Tracking highly Si-undersaturated, CO2-rich Ocean Island magmas from the surface to the depth of redox melting: a reverse approach to understand melting columns within mantle plumes

The generation and evolution of CO 2 -rich silicate melts within mantle plumes are critical to understanding intraplate magmatism. We employ a novel reverse experimental approach to track the compositional evolution of a primitive Cape Verde basanitic magma down the asthenospheric melting column at adiabatic temperatures (for a potential temperature T P = 1350°C). Melt evolution is reconstructed from 3 to 7 GPa, 1380–1420°C, i.e. from the lithosphere-asthenosphere boundary (LAB) at ∼100 km to the redox melting front at ∼230 km. Using "forced multiple saturation experiments", we equilibrate the melt with a four-phase garnet-lherzolite (olivine–orthopyroxene–clinopyroxene–garnet), starting with 5 wt.% CO 2 and employing H 2 O contents as constrained by mantle/melt hydrogen partitioning. The experimental melts evolve from olivine melilititic at 3–4 GPa to kimberlitic at 6–7 GPa, characterised by decreasing SiO 2 (38.6 to 23.9 wt.%) and Al 2 O 3 (8.9 to 1.5 wt.%) alongside increasing MgO (16.4 to 24.5 wt.%) and volatiles (CO 2 + H 2 O = 6.4 to 18.7 wt.%), as the expanding stability fields of orthopyroxene and garnet restrict silica and alumina in the melt. Comparison with prior studies on H 2 O-free carbonated mantle melting reveals that H 2 O and alkalis lower the transition temperature from carbonatitic to carbonated silicate melts, such that carbonatitic melts could occur only at sub-adiabatic temperatures. Instead, at adiabatic and realistic H 2 O mantle conditions, redox melting produces kimberlitic melts, converting solid-state plumes into melt-bearing plumes. As these melts rise through porous flow, they increase their SiO 2 content, dilute their CO 2 and decrease their alkalinity, thus evolving into ultramafic lamprophyres and melilititic basanites. The final pressure of asthenospheric equilibration controls Si-undersaturation in the primitive magmas that may ultimately erupt at the surface.