Mg-carbonate interaction with metallic iron in the lower mantle and at the core-mantle boundary

The presence of carbonates in the deep Earth strongly depends on the oxygen fugacity, controlled by oxidation state of iron in minerals and melts. A large part of the mantle is thought to be significantly reduced, with detectable amount of Fe0 presented in the lower mantle and core-mantle boundary. Therefore, subducted carbonates would interact with Fe0 dispersed in the ambient mantle. However, the mechanism of this interaction remains controversial. Here, we investigated MgCO3-Fe0 interaction at 70-145 GPa and 800-2600K. Experiments were conducted in situ in a diamond anvil cell using X-ray diffraction to observe phase transformations. MgCO3 crystals and Fe foil (99,9%) were used as the starting materials. Formation of wustite (FeO), ferropericlase (Mg0.6Fe0.4)O, carbide (Fe7C3) and diamond was observed. Three different modifications of FeO were detected: B1 - at T = 1100-2600K and 70-145 GPa, rB1- at T<1100 K and P<136GPa; and B8- at P = 143-145 GPa. Interestingly, we observed coexistence of wüstite and ferropericlase, which may suggest an existence of immiscibility gap in FeO-MgO system at P> 70 GPa. Mg-carbonate reduction can be schematically presented by following reaction: 3MgCO3+13Fe=6FeO+3MgO+Fe7C3. Formation of high-pressure modifications of Mg-carbonate above 80 GPa does not change the reaction path, and same products were observed in all experimental runs. The studied carbonate-iron reaction supports formation of the (Fe,Mg)O and carbide phases in the lower mantle and at the Earth's core-mantle boundary. The effectiveness of the reaction and its influence on carbonates stability and carbon speciation in the lower mantle also depends on diffusivity in silicates, which is extremly slow. At the same time, if carbonates could be transported to the core-mantle boundary, where concentration of iron is significantly higher, they would be completely reduced to carbide or diamond.