Abstract
Methane (CH(4)) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH(4) produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH(4)-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon-hydrogen isotopic compositions confirm the abiotic origin of this CH(4). Reconstructed P-T-fO(2)-fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH(4) was generated during cold subduction at depths of 50-120 km, whereas CO(2) was produced during exhumation. The massive production of abiotic CH(4) in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH(4) that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH(4) on Earth.