Abstract
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.
Highlights
The global carbon cycle is of great importance, due to its influence on climate change (Dasgupta and Hirschmann, 2010)
Tests showed that the difference in the vibrational free energies calculated using the density functional theory (DFT)+U method and modified HSE06 exchange-correlation functional was no more than 10 meV/atom
This is seen at 120 GPa, where the formation of Fe4C3O12 + C occurs at about 1500 K if magnetic entropy is accounted for, but does not occur up to 3000 K if it is neglected (Fig. 4(a))
Summary
The global carbon cycle is of great importance, due to its influence on climate change (Dasgupta and Hirschmann, 2010) It describes the distribution and exchange of carbon between major reservoirs, such as the atmosphere, crust, mantle and core. Studies of the global carbon cycle have often focused primarily on the atmosphere, oceans, and shallow crustal environments, referred to as the “near-surface cycle” (Hazen and Schiffries, 2013). While this works well for short time scales, to predict long-term changes in the concentration of CO2 in the atmosphere it is necessary to include exchange between the surface and interior of the Earth. Low solubility of carbon in mantle minerals, suggests that, once in the mantle, carbon is stored as either carbonates or diamond (Panero and Kabbes, 2008; Shcheka et al, 2006), depending on the local oxidation state
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