Deep mantle melting marks the onset of Earth differentiation 1 , yet a unifying framework for how buoyancy-driven mantle upwellings initiate melting and how such incipient melts evolve within the asthenosphere has remained elusive. Here we show that the first melts generated in any solid-state mantle upwelling are kimberlitic CO 2 -rich silicate melts that form at about 250 km depth through oxidation of elemental carbon to CO 2 (refs. 2 , 3 ). Our experiments force a range of surface melts, derived from mantle plumes 4 or broad upwellings 5 (kimberlites, ocean island basalts and mid-ocean ridge basalts), into equilibrium with fertile mantle at adiabatic and super-adiabatic conditions at 7 GPa. The results define a framework in which redox melting at depth universally yields kimberlitic melts, which, while ascending through the asthenosphere by reactive porous flow 6 , 7 , evolve to higher degrees of melting, lesser volatiles and incompatible elements, but higher SiO 2 . Channelized flow 7 in the lithosphere may then enable direct extraction of these melts, leading to kimberlites, where the lithosphere commences just above the C → CO 2 redox front, to alkaline Si-undersaturated intraplate magmas where lithospheric thicknesses are 150–100 km, and to tholeiitic basalts below mid-ocean ridges where voluminous ‘dry’ melting becomes overwhelming. This framework is consistent with the widespread seismic low-velocity zone at about 250 km beneath mid-ocean ridges 8 , 9 and aligns with ocean island and mid-ocean ridge basalts sampling the various geochemical mantle components at different degrees of melting in different proportions 10 , 11 .
Schmidt et al. (Wed,) studied this question.