One accepted scientific hypothesis is that the Earth was formed by accretion of material with a chondritic composition through impacts with differentiated planetesimals. During this accretionary phase, planetary differentiation separated the Earth's core, where heavy metallic siderophile elements accumulated, from the surrounding undifferentiated primitive mantle. Further differentiation would take place later, creating the different chemical reservoirs of crust and mantle material, with incompatible elements accumulating in the crust.
Today, differentiation still continues in the upper mantle, resulting in two types of mantle reservoirs: those depleted in lithophile elements (depleted reservoirs), and those composed of "fresh" undifferentiated mantle material (enriched or primitive reservoirs). Volcanic rocks from hotspot areas often have a primitive composition, and because the magma at hotspots is supposed to have been taken to the surface from the deepest regions of the mantle by mantle plumes, geochemists assume there must be a relatively closed and very undifferentiated primitive reservoir somewhere in the lower mantle. One hypothesis to describe this assumption is the existence of the D"-layer at the core-mantle boundary.