The fate of carbon in subduction is explored by considering fluid-rock interactions in a body of metamorphosed ultramafic rocks from the Appalachian belt as potential proxies for metasomatism and carbon recycling (e.g., infiltration, solution, precipitation) in the mantle wedge at convergent margins. Ultramafic rocks may record intense redox variations affecting the exchanges between solid and fluid carbon-bearing phases. The Belvidere Mountain Complex (BMC) in Vermont, USA is an ultramafic body that underwent Ordovician Taconic subduction metamorphism up to 510-520 °C and 0.9-1.3 GPa. Previous investigations indicate that the BMC experienced partial serpentinization and carbon recycling in the subduction zone. Here bulk δ¹¹B, ⁸⁷Sr/⁸⁶Sr and δ¹³C, data, in-situ microRaman spectroscopy and bulk δ¹³C_CH4 and δ²H_CH4 on fluid inclusions, and numerical modeling results aim to constrain the origin and formation mechanisms of carbon-bearing solid and fluid phases in the BMC. Bulk ⁸⁷Sr/⁸⁶Sr ratios and δ¹¹B of the BMC rocks suggest infiltration of metasediment-derived fluids inside the mafic/ultramafic body. Carbonates present in the studied ultramafic and mafic rocks have δ¹³C values ranging from -7.26 to -1.27 ‰. This range was found to record progressive reduction of an initial isotopically light carbonate to methane under reducing conditions. MicroRaman spectroscopy on fluid inclusions confirms CH₄-rich gaseous compositions, along with N₂, NH_3, and S-H compounds, which support a metasedimentary origin for the fluid infiltrating the BMC. Methane-rich fluid inclusions were observed in partially serpentinized peridotites, in carbonate veins, and in amphibolite bodies associated with the ultramafic body, and as well in the surrounding metasedimentary rock units. The δ¹³C and δ²H of methane in fluid inclusions show a wide range of values in ultramafic rocks from -45.2 ‰ to -12.6 ‰ for carbon and -226 ‰ to -140 ‰ for hydrogen. The methane stable isotope data suggest the presence of two types of methane preserved in studied samples, formed by high-temperature thermogenic processes of metamorphosed graphitic carbon and by abiotic conversion of more oxidized carbon bearing species in the presence of H₂-rich fluids produced during metamorphic serpentinization. The BMC also hosts a remarkable example of graphite deposit, with up to 3.90 wt% graphite, along a ∼ 6 m-thick zone within the serpentinized ultramafic body and along lithologic contacts. The δ¹³C composition of the graphite clusters at -15 ‰ (VPBD), which is consistent with the composition of graphite precipitating from the mixing of the two types of methane-bearing fluids identified in fluid inclusions. The mixing between metasediment-derived and serpentinization-related methane-rich fluids is proposed as the mechanism leading to graphite precipitation. Further complexity is suggested by carbonate reduction evidenced in the samples, which may have locally contributed to the formation of graphite on the precursor carbonate domains. The BMC rocks highlight the variability of carbon recycling processes, including carbon mobilization from multiple solid sources, and mixing of multiple carbon-bearing fluids in a dynamic fluid-rock system, with carbon mobilization and sink adapting to evolving redox conditions.