The composition of the lower mantle—comprising 56% of Earth’s volume—remains poorly constrained. Among the major elements, Mg/Si ratios ranging from ∼0.9–1.1, such as in rocky Solar-System building blocks (or chondrites), to ∼1.2–1.3, 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)SiO3–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.
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The authors thank F. Deschamps for comments that helped to improve the manuscript. They are further grateful to A. Dziewonski for discussions on the seismic structure and dynamics of the mantle. Calculations have been performed on akua, the in-house cluster of the Department of Geology & Geophysics, University of Hawaii. M.D.B., C.H., J.W.H. and 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.
The authors declare no competing financial interests.
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Ballmer, M., Houser, C., Hernlund, J. et al. Persistence of strong silica-enriched domains in the Earth’s lower mantle. Nature Geosci 10, 236–240 (2017). https://doi.org/10.1038/ngeo2898
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