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–
f
O
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.
Massive abiotic methane can be produced during prograde HP-UHP metamorphism in cold subduction zone, which may represent one of the largest sources of abiotic CH
4
on Earth.