Abstract
Volatiles such as H (sub 2) O and CO (sub 2) is known to influence thermodynamic stability, structure, and properties of silicate melts and thus play an important role in the generation, segregation, and emplacement of partial melts in the Earth's interior. In this study, we use first-principles molecular dynamics (FPMD) simulations to explore the effect of H (sub 2) O and CO (sub 2-) in the properties of alkali rich aluminosilicate melt in the Na (sub 2) O-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O-CO (sub 2) (NASHC) system. We explored pressures and temperature range of 0-20 GPa and 2500-4000 K. Our results on the equation of state of the melts show that both H (sub 2) O and CO (sub 2) reduces the density of the melt and partial molar volume of H (sub 2) O and CO (sub 2) in the melt is lower than the molar volume of pure H (sub 2) O and CO (sub 2) . At pressures greater than 5 to 7 GPa, pressure evolution of the partial molar volume of CO (sub 2) and H (sub 2) O are quite different. This translates into their distinct role in the reduction of the melt density. The water bearing melts exhibit OH, H (sub 2) O, and hydrogen bonded clusters at higher pressures. Carbon dioxide-bearing melts are dominated by CO (sub 3) species up to the highest pressure explored in this study, i.e., approximately 20 GPa. At lower pressure, approximately 20% of the species comprises of CO (sub 2) which decreases with increasing pressure resulting into the formation of more CO (sub 3) and CO (sub 4) species. We find approximately 10% CO (sub 4) species at approximately 20 GPa, i.e., at mantle transition zone to lower mantle depths. This is in contrast with carbonate solids where CO (sub 4) species occur at pressures greater than core mantle boundary. We notice that at lower pressures, the CO (sub 2) often attaches to the alkali atoms and one non bridging oxygens (NBO). In contrast, at high pressures, CO (sub 3) and CO (sub 4) species are connected to alkali ions/network modifier cations. We find that for the melts in the NASHC system, the self-diffusion coefficients of individual species are in the following order. Comparison of self-diffusion coefficient with that of anhydrous melt shows that the mobility of each species are enhanced by the presence of water. However, the self-diffusion of the ions is largely unaffected by the presence of carbon.