Raman spectra of carbonaceous material in metasediments: a new geothermometer

Abstract

Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within‐sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustrés (Western Alps) and Sanbagawa (Japan) cross‐sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross‐section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high‐temperature, low‐pressure to high‐pressure, low‐temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = −445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.

The relationship between micro-Raman spectral parameters and reflectance of solid bitumen

“…The D band is also called D1 band (Marques et al, 2009;Wopenka and Pasteris, 1993) or A band (Kelemen and Fang, 2001), and it originates in vibrations of the disordered structure of carbonaceous material (Beyssac et al, 2002(Beyssac et al, , 2003. The G band is assigned to vibrations of the ordered (graphitic) structure (Beyssac et al, 2002(Beyssac et al, , 2003. With increasing thermal maturity, these two bands show regular changes in their position, FWHM (full width at half maximum) and intensity, which have been shown to be clearly correlated with maturity (Guedes et al, 2010;Kelemen and Fang, 2001;Kostova et al, 2012;Quirico et al, 2005).…”

“…In recent decades, numerous papers have reported that the Raman spectral characteristics and related parameters of geological solid organic matter, including coals, carboniferous fossils and carbonaceous materials, can be used as indicators for thermal maturity (i.e., Beyssac et al, 2002;Ferrari and Robertson, 2000;Guedes et al, 2010;Kelemen and Fang, 2001;Kwiecinska et al, 2010;Muirhead et al, 2012;Quirico et al, 2005Quirico et al, , 2009Rahl et al, 2005;Roberts et al, 1995;Schopf et al, 2005;Schopf and Kudryavtsev, 2009;Zeng and Wu, 2007). Detailed studies have focused on coals with different rank (Green et al, 1983;Guedes et al, 2010;Kelemen and Fang, 2001;Marques et al, 2009;Quirico et al, 2005;Sonibare et al, 2010).…”

“…The first-order characteristic bands of the Raman spectrum of carbon in coals have been found to generally occur at approximately 1350 cm −1 (D band) and 1580 cm −1 (G band) (Guedes et al, 2010;Kwiecinska et al, 2010;Wopenka and Pasteris, 1993). The D band is also called D1 band (Marques et al, 2009;Wopenka and Pasteris, 1993) or A band (Kelemen and Fang, 2001), and it originates in vibrations of the disordered structure of carbonaceous material (Beyssac et al, 2002(Beyssac et al, , 2003. The G band is assigned to vibrations of the ordered (graphitic) structure (Beyssac et al, 2002(Beyssac et al, , 2003.…”

Potential Raman parameters to assess the thermal evolution of kerogens from different pyrolysis experiments

“…Kelemen et al [12] and Liu et al [13] proposed a positive linear correlation between the inter-interval of peaks D/G and Ro. Beyssac et al [14,15] and Bernard et al [16] suggested that simple D/G band ratios are not adequate to evaluate the thermal maturity level of geologic carbonaceous materials by changes in Raman spectra. It seems that Raman spectra could be applied to evaluate the thermal maturity of carbonaceous materials, but that different reports have contradictory results and perspectives on the usefulness of different parameters.…”

Optical signatures of bulk and solutions of KC8 and KC24

“…10,11 In geology, the Raman signature of carboneous material can be used as a geothermometer. 12 The Raman signal allowed in the center of the Brillouin zone corresponding to the phase opposition motion of bonded atoms is degenerated. It is called G or E 2g mode.…”