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

摘要

Experimental modeling of the formation of graphite and diamond as a result of carbide–fluid interactions was performed in the Fe 3 C–SiO 2 –Al 2 O 3 –(Mg,Ca)CO 3 systems at 6.3 and 7.5 GPa and 1100–1650 °C. In the experiments with ?O 2 -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 CO 2 (reducing conditions), and redox reactions of magnesiowüstite and CO 2 (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 ?O 2 -gradient (6.3 GPa), decarbonation reactions with the formation of CO 2 -fluid and Fe,Mg,Ca-silicates, as well as C 0 -producing redox reactions of CO 2 -fluid with cohenite were simultaneously realized. As a result, graphite (± diamond growth) was formed in assemblage with Fe 2+ ,Fe 3+ ,Mg-silicates and magnetite (1100–1200 °C), or with Fe 3+ -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 CO 2 -fluid to FeO and Fe 3 O 4 , accompanied by the extraction of carbon from Fe 3 C and the corresponding reduction of CO 2 to C 0 .