RAMAN SPECTROSCOPY IN GEOBIOLOGICAL STUDIES: MERGING MOLECULAR AND MINERALOGICAL INFORMATION WITH MORPHOLOGICAL OBSERVATIONS

Raman spectroscopy enables instantaneous and non-destructive chemical and mineralogical identification of a large variety of geobiological sample types. Fast high-resolution Raman mapping is an ideal initial technique for characterization of small-scale features before other, more cumbersome, chemical and isotopic techniques are applied (such as TEM, SIMS, synchrotron-based microanalysis). Specific types of applications of Raman spectroscopy include mapping of microfossils, the study of crystal growth in biomineralization processes, identification of mineral assemblages in stromatolites and microbial mats, and the detailed characterization of fluid inclusions. Some examples will be discussed here of the characterization of carbonaceous structures and surrounding mineral assemblages in Archean chert and carbonate deposits. Case studies of these intriguing but often controversial remnant of early life include: 1) The recognition of two types of kerogen in the 3.5 Ga Dresser Formation, Pilbara, Western Australia; one that is related to deeply rooted chert feeder veins in a seafloor hydrothermal environment, and one that is specifically found in ankerite crystals in seafloor bedded-carbonate deposits. Raman spectroscopy was also used to identify primary Fe-rich ankerite and siderite cores associated with these kerogenous phases, and Mg-rich ankerite-rims that clearly were formed during later hydrothermal and metamorphic reworking. This integrated study on kerogen and associated carbonates shows that shallow-marine biologic remains can be distinguished from deep-seated hydrothermal carbon. 2) The controversy regarding the oldest traces of life on Earth – graphite inclusions in apatite crystals in a 3.8 Ga old quartz-pyroxene rock on Akilia Island. Raman identification of graphite in CO₂-CH₄ fluid inclusions enabled the recognition of abiologic metamorphism-related processes. Thermodynamic constraints in the COH-system indicate that the graphite formed by dehydration reactions, and temperature decrease during retrograde metamorphism, and does not represent altered organics. This observation greatly complicates any claims of early traces of life in these rocks.