Preserved carbon isotope compositions in 3.7 billion year old detrital organic matter from the Isua Supracrustal Belt

Abstract 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 remains of life on Earth. Here, we examine the carbon isotope compositions of graphite occurring 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 δ 13 C 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 displays lower δ 13 C 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.