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

Si self‐diffusion rates in Mg 2 SiO 4 polycrystalline wadsleyite were measured at 18 GPa and 1430–1630°C using an isotopic tracer ( 29 Si) 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 [m 2 /s] exp (−299 [kJ/mol]/ RT ) and δ D gb = 1.14 × 10 −17 [m 3 /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.