Abstract
Carbon di-oxide and water are two important volatiles that are often present in silicate magmas and volcanic eruptions. To address the influence of these volatiles in deep seated melts, their properties (e.g., structure, transport, thermodynamics) at relevant pressure-temperature (P-T) conditions along with compositional variance need to be explored. MgSiO (sub 3) being one of the major components of the mantle, the study of carbonated MgSiO (sub 3) melts is of great contextual relevance. In the present work we investigate the structure and thermodynamics of carbon bearing MgSiO (sub 3) melts under conditions of the entire mantle.Our first-principles molecular dynamics (MD) simulations of the MgSiO (sub 3) -CO (sub 2) system show that pressure profoundly influences the behavior of carbon-bearing silicate melts. Our results encompassing from 5 - 30 wt.% CO (sub 2) in MgSiO (sub 3) demonstrate that: (1) carbon speciation consists of distinct molecular CO (sub 2) and carbonate ions ( (CO (sub 3) ) (super 2-) ) below 15 GPa and interestingly almost all of the carbonate ions are bound to Mg polyhedra; (2) with compression they evolve to silicon-polyhedral bound carbonate (along with Mg polyhedra bound), CO (sub 4) , and di-carbonate species. Accordingly, carbon solubility in the silicate melt becomes nearly ideal and carbon remains completely miscible with increasing pressure. Carbon reduces the melt density modestly by 0.015 to 0.005 g cm (super -3) per wt.% CO (sub 2) between 15 and 140 GPa. These results imply that deep-seated silicate melts above and below the transition zone, and atop the core-mantle boundary may be able to sequester significant amounts of carbon without making melts gravitationally unstable.