Abiotic synthesis of graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks

The search for the earliest evidence of life is complicated by the high-grade metamorphism of the oldest sedimentary rocks containing organic matter, which transforms original biomass into crystalline graphite and can distort or erase morphological biosignatures. Putative microfossils and biogenic graphite have been described from Eoarchean terranes such as the Nuvvuagittuq Supracrustal Belt and the Isua and Akilia associations, but these claims are debated, particularly for the Saglek-Hebron Gneiss Complex where 13C-depleted graphite was proposed as >3.9 Ga traces of life. Metamorphism can remove heteroatoms and alter isotopic compositions, complicating the interpretation of graphite origins. Therefore, distinguishing abiotic from biogenic processes in ancient graphitic carbon requires an integrated approach considering metamorphic effects and potential abiotic pathways. This study applies correlative petrographic, geochemical and spectroscopic analyses to Eoarchean metasedimentary rocks from the Saglek-Hebron Gneiss Complex to determine specific mineral associations, crystallinity variations, and isotopic characteristics of graphitic carbon, with the aim of constraining its origin and the role of metamorphism in transforming primitive organic matter.

Previous reports document 13C-depleted graphite and purported biosignatures in Eoarchean rocks from Isua (>3.7 Ga), Akilia (>3.83 Ga), and Saglek-Hebron (~3.9 Ga). However, high-grade metamorphism is known to graphitize organic matter, remove heteroatoms (H, N, O, S), and shift isotopic compositions, potentially producing isotopically heavy residues. Some studies questioned the antiquity and biogenicity of Saglek-Hebron graphite, suggesting hydrothermal/metasomatic precipitation during later metamorphic episodes. Abiotic mechanisms proposed for graphite formation in metamorphic rocks include decarbonation of carbonates, Fischer–Tropsch-type (FTT) synthesis catalyzed by Fe-oxides during serpentinization, and deposition from C–H–O fluids with variable CO2/CH4 ratios. Raman spectroscopy of organic matter provides a geothermometer to estimate crystallization temperatures and assess metamorphic conditions. Documented occurrences in Proterozoic and Archean BIF and other metamorphic settings show multiple graphite generations and crystallinities, complicating attribution solely to biology.

Samples: Three metasedimentary rocks from the Eoarchean Nulliak supracrustal assemblage of the Saglek-Hebron Gneiss Complex were analyzed: two banded iron formations (BIF; SG-274, SG-236) and a marble (SG-275). Petrography and mineralogy: Polished thin sections were examined using an Olympus BX-51 microscope (transmitted/reflected light), prepared to 0.25 µm alumina polish. Mineralogical modal/textural mapping used a TESCAN Integrated Mineral Analyzer (TIMA FEG3) in liberation analysis mode at 25 kV, 18.7 mA, working distance 15 mm, with BSE and EDS acquisition (point spacings 2.5 µm and 7.5 µm). Calibration used a Pt Faraday cup (BSE) and Mn standard (EDS). Raman spectroscopy: A WITec alpha 300 confocal Raman microscope (532 nm, 8 mW, 100× objective, NA 0.9, 50 µm fiber) collected spectra and hyperspectral maps below the thin section surface. Mineral peak mappings targeted quartz (~465 cm−1), carbonate (~1092 cm−1), apatite (~968 cm−1), magnetite (~673 cm−1), crystalline graphite (~1585 cm−1), poorly crystalline graphite (PCG; ~1355 cm−1), and CO2 (~1389 cm−1). Spectra were background-corrected and Lorentz-fit to extract peak positions, FWHM, and areas. Raman geothermometry estimated graphite crystallization temperatures using T(°C) = 455 × D1/(D1 + G + D2) + 641 (calibrated 330–641 °C, ±50 °C). Stable isotopes: Bulk-rock total graphite δ13C was measured by EA-IRMS (Thermo-Finnigan Flash 1112 EA + Thermo Delta V via Conflo IV) after carbonate removal with 6 N HCl; reproducibility better than 0.2‰, reported vs VPDB. Carbonate δ13C was measured on a Finnigan MAT 253 (CO2 extracted with H3PO4 at 75 °C), precision better than 0.15‰. Fluid inclusion gases (CH4, CO2) were extracted by vacuum heating quartz at 600 °C for 15 min, sequential cold traps (dry ice/ethanol, liquid N2), and oxidation of residual gases with CuO at 780 °C; δ13CCH4 and δ13CCO2 measured on MAT 253 with ±0.024‰ and ±0.037‰ precision, respectively. Electron microscopy: SEM-EDS with JEOL JSM-6480L at 15 kV, 1 nA, 10 mm working distance characterized morphology and composition. FIB-TEM: Site-specific sections were prepared with a Helios 5 CX FIB (30 kV Ga+; protective Pt; ~1 µm thick lamellae thinned to ~100 nm at 43 pA) and analyzed by JEOL 2100 TEM at 200 kV for lattice imaging and EDS. NanoSIMS: Elemental maps on 30-µm thick thin sections were acquired using a CAMECA NanoSIMS 50L (Cs+ primary beam, −1.6 pA; pre-sputter 1.5–3 nA, ion dose 5×10^16 ions/cm^2), collecting 12C−, 13C−, 12CH−, 13CN−, O−, P−, 32S− in multi-collection mode; 512×512 px images acquired in 24 min with electron flood for charge compensation.

1. Petrography and occurrences: Four graphite types were identified based on texture and associations: Gra1 (pure solid inclusions in coarse quartz), Gra2 (thin coatings on calcite grains), Gra3 (graphite associated with C–H–O fluid inclusions and PCG), and Gra4 (graphite intertwined with magnetite in marble). In BIF SG-274, 640 calcite grains were mapped in quartz-rich bands; 64 (10%) had calcite + graphite associations, lower than apatite + graphite proportions reported in Akilia and Nuvvuagittuq. Graphite sizes in Saglek-Hebron BIF are generally ~1–12 µm, mainly as coatings on microscopic calcite within millimeter quartz. In marble SG-275, graphite occurs as micrometer-scale opaque patches closely associated with magnetite within Fe-carbonate (dolomite/ankerite) grains and as filamentous textures (~24 µm). 2) Raman crystallinity: All graphite shows G bands at 1583–1589 cm−1 (FWHM 18–48 cm−1), D bands at 1352–1368 cm−1 (FWHM 21–80 cm−1), and 2D bands at 2683–2714 cm−1 (FWHM 45–86 cm−1). Gra3 commonly has stronger D and 2D intensities and lower-frequency 2D peaks, consistent with PCG. 2D/G intensity ratios: Gra1=0.22–0.67; Gra3=0.29–0.83; Gra2=0.44–0.97; Gra4=0.25–0.70. D/G ratios: Gra1=0.04–0.78; Gra3=0.05–0.80; Gra2=0.30–1.70; Gra4=0.09–1.09. HRTEM shows (002) spacings 3.23–3.84 Å, with expanded spacings indicating poorly crystalline graphite. 3) Raman geothermometry: Estimated graphite crystallization temperatures (±50 °C): Gra1 425–627 °C; Gra2 415–610 °C; Gra3 336–498 °C; Gra4 368–583 °C. Higher temperatures align with upper amphibolite-facies metamorphism of host rocks (>500 °C), while lower ranges indicate secondary deposition. 4) Isotopes (bulk): Graphite δ13Cgra ranges from −27.0‰ to −22.7‰ (average −25.5‰, n=8). Carbonate δ13Ccarb ranges from −11.2‰ to −1.4‰ (average −6.3‰, n=8). The large negative carbonate values indicate formation from 13C-enriched CO2 produced by oxidation of rock organic carbon. 5) Fluid inclusions: Quartz-hosted inclusions contain CO2 + CH4 ± PCG ± graphite. δ13CCO2 ≈ −21.8‰ and δ13CCH4 ≈ −30.3‰. Graphite δ13C values plot between these, consistent with deposition from mixed CO2–CH4 C–H–O fluids with variable ratios. 6) Elemental composition: NanoSIMS and TEM-EDS show graphite comprises C and H, lacking N, P, S; apatite + graphite associations common in biogenic contexts are scarce, supporting abiotic processes. 7) Genetic interpretations: In BIF, poorly crystalline graphite precipitated from retrograde C–H–O fluids generated by thermal decomposition and oxidation of syngenetic organic matter, while more crystalline graphite records prograde metamorphism of that biomass. In marble, graphite formed via decarbonation of Fe-carbonates, co-occurring with magnetite and calcite within Fe-dolomite grains; FTT reactions are unlikely given temperature constraints and lack of catalytic associations.

The integrated petrographic, spectroscopic, and isotopic evidence distinguishes abiotic and metamorphosed biogenic contributions to graphitic carbon in Eoarchean Saglek-Hebron metasediments. In BIF, the close spatial association of graphite with CO2–CH4-bearing fluid inclusions within quartz, the range of crystallinities (including PCG), and graphite δ13C values intermediate between CH4 and CO2 from inclusions indicate fluid deposition from C–H–O fluids. These fluids likely derived locally from syngenetic organic matter: thermogenic CH4 (δ13C ≈ −30.3‰) and CO2 from its oxidation (δ13C ≈ −21.8‰). Cooling during retrograde metamorphism reduced carbon solubility and promoted graphite precipitation (PCG), also enabling co-precipitation with calcite. Simultaneously, syngenetic organic matter underwent prograde graphitization to crystalline graphite, explaining coexisting graphite generations and crystallization temperatures. In marble, graphite’s strict association with magnetite and calcite within Fe-carbonate grains, filamentous textures, and absence of graphite tied to serpentine-vein magnetite support decarbonation of Fe-carbonate as the dominant mechanism, rather than FTT synthesis. Collectively, these relationships resolve the origin of variably crystalline graphite as predominately abiotic products from C–H–O fluids (BIF) and decarbonation (marble), with a preserved record of metamorphosed syngenetic biomass in the BIF. This framework addresses the central problem of distinguishing abiotic from biogenic signals in highly metamorphosed Eoarchean rocks and refines criteria for interpreting graphite biosignatures.

Graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks record multiple origins. In BIF, crystalline graphite reflects prograde metamorphism of syngenetic organic matter, whereas poorly crystalline graphite precipitated from retrograde C–H–O fluids produced by thermal cracking and oxidation of that biomass. In marble, graphite formed abiogenically via decarbonation of Fe-carbonate, coexisting with magnetite and calcite within Fe-bearing carbonate grains. These results provide robust evidence for abiotic synthesis of graphitic carbon in Eoarchean rocks and clarify how metamorphism transforms and overprints primary organic signals. The study establishes petrographic and spectroscopic criteria (mineral associations, inclusion chemistry, Raman metrics) for recognizing abiotic versus metamorphosed biogenic carbon, informing the search for early life on Earth and potentially other planetary bodies. Future work could include in situ micro-isotopic analyses of individual graphite generations, tighter geochronological constraints on metamorphic episodes, broader sampling across Eoarchean terranes, and experimental calibration of graphite precipitation from C–H–O fluids and decarbonation under relevant P–T–X conditions.

Interpretations rely on highly metamorphosed rocks where primary biosignatures are transformed or obliterated. Bulk δ13C measurements average mixed graphite generations and cannot resolve micro-scale isotopic heterogeneity. Temporal constraints on the timing of graphite formation relative to metamorphism are indirect. The dataset focuses on a limited number of samples and areas, and the scarcity of unequivocal biogenic mineral associations (e.g., apatite + graphite) restricts direct biogenic attribution. While alternative abiotic mechanisms (e.g., FTT) are argued against, fully excluding them in all micro-environments remains challenging without exhaustive catalyst-phase mapping and experimental analogs.