Carbonic Fluids Drive Continental Carbon Cycling as Revealed by the Geochemistry of the Eclogite‐Garnet Peridotite Interface

Abstract Subduction zones regulate Earth's carbon distribution, yet the mechanism of carbon transfer from continental crust to mantle remains elusive. We examined an eclogite‐garnet peridotite interface from the Chinese Continental Scientific Drilling Program in the Sulu orogen, representing the slab–mantle wedge boundary formed during continental subduction. Whole‐rock magnesium (Mg) isotopic and major‐trace element data, together with in situ mineral analyses, identify the presence of carbonic fluids characterized by notably light Mg isotopic compositions (−0.54 to −0.36‰) and elevated Ca, Mg, Sr, and rare earth elements contents. These fluids, generated by slab decarbonation during prograde metamorphism, mobilized carbon from the subducted crust and enriched the mantle wedge. Modeling indicates that continental subduction rivals oceanic systems in transporting carbon to mantle. However, the paucity of mantle‐derived magmatism limits carbon return, promoting long‐term retention of continental carbon in mantle and establishing continental subduction as a major sink in the global carbon budget.