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Most of the geologic CO₂entering Earth’s atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO₂and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite–talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul’s transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO_2(aq)concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ¹³C of −3.40 ± 0.04‰, and the enrichment of CO₂in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO₂. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul’s Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO₂. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO₂-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO₂.