CALCULATED PHASE EQUILIBRIA FOR GRAPHITIC METAPELITE: A THEORETICAL APPRAISAL OF INTERNAL BUFFERING AND GRAPHITE PRECIPITATION

The phase relations among C-O-H fluid, graphite, and silicate mineral assemblages have been intensively discussed in the petrologic literature. When graphite is present, carbon-bearing species dissolve in the C-O-H fluid and lower the activity of water (a (H₂O)). Accordingly, metamorphic reactions that involve water, namely dehydration and partial melting, adjust their pressure-temperature (P–T) positions to accommodate the change in a (H₂O). In addition to the merits of petrogenetic grids, the pseudosections presented in this study enable researchers to quantitatively investigate the evolution of phase modes, including graphite, along specific metamorphic P–T paths in carbon-bearing systems. The P –T pseudosections are calculated in closed systems with fixed bulk composition, so the oxygen fugacity (f (O₂)) is internally buffered. As the result of the lowered a (H₂O) in carbon-bearing systems, the temperature displacements of the solidus can be as large as 50 °C at low pressures (< 4 kbar), but are less than 20 °C at higher pressures (4 to 8 kbar). In the suprasolidus region, the phase relations among silicate minerals + melt are very similar to those in carbon-free systems. f (O₂) – T pseudosections illustrate the intrinsic characteristics of internally-buffered metamorphic reaction paths. Dehydration and partial melting reactions are parabola shaped in f (O₂) – T space, centering on the axial trace [H₂O]_max (the f (O₂) – T series of x (H₂O) maxima). Dehydration reactions drive fluid compositions toward [H₂O]_max, while at higher temperatures the coexistence of the C-O-H fluid, graphite, and silicate melt drives fluid compositions away from [H₂O]_max due to increases in the carbon dioxide mole fraction. The mode of graphite is subject to thermodynamic and bulk compositional constraints. At high temperatures in the suprasolidus region, graphite is predicted to precipitate as the temperature decreases or pressure increases. Open-system mixing between CO₂-dominant and CH₄-dominant fluids can also precipitate graphite. Natural graphite may be the product of a combination of the above three mechanisms; additional petrologic information is needed to distinguish the dominant process.