The evolution of subduction zone metamorphic fluids: Constraints from CH4-bearing fluid inclusions in eclogites from Western Tianshan, China

Subduction zone metamorphic fluids are crucial in the dynamics of volcanic eruptions, earthquakes and the deep carbon cycle. However, the carbon species and their evolution during subduction and exhumation remain poorly understood. This study investigates CH₄-bearing fluid inclusions in eclogite samples with different metamorphic grades from the Western Tianshan High pressure (HP) - Ultrahigh pressure (UHP) metamorphic belt, China. Our petrological and Deep Earth Water (DEW) modeling studies reveal that CH₄-bearing reduced fluids are commonly generated during the subducted prograde HP-UHP metamorphic stage, and gradually transform to oxidized fluids during the decompression and exhumation stage. Employing 3D Raman spectroscopy, we quantitatively analyzed the CH₄: H₂O ratios in fluid inclusions. We discovered that fresh eclogite samples maintain a consistent CH₄:H₂O ratio of approximately 1:3 and stable CH₄ weight percentages ranging from 0.97 to 1.48 wt%. In contrast, retrograde eclogites display greater variation in CH₄: H₂O ratios, spanning from 1:1 to 1:6, and a broader range of CH₄ content (0.57–11.45 wt%). These findings highlight the evolutionary transition in metamorphic C-O-H fluids, most likely triggered by transformation from the miscibility to immiscibility of CH₄ and H₂O during decompression and exhumation processes.