Complexity of graphite formation in response to metamorphic methane generation and transformation in an orogenic ultramafic body

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 degrees 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 delta B-11, Sr-87/Sr-86 and delta C-13, data, in-situ microRaman spectroscopy and bulk delta C-13(CH4) and delta H-2(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 Sr-87/Sr-86 ratios and delta B-11 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 delta C-13 values ranging from -7.26 to -1.27 parts per thousand. 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 N-2, NH3, 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 delta C-13 and delta H-2 of methane in fluid inclusions show a wide range of values in ultramafic rocks from -45.2 parts per thousand to -12.6 parts per thousand for carbon and -226 parts per thousand to -140 parts per thousand 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 H-2-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 similar to 6 m-thick zone within the serpentinized ultramafic body and along lithologic contacts. The delta C-13 composition of the graphite clusters at -15 parts per thousand (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.