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

Abstract

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 tempertaures 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 the four most common Raman spectral parameters and ratios 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 I[d]/I[g] and 0.11 for R2, while 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 apply three equations used to derive the peak temperatures from Raman spectra to our data to investigate the implications on predicted temperatures between strained and unstrained samples. All three equations produce different temperature gradients with depth in unstrained samples. We observe that individual equations exhibit apparently varying sensitivities to strain, but calculated temperatures can be up to 140 °C different for adjacent strained and unstrained samples using the same temperature equation. These results have implications for how temperatures are determined in strained rock samples from Raman spectra.

Towards a kerogen-to-graphite kinetic model by means of Raman spectroscopy

“…5 a) is also observed on graphitic carbon (i.e., carbon that suffered at least greenschist metamorphism) from Monti Romani (Italy) and Internal Rif (Morocco) by Muirhead et al (2021) and by Ito et al (2017). Working on zones of concentrated aseismic strain, other authors (Kuo et al, 2017 in low slip rate experiments; Kuo et al, 2018aKuo et al, , 2018bKedar et al, 2020Kedar et al, , 2021Muirhead et al, 2021) find a decrease of the R1 values (Fig. 5 a).…”

“…This is the case of the Raman shift of amorphous carbon (Fig. 5), ascribed to aseismic shear strain by Kedar et al (2020Kedar et al ( , 2021 and Muirhead et al (2021) and possibly in the damage, breccia and gouge zones of Hirono et al (2015). From Fig.…”

“…Many authors have tried to quantify the frictional heating on a fault plane (Hirono et al, 2015;Ito et al, 2017;Kouketsu et al, 2017;Kuo et al, 2018aKuo et al, , 2018bNakamura et al, 2015) or the effect of the additional strain in aseismic shear zones on CM ordering (Kedar et al, 2020(Kedar et al, , 2021Muirhead et al, 2021). In both cases, similar approaches were utilized, analysing samples in the less deformed host rocks, moving progressively toward subzones of increasing deformation (e.g., damage zone, breccia zone and gouge zone).…”

The impact of melt versus mechanical wear on the formation of pseudotachylyte veins in accretionary complexes

“…Alternately to temperature, strain might be the factor controlling RSCM signal variations. The RSCM intensity ratio used in this work has been shown in natural and experimental examples to be sensitive to deformation 30,32,52,53,35 .…”