Graphite and Diamond Formation in the Carbide–Oxide–Carbonate Interactions (Experimental Modeling under Mantle P,T-Conditions)

Experimental modeling of the formation of graphite and diamond as a result of carbide–fluid interactions was performed in the Fe₃C–SiO₂–Al₂O₃–(Mg,Ca)CO₃ systems at 6.3 and 7.5 GPa and 1100–1650 °C. In the experiments with ƒO₂-gradient (7.5 GPa, 1250–1350 °C), graphite + magnesiowüstite + garnet ± cohenite assemblage was formed. Graphite was produced through the redox interactions of carbide with carbonate or CO₂ (reducing conditions), and redox reactions of magnesiowüstite and CO₂ (oxidizing conditions). At 1450–1650 °C, crystallization of graphite, garnet, magnesiowüstite and ferrospinel, as well as generation of Fe^2+,3+-rich carbonate–silicate melt occurred. This melt, saturated with carbon, acted as a medium of graphite crystallization and diamond growth on seeds. In the experiments without ƒO₂-gradient (6.3 GPa), decarbonation reactions with the formation of CO₂-fluid and Fe,Mg,Ca-silicates, as well as C⁰-producing redox reactions of CO₂-fluid with cohenite were simultaneously realized. As a result, graphite (± diamond growth) was formed in assemblage with Fe²⁺,Fe³⁺,Mg-silicates and magnetite (1100–1200 °C), or with Fe³⁺-rich garnet and orthopyroxene (1300–1500 °C). It has been established that a potential mechanism for the crystallization of graphite or diamond growth is the oxidation of cohenite by CO₂-fluid to FeO and Fe₃O₄, accompanied by the extraction of carbon from Fe₃C and the corresponding reduction of CO₂ to C⁰.