Raman spectroscopy has been applied to determine the density and pressure of formation of CO2 fluid inclusions in mantle xenoliths, carried to the surface at Wilcza Góra in southwestern Poland by early Miocene alkaline magmas. The magmas were generated by partial melting in the transition zone between the spinel and garnet stability fields. Determination of the densities of CO2 inclusions allows calculation of the partial pressures and reconstruction of the depth of xenolith origin as well as their history en route to the surface. The density of CO2 inclusions ranges from 0.06 to 1.10 g/cm3 in olivines and 0.17 to 1.11 g/cm3 in orthopyroxenes. Only inclusions with a density above 0.8 g/cm3 reflect lower crust (ca. 15–30 km) and upper mantle (ca. 30–38 km) conditions. Slight differences in density of the inclusions between olivines and orthopyroxenes can be attributed to their different rheological properties during magma ascent. Modelling of pressure and depth within the temperature range in which the xenoliths equilibrated with the magmas yields information about the complex eruption history of basanitic volcanoes. Our estimates are consistent with seismic data that show a regional high-velocity layer at the Moho (ca. 30–35 km deep) with p-wave velocities up to 8 km/s which have been attributed to mafic and ultramafic lithologies.