The formation of graphite-rich eclogite vein in SW Tianshan (China) and its implication for deep carbon cycling in subduction zone

The balance of carbon flux in subduction zones is critical to the deep carbon cycle. Carbonate-bearing lithologies are the major carbon carriers transported from Earths surface into its interior at subduction zones. Recently, a number of studies have showed that carbon can be released from the subducting slab through metamorphic decarbonation and dissolution into C-H-O fluids. However, the evolution of the released C-H-O fluids during subduction-zone metamorphism is ambiguous and poorly explored. In this study, we found graphite-rich eclogite veins (VE) in the carbonated eclogites from the Southwestern (SM.) Tianshan subduction zone. The observed graphite with high crystallinity and graphite-bearing fluid inclusions indicate the fluid-deposited origin. Phase equilibrium modelling for the host carbonated-eclogite (HE) in a closed system indicates that it has experienced a retrograde P-T path involving decompression with heating from 26.5 kbar at 487 degrees C to 20.6 kbar at 565 degrees C. The calculation showed that about 0.92-2.03 wt% of CO2 (0.25-0.55 g C per 100 g rock) could be released from the carbonated eclogite during its exhumation process, which is enough to provide the carbon source for graphite precipitation in the VE. Combined with petrological and isotopic results, we suggest that the graphites in the VE were precipitated from carbon-bearing fluids derived from the carbonated eclogites during exhumation metamorphism. The overall redox reaction is: FeO (in silicate, Grt, Omp or Gln) + FeS + (H2O + CO2) (released from Lws and Dol) -> Fe2O3 (in Ep, Andradite or Hematite) + C (graphite) + SO42- + HCO3 + CO32-. Mass balance calculation indicates that carbonates could also be re-precipitated in the VE during fluid-rock interaction, in addition to the graphite precipitation. The finding of fluid-deposited graphite in the carbonated eclogites provides new insights into the fate of carbonic fluids formed in the subducted oceanic crust. We suggest that carbonic fluids formed in the carbonated eclogites by decarbonation or carbonate dissolution may also precipitate abiotic graphite or carbonates under favorable conditions during their migration in addition to the commonly recognized transportation to the mantle wedge.