Home – Home – Jornals – Russian Geology and Geophysics 2025 number 8

Metasomatic reactions involving garnet lherzolite and SiO₂, Al₂O₃, and alkali-rich supercritical fluids or melts have been experimentally studied at pressures of 3.0 and 5.5 GPa and a temperature of 1200 °C. These fluids and melts might have formed during the dehydration, decarbonation, and melting of metasediments in subduction zones. The reaction of garnet lherzolite with model subduction-related mobile phase is shown to lead to changes typical of modal metasomatism in the lithospheric mantle. The reaction with the melt at 5.5 GPa yields phlogopite-bearing garnet lherzolite, while at 3.0 GPa it produces phlogopite-bearing garnet harzburgite. The reaction with a fluid rich in volatiles, mainly CO₂, at 3.0 and 5.5 GPa proceeds through intense carbonation of peridotite, leading to the consumption of olivine and the formation of orthopyroxene and magnesite, which results in the transformation of lherzolite into carbonated pyroxenite. At 3.0 GPa and 1200 °C, a carbonate-silicate melt rich in alkalies forms. In general, the presence of a significant amount of dissolved CO₂ in SiO₂-rich fluid or melt induces metasomatic transformations in peridotite, which are very similar to those occurring during its reaction with carbonatite melts. Specifically, at the molar CO₂ /(CO₂ + H₂O) ratio of <0.23, phlogopite forms, while at its value of >0.51, magnesite is produced. Phlogopite and magnesite do not form simultaneously in metasomatic reactions.