Immiscible C-H-O fluids formed at subduction zone conditions

Earth's long-term carbon cycle, which is regulated by subduction and volcanism, is critical for understanding Earth's structure, dynamics, and climate change. However, the mechanisms for carbon mobility in subduction zones remain largely unresolved. Aqueous fluids produced by slab devolatilisation may dissolve a considerable amount of carbon, but it is usually assumed that aqueous C-H-O fluids in subduction zones are fully miscible. In order to constrain the nature of aqueous C-H-O fluids in subduction zones, experiments were performed at 0.2 to 2.5 GPa and 600 to 700 degrees C to study the phase relations of C-H-O fluids in the presence of 3 wt. % NaCl, using the synthetic fluid inclusion technique. The results show that at 0.2 GPa and 700 degrees C, one single C-H-O fluid phase was present; however, at 1.5 to 2.5 GPa and 600 to 700 degrees C, H2O and gases of CH4+ H-2, CH4+CO2, or CO2 coexisted as immiscible fluid phases. These results demonstrate that pressure can significantly expand the miscibility gap of C-H-O fluids and immiscible C-H-O fluids may occur in subduction zones. The likely occurrence of immiscible C-H-O fluids in subduction zones may cause extensive decarbonation and the formation of immiscible CO2-rich fluids, providing an important mechanism for the transfer of slab carbon to the mantle wedge.