Aqueous fluids derived from a subducting slab: Observed high 3 He emanation and conductive anomaly in a non-volcanic region, Kii Peninsula southwest Japan Koji Umeda a, * , Yasuo Ogawa b , Koichi Asamori a , Teruki Oikawa a a Tono Geoscience Center, Japan Nuclear Cycle Development Institute, 959-31, Jorinji, Izumi-Cho, Toki, 509-5102, Japan b Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo, 152-8551, Japan Received 2 August 2004; received in revised form 29 April 2005; accepted 14 June 2005 Abstract It has long been recognized that the Kii Peninsula in the southwest Japan arc is peculiar in a non-volcanic region, indicated by the presence of high temperature hot springs, high terrestrial heat flow and high 3 He content in hot spring gases. Geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the Kii Peninsula. Most of the measured 3 He/ 4 He ratios are similar or higher than air, indicating wide spread incorporation of mantle-derived helium into meteoric water. A region with rather high 3 He/ 4 He ratios (N 4R A ) on the west side of the Omine Mountains coincides with the occurrence of high temperature hot springs. A deep crustal resistivity structure across the Omine Mountains was imaged by wide- band magnetotelluric soundings. A 2-D inversion with N–S strike using both TM and TE modes reveals two conductors, one in the upper (3–7 km depth) and the other in lower crust (25–35 km depth) to the west of the Omine Mountains. The distribution of microearthquakes and low-frequency tremors, and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the Philippine Sea plate. The upper-crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust, which are presumably derived from the subducting slab. Considering the occurrence of seismic events in the subducting slab beneath the southern Kii Peninsula, the aqueous fluids generated by dehydration of the slab mantle could plausibly include MORB-type helium derived from the residual lithospheric mantle. Therefore, the high temperature hot springs and high 3 He emanations in hot spring gases and other geotectonic events in the southern Kii Peninsula may be due to heat flux and mantle-derived helium discharged from aqueous fluid in the upper crust. D 2005 Elsevier B.V. All rights reserved. Keywords: 3 He/ 4 He ratio; hot spring; magnetotelluric soundings; aqueous fluid; non-volcanic region 1. Introduction The Kii Peninsula is located in the fore-arc region of the southwest Japan arc, where the young Philip- pine Sea plate (~20 M; Okino et al., 1994) is subduc- 0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.06.005 * Corresponding author. Fax: +81 572 55 0180. E-mail address: umeda@tono.jnc.go.jp (K. Umeda). Journal of Volcanology and Geothermal Research 149 (2006) 47– 61 www.elsevier.com/locate/jvolgeores