Physical properties of carbonated melts in the Earth's mantle (Invited)

There are compelling evidences that carbonated melts play and have played a key role in mantle geodynamics and degassing of the Earth's mantle. Partial melting experiments dealing with silicate-carbonate assemblages show that a large composition range of CO2 bearing melts can be produced depending on the initial composition of the rocky material, the pressure and the temperature. Thus at mantle conditions (1350-1550 C and 3-20 GPa) carbonatite, kimberlite and CO2-rich silicate melts can be formed. In contrast with silicate magmas whose physical properties have been thoroughly investigated, those of carbonated melts are still badly known. The rare available data on pure molten carbonates (e.g. CaCO3) indicate that they are characterized by a very low viscosity and a high electrical conductivity in contrast with silicate melts. However, the evolution of the physical properties of a silicate-carbonate melt with the weight fraction of the silicate component (or alternatively that of the carbonate component) is essentially unknown. Do these properties evolve gradually with the composition or a threshold effect is showing up ? The purpose of the present study is to evaluate the properties of carbonated melts by molecular dynamics simulation techniques and to investigate their evolution with the melt composition. In order to do so, we develop a force field which accounts for the reactivity of CO2 in silicate melts and permits to model a large composition range from carbonatites (<10wt%SiO2) to CO2-rich basalts (45wt%SiO2). The equation of state, the viscosity and the electrical conductivity of different melt compositions have been evaluated as function of temperature and pressure along an adiabat. Geochemical implications of the simulation results will be discussed and put in perspective with currently debated views of the field.