Home – Home – Jornals – Russian Geology and Geophysics 2015 number 1-2

Experimental modeling of the processes of formation of ferric carbonate-silicate melts through the carbonate-oxide-metal interaction is performed in the system (Mg,Ca)CO3-SiO2-Al2O3-Fe0 at 6.3 and 7.5 GPa and within 1150-1650 ºC, using a multianvil high-pressure apparatus of «split-sphere» type (BARS). Two parallel reactions run in the subsolidus region (1150-1450 ºC): 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 magnesiowüstite (Fe# = 0.75-0.85). It is shown that the reduction of carbonate or CO2 fluid by iron carbide and parallel redox interaction of magnesiowüstite tite with CO2 produce graphite in assemblage with Fe3+-containing magnesiowüstite. In the temperature range 1450-1650 ºC, generation of carbonate-silicate melts coexisting with pyrope-almandine, magnesiowüstite, magnetite, ferrospinel, and graphite takes place. The composition of the produced melts is as follows: SiO2 ≈ 10-15 wt.%, ∑Fe(FeO + Fe ₂O ₃) = = 36-43 wt.%, and Fe3+/∑Fe ≈ 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+/∑Fe 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.