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 subduction(3), accelerates plume ascent(4) and inhibits chemical mixing(5). 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 chondrites(8), the anomalous (142)Nd:(144)Nd, (182)W:(184)W and (3)He:(4)He isotopic ratios in hot-spot magmas(9,10), the plume deflection(4) and slab stagnation in the mid-mantle(3) 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.