The composition of the lower mantle-comprising 56% of Earth's volume-remains poorly constrained. Among the major elements, Mg/Si ratios ranging from â 1/40.9-1.1, such as in rocky Solar-System building blocks (or chondrites), to â 1/41.2-1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing, and thus homogenous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity associated with a 20-fold change in viscosity, such as due to the dominance of intrinsically strong (Mg, Fe)SiO 3-bridgmanite in low-Mg/Si domains, is sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize mantle convection patterns, and coherently persist at depths of about 1,000-2,200 km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such bridgmanite-enriched ancient mantle structures (BEAMS) may reconcile the geographical fixity of deep-rooted mantle upwelling centres, and geophysical changes in seismic-Tomography patterns, radial viscosity, rising plumes and sinking slabs near 1,000 km depth. Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.
Bibliographical noteFunding Information:
K.H. were supported by the WPI-funded Earth-Life Science Institute at Tokyo Institute of Technology. C.H., J.W.H., and K.H. received further support through MEXT KAKENHI grant numbers 15H05832 and 16H06285. R.M.W. was funded through NSF grants EAR-1319361 and EAR-1348066.