Deep-UV micro-Raman spectroscopy estimates of peak metamorphic temperatures using rock chips bearing carbonaceous material

A novel analytical method has been developed for non-destructive peak metamorphic temperature (PMT) estimates of carbonaceous material (CM)-bearing rock chips using deep-UV micro-Raman spectroscopy. CM exhibits a resonantly enhanced Raman scattering of the G-band when excited by a deep-UV laser (266 nm), allowing analysis with much-lower laser fluence of 0.06 to 0.58 J cm−2 (2.4–23 μW) compared with other silicate minerals. This allows for the acquisition of high-precision CM Raman spectra with minimal fluorescence interference from bulk-rock surfaces, without sample preparation. The spectral evolution of CM to graphite was assessed by analysis of 22 rock chips recording PMTs of <38 to ∼600 °C, with strong linear relationships observed between the PMT and Raman parameters such as G-band position, G-band full-width at half-maximum (FWHM), and Raman band separation (RBS). The linear correlation between the G-band FWHM and the PMT was strongest at 83 to 555 °C with an estimation error of 50 °C [T (°C) = −9.105 * (G-band FWHM, cm−1) + 712.5; R2 = 0.966], indicating its reliability as a geothermometer for rock chips. A similarly strong linear correlation between mean random vitrinite reflectance (VRr, %) and RBS was observed at 0.14–5.12 % VRr [VRr (%) = −0.026 * (RBS, cm−1) + 7.158; R2 = 0.977]. A key advantage of our method is that it requires no preparation of polished thin sections for PMT estimation and no selection of complex curve fitting methods, thereby eliminating uncertainties associated with sample damage during polishing, personal bias related to peak deconvolution, and analytical bias resulting from the orientation of CM grains. The method potentially has broad application as a reliable and robust thermal indicator in investigations of complex crustal evolution in various tectonic settings.