Raman spectroscopic measurements of CO_2 density: Experimental calibration with high-pressure optical cell (HPOC) and fused silica capillary capsule (FSCC) with application to fluid inclusion observations

摘要

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 0.75g/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 1.0060g/cm~3. The measured CO_2 Fermi diad splits were calibrated with two well established Raman bands of benzonitrile at 1192.6 and 1598.9cm~(-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=0.0253g/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_2O and CH_4 on this relationship were also examined.