ALBERT

Notes: Abstract Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of δ 13C values determined for flake graphite in the igneous rocks (−26.5 to −13.8‰) overlaps the range for flake graphite in the wall rocks (−26.0 to −16.7‰), and spatial correlation of some δ 13C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9‰ higher to 1.4‰ lower in both igneous and metasedimentary samples. The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO2/CH4 depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of 13C between secondary graphite and the evolving fluid because 13C is preferentially concentrated into CO2 relative to CH4. Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in 13C (−15.7 to −11.6‰) relative to both the igneous and wall-rock δ 13C values. Values of δ 13C vary by up to 3.4‰ within veins, with samples taken only 3 cm apart differing by 2.0‰ These variations in δ 13C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).