The currently accepted scientific hypothesis is that the Earth formed by accretion of material with a chondritic composition. Still during the accretionary phase planetary differentiation started, giving rise to the Earth's core, where heavy metallic siderophile elements accumulated. Around it was a (in this stage) undifferentiated mantle, the primitive mantle. Further differentiation would take place later, creating the different chemical reservoirs of crust and mantle, with incompatible elements accumulating in the crust.
Today differentiation still continues in the upper mantle. Reservoirs depleted in lithophile elements are called depleted, "fresh" undifferentiated parts of the mantle are called enriched or primitive. The last name is confusing but derives from the fact that such reservoirs are comparable in composition to the primitive mantle. Volcanic rocks from hotspot areas often have a primitive composition. 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 reservoir of very primitive composition somewhere in the lower mantle. One of the hypotheses is this is the so-called D"-layer at the core-mantle boundary.
- Kremer, William (19 September 2013). "Does gold come from outer space?". BBC Magazine (BBC World Service). Retrieved 19 September 2013.