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

The search for early life evidence is hampered by the highly metamorphosed nature of the oldest rocks containing organic matter. Metamorphism alters organic matter into crystalline graphite, obscuring original morphology and raising questions about the origin of purported microfossils in highly metamorphosed rocks. This necessitates a broader search for chemical and isotopic biosignatures, defined as potential life signatures, alongside morphological evidence. Previous studies reported biosignatures of graphitic carbons in >3.6 Ga rocks from locations like Greenland and Canada's Saglek-Hebron Gneiss Complex. High-grade metamorphism leads to biomass losing heteroatoms (H, N, O, S) and lighter isotopes (¹³C, ¹⁴N), leaving behind isotopically heavy residual graphitic carbons. Determining the precursor carbon requires understanding the influence of metamorphism and abiotic processes. The Saglek-Hebron Gneiss Complex, containing ~3.9 Ga TTG gneisses and Eoarchean to Paleoarchean supracrustal rocks, has yielded C-depleted graphite interpreted as ancient life, but this interpretation has been questioned, suggesting possible precipitation from hydrothermal fluids during later metamorphic episodes. This study aims to distinguish between abiotic and biological processes in the formation of these graphitic carbons.

Extensive research has focused on identifying biosignatures in ancient rocks, particularly graphite. Studies on the Isua Supracrustal Belt (>3.7 Ga), Akilia association (>3.83 Ga), and the Saglek-Hebron Gneiss Complex (~3.9 Ga) have reported graphitic carbons with isotopic compositions suggestive of biological origin. However, the impact of metamorphism on these signatures is debated. Some researchers argue that the isotopic signatures could be the result of abiotic processes, such as hydrothermal alteration, while others maintain a biological origin. This uncertainty highlights the need for detailed analysis of the mineral associations and petrographic context of the graphite to better constrain its origins. Previous work has shown that the degree of crystallinity in graphite can be related to the temperature of metamorphism and may help discriminate between biogenic and abiogenic origins.

This study investigated graphitic carbons in three samples from the Eoarchean Nulliak supracrustal assemblage of the Saglek-Hebron Gneiss Complex: two banded iron formations (BIFs; SG-274 and SG-236) and a marble (SG-275). Optical microscopy, TESCAN Integrated Mineral Analyzer (TIMA), micro-Raman spectroscopy, isotope ratio mass spectrometry (IRMS), scanning electron microscopy (SEM), focused ion beam milling-transmission electron microscopy (FIB-TEM), and nano-scale secondary ion mass spectrometry (NanoSIMS) were employed. TIMA provided mineralogical maps, while micro-Raman spectroscopy characterized the graphite's crystallinity using peak intensity ratios (D/G, 2D/G) and determined crystallization temperatures. IRMS analyzed carbon isotope compositions of both graphite and carbonate to assess potential biosignatures. SEM and FIB-TEM provided high-resolution images of graphite morphology and microstructure. NanoSIMS generated high-resolution elemental maps to detect the presence of other bioessential elements. A geothermometer based on Raman spectroscopy was used to estimate the crystallisation temperatures of the different types of graphite. The carbon isotopic compositions of total graphite were determined on bulk rock powders after acidification to remove carbonates. Carbon isotopic compositions of CH4 and CO2 in fluid inclusions were determined by heating the sample under vacuum and analyzing the released gases.

Graphite in the Saglek-Hebron metasedimentary rocks was classified into four petrographic types (Gra₁-Gra₄) based on texture, occurrence, and associated minerals. Gra₁ is pure graphite inclusions in quartz; Gra₂ is calcite + graphite associations; Gra₃ is graphite associated with C-H-O fluid inclusions; and Gra₄ is graphite associated with magnetite in marble. Micro-Raman spectroscopy revealed varied crystallinity, with Gra₃ showing poorer crystallinity than others. The D/G and 2D/G ratios varied significantly between the types. HRTEM confirmed the presence of poorly crystalline graphite (PCG). Carbon isotope values (δ¹³C_gra) ranged from -27.0‰ to -22.7‰ (average -25.5‰), while carbonate (δ¹³C_carb) values ranged from -11.2‰ to -1.4‰ (average -6.3‰). The negative δ¹³C_gra values are consistent with biological fractionation, but the lack of other bioessential elements (N, P, S) in graphite suggests abiotic processes. In the BIF, the association of graphite with C-H-O fluid inclusions and the δ¹³C values falling within the range of CO₂ and CH₄ in fluid inclusions suggest deposition from C-H-O fluids derived from the thermal decomposition of organic matter. In the marble, the close association of graphite with magnetite and carbonate is attributed to decarbonation reactions. Crystallisation temperatures estimated from Raman spectroscopy suggest that the high-temperature graphitic carbons originated from syngenetic organic matter, while the lower-temperature ones are attributed to secondary processes.

The findings demonstrate that the graphite in the Saglek-Hebron metasedimentary rocks has a dual origin. The crystalline graphite in the BIF likely represents the metamorphic alteration of syngenetic organic matter, while the poorly crystalline graphite in both the BIF and the marble likely formed from abiotic processes. The isotopic signatures previously interpreted as biosignatures could be a result of the combination of primary organic carbon and the secondary abiotic formation of graphite. This study challenges the interpretation of ¹³C-depleted graphite as unequivocal evidence of early life, highlighting the importance of considering abiotic processes, specifically the contribution of C-H-O fluids and decarbonation reactions, when interpreting the isotopic composition of graphite in ancient rocks. The study's implications extend to understanding geochemical cycling of carbon on early Earth and the preservation of prebiotic organic matter through metamorphism.

This research demonstrates the abiotic synthesis of graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks through two primary mechanisms: fluid deposition and decarbonation. The graphite's varied crystallinity highlights the complex interplay between primary organic matter and metamorphic processes. This work underscores the necessity for a comprehensive approach, considering both isotopic and petrographic analyses, when assessing potential biosignatures in ancient rocks. Future research could focus on expanding this analysis to other ancient rock formations and exploring the potential for abiotic carbon synthesis pathways under varying conditions to refine the understanding of early Earth geochemistry and the search for early life.

The study focused on a limited number of samples from the Saglek-Hebron Gneiss Complex. While the methods employed provided high-resolution data, the interpretation relies on the accuracy of the geothermometer used to estimate crystallisation temperatures and the assumption of equilibrium conditions during the isotopic fractionation. Further research with a larger sample size and incorporating other analytical techniques might refine the understanding of the precise mechanisms involved.