Publication 100
Temperature of the upper mantle beneath the Intermontane Belt, northern Canadian Cordillera, determined from combining
mineral composition, electrical conductivity laboratory studies and magnetotelluric field observations
Ledo, J. and A.G. Jones
Abstract
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.
Source
Earth and Planetary Science Letters, 236, 258-268, 2005.
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Alan G Jones / 21 April 2006 /
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