Silicon and oxygen self-diffusion in stishovite: Implications for stability of SiO2-rich seismic reflectors in the mid-mantle

Diffusion of Si and O in single crystal stishovite was examined at pressures of 14.0-21.5 GPa and temperatures of 1673-2073 K. Self-diffusion coefficients of Si (DSi) and O (DO) were determined as DSi[m2/s]=2.4×10−12exp⁡{−(237[kJ/mol]+6.0[cm3/mol]×P)/RT} and DO [m2/s]=7.2×10−11exp⁡{−(263[kJ/mol]+4.8[cm3/mol]×P)/RT}, respectively, where P is pressure (in GPa), T is absolute temperature (in K) and R is the ideal gas constant. It was revealed that diffusion of Si is approximately one order of magnitude slower than that of O and, thus, Si is the rate-controlling element for plastic deformation of stishovite. Si diffusion in stishovite is assessed to be at least three orders of magnitude slower than that in bridgmanite under mid-mantle conditions. Therefore, it is anticipated that highly viscous SiO2-rich components subducted into the lower mantle persist as the seismic reflectors for long term without mixing up with the bridgmanite-dominated surrounding mantle.