Fluid-deposited graphite in metapelites from the southwestern Tianshan orogen (China): Implications for carbon cycling in subduction zones

Abstract

Graphite is essential to characterize biogenic (organic) and abiogenic (inorganic) sources of metamorphic carbon. This study includes a detailed petrological, Raman spectroscopic, and stable carbon isotopic analysis of graphite from the high-pressure (HP) and ultrahigh-pressure (UHP) metapelites in the southwestern Tianshan orogenic belt, China. Graphite occurs as foliation-parallel bands, a feature described for the first time in Tianshan, and has relatively high δ¹³C values (δ¹³C_graphite of −14.8 to −12.5‰), indicating an abiogenic precursor. The structural characterization of the graphite morphologies using backscattered electron images and Raman spectroscopy reveals very high crystallinity. Thermodynamic modeling and zirconium in rutile thermometry constrained the P–T conditions of graphite formation during eclogite-facies peak metamorphism at approximately 27 kbar and 530 °C. The banded occurrence of graphite, the CH₄-rich fluid inclusions associated with graphite, and thermodynamic modeling (by GFluid) suggest that graphite precipitated oxidatively from a CH₄-rich fluid. This CH₄-rich fluid may stem from the (U)HP carbonate-bearing eclogites that have been reported in previous studies. The oxidative precipitation of the studied graphite from CH₄-rich fluid suggests that the ambient redox state is one of the key factors that control the fate of COH fluids in the deep carbon cycle.