SIMS carbon isotope and Raman data from Isua metasediments

Graphite found along contiguous horizons in the metamorphosed detrital sediments from the ≥ 3.7 Ga Isua Supracrustal Belt in Southwest Greenland may represent the oldest re-mains of life on Earth. Here, we examine the carbon isotope compositions of graphite occur-ring in situ as inclusions in minerals in both metasediment and secondary vein lithologies by secondary ion mass spectrometry. The carbon isotope composition of the metasediment-associated graphite displays a significantly narrower spread in d13C values (average –20.6 ± 1.8 ‰, 1 standard deviation) than vein-associated graphite does (–20.4 ± 3.8 ‰), which can be explained by different origins, respectively interpreted as detrital biogenic matter and abiotic fluid-precipitated graphite. Additionally, graphite within metasedimentary porphyroblasts dis-plays lower d13C values (–20.9 ± 1.6 ‰, down to –26.4 ± 0.5 ‰) than graphite in the surrounding matrix (–18.1 ± 1.7 ‰), indicating the former to be less altered than the latter.