Seminar

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 “chondritic” solar-system building blocks, to ∼1.2–1.3, such as in “pyrolitic” upper mantle rocks, have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing and thus homogeneous 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 with intrinsic viscosity variations of ∼20×, such as due to the dominance of (Mg,Fe)SiO3−bridgmanite in low-Mg/Si domains, are sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize degree-two mantle-convection patterns, and coherently persist at depths of ∼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 centers, and fundamental geophysical changes near 1,000 km depth (e.g. in terms of mantle viscosity, seismic patterns, deflections of rising plumes and sinking slabs). Moreover, BEAMS may provide a reservoir to host primordial geochemical reservoirs.