Reassessing early traces of life from the Eoarchean metasedimentary rocks in Labrador, Canada: carbon isotope compositions for individual graphitic grains

Abstract

Graphite with low carbon isotope ratios (δ¹³C_gr) in the Eoarchean metasedimentary rocks from the Labrador region of northern Canada has attracted attention as possible evidence for the oldest traces of life on the Earth. However, δ¹³C_gr values were primarily determined through whole-rock analysis, and maturation degree was assessed in a limited number of samples. Whether all ¹³C-depleted carbons existed before the peak metamorphism remains unclear. Here, we performed in situ micro-Raman spectroscopy and nanoscale secondary ion mass spectrometry on graphitic grains to determine their crystallinity and carbon isotopic compositions. Based on the occurrence of graphitic grains in thin sections and Raman spectra, we classified the grains into four types: (1) inclusion graphite within matrix minerals, (2) inclusion poorly crystallized graphite (PCG), (3) boundary graphite among mineral grains, and (4) boundary PCG. Inclusion graphite is considered the most primary. The δ¹³C_gr values ranged from −35‰ to 7‰ and were heterogeneous within single hand specimens of pelitic rocks, conglomerates, and chert nodules, regardless of whether grains were inclusions or boundary types, or whether they were graphite or PCG. Carbonate rocks contain grains with relatively homogeneous δ¹³C_gr values. Overall, the δ¹³C_gr values correlate with lithology, increasing from pelitic rocks to conglomerates and carbonate rocks with chert nodules. These data suggest that determining the origin of all graphitic grains from a single source is difficult. Graphitic grains with positive δ¹³C_gr values may have an abiological origin; however, the wide range of δ¹³C_gr values, together with their geological occurrence, does not allow a unique interpretation, but is consistent with multiple origins, including both abiotic and biotic organic carbon.