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The composition and evolution of carbonated-silicate melts as they ascend through the asthenospheric melting column remain controversial. To address this goal, we adopt a reverse experimental strategy with a multi-anvil apparatus, forcing a primitive ocean island basalt from Cape Verde into saturation with four-phase garnet lherzolite at adiabatic temperatures (1380–1420 °C) and pressures of 3–7 GPa, employing H2Omelt contents calculated from mantle/melt hydrogen partitioning.

Our forced multiple saturation experiments back-track the downward evolution of intraplate alkaline magmas erupted at the surface, from their last equilibration depth with mantle peridotite at the lithosphere-asthenosphere boundary down to their origin at redox melting depths. With increasing pressure, the experimental melt composition in equilibrium with four-phase garnet lherzolites shifts from a silica-undersaturated olivine melilitite (3–4 GPa) to a kimberlite (6–7 GPa), as defined by decreasing SiO₂ (38.6–23.9 wt.%) and Al₂O₃ (8.9–1.5 wt.%), and increasing MgO (16.4–24.5 wt.%) and volatiles (CO₂ + H₂O = 6.4–18.7 wt.%).

Notably, the presence of H₂O in our complex natural system significantly lowers the temperature at which the carbonatite-silicate transition occurs, compared to H₂O-free systems, leading to higher silica and lower CaO and CO₂ compared to carbonatitic melts produced at sub-adiabatic temperatures. At redox melting depths, where a diamond-bearing mantle upwelling converts into a carbonated silicate melt-bearing system, a kimberlitic melt is thus expected to be generated. As these kimberlites rise through the asthenospheric column, they dilute their volatile contents and become increasingly less silica-undersaturated, the last pressure of equilibration dictating the degree of Si-undersaturation of the most primitive magma to erupt at the surface.