  The composition of the lower mantle $-$ comprising 56% of Earth's volume $-$
remains poorly constrained. Among the major elements, Mg/Si ratios ranging from
$\sim$0.9$-$1.1, such as in rocky solar-system building blocks (or chondrites),
to $\sim$1.2$-$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 $\sim$20$\times$, such as due to the dominance of
intrinsically strong (Mg,Fe)SiO$_3-$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
$\sim$1,000$-$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.
