Geothermometry by Raman spectroscopy of dispersed organic matter

Abstract

Raman spectroscopy of carbonaceous material (RSCM) is frequently used to determine peak metamorphic temperature or to infer the coal rank as well as the degree of organic maturation. Several temperature calibrations exist, but methodical aspects limit the portability of these calibrations among laboratories and reduce overall comparability of the method. By identifying the subjectivity of spectral evaluation, experimental setup and sample heterogeneity as major sources of bias in the method, an outline to increase comparability could be established. To reduce the subjectivity during spectral evaluation the automated, user-input independent curve-fitting software ’IFORS’ (Iterative Fitting Of Raman Spectra) has been written. To reduce the bias due to the experimental setup, a reference sample series has been collected that covers a temperature range of 160 °C to 600 °C. Multi-wavelength resonance Raman spectroscopy was performed during sample preparation on dispersed vitrinites that experienced diagenetic to epizonal pressure and temperature conditions to infer the analogy between the RSCM-method and reflectance of dispersed organic matter. The IFORS software allowed to derive the scaled total area (STA) Raman parameter which accurately characterizes Raman spectra of carbonaceous matter. Based on the resonance Raman data it could be shown that STA-RSCM method can be used in analogue to vitrinite reflectance, that this method is robust to sample preparation, especially polishing, and that the resonance Raman spectra of vitrinite reflect a two-stage molecular evolution during coalification and graphitization. During the first stage, which ends approximately after the CM passed through the gas-window, linear polycyclic aromatic structure grow, while the second stage indicates growth of condensed polycyclic aromatic structures. The STA-RSCM method has been extended to describe the Raman spectra of metamorphic CM and was successfully calibrated to the reference sample series. Thus, a revised RSCMgeothermometer valid from 160 °C to 600 °C is proposed. The sample series is available to the public and is supposed to be extended by the scientific community to further increase the quality of the reference series. When used in combination, the STA-RSCM method and the reference sample series will improve the overall comparability among laboratories and will advance the general applicability of this geothermometric method.