Abstract
The earth is a unique terrestrial planet with abundant water on its surface. This surface water and other volatiles have played a vital role in stabilizing continents by producing silica-rich granitic magma. Understanding the density, compressibility, and transport properties of silica-rich crustal and deep crustal melts is crucial for gaining insight into the formation of continents over geological time. We used first principles molecular dynamics simulations to understand the behavior of aluminosilicate melts 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 examined how pressure, temperature, and volatile content affect the atomistic scale structure and how that in turn affects density, compressibility, diffusion, and viscosity of the melts. In our study, we explored pressures up to 20 GPa and a range of temperatures between 2500-4000 K. Our results on the equation of state of the aluminosilicate melt compositions show that both H (sub 2) O and CO (sub 2) reduce the densities of the melts. At different pressures and temperatures, the hydrous aluminosilicate melts exhibit a variety of species that include hydroxyls (OH-), molecular water (H (sub 2) O), and hydrogen bonded clusters. At most PT conditions, carbon bearing aluminosilicate melts are dominated by carbonate (CO (sub 3) ) species. Some fraction of molecular carbon dioxide (CO (sub 2) ) were found at low pressures. Distorted tetrahedrally coordinated CO (sub 4) species were found at conditions relevant for the mantle transition zone and lower mantle. In contrast to our findings on melts, experimental studies on solid mantle carbonates show CO (sub 4) species only at Core Mantle boundary pressures. At lower pressures, we found that the molecular carbon dioxide is often attached to the alkali (Na) atoms. In contrast, at high pressures, carbonate ions and tetrahedrally distorted species are connected to alkali ions/network modifier cations. We found that for the melts in the NASHC system, hydrogen is the fastest diffusing species followed by alkali and then carbon ions. We found that oxygen, aluminum, and silicon all show similar diffusion and are often slower than the volatiles and the alkali cations. Water enhance the diffusion of cations and anions. In contrast, the effect of carbon on diffusion is negligible.