Favorable P–T–ƒO2 conditions for abiotic CH4 production in subducted oceanic crusts: A comparison between CH4-bearing ultrahigh- and CO2-bearing high-pressure eclogite

Abstract

Subduction is a key element in the carbon cycle between the Earth’s surface and deep interior. In addition to pressure (P) and temperature (T), oxygen fugacity (ƒO2) is an intensive variable that can change the speciation, stability, and mobility of carbon-bearing phases in subduction zones. However, tracking redox evolution and associated carbon speciation in high-pressure metamorphic rocks from subduction zones remains challenging. In this study, we identified CH4-rich fluid inclusions in ultra-high-pressure (UHP; i.e., coesite-bearing) eclogites (the UHP-CH4 eclogite) and CO2-rich fluid inclusions in high-pressure (HP; i.e., quartz-bearing) eclogites (the HP-CO2 eclogite) from the Chinese southwestern (SW) Tianshan palaeosubduction zone, respectively. Combining the garnet-clinopyroxene oxybarometry and comprehensive thermodynamic calculations, we revealed that the ƒO2 of the UHP-CH4 eclogite decreases to a minimum of ∼FMQ − 3 at peak P–T conditions (2.8 GPa, 525 °C) and CH4 is expected to form in graphite-saturated CHO fluids. Whereas during the early-exhumation stage (2.5 GPa, 600 °C), the UHP-CH4 eclogite is predicted to buffer higher ƒO2 conditions of ∼FMQ − 0.9. In contrast, the HP-CO2 eclogite, which has a higher bulk CO2 content (∼10 wt%) and elevated Fe3+/ΣFe ratio (0.24) than the UHP-CH4 eclogite, suggesting a stronger oxidative seafloor alteration of the HP-CO2 eclogite protolith before subduction, stabilizes graphite and CO2-rich fluids at peak ƒO2 conditions between FMQ and FMQ + 1 (2.6 GPa, 500 °C). These data demonstrate that the thermal structure (P–T conditions) of the subduction zone and the composition of the slab are first-order controls on the redox evolution of subducting slabs and the changing speciation of carbon in slab fluids buffered by the metamorphic rock system. Therefore, we propose that the cold subduction of a weakly altered oceanic crust (e.g., UHP-CH4 eclogites from the SW Tianshan) provides ideal conditions for the formation of ultra-deep abiotic CH4.