Electrical conductivity of stishovite as a function of water content

The electrical conductivity of stishovite with various Al₂O₃ and H₂O contents was measured at 12GPa of pressure (P) and temperatures (T) up to 1900K in a Kawai-type multi-anvil apparatus. Starting materials were pre-synthesized at 12GPa and 1673K from various mixtures of SiO₂, Al₂O₃ and Al(OH)₃. The synthesized stishovite aggregates contained various H₂O concentrations up to 0.25wt.%. 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=σ0CWexp{-[δH0-αCW1/3]/kT}, where σ₀ is the pre-exponential term, C_W 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 0.90eV with increasing H₂O content from 0.01 to 0.22wt.%. 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 660km seismic discontinuity.