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 Nd-142:Nd-144, W-182:W-184 and He-3:He-4 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.

Automated summary

A substantial viscosity difference is observed in the lower mantle of Earth at a depth of 800-1,200 km, known as the mid-mantle viscosity jump. This viscosity jump plays a crucial role in the dynamics and evolution of the mantle, affecting slab subduction, plume ascent, and chemical mixing. The origin of this viscosity jump has remained unknown, but this study suggests that rocks enriched with bridgmanite in the deep lower mantle have significantly larger grain size and higher viscosity compared to overlying pyrolitic rocks, providing an explanation for the mid-mantle viscosity jump.