The role of rocks saturated with metallic iron in the formation of ferric carbonate-silicate melts: Experimental modeling under PT-conditions of lithospheric mantle

Experimental modeling of the processes of formation of ferric carbonate-silicate melts through the carbonate-oxide-metal interaction is performed in the (Mg,Ca)CO₃-SiO₂-Al₂O₃-Fe⁰ system at 6.3 and 7.5GPa and within 1150-1650°C, using a multianvil high-pressure apparatus of "split-sphere" type (BARS). Two parallel reactions run in the subsolidus region (1150-1450°C): decarbonation, producing pyrope-almandine (Fe#=0.40-0.75) and CO₂ fluid, and redox interaction between carbonate and Fe⁰, resulting in the crystallization of iron carbide in assemblage with magnesiowustite (Fe#=0.75-0.85). It is shown that the reduction of carbonate or CO₂ fluid by iron carbide and parallel redox interaction of magnesiowustite with CO₂ produce graphite in assemblage with Fe³+-containing magnesiowustite. In the temperature range of 1450-1650°C, generation of carbonate-silicate melts coexisting with pyrope-almandine, magnesiowustite, magnetite, ferrospinel, and graphite takes place. The composition of the produced melts is as follows: SiO₂ ≈ 10-15wt.%, X(FeO+Fe₂O₃)=36-43wt.%, and Fe³+/XFe ≈ 0.18-0.23. These Fe³+-enriched carbonate-silicate melts/fluids are saturated with carbon and are the medium of graphite crystallization. Oxide and silicate phases (almandine, ferrospinel, and magnetite) coexisting with graphite are also characterized by high Fe³+/XFe values. It has been established that Fe³+-enriched carbonate-silicate melts can result from the interaction of Fe⁰-containing rocks with carbonated rocks. In the reduced mantle (with the presence of iron carbides or oxides), melts of this composition can be the source of carbon and the medium of graphite crystallization at once. After separation and ascent, these ferric carbonate-silicate melts can favor oxidizing metasomatism in the lithospheric mantle.