A new approach to develop the Raman carbonaceous material geothermometer for low‐grade metamorphism using peak width

Abstract

The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150-280°C), medium-grade CM (c. 280-400°C), high-grade CM (c. 400-650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band (G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low-to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum (FWHM) of the D1-and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150-400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer.

Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula, SW Japan: Application of the carbonaceous material Raman geothermometry

“…Peak metamorphic temperatures of the samples were estimated by the RSCM geothermometer using full-width at half maximum (Γ) of the disordered peaks (Kouketsu et al, 2014). Raman spectroscopy of CM grains were carried out by a laser Raman spectrophotometer (JASCO, NRS-3100) at the Department of Geology and Mineralogy, Kyoto University, using a diode-pumped solid-state (DPSS) laser emitting at 532 nm.…”

“…Recent progress on the Raman microscopy utilizes carbonaceous material (CM) as a convenient in situ temperature indicator (e.g., Beyssac et al, 2002). A variety of geologic situations become available for the application of Raman spectra of carbonaceous material (RSCM) geothermometer: regional metamorphism (Beyssac et al, 2002), contact metamorphism (Aoya et al, 2010), lowgrade metamorphism down to the zeolite-facies (Rahl et al, 2005;Lahfid et al, 2010;Kouketsu et al, 2014), and even for the extraterrestrial materials whose peak metamorphic temperature is below 100°C (Homma et al, 2015). Notably, the RSCM geothermometer is applicable for low-temperature conditions (approximately <400°C) where conventional thermodynamic approaches have difficulties due to the slow rate of metamorphic reactions (i.e., chemical equilibrium is not always attained: Cho and Liou, 1987).…”

“…1), by means of RSCM geothermometer calibrated for low-grade metamorphic rocks (Kouketsu et al, 2014). The study area is divided into the Sanbagawa metamorphic belt, Mikabu belt, Chichibu belt, Kurosegawa belt, and Shimanto belt from the north to the south (e.g., Kurimoto, 1986).…”

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

“…If so, they can be interesting material for further study, giving us a new insight into the role of regional metamorphism in the Holy Cross Mts. Bourdelle et al (2013) and Kouketsu et al (2014) give reliable results in the low temperature hydrothermal system. 9.…”

“…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.…”