Carbon isotopic variations in fluid-deposited graphite: evidence for multicomponent Rayleigh isotopic fractionation

Carbon isotopic composition of fluid-deposited graphite in a metamorphic terrain can be used as a tracer for sources of fluids, their compositions and temperatures of graphitization. However, 13C of graphite almost always shows a large spread, mostly because of isotope fractionation during precipitation, making it necessary to understand these processes. In this work, a novel quantitative approach, involving Rayleigh isotope fractionation during formation of graphite from a realistic multicomponent metamorphic fluid, is proposed to explain the observed 13C variations in fluid-deposited graphite. Results of this study reveal that most graphite crystallizes from a mixed CO2-CH4 fluid that can acquire its isotopic composition from a variety of sources like organic matter, carbonate, mantle degassing or variable mixtures of these.