Variable mantle redox states driven by deeply subducted carbon

Slab subduction transports carbonates into the reduced, metallic iron (Fe⁰)–bearing sublithospheric mantle (>250 kilometers), leading to heterogeneous mantle redox states and sublithospheric diamond formation beneath cratons. To elucidate the drivers of mantle redox variation, we performed mixed reaction experiments between carbonatite melt and Fe⁰-bearing peridotite at 9 to 21 gigapascals under varying redox conditions. Comparing our results with sublithospheric diamond inclusions, we find that majorite and ferropericlase inclusions from the Amazonia Craton reflect a predominantly reduced, nonplume mantle environment, while majorites from the Kaapvaal Craton indicate a fully oxidized plume setting. In nonplume environments, carbonatite melts are progressively consumed until fully frozen as reduced carbon. Attachment of these materials to the cratonic keel further enhances craton stability. In plume environments, carbonatite melts surpass the redox buffering capacity of Fe⁰, leading to an oxidized, CO₂-rich melt-bearing mantle. Impregnation of these melts into the lithosphere weakens the cratonic keel, resulting in lithosphere delamination, surface uplift, and widespread volcanism.