Towards a kerogen-to-graphite kinetic model by means of Raman spectroscopy
Andrea Schito, David Muirhead, John Parnell
Abstract
Raman spectroscopy on carbonaceous material has become the most used geothermometer in Earth science studies. However, its application in very different settings, associated with different heating rates, demands an updated review to understand the different paths of maturation that can occur in the geological record. A comparison between organic matter matured under slow (diagenesis and regional metamorphism) or fast (around shallow intrusion or under artificial pyrolysis) heating rates highlight a mismatch between Raman spectra at the same thermal maturity. Such differences are probably due to the interplay of kinetics and strain, and highlights that classical kinetics based on vitrinite reflectance is not appropriated when Raman parameters are used. In the controversial application to faults and shear zones a review of existing literature indicates that a distinction is needed between strain and frictional heating effects since they lead to different spectra evolution. The effect of strain enhances organic matter aromaticity, while data from frictional heating experiments show strong analogies with charcoal spectra even if the kinetics of the process still need to be understood. All this evidence emphasizes that Raman spectroscopy is a powerful tool to describe the aromatization experienced by carbon material in most of the natural conditions and is a good candidate for the development of a universal geothermometer based on a new kinetic model for Earth and planetary sciences.