We have measured the electrical conductivity (EC) of multiphase pyrolite at lower-mantle pressures (25–80 GPa) using impedance spectroscopy coupled to laser-heated diamond anvil cells. Electrical conductivity is more sensitive to temperature and mineral chemistry than more commonly used elasticity measurements, but has historically been an underused method due to the increasingly noisy nature of its data with depth and to the sharp disagreements between existing datasets of high-pressure laboratory-measured values. Yet, the resolution of EC sounding techniques improves over time, as longer periods penetrate deeper into the Earth, thus promising better and more extensive data down the line. Because this observational data is calibrated against experimentally derived data to yield thermochemical models, a reevaluation of the experimental data is critically needed.

From our measurements we have identified previously unreported dependences of impedance on voltage and on time following compression, that further compromise the usefulness and interpretation of existing experimental data, particularly at lower pressures where the voltage dependence appears most pronounced. Moreover, we find that the EC of pyrolite rises monotonically with pressure, which is contrary to the only other EC measurements of pyrolite, where the conductivity drops between 50 and 100 GPa before recovering as in monophase ferropericlase. This significant discrepancy is attributed to different synthesis conditions (pre-synthesis in the multianvil press vs. in situ synthesis in the diamond anvil press) and textures (fine grained, equigranular, homogeneous vs. likely dominated by banding and lattice-preferred orientation of ferropericlase due to strongly uniaxial compression), and therefore represents different states being measured—bulk pyrolite vs. essentially ferropericlase. With the new EC-temperature equations of state we can generate a model of the lower mantle through inversion of new satellite EC data that provides tighter constraints on its thermal state.