The interaction between Fe–metal and magnesite was studied in multianvil experiments at 6 GPa and 1273–1873 K using different capsule materials: Fe, BN, and MgO. It was observed that at subsolidus conditions reaction proceeds with the formation of Fe3C and magnesiowüstite in the stoichiometric proportions according to relation: MgCO3 + 5Fe = 3(Fe0.66Mg0.33)O + Fe3C. At melting conditions (1673–1873 K) magnesite and iron react in nearly equivalent molar proportions with formation of Mg–Fe–carbonatite melt, Fe–C alloy, magnesiowüstite and graphite. The reactions clearly show that free carbon and metallic iron phases cannot coexist in the upper mantle and presumably in transition zone and will always form Fe–carbide. The carbon content in Fe–C alloy and its coexistence with diamond will be strongly dependent on the oxygen fugacity. The studied reactions can be considered as intermediate processes in the reduced mantle domains at the contact with submerging subduction slabs and have further implication to the processes at the core–mantle boundary.