Hydrogen self-diffusivity in single crystal ringwoodite: Implications for water content and distribution in the mantle transition zone

Hydrogen lattice diffusion in Fe-bearing ringwoodite was investigated through hydrogen and deuterium interdiffusing in a pair of synthesized single crystals at various temperatures (1000-1300 K) at 21 GPa. Diffusion profiles were investigated by secondary ion mass spectrometer to determine the hydrogen self-diffusivity in ringwoodite. Temperature dependences of hydrogen diffusion in ringwoodite were determined to be D-H=10(-7.29(+/- 0.46)) exp[-101(+/- 10) kJ mol(-1)/RT] m(2)/s in ringwoodite at 21GPa. The proton conductivities of ringwoodite estimated from the present diffusion coefficients are similar to those of Yoshino et al. [2008] at the transition zone condition at low water content (<1000 ppm by weight (ppmw)) but lower at higher water content range (>1000 ppmw). If the proton-vacancy mechanism is assumed to be a main controlling mechanism, contribution of water to the electrical conductivity of ringwoodite is insignificant due to large contribution of hopping conduction at the transition zone condition, and global average water concentration in the lower part of transition zone is less than 1000 ppmw.