Speciation and dynamical properties of hydrous MgCO3 melt studied by ab-initio molecular dynamics

Abstract

Understanding the effects of water on the geochemical behavior of carbonate melts in the Earth's interior requires thorough knowledge of their speciation and dynamical properties. However, the mechanisms of water dissolution in carbonate melts are unknown due to experimental challenges. Here we used ab-initio molecular dynamics simulations as a complementary approach to study the speciation and element diffusivities of MgCO3 melt containing 10 wt.% water at conditions up to 1765 K and ∼3.9 GPa. The simulations revealed a diverse COH speciation including CO32−, HCO3−, H2O, OH−, and CO2. The structural analysis was complemented by the calculation of vibrational density of states, and anharmonic infrared and Raman spectra. Good agreement between theoretical Raman spectra and experimental spectra from the literature supports the finding that HCO3− is an important species in hydrous MgCO3 melt. Mean square displacement analysis revealed rapid diffusional motion of hydrogen. However, the contribution of hydrogen to the total ionic conductivity is estimated to be relatively low compared to the already very high conductivity of anhydrous carbonate melts. In addition to fundamental insights into molecular structure and transport properties, our results provide a valuable basis for the development of thermodynamic speciation and solubility models, which are essential for a quantitative description of hydrous carbonate melts in the deep Earth.