Incorporating RSCM temperature data into inverse thermal models of low-temperature thermochronometric data

Abstract

Inverse thermal modeling is an established method to resolve time-temperature paths from low-temperature thermochronometric data (U-Th/He, fission-track, and Ar/Ar methods, particularly). These models use the temperature and time dependent characteristics of diffusion (U-Th/He and Ar/Ar) or annealing (fission-track) to resolve thermal histories consistent with the observed radiometric age and fission-track length characteristics of analyzed samples. A limitation of inverse thermal models is that they cannot resolve details of thermal histories, from thermochronometric data alone, at temperatures above the closure temperature(s) of the thermochronometric methods of interest, where diffusion or annealing is geologically instantaneous. In cases where peak temperatures substantially exceed the closure temperature of analyzed thermochronometric systems, incorporation of peak temperature information into inverse thermal models can narrow the range of time-temperature histories consistent with observered thermochronometric data.Low temperature geothermometers (e.g. irreversible reactions sensitive to temperature such as  illite crystallinity, illite/smectite ratio, vitrinite reflectance, Tmax, and Raman Spectroscopy of Carbonaceous Material (RSCM)) are, therefore, potentially highly complementary to the inverse thermal modeling of thermochrometric data. Of these methods, vitrinite reflectance (%Ro) has been most widely incorporated into inverse thermal models (e.g. QTQt), because such data is copious in the petroleum industry, and because kinetic models of vitrinite maturation are well-correlated with temperature from ~0 – 200°C, a range of temperature applicable to low-temperature thermochronometry; however, vitrinite reflectance has seen lessened use due to concerns about reproducibility and cost of analyses. Raman Spectroscopy of Carbonaceous Materials (RSCM) has become a far more prevalently used geothermometer in the past decade, with recent studies calibrating the RSCM thermometer to temperatures as low as ~150°C. Kinetic models to describe the evolution of Raman spectra are being explored, but none have yet been proposed. However, multiple studies have reported a high degree of correlation between the Raman parameters used for geothermometry and vitrinite reflectance values.We compiled >200 samples on which both RSCM temperatures and vitrinite reflectance measurements were made, a number of which were collected in concert with low-temperature thermochronometric data. We perform inverse thermal models of the low-temperature thermochronometric data (1) without including any geothermometry data; (2) including the vitrinite reflectance measurements as a data input; (3) including an “equivalent” vitrinite reflectance measurement derived from the RSCM temperature as a data input; and (4) including the RSCM temperature as a constraint within the inverse model, as opposed to a data input. We explore the differences in thermal histories predicted by each of these approaches and suggest best practices for including RSCM geothermometry data in inverse thermal models.