Raman spectroscopic thermometry of carbonaceous material in chondrites: Four-band fitting analysis and expansion of lower temperature limit

Abstract

Raman spectra of carbonaceous materials were observed for 20 chondrite samples (one R, four CV, four CO, two CM, one CR, one CB, one C-ung, three LL, one L, and two H chondrites). The observed specimens were chips (fifteen samples) and thin sections (five samples). The obtained spectra were decomposed into four components: GL, D1, D3, and D4 bands at 1600, 1350, 1510, and 1245 cm-1, respectively. The full width at half maximum of the D1 band (ΓD1) decreased with increasing peak metamorphic temperature (PMT) of the parent body, as published previously. By increasing the number of band components to four, the applicable range of the Raman spectroscopic thermometer using GD1 extended its lower limit to 20-30 °C. The upper limit of the present Raman spectroscopic thermometer is 540 °C, which is derived from the bottoming out of GD1. This thermometer was applied to chondrites whose PMT are unknown. The obtained temperature values were consistent with the order of petrological type.