Raman spectroscopic measurements of CO2 density: Experimental calibration with high-pressure optical cell (HPOC) and fused silica capillary capsule (FSCC) with application to fluid inclusion observations
Raman spectroscopy is a powerful method for the determination of CO 2 densities in fluid inclusions, especially for those with small size and/or low fluid density. The relationship between CO 2 Fermi diad split ( Δ , cm −1 ) and CO 2 density ( ρ , g/cm 3 ) has been documented by several previous studies. However, significant discrepancies exist among these studies mainly because of inconsistent calibration procedures and lack of measurements for CO 2 fluids having densities between 0.21 and 0.75 g/cm 3 , where liquid and vapor phases coexist near room temperature. In this study, a high-pressure optical cell and fused silica capillary capsules were used to prepare pure CO 2 samples with densities between 0.0472 and 1.0060 g/cm 3 . The measured CO 2 Fermi diad splits were calibrated with two well established Raman bands of benzonitrile at 1192.6 and 1598.9 cm −1 . The relationship between the CO 2 Fermi diad split and density can be represented by: ρ = 47513.64243 − 1374.824414 × Δ + 13.25586152 × Δ 2 − 0.04258891551 × Δ 3 ( r 2 = 0.99835, σ = 0.0253 g/cm 3 ), and this relationship was tested by synthetic fluid inclusions and natural CO 2 -rich fluid inclusions. The effects of temperature and the presence of H 2 O and CH 4 on this relationship were also examined.