Upper mantle temperature determined from combining mineral composition, electrical conductivity laboratory studies and magnetotelluric field observations: Application to the intermontane belt, Northern Canadian Cordillera

Using laboratory-derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws, we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine-orthopyroxene-clinopyroxene (Ol-Opx-Cpx) system. We calculate physical property bounds (Hashin-Shtrikman bounds) as a function of the fraction of different phases present; these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected. The relationships we present between temperature, mineral composition and bulk electrical conductivity allow constraining one of them given the other two. We apply this approach to an area of the North American Cordilleran Intermontane Belt in the Yukon Territory, northern Canada, where xenolith evidence indicates bimodal upper mantle mineral assemblages (harzburgite and lherzolite). This locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data. Given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity, deduced by non-linear model appraisal, we determine the permitted extremal temperature bounds of the Intermontane belt mantle rocks directly below the Moho to a depth of some 80 km. We show that the mantle in this region is at a minimum temperature of 820 °C and a maximum temperature of 1020 °C; the latter is some 200 °C colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey.