The electrical conductivity and thermal profile of the earth's mid-mantle.

Electrical conductivity in the Earth's mantle is sensitive to temperature a nd chemical environment. Recent laboratory measurements of electrical condu ctivity are combined with candidate mantle geotherms to produce synthetic e lectrical conductivity profiles. These profiles are used to forward model t he Earth's geomagnetic response function C, results of which are compared w ith the observed globally averaged response function at periods of 3.5 days to 4 months. Candidate lower mantle geotherms, representing whole-mantle a nd layered convection end-members, are compared using published electrical conductivity measurements on alumina-bearing and alumina-free perovskite in the conductivity models. Comparison of the predicted response functions wi th the observed geomagnetic response of the Earth shows that a) if lower ma ntle alumina is incorporated into perovskite, then the lower mantle must be cool, and b) if the alumina is not incorporated in perovskite then the res ults are only consistent with a hot lower mantle. In addition, the maximum alumina content of lower mantle MgSiO3 perovskite is constrained at 4%.