Micro-Raman Densimeter for CO 2 Inclusions in Mantle-Derived Minerals

We investigated the applicability of Raman microprobe spectroscopy for determining the density of CO₂ in fluid inclusions in minerals of mantle-derived xenolith samples. A separation (Δ) between two Raman bands of CO₂ due to Fermi resonance can be a reliable densimeter for CO₂ fluid. The relationship between the density of CO₂ (g/cm³) and Δ(cm⁻¹) can be expressed as: d = −0.03238697Δ³ + 10.08428Δ² – 1046.189Δ + 36163.67. This equation was obtained from the Raman data on CO₂ fluid with densities from 0.1 to 1.21 g/cm³, including super critical fluids at 58–59 °C. The Δ value was constant with increasing temperature from room temperature to 200 °C. This indicates that the Raman densimeter is not affected by a possible rise in temperature, an artifact induced by the high flux of the incident laser. The minimum size of measurable inclusions is 1 μm, and the precision in the determination of Δ is 0.1 cm⁻¹, corresponding to 0.02 g/cm³ for inclusions of 1 μm in size. The precision can be better for larger inclusions. The micro-Raman densimeter can determine the density of CO₂ fluid inclusions over a wide range. In particular, densities of gas and mixtures of gas and liquid phases, which cannot be measured by microthermometry, can be determined.