Electrical conductivity of pyrope-rich garnet at high temperature and high pressure

Abstract

Garnet is one of the important constituent minerals in the upper mantle and the transition zone of the Earth's mantle. However, there were very few previous works on its electrical conductivity. We have measured the electrical conductivity of single crystal of pyrope-rich garnet (∼Py 73-Alm 14-Grs 13) under the conditions of 4–16 GPa, 873–1473 K and frequency range from 10 −2 to 10 6 Hz, with a range of water content (from less than 10 to 7000 H/10 6 Si). A KAWAI-type multi-anvil apparatus and a Solarton-1260 Impedance/Gain Phase analyzer were used in this study. The impedance spectra showed two circles correspondent to the intrinsic resistivity of the crystal and to the effects of charge accumulation at the electrodes. The DC electrical conductivity was determined by the impedance spectroscopy. Molybdenum and molybdenum oxide solid buffer was used to control the oxygen fugacity. Results on hydrous and anhydrous samples were compared to determine the influence of water content on the electrical conductivity of single crystal garnet. Under anhydrous conditions, the electrical conductivity of garnet increases with temperature and decreases with pressure. When we used a thermal activation parameterization, we obtain the following relationship: σ = A exp(−( E * + PV *)/ RT), A = 1036(±236) (1 − 0.044(±0.007) P(GPa)) S/m or A = exp[7.16(±0.37) (1 − 0.012(±0.009) P(GPa))] S/m, E * = 128 ± 6 kJ/mol and V * = 2.50 ± 0.48 cm 3/mol. Hydrous garnet crystals have significantly higher electrical conductivity with different temperature and pressure sensitivity, and the conductivity in these samples increases with the water content. The results can be summarized as σ = A ⋅ C w r exp [ − ( E * + P V * ) / R T ] with A = 1950 (+870, −600) S/m, r = 0.63 ± 0.19, E * = 70 ± 5 kJ/mol and V * = −0.57 ± 0.05 cm 3/mol. These results are similar to those obtained by Wang et al. [Wang, D.J., Mookherjee, M., Xu, Y.S., Karato. S., 2006. The effect of water on the electrical conductivity of olivine. Nature 443, 977–980] for olivine, Huang et al. [Huang, X.G., Xu, Y.S., Karato, S., 2005. Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite. Nature 434, 746–749] for wadsleyite and ringwoodite and we suggest that the mechanism of hydrogen conduction is likely common to these minerals. We conclude that hydrogen enhances the electrical conductivity of pyrope-rich garnet and its effect increases with pressure but decreases with temperature. At a typical pressure and temperature in the upper mantle, the influence of water is substantial.