Water‐Facilitated Diamond Formation in the Deep Mantle

As critical volatiles in the deep Earth, carbon and water significantly modify magma activities and mantle dynamics. However, it remains highly controversial how hydrous minerals affect carbonate stability and carbon speciation in the deep mantle. Here, we synthesized 13 C‐enriched dolomite (MgCa(CO 3 ) 2 ) and experimentally investigated hydrous interactions with the synthetic carbonate at 14–120 GPa and 1,500–2,800 K. Compared to dry conditions with low decarbonation efficiency, hydrous environments significantly boost decarbonation rate, facilitating the formation of superdeep elemental carbon (i.e., graphite, diamond) under deep‐mantle conditions. In principle, water polarizes the CO 3 2− electron cloud, depleting electron density in C–O bonds and weakening their structural integrity. This water‐driven breakdown represents a viable mechanism for carbon mobilization and enrichment via mantle fluids. Our findings highlight tight coupling between deep water and carbon cycles, establishing a mechanistic framework for superdeep diamond genesis.