The problem
Earthquake waves reveal two vast regions of slow shear-wave travel in the lowermost mantle beneath Africa and the Pacific, above the metallic core. What combination of temperature and material causes the slowing, and how long have the regions persisted?
A hot region and a chemically different region can both alter wave speeds. In the 2025 circulation simulations, provinces with different amounts and ages of recycled oceanic crust produce similar slowing because temperature dominates the modeled signal. A seismic map therefore does not supply a unique chemical recipe.[1] [2]
Scope
The composition and evolutionary history of the major low-seismic-velocity provinces beneath Africa and the Pacific, separate from Earth’s metallic core.
What remains open
The circulation models replenish the Pacific province with younger oceanic crust while the African province contains older, more mixed material. Those histories depend on reconstructed plate motions and model assumptions. A separate reconstruction uses isotope signatures and the locations of ancient large volcanic provinces to suggest partial replenishment and migration across supercontinent cycles. It does not directly image the ancient mantle. Neither study uniquely establishes present chemistry or a definitive single-impact origin.[1] [2]
Why it matters
Ocean floor can be carried into the deep mantle. Whether these regions preserve or continually remix that material connects the history of surface plates to volcanism fed from below.[1]
If recycled oceanic crust persists at depth, later volcanism can carry evidence of earlier surface processes. If the deep regions migrate or regenerate, their connection to supercontinent breakup also changes through time. Comparing seismic images with chemical records of volcanic rocks tests this link between moving surface plates and deep circulation, rather than assuming today’s map has always applied.[1] [2]
Definitions
2 terms
mantle
The rocky interior between Earth’s crust and metallic core. These slow-wave provinces occupy its lowermost part; they are distinct from the core.[1]
isotope signatures
Ratios of isotopes measured in rocks. In the cited reconstruction, neodymium isotopes help infer changing volcanic-source mixtures and recycling; they do not directly sample an ancient deep-mantle province.[2]
Sources
- Unique composition and evolutionary histories of large low velocity provinces
Notes on Unique composition and evolutionary histories of large low velocity provinces
Original mantle-circulation simulations explicitly identify recycled-crust concentration as open and give model-dependent different histories for the two provinces.
- Basal mantle structure regenerated through supercontinents
Notes on Basal mantle structure regenerated through supercontinents
Original geochemical and paleogeographic reconstruction supports partial replenishment and migration between supercontinent cycles, contrasting a permanently fixed interpretation.