Physics of the Earth and Planetary Interiors, 42 (1986) 215-226 215 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Conductivity and fluids in the oceanic upper mantle P. Tarits Laboratoire de Gbomagnktisme, Institut de Physique du Globe, Tour 14, 4 Place Jussieu, 75230 Paris Cedex 05 (France) (Received February 12, 1985; revision accepted August 27, 1985) Tarits, P., 1986. Conductivity and fluids in the oceanic upper mantle. Phys. Earth Planet. Inter., 42: 215-226. Three deep electromagnetic soundings in the North Pacific Ocean were re-examined. Our inversion of existing data showed the presence of conductors below the three sites, in very good agreement with previous results. We showed that H20-rich fluids as well as partial melting account for the conductivities observed. This led us to determine temperatures ranging firstly, from 1050 to 1150°C, when only H20-rich fluids were present, and secondly, from 1300 to 1350°C for partial melting only. We suggest that the upper mantle below the two oldest sites (30 and 72 Ma) is characterized by about 1% of partial melting in the presence of about 1% of H20-rich fluids. Below the third site, that nearest to the ridge zone (1 Ma), the degree of partial melting might reach 10%. The presence of 1% of H20-rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism. This agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere. The weakening effect of fluids, particularly water, on upper mantle rheology is also discussed. We suggest that the asthenosphere and lithosphere might be decoupled mechanically, which, if true, would affect small scale convection below the oceanic lithosphere. 1. Introduction Electrical conductivity is one of the most sensi- tive parameters of upper mantle thermodynamics. Numerous laboratory studies have shown that conductivity greatly increases when the tempera- ture rises, partial melting occurs and fluids are present (e.g., Tozer, 1959; Shankland and Waft, 1977; Shankland and Ander, 1983). In addition, pressure, mineralogical composition, fugacity of oxygen may also influence deep-seated conductiv- ity (Duba et al., 1974; Shankland, 1975). Numerous factors may, therefore, strongly af- fect conductivity in the Earth. For instance, small variations in parameters like temperature or the degree of melting may cause conductivity to vary by several orders of magnitude. This possibility therefore limits considerably the range of varia- tions for a given thermodynamic parameter. Contribution IPGP no. 892. 0031-9201/86/$03.50 © 1986 Elsevier Science Publishers B.V. Bulk conductivity of up to about 150-200 km in depth can only be derived from low frequency magnetotelluric soundings (MTS) or geomagnetic deep soundings (GDS) (Jiracek et al., 1983). The electromagnetic ocean soundings made by pioneers were reviewed by Filloux (1973). Most of the MTS and GDS were done in the North Pacific Ocean (Law, 1983), and their number is now increasing fast (Delaurier et al., 1983; Segawa et al., 1982; Filloux, 1982a, b; Chave and Cox, 1982). In this paper, we re-examine three deep sound- ings in the Pacific Ocean. The data were collected by Filloux (1977, 1980) and Law and Greenhouse (1981). The age of the underlying oceanic litho- sphere differs from one site to another. Filloux (1980), Oldenburg (1981) and Law and Greenhouse (1981) showed the presence of a con- ductor below each site. This conductor deepens as the age of the lithosphere increases. The strong conductivities observed were believed to reflect the partial melting of pyrolite (Oldenburg, 1981).