5:15 PM - 6:45 PM
[SCG45-P02] Evaluation of the influence of laser irradiation for Raman spectra of low-grade carbonaceous material
Keywords:Carbonaceous material, Raman spectroscopy, Raman CM geothermometer, Laser irradiation
Measurement targets for Raman carbonaceous material (CM) geothermometry are limited to CM inclusions in other transparent minerals within thin sections or polished slabs. Therefore, the aim of this study was to confirm that new sample forms can be applied to the Raman CM geothermometer to expand the range of samples. However, to compare the differences among sample forms, the influence during Raman spectra measurement was confirmed. Although Raman spectroscopy is considered a non-destructive analysis, it has been reported that even relatively low laser irradiation power can induce the change in Raman spectra of CM because of its opacity and color, black. This change in Raman spectra is caused by two major factors. The first is the effect of heat due to laser irradiation, which is known to change the peak position of the Raman spectra as the temperature of the CM increases [1]. The second is that laser irradiation causes irreversible damage to the CM [2]. However, most of the previous studies have focused on high-grade CM (i.e., graphite), and the effects of laser irradiation on low-grade CM have not been fully evaluated thus further verification is needed.
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℃. The changes in Raman spectra induced by laser irradiation were measured and compared at different laser irradiation power. The changes in Raman spectra of CM caused by laser irradiation were then evaluated separately for heat and damage. As a result, the peak position of the CM Raman bands which reflect the defect structure (D1 band and D2 band) tended to decrease, especially with heat effects, suggesting that the decrease in peak position occurred due to an increase in temperature of CM as in the graphite [1]. In contrast, the full width at half maximum (FWHM) of the D1 band tended to increase especially with damage effects. This suggests that the FWHM of the D1 band, a parameter of the Raman CM geothermometer [3], well reflects the change in crystal structure. These changes may affect the temperature estimation by the Raman CM geothermometer. New sample forms, such as rock cut surfaces, fractured surfaces, and after hydrofluoric acid decomposition, will also be examined.
[1] Kagi et al. (1994) Geochimica et Cosmochimica Acta, 58, 3527–3530. [2] Niwase (1995) Physical Review B, 23, 15785–15798. [3] Kouketsu et al. (2014) Island Arc, 52, 33–50.
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℃. The changes in Raman spectra induced by laser irradiation were measured and compared at different laser irradiation power. The changes in Raman spectra of CM caused by laser irradiation were then evaluated separately for heat and damage. As a result, the peak position of the CM Raman bands which reflect the defect structure (D1 band and D2 band) tended to decrease, especially with heat effects, suggesting that the decrease in peak position occurred due to an increase in temperature of CM as in the graphite [1]. In contrast, the full width at half maximum (FWHM) of the D1 band tended to increase especially with damage effects. This suggests that the FWHM of the D1 band, a parameter of the Raman CM geothermometer [3], well reflects the change in crystal structure. These changes may affect the temperature estimation by the Raman CM geothermometer. New sample forms, such as rock cut surfaces, fractured surfaces, and after hydrofluoric acid decomposition, will also be examined.
[1] Kagi et al. (1994) Geochimica et Cosmochimica Acta, 58, 3527–3530. [2] Niwase (1995) Physical Review B, 23, 15785–15798. [3] Kouketsu et al. (2014) Island Arc, 52, 33–50.