Organic geochemistry of artificially matured conodonts

Abstract

Organic geochemical methods for conodont analysis have been developed. Coniform elements of the conodont genus Drepanodus from the Ordovician Emanuel Formation and pectiniform elements of the conodont genus Polygnathus from the Devonian Napier Formation both from the Canning Basin, Australia have been shown to contain different organic matter. Both conodont samples have been artificially matured and the structure of organic matter present related to the conodont alteration index (CAI) and maturation temperature (TM). The carbon 1s X-ray photoelectron spectra (XPS) acquired from both sample sets contain six peaks. These peaks are assigned to highly ordered graphitic like carbon (282.9 eV), sp 3 hybridised carbon bonded to only carbon or hydrogen (284.7 eV), ether (286.1 eV), alcohol (287.0 eV), carbonyl (287.6 eV) and carboxyl (289.0 eV) functional groups. Alkene and aromatic carbon is also probably present but is difficult to delineate. During artificial maturation, XPS analysis monitors the main chemical modifications expressed by the carbon–oxygen functional groups. The first stage of maturation begins with the removal of carboxyl and alcohol groups before a CAI of 2 (TM 450 °C). Between a CAI of 3–6 (TM 450–800 °C) an increase of sp 3 hybridised carbon bonded to only carbon or hydrogen is found, which is accompanied by a decrease in ether and carbonyl functional groups. From a CAI of 6–7 (TM 800–950 °C), sp 3 hybridised carbon bonded to hydrogen or carbon and carbonyl decreases while the development of highly ordered graphitic like carbon is observed. The first order laser Raman spectra recorded for both sample sets show a low degree of structural order from a CAI of 2–3, and from CAI 4–6 show a progressive increase in structural disorder of conodont organic matter. The D/G band (band at approximately 1345–1365/band at 1610 cm −1) line width ratio correlates with CAI. Conodont colour during artificial maturation occurs by migration of compounds to the surface. As the nitrogen compounds are volatilised from the surface, colour disappears.