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

Mantle convection causes changes in the grain sizes of mantle minerals due to dynamic recrystallization, phase transitions, grain growth and Ostwald ripening. These processes are highly non-uniform and may produce large spatio-temporal variations in the grain size and the viscosity. Various estimates suggest that the grain size is likely to vary from 0.01 to 1 cm, which is consistent with observations of xenoliths and recent seismic constraints. The primordial crystal sizes established upon crystallization of the early magma ocean can reach several centimeters in diameter. No subsolidus or supersolidus process seems to be capable of producing a locally averaged grain size larger than 10 cm. The spinel-perovskite phase transformation is the most significant one for grain size reduction. Other phase transformations cause a transient grain size reduction over length scales of about several kilometers within the phase boundary, although nanometer-size grains can be produced over large regions in the cold subducting slabs. Numerical simulations of mantle convection with grain size evolution shows that while the spinel-perovskite phase transformation keeps the grain size in the most of the lower mantle small, some material can preserve its original grain size. The large viscosity of these regions prevents them from mixing and preserves their primordial composition. The amount of the preserved heterogeneities depends on various factors including the vigor of convection, the compositional buoyancy of the heterogeneities and the kinetics of grain growth.