3:30 PM - 3:45 PM
[SCG53-07] Evaluation of the influence of laser irradiation power on Raman spectroscopic analysis of low-grade carbonaceous material
Keywords:Carbonaceous material, Raman spectroscopy, Raman CM geothermometer, Laser irradiation
The Raman Carbonaceous Material (CM) geothermometer [1][2][3] is a method for estimating the maximum heating temperature of CMs in rocks by obtaining their Raman spectra and analyzing their crystal structure. In recent years, the detailed thermal structure of metamorphic terrains was estimated by using this method.
It has been reported that even relatively low laser irradiation power can induce the change in Raman spectra of CM. On the other hand, low laser power causes decrease in the Signal-to-Noise Ratio (SNR) of the Raman spectra, resulting in increased measurement errors. Previous studies have focused on high-grade CM (i.e., graphite), and the effects of laser irradiation on low-grade CM have been sufficiently evaluated[4][5].
Therefore, this study evaluates the changes in Raman spectra of CM due to noise and laser irradiation to determine the appropriate laser power and other measurement conditions for low-grade CM.
Studied materials are mudstones and pelitic schists from three different localities, the Osawa Formation of South Kitakami Belt (Lower Triassic), the Kuzuryu Supergroup (Middle Jurassic) and the chlorite zone of Asemi-gawa area of Sanbagawa Belt (Cretaceous). These samples reflect unique diagenesis and metamorphic history, adequate to evaluate changes in the Raman spectra of low-grade CM. The diagenetic and metamorphic temperatures of the three rocks measured by the Raman CM geothermometer were approximately 250℃, 270℃, and 300℃. To assess the effects of both low and high laser power on the Raman spectra of CM, we conducted the following evaluations:
(1)Low laser power: To assess the influence of noise on spectral parameters and estimated temperatures, the SNR was calculated and compared with the measurement errors obtained from multiple measurements of the same CM. SNR was calculated from the peak intensity of the Raman spectra and the standard deviation of the background noise. Additionally, the changes in SNR were examined by varying the exposure time.
(2)High laser power: Measurements were conducted with different laser powers and exposure times for comparison. Furthermore, measurements were performed using different combinations of objective lenses and spectrometers, under varying laser power conditions.
The evaluation results showed that under condition (1), when the SNR was 20 or higher, sufficient spectral intensity was obtained, and the measurement error was reduced. In the Raman spectrometer used in this study, a laser power of approximately 0.5 mW was sufficient to obtain spectra with an SNR of 20 or higher.
Under condition (2), when using a 50× objective lens with an exposure time of 10 seconds × 3 measurements, spectral changes in CM were observed at laser power of 2 mW or higher.
Based on these results, in the case of applying the Raman CM geothermometer to low-grade CM, an appropriate laser power for measurement is 0.5–2 mW. This result differs from the empirically determined range of 1–3 mW proposed by Kouketsu et al. (2014)[3], suggesting that lower laser power is required for the measurement of low-grade CM.
[1] Beyssac et al. (2002) J. Metamorph. Geol., 20, 858–871. [2] Aoya et al. (2010) J. Metamorp. Geol., 28, 895–914. [3] Kouketsu et al. (2014) Isl. Arc, 52, 33–50. [4] Kagi et al. (1994) Geochim. Cosmochim. Acta, 58, 3527–3530. [5] Niwase (1995) Phys. Rev. B, 23, 15785–15798.
It has been reported that even relatively low laser irradiation power can induce the change in Raman spectra of CM. On the other hand, low laser power causes decrease in the Signal-to-Noise Ratio (SNR) of the Raman spectra, resulting in increased measurement errors. Previous studies have focused on high-grade CM (i.e., graphite), and the effects of laser irradiation on low-grade CM have been sufficiently evaluated[4][5].
Therefore, this study evaluates the changes in Raman spectra of CM due to noise and laser irradiation to determine the appropriate laser power and other measurement conditions for low-grade CM.
Studied materials are mudstones and pelitic schists from three different localities, the Osawa Formation of South Kitakami Belt (Lower Triassic), the Kuzuryu Supergroup (Middle Jurassic) and the chlorite zone of Asemi-gawa area of Sanbagawa Belt (Cretaceous). These samples reflect unique diagenesis and metamorphic history, adequate to evaluate changes in the Raman spectra of low-grade CM. The diagenetic and metamorphic temperatures of the three rocks measured by the Raman CM geothermometer were approximately 250℃, 270℃, and 300℃. To assess the effects of both low and high laser power on the Raman spectra of CM, we conducted the following evaluations:
(1)Low laser power: To assess the influence of noise on spectral parameters and estimated temperatures, the SNR was calculated and compared with the measurement errors obtained from multiple measurements of the same CM. SNR was calculated from the peak intensity of the Raman spectra and the standard deviation of the background noise. Additionally, the changes in SNR were examined by varying the exposure time.
(2)High laser power: Measurements were conducted with different laser powers and exposure times for comparison. Furthermore, measurements were performed using different combinations of objective lenses and spectrometers, under varying laser power conditions.
The evaluation results showed that under condition (1), when the SNR was 20 or higher, sufficient spectral intensity was obtained, and the measurement error was reduced. In the Raman spectrometer used in this study, a laser power of approximately 0.5 mW was sufficient to obtain spectra with an SNR of 20 or higher.
Under condition (2), when using a 50× objective lens with an exposure time of 10 seconds × 3 measurements, spectral changes in CM were observed at laser power of 2 mW or higher.
Based on these results, in the case of applying the Raman CM geothermometer to low-grade CM, an appropriate laser power for measurement is 0.5–2 mW. This result differs from the empirically determined range of 1–3 mW proposed by Kouketsu et al. (2014)[3], suggesting that lower laser power is required for the measurement of low-grade CM.
[1] Beyssac et al. (2002) J. Metamorph. Geol., 20, 858–871. [2] Aoya et al. (2010) J. Metamorp. Geol., 28, 895–914. [3] Kouketsu et al. (2014) Isl. Arc, 52, 33–50. [4] Kagi et al. (1994) Geochim. Cosmochim. Acta, 58, 3527–3530. [5] Niwase (1995) Phys. Rev. B, 23, 15785–15798.