Metasediments decarbonate more easily in deeper continental crust

Storage and remobilization of carbon bound in crustal carbonates critically regulate the global carbon cycle. These processes also influence ore formation associated with CO2-rich fluids, such as orogenic gold deposits. To investigate the thermal stability of deep crustal carbonates, we conducted decarbonation experiments on average continental margin sediment (CMS) and its carbonate-depleted counterpart at 0.6−1.0 GPa and 750−850 °C. Our results show higher carbonate fractions in CMS at 0.6 GPa than at 1.0 GPa for a given temperature, and a negative Clapeyron slope for the solidus of CMS. These findings partially explain the prevalence of CO2-rich fluids in the lower crust versus H2O-dominated fluids common in the upper−middle crust. Modeling of CMS decarbonation behaviors across three representative tectonic settings demonstrates that continental regions with a high geothermal gradient and thick crust tend to exhibit enhanced decarbonation efficiency. Consequently, significant CO2 degassing is expected during orogenic crustal shortening, whereas it is suppressed in extensional tectonic settings such as rifts, even with a hot thermal structure. This aligns with observations of elevated crustal CO2 fluxes in the Himalaya-Tibet orogen and of mantle-dominated CO2 degassing in the Kenya rift. Our study underscores the pivotal role of burial depth and tectonic setting in the fate of crustal carbonates and highlights the dynamic nature of continental crust as a carbon sink or source during orogenic evolution. CO2 release from deep crustal decarbonation during orogeny provides a plausible explanation for the CO2-rich fluids associated with orogenic gold deposits, offering new insights into the tectonic-fluid-ore connections in continental settings.