Electrical conductivity of the Earth's lower mantle

Abstract

ELECTRICAL conductivity is an important physical property of the Earth's mantle because it controls the transmission of geomagnetic signals from the core to the surface. The lower mantle, from a depth of 670 km down to the core–mantle boundary (2,990 km), is probably composed of (Mg,Fe)SiO₃ perovskite and magnesiowüstite, (Mg,Fe)O. Analysis of the transient and secular variations of the geomagnetic field yields values of the lower-mantle conductivity of the order of 1 S m⁻¹ at a depth of 1,000 km, increasing to ∼100 S m⁻¹ at the core–mantle boundary^1,2. Information about the physical mechanism of the conductivity and its dependence on temperature and pressure would help to constrain the temperature profile in the Earth. In the only study published so far, Li and Jeanloz^3,4 reported values of the conductivity of the silicate perovskite and of a mixture of perovskite and magnesio-wiistite lower than 10⁻³ S m⁻¹ and concluded that the lower mantle is an insulator, thus casting doubt on the geomagnetic results. We report here the results of measurements of the d.c. electrical conductivity of a mixture of perovskite and magnesiowiistite resulting from disproportionation of olivine, and of pure perovskite. The measurements were made in an externally heated diamond-anvil cell at pressures of ∼40 GPa and temperatures from 25 ^°C to ∼400 ^°C. Conductivity increases with increasing iron content, increasing temperature and increasing pressure. The activation energy (0.35 eV for 11% Fe) decreases with increasing iron content. The results are compatible with an electron-hopping conduction mechanism. Extrapolation to the temperature appropriate for a depth of 1,100 km, which corresponds to the pressure of our experiments, yields a conductivity of the lower mantle of the order of 1 S m⁻¹; extrapolation to the temperature and pressure of the core-mantle boundary yields values between 50 and 100 S m⁻¹, in agreement with geomagnetic determinations.