Redox reactions in subducted CaCO₃-Fe-SiO₂ as a potential source of superdeep diamond inclusions

CaCO₃ is one of the most abundant carbonates at the Earth's surface. It is transported into the mantle in substantial quantities via subducted slabs and participates in the deep mantle carbon cycle along with other mantle materials. Our experiments utilized the CaCO₃-Fe-SiO₂ system to simulate the environment of subducted oceanic crust entering the deep mantle. In situ X-ray diffraction experiments were performed on the CaCO₃-Fe-SiO₂ system at pressures of 25-53 GPa and temperatures of 1200-2250 K using a laser-heated diamond anvil cell. The results indicate that redox reactions occur within the pressure range of 25-53 GPa and the temperature range of 1500-2000 K, leading to the formation of CaSiO₃ + FeO + Fe₃C. At higher temperatures of 2000-2250 K and pressures of 25-53 GPa, the products are CaSiO₃ + Fe₃O₄ + Fe₇C₃. The mineral assemblage of Fe₃O₄ and Fe₇C₃, crystallizing under temperature conditions corresponding to the slab-mantle interface, can be encapsulated as inclusions in superdeep diamonds, such as those from the Juina area of Brazil. This assemblage may serve as an indicator of diamond formation at depths of approximately 700-1300 km in the deep mantle.