[M-TT51] Frontiers in Geochemistry

[MTT51-P05] Pressure dependence of micro-Raman mass spectrometry for carbon isotopic composition of carbon dioxide fluid

*Lena Yokokura¹, Yuuki Hagiwara¹, Junji Yamamoto² (1.Hokkaido University, 2.Hokkaido University Museum)

Keywords:carbon isotope ratio, Micro-Raman spectroscopy, CO2 fluid, fluid inclusion

Micro-Raman spectroscopy can find the carbon isotopic ratio of CO₂ fluid from the ratio of intensity or area of a ¹³CO^₂ peak to that of a ¹²CO₂ peak. We exam- ined the precisions of carbon isotopic ratios (δ¹³C) of CO₂ at constant room temperature and pressure of 10–150 MPa. Measurement of the intensity ratio has precision of 2.8–8.7‰, which is better than that of the area ratio of 4.5–14.7‰. We also investigated the pressure dependence of the Raman inten- sity ratios and area ratio by changing fluid pressure. When changing fluid pres- sure from 10 to 150 MPa, the ratios of intensity and area both show negative correlation with fluid pressure (CO₂ density). Pressures of two types affect the Raman spectrum of CO₂ peaks, affecting the peak position and peak shape. To evaluate effects on the peak position, we repeatedly measured the intensity ratio at constant CO₂ pressure (10 MPa) with movement of the grating center position, which is defined as the center value of the analyzed wave number range. Although we moved the grating center position from 1248.5 to 1251.5 cm⁻¹, no significant correlation was observed for either ratio of inten- sity or area. The pressure effect on the ratios can be corrected by ascertaining the CO2 pressure. Combination with the Raman spectroscopic barometry for CO₂ enables analyses of δ¹³C of CO₂ respectively using the intensity ratio and the area ratio of CO₂ Raman peaks within 8.7 and 14.7‰.