Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800-1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump^1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction³, accelerates plume ascent⁴ and inhibits chemical mixing⁵. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump^1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection^5-7. The high Mg:Si ratio of the upper mantle relative to chondrites⁸, the anomalous ¹⁴²Nd:¹⁴⁴Nd, ¹⁸²W:¹⁸⁴W and ³He:⁴He isotopic ratios in hot-spot magmas^9,10, the plume deflection⁴ and slab stagnation in the mid-mantle³ as well as the sparse observations of seismic anisotropy^11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.