The grain growth kinetics of bridgmanite at the topmost lower mantle.

• The grain size exponent for growth of bridgmanite in post-spinel is 5.2 ± 0.3. • The activation enthalpy for grain growth of bridgmanite is 260 ± 20 kJ/mol. • Grain size of bridgmanite is ∼30-45 μm in cold subducted slabs with 1-10 Myr age. The grain size of minerals is an important parameter that controls mantle dynamics. We investigated the grain growth kinetics of bridgmanite that coexists with ferropericlase by multi-anvil experiments at 27 GPa, 1400-2400 K, corresponding to the topmost lower mantle conditions. The experimental results indicate that the grain size of bridgmanite systematically increases with increasing duration, with a grain size exponent of 5.2 ± 0.3. The grain growth rate increases with increasing temperature with an activation enthalpy of 260 ± 20 kJ/mol. The grain size of bridgmanite is ∼30-45 μm in young subducted slabs at the topmost lower mantle conditions. The small grain size in subducted slabs may significantly reduce their creep strength in relatively to ambient mantle, and may cause the slab stagnation at the topmost lower mantle. [ABSTRACT FROM AUTHOR]