Iron-rich carbonates stabilized by magnetic entropy at lower mantle conditions

Constraining the flux of carbon in and out of the interior of the Earth due to long-term geological processes is important, because of the influence that it has on climate change. On timescales of billions of years, host minerals such as carbonate phases could play a significant role in the global carbon cycle, transporting carbon into the lower mantle as a component of subducting slabs. We use density functional theory based calculations to study the high-pressure, high-temperature phase stability of Mg1-xFexCO3. Our results show that iron-rich phases, where carbon is in tetrahedral coordination, are only stable at lower mantle conditions due to their magnetic entropy, which is also responsible for the unusual shape of their phase boundary. Low-pressure carbonate phases are found to be highly anisotropic, but high-pressure carbonate phases are not, which has important implications for their seismic detectability. Our work confirms that future discussions of the global carbon cycle should include the deep Earth.