A universal concept for melting in mantle upwellings.

Abstract

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 flow^6,7, 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 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.