Silicon self-diffusion in wadsleyite: Implications for rheology of the mantle transition zone and subducting plates

Si self-diffusion rates in Mg₂SiO₄ polycrystalline wadsleyite were measured at 18 GPa and 1430-1630°C using an isotopic tracer (²⁹Si) and secondary ion mass spectrometry. The volume diffusion coefficient (D_v) and grain-boundary diffusion coefficient (D_gb) were determined to be D_v = 3.44 × 10^-11 [m²/s] exp (-299 [kJ/mol]/RT) and δD_gb = 1.14 × 10^-17 [m³/s] exp (-248 [kJ/mol]/RT), respectively. Si diffusion rates in wadsleyite are about 5 orders of magnitude slower than Mg-Fe interdiffusion rates at 1400°C. Assuming that Si is the slowest diffusing species in wadsleyite, the geophysical model of the viscosity in the mantle transition zone can be explained by diffusion creep in wadsleyite for a grain size of about 0.5-5 mm. Some portions in cold subducting slabs, where the grain size reduces to less than 1 μm after the olivine-spinel transformation, become weaker than the surrounding mantle.