The primitive nature of large low shear-wave velocity provinces

The lowermost (>2400 km) Earthʹs mantle mapped by seismic tomography is strongly heterogeneous, the most striking feature being two large regions where shear-wave velocity drops by a few percent compared to averaged mantle. Additional seismic observations indicate that these structures cannot result from purely thermal effects. Compositional anomalies are required to fully explain seismic observations, but their exact nature is still debated. Here, we show that low shear-wave velocity provinces unlikely consist of recycled oceanic crust (MORB). We calculated seismic sensitivity to high-pressure MORB, and found that in the lowermost mantle shear-wave velocity increases with increasing fraction of MORB. Therefore, unless they are heated up to unrealistic temperatures, high-pressure MORB would induce high shear-wave velocity, in contradiction with the observations. Instead, material enriched in iron by ∼3.0% and in (Mg,Fe)-perovskite by ∼20% compared to regular mantle provides a good explanation for the low shear-wave velocity provinces and for the high bulk-sound velocities observed in the same areas. In addition, several geochemical and geodynamical arguments support a primitive origin for this material. Low shear-wave velocity provinces may thus consist of reservoirs of primitive material that have differentiated early in the Earthʹs history.