Paleo-Moho depth determined from the pressure of CO₂ fluid inclusions: Raman spectroscopic barometry of mantle- and crust-derived rocks

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.