The Fate of Subduction Zone C-O-H Fluids Revealed by Eclogite-Hosted Fluid Inclusions

Metamorphic fluids in subduction zones play a critical role in arc volcanism, seismicity, and deep carbon cycling. However, the speciation and evolution of carbon during subduction and exhumation remain poorly constrained. This study investigates fluid inclusions in eclogites from the Western Tianshan HP–UHP metamorphic belt, China. Abundant CH₄-rich fluid inclusions are identified in high- to ultrahigh-pressure eclogites and associated veins. Petrological features and C–H isotopic data confirm an abiotic origin for this methane. Reconstruction of P–T–fO₂–fluid trajectories, combined with Deep Earth Water modeling, demonstrates that prograde metamorphism at 50–120 km depth promotes large-scale CH₄ synthesis via redox reactions during slab dehydration. In contrast, retrograde exhumation leads to CO₂-dominated fluid production. Quantitative flux estimates highlight eclogite-hosted methane as a globally significant yet previously overlooked abiotic CH₄ source.Furthermore, two distinct types of fluid inclusions are identified both in eclogites and their veins: Type‑I (water-rich, with CH₄ vapor) and Type‑II (CH₄-rich, with little or no H₂O). Their coexistence indicates fluid immiscibility under high-pressure conditions. Quantitative 3D Raman spectroscopy analysis of CH₄:H₂O ratios underscores an evolutionary transition in C–O–H fluids during decompression and exhumation, driven by progressive immiscibility between CH₄ and H₂O. This phase separation enhances carbon transfer from the subducting slab to the mantle wedge, improves decarbonation efficiency, and may contribute to the formation of abiogenic natural gas accumulations.