High pressure diversity of solid organic compounds in subduction zones, a window to the deep organic carbon cycle.

The nature and the fraction of carbon that are recycled to the deep Earth via the subduction factory remain an active and controversial research frontier. While many studies have attempt to establish the budgets and distribution of inorganic carbon in the subducting slab at high pressure (HP), relatively little is known about the organic counterpart. Here, we explore the nature and diversity of solid organic compounds trapped in HP ultramafic rocks from the Monviso meta-ophiolite. We show that the eclogitic ultramafic rocks record strong variations of redox conditions (i.e., oxygen fugacity, fO₂) during subduction. Such variations influenced carbon distribution and redox state. In particular, reducing conditions associated with brucite breakdown in meta-ophicarbonate promoted the formation of an unexpected diversity of abiotic carbon-based materials, ranging from disordered carbonaceous matter and organic minerals, never described in HP environments, to nanodiamonds. These newly-formed organic compounds could subsequently be recycled in the deep mantle, with the potential to play a major role on the deep carbon cycle, therefore calling for a thorough examination of the diversity, abundance and stability of solid organic phases under deep Earth.