Electrical conductivity of the Earth's lower mantle

Abstract

THE electrical conductivity of the Earth's lower mantle constrains both the propagation to the surface of geomagnetic disturbances in the core and the nature of core–mantle coupling. Extrapolations of laboratory measurements on materials representative of the lower mantle agree weakly^1,2 or not at all^3,4 with recent geophysical models^5–8 of lower-mantle electrical conductivity based on variations of magnetic and electrical fields measured at the Earth's surface. Here we report d.c. conductivity measurements on samples with compositions approximating that of the lower mantle, at pressures of 1.2 to 40 GPa and temperatures in the range 20 to 400 °C. Our results agree with some of those obtained previously^1,2. But in contrast to this previous work, we extrapolate the results to lower-mantle conditions by adopting a functional form for the conductivity that incorporates the effect of pressure as well as temperature. The resulting estimates of conductivity are in agreement with the geophysical determinations^5–8. We find that, because of a very weak dependence on temperature, pressure and composition, the conductivity is likely to vary by no more than about a factor of five across the entire lower mantle, reaching a maximum value of only 3–10 S m⁻¹. Lateral temperature variations as large as a few hundred degrees will therefore be hard to detect geophysically, and the compositionally distinct D″ layer at the base of the lower mantle remains the only possible location for a highly conducting layer.