180 / 2025-04-14 09:56:46

Microscopic evidence for abiotic graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks

Saglek-Hebron, graphite, metamorphism, banded iron formation, early life

Theme 1: Novel Tools and Techniques in Geobiology > Session 1b: Advances in Geobiology Catalysed by Correlated Microscopy at the Nanoscale

Graphite in metasedimentary rocks of the Eoarchean Saglek-Hebron Gneiss Complex (Canada) is depleted in ¹³C and has been interpreted as one of the oldest traces of life on Earth. The variation in crystallinity of this oldest graphitic carbon could possibly confirm the effect of metamorphism on original biomass, but this is still unexplored. Here, we report specific mineral associations with graphitic carbons that also have a range of crystallinity in the Eoarchean Saglek-Hebron metasedimentary rocks. Microscopic observations show that the granular quartz most away from iron oxides in banded iron formation (BIF) contains pure graphite particles in quartz , graphite + calcite, and graphite + fluid inclusions. In comparison, in a granulite marble, disseminations of graphite co-occur with magnetite disseminations, which is more variably crystalline and morphologically shaped, including filaments and arborescent structures. Nanoscale petrographic observations and high-resolution elemental compositions acquired by NanoSIMS and TEM-EDS suggest abiotic synthesis for these graphitic carbons. The close association of graphite with fluid inclusions and similar δ¹³C values between them reveal that graphite in the Saglek-Hebron BIF is likely deposited from C-H-O fluids, while graphite associated with magnetite and carbonate in the Saglek-Hebron marble arises from decarbonation. The crystallization temperatures calculated using Raman peaks range from 336°C to 627°C, which suggests that crystalline graphite may represent syngenetic organic matter during prograde metamorphism, and function as a precursor and substrate for the formation of poorly crystalline graphite during retrograde metamorphism. Our results thus highlight that variably crystalline graphitic carbons in the Saglek-Hebron metasedimentary rocks are potential abiotic products from primitive sedimentary organic matter on early Earth, which lay the groundwork for identifying the preservation of prebiotic organic matter through metamorphism on Earth and beyond.