Redox processes at the slab-mantle interface: Evidence from reduced carbon inclusions in mantle wedge peridotites

Carbonate-bearing fluids derived from subducting slabs are important agents in crust-mantle interactions that can modify the composition and oxidation state of the overlying mantle wedge. Here we use magnesite-bearing metasomatic veins as well as fluid inclusions in the mantle wedge harzburgite to investigate multiple stages of fluid-mantle interactions which initialize the mantle oxidation and chemical transformation of carbon-bearing species. We find that compositional growth zones in metasomatic minerals record three stages of changes in fO₂, fluctuating from FMQ-1 to FMQ-5 and back to FMQ-1.5. Such fO₂ fluctuation corresponds to the presence of carbon-bearing species with varying valence states, including magnesite, amorphous carbon, graphite, nanosized diamond, as well as methane and organic compounds. We suggest that multiple stages of fluid-mantle interactions, regulated by oxidation states of the evolving carbonaceous fluid, provide a mechanism for the abiotic formation of organic carbon at the slab-mantle interface.