In situ Raman spectroscopic measurement of the 13C/12C ratio in CO2: Experimental calibrations on the effects of fluid pressure, temperature and composition

Abstract

The carbon isotope (δ13C) signature of CO2 within fluid inclusions is of significance to investigate because it can be used to decipher the carbon source of geological fluids. Raman spectroscopy has been applied to determine the δ13C of pure CO2. However, whether this approach can be applied to natural inclusions with complex compositions remains unknown. Here we present Raman spectra of CO2 ± N2 ± CH4 systems with known 13C/12C ratio (0.01–1.12) at temperatures (T) of 21 °C and 40 °C and at pressures (P) ranging from 0.5 to 50 MPa. Peak areas and peak heights of the 13CO2 (∼1370 cm−1) and 12CO2 (∼1285 cm−1) bands were determined to calculate the Raman spectroscopic quantification factors (F- and G-factor ratio). The results show that the quantification factors vary significantly with rising pressure in the low pressure range (e.g., <10 MPa), especially for pure CO2 system. Different fluid systems are characterized by pressure dependences. Specifically, both F- and G-factor ratios for CO2 fluid with a low 13C/12C ratio (e.g., 0.01) are different from that with a high 13C/12C ratio (e.g., 0.13) under the same T-P conditions. Fluid temperature does not result in significant variation in F- or G-factor ratios at the investigated temperatures. Based on these calibrations, we suggest that fluid pressure and composition (x) are prerequisites for Raman spectroscopic measurements of the 13C/12C ratio of CO2. However, the 13CO2 (∼1370 cm−1) band for CO2 from natural fluid inclusions is characterized by low Raman intensity, resulting in large errors in calculated peak area and peak height. Thus, accurate determination of the 13C/12C ratio of fluid inclusion CO2 is not feasible for inclusions with low CO2 density. Nevertheless, this method can be applied to investigate the carbon isotope fractionation in 13C labelled hydrothermal experiments, where CO2 with high a 13C/12C ratio is produced.