Grain growth kinetics of ringwoodite and its implication for rheology of the subducting slab

Grain growth rates of Mg2SiO4- and (Mg0.9Fe0.1)2SiO4-ringwoodites have been studied under the conditions of the mantle transition zone at a pressure of 21 GPa and temperatures ranging from 1473 to 2023 K, using a Kawai-type high-pressure apparatus. The grain growth rates were expressed by Gn − G0n = k0texp (− H* / RT) where G (m) is grain size at time t (s), and G0 is the initial grain size at t = 0, with n = 4.5 ± 0.8, and H* = 414 kJ/mol and log k0 = − 20.4 ± 1.6 m4.5/s for (Mg0.9Fe0.1)2SiO4-ringwoodite, and H* = 456 kJ/mol and logk0 = − 20.2 ± 1.4 m4.5/s for Mg2SiO4-ringwoodite. Using these kinetic parameters for grain growth, we estimated the grain size of ringwoodite in the cold subducting slabs to be less than ∼100 μm. This suggests that the dominant deformation mechanism in the subducting slab is diffusion creep. It is also likely that some cold slabs are softer than the warmer surrounding mantle.