Tracing life in the early Archean: The 3800 Ma old Isua Supracrustal Belt, southern West Greenland

Researchers have long alleged that graphite contained in amphibolite grade rocks from the 3.8 Ga Isua Supracrustal Belt (ISB) in southern West Greenland could be biogenic in origin. This conclusion was based on some low carbon isotope ratios of reduced carbon found in Isua rocks. In addition, carbonaceous matter was found to contain traces of hydrogen and nitrogen, elements that supported the biogenic interpretation. Some of these rocks were previously interpreted as shallow marine carbonate deposits, which suggested a suitable setting for preservation of remnants of possible ancient life.; This dissertation presents a systematic study of ancient traces of life in the ISB. Each chapter focuses on a specific characteristic of graphite; thermodynamic constraints of formation, petrography and geological setting, degree of crystallinity, carbon isotope composition, and isotope composition of trapped gases (nitrogen and argon).; Field evidence, micropetrographic analysis and thermodynamic constraints indicate that graphite formed epigenetically either by disproportionation of Fe-bearing carbonates in secondary metacarbonate veins or by serpentinization of ultramafic rocks. Both processes are associated with metasomatic activity in the ISB under amphibolite-facies conditions. The observed carbon isotope composition of graphite in these rocks is in accord with isotope fractionation models for these metasomatic processes and CO₂ in the mantle was a likely ultimate carbon source. Stepped-heating/combustion-mass spectrometry of isolated graphite samples revealed three different reservoirs of trapped nitrogen; (1) organic contamination and nitrogen adsorbed to the graphite surface, (2) intercalated nitrogen, (3) nitrogen chemically bound at intrinsic defects within the graphite structure. The isotope composition of this bound nitrogen component in carbonate-derived graphite samples, suggests that the associated metamorphic fluids leached basaltic rocks in Isua. Graphite could not be found in chemical sediments, like banded iron formations (BIFs) and metacherts. Carbon isotope analysis of these metasediments by stepped-heating/combustion-MS indicates that the tiny amounts of reduced carbon are derived from recent organic contamination. The conclusions of this study question some of the earlier inferences about traces of life in the ancient Isua rocks.; Graphite in slates and graded beds (interpreted as a turbidite) appears not to be related to either Fe-carbonate disproportionation or serpentinization reactions. A biogenic origin of this graphite can therefore not be excluded.