Deep mantle melting marks the onset of Earth differentiation1, 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 CO2-rich silicate melts that form at about 250 km depth through oxidation of elemental carbon to CO2 (refs. 2,3). Our experiments force a range of surface melts, derived from mantle plumes4 or broad upwellings5 (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 flow6,7, evolve to higher degrees of melting, lesser volatiles and incompatible elements, but higher SiO2. Channelized flow7 in the lithosphere may then enable direct extraction of these melts, leading to kimberlites, where the lithosphere commences just above the C → CO2 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 ridges8,9 and aligns with ocean island and mid-ocean ridge basalts sampling the various geochemical mantle components at different degrees of melting in different proportions10,11.
