The Electrical Conductivity of the Lower Mantle From High-Pressure/High-Temperature Measurements of Pyrolite and Ten Years of Geomagnetic Observations

Abstract

Electromagnetic sounding coupled with experimental measurements of electrical conductivity (EC) can provide valuable constraints on the thermochemical state of Earth's lower mantle. Here, we report a new set of experimentally determined EC values of pyrolite, a candidate composition of the lower mantle, between 25 and 80 GPa at room temperature and high temperature between 1,200 and 2,300 K using a laser-heated diamond anvil cell combined with impedance spectroscopy. To maintain and better constrain the geometry of the experimental assembly, we used focused ion beam-cut discs of a pre-synthesized pyrolitic sample. Our results reveal a monotonic increase in EC with pressure, in contrast to earlier measurements that suggested a spin-transition-induced conductivity drop in the mid-lower mantle. We also identify intrinsic voltage- and pressure-related time-dependent behavior that are likely expressions of nonlinear grain-boundary conduction and stress-driven relaxation processes, respectively. To our knowledge, this is the first report of such behavior in geological materials and may represent a previously unrecognized source of uncertainty in earlier high-pressure EC data sets. Finally, we compare predictions based on our experimental findings against the radial EC structure derived from analysis of 10 years of satellite magnetic data from the ESA Swarm mission, with improved resolution in the lower mantle down to 2,200 km. We find that the measured EC of pyrolite is consistent with the geophysically inferred conductivity structure.