—Carbon dioxide (CO2) is one of the main volatile components of natural magmas, but estimation of its initial contents remains a challenge. Study of melt inclusions in minerals permits a direct estimation of the content of CO2 in the melts. For the precise determination of its content in melt inclusions, it is necessary to analyze the contents of CO2 both in glass and in the fluid daughter phase of the inclusions. In this work, we constructed a calibration dependence of the density of CO2 in the range 0.01–0.22 g/cm3 on the distance between its characteristic peaks in Raman spectra (Fermi diads). The accuracy of density determination is ±0.03 g/cm3. The calibration plot was used to estimate the density of CO2 in the gas phase of melt inclusions in magnesian olivine (Fo84.8-88.5) from basalts of the Karymskii Volcano, eastern Kamchatka. The estimated density was 0.03–0.21 g/cm3. Using these values, we have first evaluated the minimum initial content of CO2 in the parental magmas of the Karymskii Volcano, 0.45 wt.%. These data, along with the known initial content of water (~4.5 wt.%), indicate that the parental magmas began to crystallize at a pressure of at least 7 kbar (depth of >25 km). To increase the reliability of the above method of estimation of the CO2 content in the gas phase of olivine-hosted melt inclusions, we propose to carry out preliminary experimental reheating of inclusions for the complete homogenization of the fluid phase and determination of the 3D size of melt inclusions. The performed study provides a reliable evaluation of the content of CO2 in parental magmas, the depth of crystallization, and the degree of magma degassing and permits a comparison of the compositions of magmatic fluids and high-temperature volcanic gases.