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

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Silicate liquids are known to play an important role in planetary evolution and magma oceans. 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 Earth's 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 one of the most efficient geological means of mass and heat transport. Silicate liquids may have played an even more important role in the Earth?s earlier history. Models of the Earth's thermal history and petrological studies of ancient samples suggest that melting may have been more widespread and may have extended to greater depths. Because of their strong influence on geochemical and geodynamical processes, a better understanding of planetary evolution requires major advances in our knowledge of relevant melt properties at mantle 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 a broad range of pressure, temperature and composition. This type of approach, being parameter free in the nature, is expected to provide the ideal complement to the experimental approach and provide important insight into the fundamental origins of physical properties and behavior in structure and bonding. One of the goals of the study will be to expand the range of compositions to include a model basalt system (diopside-anorthite). Theoretical results on the density, enthalpy, and structure as a function of pressure and temperature are expected to enhance our understanding of buoyancy, bonding, polymorphism, and thermodynamics of mixing in silicate liquids. In addition, it is planned to investigate transport properties of silicate melts through ab initio predictions of the self-diffusion coefficients and viscosities. Atomistic visualization of the position-time series data will be exploited to gain insight into the microscopic mechanisms of transport and compression, and into dependence of diffusion on temperature, pressure and composition. Studies of the structure and compression mechanisms of silicate glasses are a valuable approach to gain additional insights into the energetics underlying liquid structure, and to enrich contact with the extensive experimental literature on geologically relevant compositions in the vitreous state. A unifying theme of the proposal is thus the first-principles computer simulations of large systems that are necessary to explore realistic melt compositions, to accurately compute dynamical properties and to successfully capture the essence of glass structures. The project will have impact on a number of fields including geochemistry, petrology, geophysics, computational materials physics, and scientific visualization, and it will train new scientists to have a multidisciplinary expertise.