MgCO3 + SiO2 Reaction at Pressures up to 32 GPa Studied Using in-Situ X-Ray Diffraction and Synchrotron Radiation

Abstract

The paper reports results of an experimental study of decarbonation and melting reactions in the MgCO₃–SiO₂ system at pressures up to 32 GPa, using multianvil technique and in-situ X-ray diffraction with synchrotron radiation. 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 associated with the formation of wadsleyite and, at a higher temperature, by the generation of carbonated melt (with Mg/Si ratio close to wadsleyite) stishovite, and CO₂ fluid. At this pressure, which coincides with the stability field of the wadsleyite–stishovite assemblage in the MgSiO₃ phase diagram, the reaction temperature decreases by about 100 K. 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 the carbonate–silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO₃ with increasing pressure. The reaction marks the upper temperature limit for the stability of magnesite and a free SiO₂ phase in the Earth’s mantle and generally coincides with the mantle adiabat at depths of 300–900 km.