Chemical degradation of thermally altered silicified organic matter during acid maceration: A case study from the Lower Devonian Rhynie chert. (June 2021)

• Record Type: Journal Article
• Authors: Delarue, Frédéric; Tu, Thanh Thuy Nguyen; Duhamel, Rémi; Paris, Céline; Baudin, François
• Is Part Of: Organic geochemistry. Volume 156(2021)
• Publication Date: 2021-06
• Subjects: Acid maceration -- Hydrocarbons -- Raman spectroscopy -- Rock-Eval Pyrolysis; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2021.104246
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: Organic matter from the Devonian Rhynie chert was chemically isolated. Standard acid maceration resulted in a rise in the carbon structural order. Hydrocarbons were lost during acid maceration in significant amounts. The soluble/insoluble definition may limit the study of silicified organic matter. In situ molecular analyses can overcome issues related to chemical degradation. Abstract: The effect of standard acid maceration on organic matter (OM) from ancient silicified sediments remains undocumented. Early silicification favours preservation of organic moieties against thermal alteration over time. In this study, we investigated the effects of acid maceration on the structure of OM isolated from the Lower Devonian Rhynie chert. The structure of OM was investigated by combining Rock-Eval pyrolysis and Raman spectroscopy. Besides a loss of thermolabile organic matter owing to solvent extraction, Rock-Eval pyrolysis showed that standard acid maceration also causes a loss of CH emissions at high pyrolysis temperature (>500 °C). The standard acid maceration procedure was also associated with the disappearance of the D4 and D5 Raman spectrum shoulders assigned to CH bonds in aliphatics and bitumens, respectively, entrapped in the macromolecular network. Taken together, Rock-Eval pyrolysis and Raman spectroscopy indicate that standard acid maceration can lead to the chemical degradation of syngenetic hydrocarbonaceous moieties of OM isolated from ancient silicified and thermally altered sediments. In sediments having experienced early silicification, which hampers bitumen migration and favours pyrobitumen formation, we suggest that novel in situ molecular analytical techniques are required to provide a thorough examination of the syngenetic molecular content independent of the soluble/insoluble operational definition.