Mg lattice diffusion in iron-free olivine and implications to conductivity anomaly in the oceanic asthenosphere

Mg lattice diffusion coefficients in iron-free olivine aggregates were measured as a function of pressure from 1 to 13 GPa, temperature from 1100 to 1300 K, and bulk water content from less than 1 up to 350 wt. ppm using multi-anvil apparatus and secondary ion mass spectrometer in depth profiling mode. The water contents in the samples were analyzed by Fourier transmission infrared spectrometer. The results show that Mg lattice diffusion coefficient increase with increasing temperature and water content, and decrease with pressure. The activation energy, water content exponent, and activation volume are 250 ± 30 kJ/mol, 1.2 ± 0.2, and 4.3 ± 0.3 cm3/mol, respectively. Since Mg lattice diffusion controls the ionic conduction in olivine based on the Nernst-Einstein relation, the ionic conductivity in the upper mantle appears as a maximum at the top of oceanic asthenosphere due to the negative correction for pressure and positive correction for water content, which well explains the high conductivity anomaly observed at ∼70-120 km depth beneath young oceanic plates.