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 Poirier 1 extrapolated high-pressure conductivity measurements of the lower-mantle assemblage (Mg,Fe)SiO3 perovskite plus (Mg,Fe)O magnesiowustite from 673 K to lower-mantle temperatures (2,200-2,500 K) to obtain conductivities consistent with geomagnetic observations (approximately 1 S m-1 at 1,000-km depth). The high-pressure study of Li and Jeanloz 2 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 magnesiowustite. For plausible mantle values of pO2, temperature and bulk composition, we obtain results in good agreement with those of ref. 1, indicating that magnesiowustite is the dominant conductor in the lower mantle and that observed upper- and lower-mantle conductivities can be produced by the same bulk composition.