Raman spectroscopy in thrust-stacked carbonates: an investigation of spectral parameters with implications for temperature calculations in strained samples

Abstract

Raman spectroscopy is commonly used to estimate peak temperatures in rocks containing organic carbon. In geological settings such as fold–thrust belts, temperature constraints are particularly important as complex burial and exhumation histories cannot easily be modelled. Many authors have developed equations to determine peak temperatures from Raman spectral parameters, most recently to temperatures as low as 75 ∘C. However, recent work has shown that Raman spectra can be affected by strain as well as temperature. Fold–thrust systems are often highly deformed on multiple scales, with deformation characterised by faults and shear zones, and therefore temperatures derived from Raman spectra in these settings may be erroneous. In this study, we investigate how some of the most common Raman spectral parameters (peak width, Raman band separation) and ratios (intensity and area) change through a thrust-stacked carbonate sequence. By comparing samples from relatively low-strain localities to those on thrust planes and in shear zones, we show maximum differences of 0.16 for ID / IG and 0.11 for R2, while full width at half-maximum (FWHM[d]) and Raman band separation show no significant change between low- and high-strained samples. Plausible frictional heating temperatures of faulted samples suggest that the observed changes in Raman spectra are not the result of frictional heating. We also consider the implications of these results for how temperatures are determined using Raman spectra in strained and unstrained rock samples.