The geochemical concept of a “Primitive Mantle”

The "Bulk Silicate Earth" (BSE) is a conceptual reservoir whose estimated composition is critical for evaluating the mass fluxes between Earth's interior and its surface reservoirs, the crust, oceans and atmosphere. As a physical entity, the BSE is the compound of Earth's heterogeneous mantle and crust. It remains questionable, however, if the BSE ever existed as a homogeneous reservoir, the so-called "Primitive Mantle" (PM) during Earth's juvenile phase. If so, it is unlikely that potential remnants of it have survived >4.5 Ga of crust-mantle cycling, consistent with the lack of evidence for sampling such material. Although a homogeneous BSE-like PM thus remains a purely conceptual reservoir, refined estimates of BSE's composition are critical for better understanding the >4.5 billion-year-long compositional evolution of the silicate Earth.