Decarbonation and melting in MgCO₃–SiO₂ system at high temperature and high pressure

Abstract

The decarbonation reaction boundary and the melting temperature in the MgCO₃–SiO₂ system were investigated up to 26 GPa using a multi–anvil apparatus. It was found that the decarbonation reaction (MgCO₃ + SiO₂ → MgSiO₃ + CO₂) occurs below 8 GPa, but it becomes a melting reaction above 8 GPa. The melting system shows a simple eutectic relation. The eutectic point shifts to a higher temperature and more MgCO₃–rich composition with increasing pressure. The eutectic temperatures at ~ 9 GPa and ~ 26 GPa were found to be ~ 1700 °C and ~ 2000 °C, respectively. Our results show that magnesite remains stable through the mantle geotherm, at least up to uppermost lower mantle, and that carbon can be transported to the lower mantle as magnesite. If a hot plume brings magnesite back to the upper mantle, it generates carbonatite magma or CO₂ fluid.