Preprints
https://doi.org/10.5194/se-2021-70
https://doi.org/10.5194/se-2021-70

  14 Jun 2021

14 Jun 2021

Review status: this preprint is currently under review for the journal SE.

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

Lauren Kedar, Clare E. Bond, and David Muirhead Lauren Kedar et al.
  • University of Aberdeen, School of Geoscience, Aberdeen, AB24 3UE, UK

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.

Lauren Kedar et al.

Status: open (until 01 Sep 2021)

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Lauren Kedar et al.

Data sets

Raman spectral data from carbonates in the Haut Giffre, French Alps Kedar, L., Bond, C. E. and Muirhead, D. https://doi.org/10.5281/zenodo.4771951

Lauren Kedar et al.

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Short summary
Raman spectroscopy of carbon-bearing rocks is often used to calculate peak temperatures and therefore burial history. However, strain is known to affect Raman spectral parameters. We investigate a series of deformed rocks that have been subjected to varying degrees of strain and find that there is a consistent change in some parameters in the most strained rocks, while other parameters are not affected by strain. We apply temperature calculations and find that strain affects them differently.