Stability of magnesite (MgCO3) in the presence of a hydrous fluid in the upper mantle

Understanding the stability of carbonates under high pressure and temperature is essential for modelling the carbon balance and cycle in the deep Earth. In the presence of an H2O-containing fluid, the melting curve of carbonates can be strongly reduced to a lower temperature. Since magnesite is an important carbonate host in the Earth's mantle, the melting curve of magnesite in the presence of an H2O-containing fluid is of particular interest. Here we report results from in situ synchrotron energy dispersive X-ray diffraction experiments and use texture observations from ex situ falling sphere experiments in the brucite-magnesite system between 1 and 12 GPa. We define the dehydration and melting curve of brucite and elucidate the stability of magnesite in the presence of a liquid and periclase. The observed liquidus provides information on the fate of magnesite-bearing rocks in subduction zones. Our results show that magnesite remains stable under typical subduction zone gradients even when infiltrated by hydrous fluids released by dehydration reactions during subduction. We conclude that magnesite can be subducted to depths below the arc and beyond. Our results therefore have important implications for the carbon budget of the Earth's mantle and its role in the regulation of the carbon cycle.