Experimental investigation of a deep organic carbon cycling promoted by carbonate reduction in subduction zones

Recent studies show that subduction zones host both biogenic and abiotic organic carbon. The latter can form under high-pressure and high-temperature conditions, yet the mechanisms and redox controls remain poorly constrained. We present piston-cylinder experiments at 700 °C and 3 GPa simulating carbonate reduction in the presence of antigorite over a wide oxygen fugacity range, from about ΔFMQ −4 to −1 relative to fayalite–magnetite–quartz (FMQ) buffer. Graphitic carbon forms under all investigated redox conditions, and is systematically associated with chlorite derived from antigorite breakdown. Raman spectroscopy, TOF- and Orbi- SIMS analyses indicate that under the most reducing conditions investigated i.e. , approaching those imposed by wüstite–magnetite buffer (ΔFMQ ≈ −4), abundant graphitic carbon forms and is highly ordered, hydrogen-poor and dominated by aromatic structures. In contrast, less reducing conditions (buffered at nickel–nickel oxide, ΔFMQ ≈ −1), yield smaller amounts of highly disordered, partially hydrogenated and oxygenated graphitic carbon. These differences, observed under identical pressure and temperature, primarily reflects the influence of oxygen fugacity, indicating a redox-driven formation pathway distinct from the thermal maturation and graphitization of biogenic organic matter. Such redox-driven precipitation of graphitic carbon may promote transient carbon storage, facilitate volatile transport into the deep mantle and contribute to the diversity of metastable carbon species involved in the deep carbon cycling.