Carbonation of oceanic serpentinite related to extensive intraplate magmatism

Extensive intraplate magmatism that forms oceanic plateaux involves significant carbon outflux and perturbation of the global carbon cycle, potentially linking to climate change, oceanic anoxia, and mass extinction events. Hydrothermal alteration related to such magmatism can modify the major-element composition of the oceanic lithosphere around the hydrothermal system. However, the relationship between carbon mass transfer and hydrothermal systems associated with intraplate magmatism remains unclear. This study investigated carbonated serpentinites occurring along with mafic metavolcanic rocks, which were part of an oceanic plateau in the Late Jurassic Paleo-Pacific Ocean. Carbonate minerals (calcite and dolomite) occur as cement in the serpentinite breccias and as discrete veins in the serpentinite. C-O-Sr isotopic compositions and rare earth element data for the carbonates suggest the calcite cements in the serpentinite breccias formed in an oceanic setting by the mixing of seawater and hydrothermal fluids before subduction. The strontium isotopic compositions of the seawater-derived carbonates suggest that carbonation most likely occurred concurrently with the formation of an oceanic plateau, suggesting intraplate magmatism plays an important role as a carbon sink by inducing alteration and carbonation during associated hydrothermal activity. Moreover, our results suggest the calcite and dolomites formed in a subduction zone from CO2-rich fluids sourced by the devolatilization of metasediments, after the accretion of an oceanic plateau. These results may indicate that the life cycle of oceanic plateaux can cause perturbations in carbon influx and outflux, and that the interplay among oceanic plateau emplacement, subsequent alteration, and carbon uptake may have modulated atmospheric CO2 and Earth’s surface environments.