0.22 to 7.9 RA, and the highest ratio is similar to MORB-type helium (8 1 RA), indicating a significant contribution of primordial mantle-derived helium. Estimation of post-intrusive radiogenic ingrowth of He by decay of U and Th suggests that the heat source beneath the Iide Mountains is not likely ancient magma intruded in the Middle Miocene but newly ascending magma derived from mantle material. The geographical distribution of He/He ratios of hot spring gases reveals that crustal magma storage exists beneath the Kitamata-dake, one of the peaks of the Iide Mountains. This can be used to interpret the geophysical anomalies in this region such as crustal seismicity and results of magnetotelluric imaging.