Na-rich fluids in jadeitite from Kenterlau-Itmurundy (Central Kazakhstan): Implications for the generation of methane in subduction zones

Jadeite, a mineral formed under high-pressure, low-temperature conditions in subduction zones, can trap key fluids. In this study, we report methane-water fluid inclusions in jadeitite from Kenterlau-Itmurundy serpentinite mélange (Central Kazakhstan). These primary two-phase (liquid + gas) inclusions (3-20 µm) are elongated along the c-axis of jadeite crystals. They exhibit homogenization temperatures ranging from 195 to 270 ℃ and melting temperatures from -7.0 to -3.0 ℃. These values correspond to moderate estimated formation pressures of ~0.5-1.0 GPa, equivalent to the depth of a shallow forearc (~16-33 km) in subduction zones. δ¹³C_CH4 (-27.6‰ to -31.0‰) and δD_CH4 (-238‰ to -205‰) values of these fluid inclusions suggest a thermogenic origin of methane. Combined with salinities of the fluids (4.96-10.49 wt.% NaCl equiv.), the lowest δD_H2O values (-135‰ to -114‰) yet reported in jadeitites worldwide may indicate the presence of meteoric water input during the intraoceanic subduction process. Together with published data from other jadeitite localities, Na-rich, CH₄-bearing fluids are suggested to be an important fluid type not only for the jadeitite at shallow forearc depths in subduction zones, but also for high- to ultrahigh-pressure (HP-UHP) metamorphic and igneous rocks at depths extending to 120 km. Our findings have implications for understanding carbon transport and redox conditions along subduction interfaces, and evaluating the quality of jadeite jades from Kazakhstan.