Invited Lecture
Silicon, oxygen and magnesium self-diffusion of forsterite as a function of water content

Knowledge of self-diffusion coefficients of mantle minerals is vital for understanding dynamic motion in the Earth’s mantle, because very low flow rates of mantle flow imply that mineral creep will be controlled by diffusion. Forsterite, Mg₂SiO₄, is an Mg-endmember of olivine, which is the most abundant mineral in the upper mantle. Petrological studies showed that water is incorporated in olivine at high pressures and temperatures. Rock deformation experiments showed that water corporation drastically change creep rate of olivine, namely creep rate increases in proportion to 1.2 power of water content. It is also demonstrated that water incorporation also increases electrical conductivity. For these reasons, we measured self-diffusion coefficients of Mg, Si and O of forsterite as a function of water (OH^-) contents.

Measured samples were synthetic single crystals or polycrystalline aggregates of forsterite. They were annealed at a pressure of 8 GPa and temperatures of 1300-1800 K with and without doping water. They were polished and isotope enriched thin films were formed on their polished faces. They were annealed for diffusion under the same conditions as the pre-annealing. Diffusion profiles were obtained by depth analysis using SIMS.

Our results demonstrated that Si and O self-diffusions increase with 0.3 and 0 powers of water content. These values are drastically smaller than those obtained by deformation experiment, implying that effects of water on mantle dynamics will be much smaller than previously considered. In contrast, magnesium diffusion coefficient increases with 1.2 power of water content, suggesting significant enhancement of ionic conductivity by water incorporation.