Electrical conductivity of majorite garnet and its implications for electrical structure in the mantle transition zone

Abstract

Electrical conductivities of majorite garnet with compositions of pyrolite minus olivine (pyrolite majorite) and mid-ocean ridge basalt (MORB majorite) were measured under physical conditions of the mantle transition zone (18 and 23 GPa and temperatures up to 2000 K) in a Kawai-type multi-anvil apparatus. The samples with MORB composition are mainly composed of majorite, which has higher Fe and Al contents, and contain a small amount of stishovite. The conductivity of the MORB majorite is more than twice higher than those of the pyrolite majorite at the same temperature. The activation energies of these majorites are both 1.4 eV at temperature of 1000–1600 K suggesting that the dominant mechanism of charge transportation is Fe 2+–Fe 3+ hopping (small polaron) conduction. At higher temperatures (>1600 K), corresponding to temperature conditions of the transition zone, conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction. The pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite, respectively, and will not largely change the conductivity-depth profile predicted for the dry mantle transition zone. The laboratory-based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi-global conductivity-depth profile obtained from electromagnetic study beneath Pacific. On the other hand, the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of MORB majorite is significantly higher than those of wadsleyite and ringwoodite. Conductivity values of MORB majorite agree with those of the stagnant slab beneath the northeastern China.