The Redox State of the 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 and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a 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 that is based on the V/Sc exchange coefficient between olivine and the 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 Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) 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 higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly. Plain Language Summary: The oxidized form of carbon, carbonate, plays a pivotal role in the melting of the deep mantle by substantially lowering the mantle solidus and instigating the formation of carbonatite melt. The presence of a high‐conductivity and seismic low‐velocity layer at depths of 220–300 km in the asthenospheric mantle suggests the presence of carbonatite melts. Nevertheless, the traditional view posits that the asthenospheric mantle at these depths is reduced, with carbon primarily in the form of diamonds, making it unlikely to induce mantle melting and carbonatite melt formation. In this study, we present a novel oxybarometer based on the V/Sc exchange coefficient between olivine and melt and calculate the oxygen fugacity of the Southwest Indian Ridge and East Pacific Rise MORBs. Our findings reveal that global MORBs and their mantle sources are more oxidized than previously thought. In such an oxidized environment, the conversion of diamond to carbonate and the formation of carbonatite melts occur at depths of 220–250 km, providing a reasonable explanation for the high conductivity and low‐velocity layer in the deep asthenosphere. Key Points: The exchange of V/Sc between the olivine and melts can serve as a robust oxybarometerMid‐ocean ridge basalts are oxidized with oxygen fugacity of FMQ + 0.46 ± 0.26Carbonatitic melts can form at a depth of 200–250 km beneath mid‐ocean ridges [ABSTRACT FROM AUTHOR]