Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis

High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS ¹³C/¹²C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp δ¹³C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between ¹²C and ¹³C in the growth medium to the interface of graphite precipitation in different growth sectors.