Grain size variations in the Earth's mantle and the evolution of primordial chemical heterogeneities

Chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection. One of the factors which can cause viscosity variations is the grain size. Analysis of various physical processes in the Earth's mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs. The largest grains are formed in the early molten Earth where Ostwald ripening is controlled by liquid‐state diffusion. The smallest grains are formed due to recrystallization by the spinel‐perovskite phase transformation in downwelling flow and subsequent Ostwald ripening. The viscosity contrast between primordial, large grain size material and recrystallized material is likely to be many orders of magnitude. Preliminary numerical simulations show that strongly time‐dependent mantle convection can process primordial large‐grain size heterogeneities through the spinel‐perovskite phase boundary only after a few overturns. This reduces the viscosity contrasts and leads to rapid mixing by mantle convection. A more steady flow in the lower mantle may lead to the formation of stagnant, poorly mixed regions which may never have had a chance to cross the 660‐km discontinuity and be recrystallized by the spinel‐perovskite phase transformation. Heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle. In this case, they accumulate at the core‐mantle boundary.