COH fluids and melts drive mantle refertilization and organic synthesis in nascent subduction zones

Slab-derived fluids and melts released during subduction refertilize the mantle wedge, shaping arc magmatism and volatile transfer. Yet the nature and efficiency of the earliest metasomatizing agents active under the warm thermal gradients of nascent subduction zones remain poorly constrained. Here, we report multiphase and polymineralic inclusions from the high-temperature metamorphic sole of the Semail ophiolite (Oman–United Arab Emirates), which record the trapping of COH fluids and hydrous silicate melts derived from both mafic and sedimentary slab components during subduction infancy. Multiphase inclusions act as micro-reactors where fluids react with host minerals during exhumation, driving the abiotic synthesis of solid organic compounds, including nanodiamond-like structures, demonstrating that subduction zones can generate molecules of prebiotic relevance. Combined with the ophiolite record, our results indicate that such COH fluids and melts refertilize the mantle wedge from the onset of subduction, generating proto-arc magmas and promoting deep carbonation and hydration.