BELVIDERE VERMONT REVISITED: ANOMALOUS GRAPHITE AND ABIOTIC METHANE IN A SERPENTINITE

The Belvidere Mt. serpentinite in Vermont is part of a chain of Taconic-age ophiolite slices that stretch from the southern Appalachians to Newfoundland. Anomalous graphite produced within the serpentinite is more widespread than previously thought. A mechanism for graphite precipitation involves production of abiotic methane by serpentinization of relict olivine and pyroxene.

Kerper et al. (2010) reported graphite in the quarries at Belvidere Mt. associated with a high-angle fault. That study also presented limited isotopic data supporting the proposal of graphite precipitation from fluid mixing between fluids originating from the adjacent carbonaceous Ottauquechee Formation and methane-rich fluids produced within the serpentinite. Here we present, in addition to fieldwork results, new fluid inclusion and isotopic data to support a mechanism for production of methane-rich fluids by serpentinization of relict olivine and pyroxene at very low fO₂, resulting in production of hydrogen. The essence of the proposed mechanism is contained in the following model reactions:

6(Mg_1.5Fe_0.5)SiO₄ + 7H₂O = 3Mg₃Si₂O₅(OH)₄ + Fe₃O₄ + H₂

CO₂ + 3H₂ = CH₄ + H₂O

CO₂ + CH₄ = 2C + 2H₂O

Fieldwork has shown that graphite precipitation at Belvidere Mt. is more widespread that previously thought, and is not limited to the high-angle fault mentioned previously. Analogous graphite in similar metamorphic settings has been previously reported at Balangero, Italy by Vitale Brovarone et al. (2017) and in Alpine Corsica by Malvoisin et al. (2012) and Galvez et al., (2013).