Abstract
Geological evidence of carbonate melt inclusion in diamonds and CO (sub 2) -rich magma from petit-spot volcanoes hint toward the existence of carbonate melts in the upper mantle. At surficial conditions carbonate melts are known to have low viscosity, thus, it is expected that these melts could be good agents of mantle metasomatism since they are likely to be enriched with incompatible elements. Recent studies on crystalline carbonate have documented a pressure-induced transition where CO (sub 3) groups transform to CO (sub 4) at the deep lower mantle. Such transition is likely to be associated with discontinuous changes in physical properties including compressibility and viscosity. However, our understanding of the transition in melt properties is rather limited. Existing studies on the high-pressure behavior of carbonate melts are limited to pressures where CO (sub 3) is the main species. In this study, we use first-principles molecular dynamics (FPMD) simulations to explore the structure, thermodynamics, and transport properties of carbonate melt with CaCO (sub 3) stoichiometrically. We have explored pressure and temperature conditions of the entire mantle i.e., 0-140 GPa and 2000-4000 K. We find that the choice of exchange-correlation functionals has a large effect on calculated density. We have explored both Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA) methods. Our simulation results with the LDA method shows that pressure-density results at 2000 K can be adequately described by a third order Birch Murnaghan equation of state with rho (sub 0) , K (sub 0) , and K' (sub 0) being 2.43 g/cm (super 3) , 18.5 GPa, and 7.07, respectively. For example, zero-pressure density at 2000 K using the LDA method is nearly equal 20 % larger than that of the GGA method. Along the 2000 K isotherm, the viscosity of molten CaCO (sub 3) increases from nearly equal 3 x10 (super -3) Pas at ambient pressure to nearly equal 6 x 10 (super -3) Pas at nearly equal 14 GPa. Structural analysis shows that the majority of carbon atoms remain in 3-fold coordination with oxygen up to mid-mantle depths. The transition from CO (sub 3) to CO (sub 4) in carbonate melts is gradual and its transition begins at lower pressures than reported for crystalline CaCO (sub 3) . For example, we find nearly equal 5 % CO (sub 4) at 40 GPa which increases to nearly equal 10 % at 60 GPa and nearly equal 20 % at 80 GPa. Owing to the gradual structural transition, the pressure dependence of bulk properties of CaCO (sub 3) shows a continuous trend. At ambient conditions, viscosities of CaCO (sub 3) melts are at least an order magnitude lower than silicate melts with composition CaSiO (sub 3) . However, the viscosity of silicate melt increases more rapidly with increasing pressure. Our calculation at nearly equal 26 GPa and 2000 K shows that the viscosity of CaSiO (sub 3) melts is about 2 orders of magnitude greater than that of CaCO (sub 3) . Thus, the high mobility of carbonate melt is likely to have a significant influence on a variety of magmatic and deep Earth processes.