On the heterogeneous electrical conductivity structure of the Earth's mantle with implications for transition zone water content

We have investigated the lateral variations in mantle electrical conductivity structure using electromagnetic sounding data. For this purpose, we used very long time series (up to 51 years) of geomagnetic observatory data at six locations encompassing different geological settings to compute response functions that cover the broadest possible frequency range (3.9 to 95.2 days): Furstenfeldbruck (FUR), Europe; Hermanus (HER), South Africa; Langzhou (LZH), China; Alice Springs (ASP), Australia; Tucson (TUC), United States (North America); and Honolulu (HON), United States (North Pacific). We inverted the response functions beneath each observatory for a local radial conductivity profile using a stochastic sampling algorithm. Specifically, we found significant lateral variations in conductivity throughout the mantle with resolution limited to the depth range similar to 500-1200 km. At 600 km depth, conductivity varies between 0.1 and 0.4 S/m and increases to 1.3-2.0 S/m at 800 km depth beneath all stations except HER (0.5 S/m). At 900 km depth, conductivity increases further to 1.4-2.4 S/m with HER, HON, and ASP being most conductive. This trend persists to a depth of 1200 km. Comparison with conductivity profiles constructed from laboratory measurements of mantle mineral conductivities and models of Earth's mantle composition and thermal state reveal that significant thermochemical variations are at the origin of the observed heterogeneities in mantle conductivity found here. Because of the somewhat large error bounds on sampled conductivity profiles and the reduced sensitivity of the electromagnetic sounding data above 500 km depth, constraints on transition zone water content are less conclusive, although H2O contents <0.5 wt% in the midtransition zone appear less likely.