Raman spectra of gas-mixture in fluid inclusions: effect of quartz birefringence on composition measurement

International audienceThe composition of the gas mixture in fluid inclusions is one of the most important properties to obtain physical and chemical information of geofluids. Raman spectroscopy is a powerful tool for the quantitativeanalysis of the composition of the fluids trapped in inclusions. The peak area of each species is directly linked to concentration by its Raman scattering cross section (RSCS). However, previous studies showed that the peak area ratio of gases changed with gas mixture composition at the same P-T conditions 1,2 , indicating that RSCS is not a constant. Besides, previous studies proved the effect of host mineral birefringence onthe shape of the stretching vibration band of liquid water, having an impact on the quantitative determination of salinity 3 , but the influence on gas-mixture inclusions is still unknown. In this study, the Raman signals of CO2-CH4 and CO2-N2 in natural fluid inclusions in quartz were collected and compared to microthermometry data 4. It showed sinusoidal variations of the peak area ratio (ACH4/ACO2, ACO2/AN2) with the rotation of the samples on the microscope stage with a period of 45°. On the contrary, the area ratio of the two peaks of CO2 (n1380/n1280) and the Fermi diad split remained constant with sample rotation. These phenomena are probably linked to the birefringence of the host mineral as observed for liquid water 3