Abstract
We investigated the applicability of Raman microprobe spectroscopy for determining the density of CO<sub>2</sub> in fluid inclusions in minerals of mantle-derived xenolith samples. A separation (Δ) between two Raman bands of CO<sub>2</sub> due to Fermi resonance can be a reliable densimeter for CO<sub>2</sub> fluid. The relationship between the density of CO<sub>2</sub> (g/cm<sup>3</sup>) and Δ (cm<sup>-1</sup>) can be expressed as: <i>d</i> = -0.03238697Δ<sup>3</sup> + 10.08428Δ<sup>2</sup> - 1046.189Δ + 36163.67. This equation was obtained from the Raman data on CO<sub>2</sub> fluid with densities from 0.1 to 1.21 g/cm<sup>3</sup>, 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<sup>-1</sup>, corresponding to 0.02 g/cm<sup>3</sup> for inclusions of 1 μm in size. The precision can be better for larger inclusions. The micro-Raman densimeter can determine the density of CO<sub>2</sub> 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.
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