Constraints on Mantle Electrical Conductivity from Field and Laboratory Measurements

A global geomagnetic response function, sensitive to the average radial electrical conductivity structure of Earth’s mantle to depths of at least 1800 km, is obtained by averaging published, single-site response functions estimated at periods between 10 – 10 seconds from magnetic observatory records. Although the error bars on the global response function are mostly smaller than 5%, Parker’s D algorithm demonstrates compatibility with a one-dimensional model, both in terms of magnitude and distribution of data residuals. Smooth models in the sense of minimum first and second derivatives of log(conductivity) with log(depth) show conductivities increasing from 0.01 S/m 200 km deep to 2 S/m at a depth of 2000 km. Geotherms inferred from these conductivities using a laboratory model for the temperature dependence of dry subsolidus olivine yield temperatures of 1750◦C at a depth of 410 km; hotter than the 1400◦C for this depth inferred from published values for the equilibrium boundary of the olivine α → α+β transition. Inclusion of a sharp jump in conductivity at the 660 km seismic discontinuity lowers the electrogeotherm to 1600◦C at 410 km, while an explicit penalty on the conductivity at this depth demonstrates that a temperature of 1400◦ is compatible with the global response function if 1000 S of additional conductance is included above 200 km. The electrical conductivity below the jump at 660 km is 1 S/m increasing to 2 S/m at 2000 km, in excellent agreement with recent diamond anvil measurements of lower mantle materials. Extension of the global response to higher frequencies is possible using data from magnetic satellites. One such study is shown to be in general agreement with the averaged response.