Effect of Carbon and Hydrogen on Phase and Melting relations and Melt Properties at High Pressure

Eiji Ohtani

Tohoku University, Japan

Carbon and hydrogen are important elements in the Earth’s mantle. These elements have strong effects on phase and melting relations and melt properties such as density and viscosity under the mantle conditions. We studied the effect of CO2 and H2O on phase and melting relations of the mantle materials. The solidus temperature in the peridotite-CO2 system is comparable with that of the wet solidus of peridotite in the upper mantle, whereas it is lower than the wet solidus under the transition zone and lower mantle conditions. The reaction experiments of MgCO3 SiO2 and CaCO3 SiO2 revealed that decarbonation reactions and formation CO2 fluids at the lower pressure range below 6 GPa for the Mg-system and below 10 GPa for the Ca-system, whereas melting reactions forming MgSiO3 or CaSiO3 phases and CO2 rich melts occurred at higher pressures. The temperature of the melting reaction is lower than the wet solidus in the transition zone and the top of the lower mantle. Thus, the magmas enriched in CO2 is likely to be formed under the transition zone and lower mantle conditions, whereas the hydrous magmas can be formed more easily under the upper mantle conditions. The partial molar volume of the volatile components in magmas is essential for the density of the volatile rich magmas. We have determined the partial molar volume of H2O and CO2 in magmas at high pressure up to about 20 GPa. Our measurements revealed that the CO2 component dissolved in magmas is less compressible compared to the H2O component at high pressure. These compression behaviors of CO2 and H2O components in magmas indicate that CO2 reduces the magma density more effectively compared to H2O, and density crossover at the base of the upper mantle between volatile-rich magmas and surrounding peridotite mantle is less likely if the volatile is CO2 enriched in compositions. The CO2 rich fluid formed in the deep upper mantle, transition zone and lower mantle can be buoyant and tend to move upwards. The reactions among metallic iron, carbon, carbonate, and silicate/oxide in the Fe-Mg-C-O and Fe-Mg-Si-C-O systems are essential in the core formation of terrestrial and extraterrestrial planets and satellites. Our experiments revealed that the reactions MgCO3 Fe and CaCO3 Fe and formation of form Fe3C and oxide phases occurs at high pressures below 15 GPa. The change of metallic iron compounds occurs associated with a change of fO2 in the Fe-Mg-Si-C-O system at high pressure and temperature, i.e., the Fe-Si melt containing minor C coexists with carbon and olivine at reducing conditions (dlogfO2~-5), whereas a Fe-C alloy melt coexists with carbon and olivine under the oxidizing conditions (dlogfO2~-1.7). The change in iron alloy is important for the composition of the cores of the planetary bodies.