High-temperature electrical conductivity of the lower-mantle phase (Mg, Fe)O

The electrical conductivity of the lower mantle, which controls the transmission of geomagnetic signals to the Earth's surface, has been the subject of recent controversy^1,2. Peyronneau and Poirier¹ extrapolated high-pressure conductivity measurements of the lower-mantle assemblage (Mg, Fe)SiO₃ perovskite plus (Mg, Fe)O magnesiowüstite from 673 K to lower-mantle temperatures (2,200-2,500 K) to obtain conductivities consistent with geomagnetic observations (∼ 1 S m^-1 at 1,000-km depth). The high-pressure study of Li and Jeanloz² gave conductivities several orders of magnitude lower. Here we report measurements of conductivity and thermopower of the iron-rich phase (Mg, Fe)O at high temperature (1,173-1,773 K) and controlled oxygen partial pressure (pO₂). We find that pO₂ has a very large influence on conductivity: at fixed temperature and pressure, changes in pO₂ can vary the conductivity by 1.5 orders of magnitude within the stability field of magnesiowüstite. For plausible mantle values of pO₂, temperature and bulk composition, we obtain results in good agreement with those of ref. 1, indicating that magnesiowüstite is the dominant conductor in the lower mantle and that observed upper- and lower-mantle conductivities can be produced by the same bulk composition.