CO2 outgassing during collisional orogeny is facilitated by the generation of immiscible fluids

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO2) fromthe continental crust. However, the processes of carbon mobilization are still poorly explored.Here, we use thermodynamic modeling to investigate the decarbonation of sedimentsmetamorphosed under high geothermal gradients. Our modeling shows that immiscible CO2-rich vapors and hydrosaline brines are generated at these conditions, with different propertiesand mobility through the crust. The CO2-rich fluid fraction could rapidly rise toward thesurface without interacting with the host rocks by carbo-fracturing the host rocks or throughdeep faults. The denser hydrosaline brines likely permeate the source rocks. When applied tothe active Himalayan orogen, these observations reconcile measured CO2 fluxes at thesurface and positive conductivity anomalies associated with micro-seismicity at depth. Ourmodeling shows that the continental crust represents a relevant reservoir of CO2 that can beefficiently degassed during hot collisions.