First-Principles Molecular Dynamics Simulations of Silicate Liquids: Structure, Diffusion and Viscosity at Mantle Conditions

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Silicate liquids are primary agents of chemical and thermal evolution in the Earth as they form early magma ocean, and appear as magmas in the surface of the planet and as partial melts in the crust and mantle. Magmatic processes are responsible for the origin and ongoing formation of the oceanic and continental crust, and for bringing to the surface one of our primary clues to the composition of the interior in the form of xenoliths. Because of the contrast in density, chemical diffusivity, viscosity, and bulk composition between silicate liquids and their source regions, the generation and transport of magma is among the most efficient geological processes leading to mass and heat transport. Silicate liquids may have played an even more important role in the Earth's early history when melting may have been more widespread and may have extended to greater depths. A better understanding of planetary evolution thus requires major advances in our knowledge of relevant melt properties at mantle pressure-temperature conditions.In this project, it is proposed to apply a combination of first-principles computational and visualization techniques to investigate structure, diffusion and viscosity of silicate melts over broad range of pressure, temperature and composition. This approach, being parameter free in the nature, is expected to provide the ideal complement to experimental approaches, and can provide important insights into the fundamental physical properties and behavior in structure and bonding. The specific activities proposed include: 1) Expanding the range of composition towards sampling natural melts (MgO-CaO-Na2O-K2O-Al2O3-TiO2-SiO2 system) with/out volatiles (H2O and CO2). The calculated densities, enthalpies, and structures as a function of pressure and temperature are expected to enhance our understanding of buoyancy, bonding, speciation of volatile components, polymorphism, and thermodynamics of mixing in a multi-component melt system. 2) Investigating the transport properties of silicate melts through first-principles predictions of the self-diffusion and viscosity coefficients. Atomistic visualization of the position-time series will allow us gain insight into the microscopic mechanisms of compression and transport phenomena, and into the complex dependence of diffusion/viscosity on temperature, pressure and composition. 3) Continuing the study of structure and compression mechanisms of silicate glasses as a way of gaining additional insight into the energetics underlying liquid structure, and in order to enrich contact with the extensive experimental literature on geologically relevant compositions in the vitreous state. The unifying theme of the proposal is the first-principles simulations of large systems needed to explore realistic compositions, to accurately compute dynamical properties and to successfully capture the essence of glass structures. The PI has local access to sufficient resources to carry out such intensive simulations. The proposed research is essentially an exploitation of ideas and techniques of computational science to challenging problems in the investigation of Earth materials. It will have impact on a number of fields including geochemistry, petrology, geophysics, computational mineral/materials physics, and scientific visualization, and it will train students to have a multidisciplinary experience and expertise.