MgCO3 + SiO2 reaction at pressures to 32 GPa studied using in situ X-ray diffraction and synchrotron radiation

Abstract

The results of the experimental study of the decarbonation and melting reactions in the MgCO3–SiO2 system at pressures up to 32 GPa using multi-anvil technique, in situ X-ray diffraction and synchrotron radiation have been reported. At 3–7 GPa and 1400–1700 K, the reaction proceeds with the release of carbon dioxide and the formation of enstatite. At 9–13 GPa and 1850–1930 K, clinoenstatite, carbonate-silicate melt, and CO2 were found among the reaction products. At 16 GPa and 1825 K, the reaction is accompanied by the formation of wadsleyite and at higher temperature by the formation of a carbonated melt, with a Mg/Si ratio close to wadsleyite, stishovite and CO2 fluid. At this pressure, which coincides with the wadsleyite-stishovite assemblage stability field in the MgSiO3 phase diagram, a decrease in the reaction temperature by about 100 K is observed. At higher pressures, the reaction proceeds with the formation of the MgSiO3 (akimotoite or bridgmanite) + melt assemblage. The reaction temperature at 25–35 GPa does not change and is about 2000 K. With a further increase in temperature to 2100 K, bridgmanite melts incongruently, reacting with a carbonate-silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO3 with increasing pressure. The studied reaction marks the upper temperature limit of the stability of magnesite and the free phase of SiO2 in the Earth’s mantle and generally coincides with the mantle adiabat at depths of 300–900 km.