The redox state of asthenospheric mantle and the onset of melting beneath mid-ocean ridges

The redox state of the convective asthenospheric mantle governs the speciation of volatile elements, such as carbon, therefore influences the depth at which (redox) melting can occur with implications for seismic signals. Geophysical observations suggest the presence of carbonatite melts at depth of 200–250 km, however, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3–4 % of Fe³⁺/∑Fe. Here we present a new oxybarometer based on the V/Sc exchange coefficient between olivine and melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid-ocean ridge basalts (MORBs) from the Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo₂) of MORBs, corrected for the depth of formation, is 0.88±0.24 log units above the fayalite-magnetite-quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits a higher oxygen fugacity than continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO₂-bearing source at depth of 200–250 km explaining, therefore, the electrical conductivity and seismic velocity anomaly of the asthenospheric mantle.