The density, and therefore the pressure, of CO2 fluid inclusions in minerals can be estimated from the Fermi diad splitting of Raman spectra of CO2. An accurate determination of the pressure of CO2 fluid inclusions enables the estimation of the depth origin of rocks from the deep Earth. A micro-Raman densimeter was applied to ultramafic-mafic xenoliths sampled along the Ohku coast of Oki-Dogo Island in the Sea of Japan (East Sea). The density of CO2 fluid inclusions in the mafic granulite was 1.02-1.05 g/cm3, while those of lherzolites were 0.98-1.02 g/cm3. In contrast, the density of CO2 fluid inclusions measured in olivine gabbro, clinopyroxenite, and harzburgite were lower ranging from 0.86-to 0.99 g/cm3. Taking into account the temperature condition estimated using a pyroxene thermometer, the mafic granulite originated from a depth of 27-30 km and the lherzolites from 25-29 km. The overlapping depth of 27-29 km can be interpreted as the depth including the Moho discontinuity under Oki-Dogo Island 3.3 Ma. This estimation is consistent with geophysical observations.
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