Recent studies have highlighted the presence of organic carbon in subduction zones, originating not only from biological sources but also through abiotic processes, including organic synthesis under high-pressure and high-temperature (HP-HT) conditions, driven by carbonate destabilization and metamorphic reactions. The formation of such compounds, which depends on redox conditions, could significantly influence the mobility and ultimate fate of carbon in subduction zones.
To investigate the formation of abiotic organic compounds via carbonate reduction during subduction, HP-HT experiments were conducted using a piston-cylinder apparatus at 700 °C and 3 GPa. Mixtures of antigorite and carbonate (calcite or ankerite) were tested across a broad fO2 range (ΔFMQ -3.8 to -0.7) using a double capsule setup.
The results reveal that graphitic carbon forms across the entire fO2 range and is spatially associated with chlorite. Additionally, fO2 influences both the quantity and structural disorder of the resulting graphitic carbon. Raman, TOF-SIMS, and Orbi-SIMS analyses indicate that more reducing conditions favor the formation of aromatic and highly ordered graphitic carbon, whereas less reducing conditions produce disordered graphitic carbon. These findings suggest that graphitic carbon can form under a wide range of subduction redox conditions, enabling carbon sequestration in the subducting slab. Furthermore, unlike organic carbon of biological origin, the degree of disorder in graphitic carbon appears to reflect variations in fO2 rather than P-T conditions.