Extending the applicability of the Raman carbonaceous‐material geothermometer using data from contact metamorphic rocks

Abstract

The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature (T) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low-pressure (<3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact-metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c-axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532-nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5-nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2-T correlations shown by contact and regional metamorphic rocks: the former are slightly bettercrystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of ±50°C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532-nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532-nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to $ ±30°C.

Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils

“…Furthermore, as CM does not undergo retrograde metamorphism, it is a useful proxy for the peak metamorphic grade or thermal alteration experienced by the host rock (e.g., Pasteris and Wopenka, 1991;Jehlicka and Bény, 1992;Wopenka and Pasteris, 1993;Marshall et al, 2001;Beyssac et al, 2002;Jehlicka et al, 2003;Aoya et al, 2010). Thus, comparison of the lineshape and spectral parameters (i.e., I D /I G ratio) of the Raman spectra of CM found within the Apex chert to that of CM from deposits of known metamorphic grade and thermal evolution allows for a semiquantitative elucidation of the thermal alteration history of the CM (but it does not allow for determination of an absolute temperature value).…”

“…This phenomenon is attributed to a resonance enhancement effect of the D band (Pocsik et al, 1998). Recently, Aoya et al (2010) showed that there is little to no difference in I D /I G and A D /A G ratios for the same CM sample acquired from either excitation with a 514.5 or 532 nm laser. Therefore, it is valid to undertake a comparison of the above I D /I G ratios from the literature to delineate the thermal evolution of CM collected with either a 514.5 or 532 nm laser.…”

Mineral assemblages as a record of the evolutionary history of the Pepper Mts. Shale Formation (the Holy Cross Mts.).

“…Raman spectra of carbonaceous material (CM) consist of two board bands in the range of 1000-1800 cm -1 (first order region) and 2500-3100 cm -1 (second order region). The shape of the Raman spectra of CM show systematic changes due to its crystallinity (Beyssac et al 2002a, Rahl et al 2005, Aoya et al 2010, Lahfid et al 2010, especially within the first order Kouketsu (2014) defines the relation between the full width at half maximum (FWHM) of D1-band and the temperature of CM alteration.…”

Petrology and tectonic evolution of the Kiskunhalas-NE fractured hydrocarbon reservoir, South Hungary

“…The average difference between the result of Equation 1 (Beyssac et al 2002) and Equation 2 (Rahl et al 2005) is about 25 °C. The temperature values for the graphitic carbonate phyllite (n=60) with the different calibrations are 368.1±15°C (Beyssac et al 2002), 384.2±38 °C (Rahl et al 2005) and 369.8±15 °C (Aoya et al 2010), while for the graphitic gneiss mylonite (n=55) they are higher, being 411.9±45 °C (Beyssac et al 2002), 387.5±35 °C (Rahl et al 2005) and 415.6±47 °C (Aoya et al 2010). The Student's t-test, the statistical probe regularly used to compare means of independent populations, confirms that at a 0.01 significance level the estimated metamorphic peak temperatures for the two units are different.…”

“…The process of graphitization has a strictly irreversible character; thus it primarily depends on the maximum temperature reached along a given P-T path (e.g. Beyssac et al 2002;Beyssac et al 2004;Wiederkehr et al 2011;Aoya et al 2010). Therefore the CM structure is insensitive to retrograde metamorphic overprint or polymetamorphic evolution (Wiederkehr et al 2011;Beyssac et al 2002;Wopenka and Pasteris 1993).…”