Carbonated mantle peridotites represent a hidden sink for subducted CO2

Subduction of carbon rich sediments and crust at convergent plate boundaries exerts a crucial control on Earth's mantle chemistry and surface habitability. Recent attention has focused on exposures of fully-carbonated mantle rocks as these may attest to an overlooked sink for subducted carbon not sampled by arc volcanism. However, even in the best-studied example, the Semail Ophiolite, Oman, the setting for carbonation remains highly contentious, with conflicting inferences from geochemistry and geochronology. We approach this problem by combining microanalysis of halogens and detailed petrography to fingerprint the origins of carbonating fluids. Fluids were derived from both sedimentary pore fluid expulsion and deep slab decarbonation reactions in a subduction zone setting. Through mass balance modelling we show that CO₂ fluxes into the forearc from deep decarbonation (1.7-3.4 × 10¹³ gyr^-1 C) could represent up to 90% of the global flux entering subduction zones, indicating that carbonated mantle peridotites likely represent a major sink for subducted CO₂ which may have varied through geological time.