JCJC - Jeunes chercheuses & jeunes chercheurs

Fossilization and geothermometry of organic matter from advanced diagenesis to metamorphism: From Nature to experiments – Geocarbons

Submission summary

Scientific background – Solid organic matter (OM) are commonly observed in rocks, in general with a sedimentary origin. Geological OM have been the subject of extensive studies by geoscientists as well as organic (geo)chemists. Solid OM are now recognised as important actors and/or tracers in several first-order geological processes: some with economical (hydrocarbons genesis) or general interest (carbon biogeochemical cycle, origin of life), others more specific (diagenesis, metamorphism). The first originality of solid OM is their incredible, endless, variety of chemistry and structure, from amorphous-like materials up to crystalline graphite and ultimately diamond. Chemistry and structure may be a fingerprint not only of their origin, either biogenic or abiotic, but also of the thermodynamic conditions (pressure P, temperature T, time t) under which these OM have evolved. Based on structural and chemical characterization, we propose to investigate the fossilization and eventual preservation of OM during advanced diagenesis and metamorphism. We will perform a multiscale characterization of undoubtedly biogenic fossils which were submitted to various but well-constrained diagenetic and metamorphic conditions. Not only the organic phases will be characterized, but also we will track possible interactions with minerals by using in situ techniques allowing retention of textural information. In the meantime, we will carry out an experimental study of the processes observed in natural samples in order to decipher the respective roles of P, T, t and chemistry (precursor, fluids). The objective is to identify some chemical (molecular, functional groups…) or structural (biomineralization…) criteria of biogenicity and the conditions under which they might be preserved. In parallel, we would like to revisit the potentiality of OM transformations (carbonification) to be used as quantitative thermometers of advanced diagenesis and low-T metamorphism (T<300°C). Under medium to high-grade metamorphism (~300-700°C), the degree of graphitization estimated from Raman spectroscopy of carbonaceous material (RSCM) is a sensitive geothermometer of peak metamorphic conditions, and our preliminary data suggest that RSCM thermometry may hold at lower temperatures. An advanced experimental study of kinetic aspects under P and T is also scheduled in the frame of this proposal for a better understanding of these transformations at low-temperatures. Natural targets – For the fossilization aspects, we have identified several fossil targets with a well-known geological history (French and Austrian Alps, Corsica and Alsace in France), from low- to high-grade metamorphism. We also plan to apply our approach to some fossil samples from the emblematic Burgess shales (Canada) which are now recognised as relatively metamorphic (greenschist facies). Regarding the RSCM calibration, we will complete our Alpine sampling, mostly in Switzerland, but also study new samples from the Franciscan Complex (California, USA) and deep drilling boreholes from the Gulf Coast (Texas, USA). Methodology – A systematic characterization at a multitude of length scales of the chemistry and the structure of OM and minerals will be performed in situ for the fossils samples and experimental analogues. We will combine observations at the nm scale (Transmission electron microscopy, synchrotron-based Scanning Transmission X-ray Microspectroscopy) with µm scale (Raman spectroscopy, Scanning Electron microscopy) and bulk techniques (Fourier Transform Infrared Spectroscopy, X-ray diffraction). When necessary, we will solicit access and expertise from the National INSU facilities for isotopic geochemistry. Regarding RSCM thermometry at low-T, we will explore various fitting procedures of the Raman spectra and estimate associated uncertainties to extract the most reliable quantitative information. We will perform high-T pyrolysis at ambient P under relevant and controlled oxygen fugacity as well as high-P, high-T experiments (up to 40 kbar, 1500°C) on various organic and organic/minerals precursors depending on the question addressed. Expected results – This study will provide a better understanding of the fossilization processes of OM from advanced diagenesis to high-grade metamorphism. Multiscale description of chemical and structural evolution of OM during metamorphism, including thermodynamic and kinetic constraints, will open new avenues for the search of life in the Geological record. The proposed work will have direct implications for the search of Early Life as the rocks from this period have all been metamorphosed. The new calibration of the RSCM geothermometer will have direct applications to decipher the thermal history of fold-and-thrust belts and deep sedimentary basins toward which the petroleum industry is now looking for new hydrocarbons reservoirs.

Project coordination

Olivier BEYSSAC (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 150,000 euros
Beginning and duration of the scientific project: - 36 Months

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