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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. 

Abstract A better characterization of subsurface processes in hydrothermal systems is key to a deeper understanding of fluid-rock interaction and ore-forming mechanisms. Vent systems in oceanic crust close to subduction zones, like at Brothers volcano and in the eastern Manus basin, are known to be especially ore rich. We measured B concentrations and isotope ratios of unaltered and altered lava that were recovered from drilling sites at Brothers volcano and Snowcap (eastern Manus basin) to test their sensitivity for changing alteration conditions with depth. In addition, for Brothers volcano, quartz-water oxygen isotope thermometry was used to constrain variations in alteration temperature with depth. All altered rocks are depleted in B compared to unaltered rocks and point to interaction with a high-temperature (>150°C) hydrothermal fluid. The δ11B values of altered rocks are variable, from slightly lower to significantly higher than those of unaltered rocks. For Brothers volcano, at the Upper Cone, we suggest a gradual evolution from a fluid- to a more rock-dominated system with increasing depth. In contrast, the downhole variations of δ11B at Snowcap as well as δ11B and δ18O variations at the NW Caldera (Site U1530) of Brothers volcano are suggested to indicate changes in water-rock ratios and, in the latter case, also temperature, with depth due to permeability contrasts between different lithology and alteration type boundaries. Furthermore, δ11B values from the NW Caldera (Site U1527) might point to a structural impact on the fluid pathway. These differences in the subseafloor fluid flow regime, which ranges from more pervasive and fluid-controlled to stronger and controlled by lithological and structural features, have significant influence on alteration conditions and may also impact metal precipitation within the sea floor.