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Micro-Raman Densimeter for CO2 Inclusions in Mantle-Derived Minerals

Volume 57, Number 11 (Nov. 2003) Page 1333-1339

Kawakami, Yoko; Yamamoto, Junji; Kagi, Hiroyuki

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.