Persistence of Strong Silica-Enriched Domains in the Earth's Lower Mantle
Abstract
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 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 viscosity variations of 20×, such as due to the dominance of intrinsically strong (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 seismic-tomography patterns, radial viscosity increase, lateral deflections of rising plumes and sinking slabs). Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.