LAUSR

Significance There is no direct access to the deep Earth mantle. Therefore, insights into the deep mantle composition rely on indirect observations from geophysics. We exploit seismic datasets of compressional and shear waves reflected at mineralogical phase transitions globally and reveal the existence of anomalous geochemical reservoirs in the mantle transition zone (MTZ). This first-order compositional pattern is linked to the 100 My old history of tectonic subduction. The existence of basaltic reservoirs and their estimated volumes require mechanisms that allow segregation of the subducted crust from the harzburgitic mantle at mid-mantle depths, regardless of the age of the subducted oceanic lithosphere. Our new constraints on compositional patterns in the MTZ help to understand heat and material flow through the mantle.