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.
KeywordsGraphite Carbon isotopes Migmatite SIMS Sector isotope zoning Kerala khondalite belt
MS-K is grateful to H. Wada for constant support and encouragements. R. Rambeloson is thanked for his kind help in collecting graphite samples from Madagascar. We thank John Jaszczak, Ernst Zinner and an anonymous reviewer for their comments on an earlier version of the manuscript. MS-K was supported by the grant-in-aid for encouragement of young scientists (Nos.18740319 and 20684022) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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