资源环境科技发展态势分析平台

The composition of the lower mantle-comprising 56% of Earth's volume-remains poorly constrained. Among the major elements, Mg/Si ratios ranging from similar to 0.9-1.1, such as in rocky Solar-System building blocks (or chondrites), to similar 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 effcient mixing, and thus homogenous Mg/Si across most of the mantle. However, previous models did not consider the effect of variable Mg/Si on the viscosity and mixing effciency 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 suffcient to prevent effcient 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.