Micro-raman densimeter for CO2 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 (Delta) 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/cm(3)) and Delta(cm(-1)) can be expressed as: d = -0.03238697Delta(3) + 10.08428Delta(2) - 1046.189Delta + 36163.67. This equation was obtained from the Raman data on CO2 fluid with densities from 0.1 to 1.21 g/cm(3), including super critical fluids at 58-59 degreesC. The Delta value was constant with increasing temperature from room temperature to 200 degreesC. 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 mum, and the precision in the determination of Delta is 0.1 cm(-1), corresponding to 0.02 g/cm(3) for inclusions of 1 mum 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.