Physics of the Earth and Planetary Interiors 155 (2006) 1–15 Water content and geotherm in the upper mantle above the stagnant slab: Interpretation of electrical conductivity and seismic P-wave velocity models Masahiro Ichiki a, ∗ , Kiyoshi Baba a , Masayuki Obayashi a , Hisashi Utada b a Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima 2-15, Yokosuka, Kanagawa 237-0061, Japan b Earthquake Reseach Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo 113-0032, Japan Received 6 December 2004; received in revised form 6 June 2005; accepted 22 September 2005 Abstract Geotherm and water content profiles in the upper mantle above the stagnant slab of the Pacific back-arc were estimated from the electrical conductivity and seismic P-wave velocity (V p ) structures. The geothermal profiles were determined by using the electrical conductivity and seismic V p structures, which, assuming a dry hartzburgite or a dry pyrolite composition, are designated as electrical and seismic geotherms, respectively. In a deeper part of the upper mantle, neither the dry pyrolite nor the dry harzburgite condition provides consistent electrical and seismic geotherms. This discrepancy can be explained by allowing for a small amount of water (500–1000 ppm H/Si) with the seismic geotherm. In a shallower part of the upper mantle, the electrical and seismic geotherms are consistent with each other within 1500–1700 ◦ C under the dry harzbutgite condition, whereas they are inconsistent by more than 100 ◦ C under the dry pyrolite condition. Alternatively, the wet pyrolite condition applied to the deeper part of the upper mantle also satisfies the electical conductivity and seismic V p structures in the shallower part. © 2005 Elsevier B.V. All rights reserved. Keywords: Water; Back-arc volcanism; Stagnant slab; Harzburgite; Pyrolite; Electrical conductivity; Seismic P-wave velocity 1. Introduction The presence of active volcanism (Miyashiro, 1986) and stagnant slab in the mantle transition zone (Fukao et al., 1992) are the most outstanding features in the north- eastern part of China (NEC), Pacific back-arc. The origin of the back-arc volcanism has not been resolved, nor is it clear whether the back-arc volcanism is associated with the stagnant slab. Fig. 1 shows schematic images of the proposed hypotheses on the origin of the volcanism. The hot region (Miyashiro, 1986) and hot asthenospheric up- ∗ Corresponding author. E-mail address: ichiki@jamstec.go.jp (M. Ichiki) welling hypothesis (Xu, 2001; Yang et al., 2003) propose a geothermal origin, the harzburgite plume hypothesis (Tatsumi and Eggins, 1995) as chemical/petrological ori- gin. The harzburgite plume hypothesis allows for direct association between the origin of the volcanism and the presence of the stagnant slab: the explanation is that the harzburgite and mid-ocean ridge basalt (MORB) com- ponents cause density reversal in the stagnant slab, and a large mass of harzburgite consequently flows up in the upper mantle as a plume. The wet region hypothe- sis (Iwamori, 1992) postulates that the volcanism is of volatile/water origin. To estimate geotherm and water content (or hydro- gen dissolution) profiles in the upper mantle beneath 0031-9201/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2005.09.010