Graphite precipitated from hydrothermal fluids pervades the sillimanite-grade metasedimentary and plutonic rocks of New Hampshire. Hydrothermal graphite occurs as microscopic veinlets, halos of spherulites around shear zones, and metre-thick veins. Carbon isotope analyses of the graphite range from −25‰ to −9‰ δ13C (PDB), intermediate between the two biogenic crustal reservoirs of reduced organic matter and carbonate. It is proposed that carbon was mobilized from sediments as CO2 and CH4 during metamorphic devolatilization reactions. The carbon-bearing species were transported in aqueous fluids through hydraulic fractures. Graphite precipitated when aqueous fluids with different CO2/CH4 ratios were mixed in fractures.

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