Electrical conductivity of stishovite as a function of water content

The electrical conductivity of stishovite with various Al2O3 and H2O contents was measured at 12 GPa of pressure (P) and temperatures (T) up to 1900 K in a Kawai-type multi-anvil apparatus. Starting materials were pre-synthesized at 12 GPa and 1673 K from various mixtures of SiO2, Al2O3 and Al(OH)3. The synthesized stishovite aggregates contained various H2O concentrations up to 0.25 wt.%. The conductivity of relatively dry stishovite was almost constant independently of Al content, whereas the conductivity significantly increased with increasing H2O content in stishovite. All electrical conductivity data fit the formula for electrical conductivity σ = σ 0 C W exp { - [ Δ H 0 - α C W 1 / 3 ] / kT } , where σ0 is the pre-exponential term, CW is the H2O concentration, ΔH0 is the activation enthalpy at very low H2O concentration, and k is the Boltzmann constant. The activation enthalpy decreased from 1.22 to 0.90 eV with increasing H2O content from 0.01 to 0.22 wt.%. A nearly linear correlation of the conductivity values on the H2O content suggests that the dominant mechanism of charge transport in stishovite is proton conduction. Although electrical conductivity of hydrous stishovite is higher than that of garnet in the subducted oceanic crust, small amount of hydrous stishovite is insufficient to raise conductivity. On the other hand, hydrous stishovite can contribute to the high conductivity occasionally observed at the mantle transition zone, if the subducted Archean continental crusts with tonalite–trondhjemite–granodiorite (TTG) composition were accumulated above the 660 km seismic discontinuity.