Home – Home – Jornals – Russian Geology and Geophysics 2018 number Неопубликованное

At pressures of 3.0 and 5.5 GPa and a temperature of 1200°C, metasomatic reactions involving garnet lherzolite and SiO₂-, Al₂O₃-, and alkali-rich supercritical fluids or melts have been experimentally studied. These fluids and melts could potentially be formed during the processes of dehydration, decarbonation, and melting of metasediments in subduction zones. The reaction of garnet lherzolite with model subduction mobile phase has been shown to lead to changes characteristic of modal metasomatism in the lithospheric mantle. As a result of the reaction with the melt at a pressure of 5.5 GPa, phlogopite-bearing garnet lherzolite is formed, while at a pressure of 3.0 GPa, phlogopite-bearing garnet harzburgite is produced. With an increase in the volatile content, primarily CO₂, in the fluid/melt at pressures of 3.0 and 5.5 GPa, the reaction proceeds through intense carbonation of peridotite, leading to the disappearance of olivine and the formation of orthopyroxene and magnesite, resulting in the transformation of lherzolite into carbonated pyroxenite. At a pressure of 3.0 GPa and a temperature of 1200 °C, a carbonate-silicate melt rich in alkalis is formed. Overall, the presence of a significant amount of dissolved CO₂ in a SiO₂-rich fluid or melt induces metasomatic transformations in peridotite that are very similar to those occurring during its reaction with carbonatite melts. Specifically, at a molar ratio of CO₂/(CO₂+H₂O) < 0.23, phlogopite is formed, while at >0.51, magnesite is produced. Phlogopite and magnesite do not form simultaneously in the products of metasomatic reactions.