Compositional mantle layering revealed by slab stagnation at ∼ 1000-km depth

Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at similar to 1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of similar to 660 and similar to 1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is similar to 8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of similar to 1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist in the presence of whole-mantle convection.