Understanding silicate geoliquids at high temperatures and pressures through molecular dynamics simulations

This dissertation discusses application of the Molecular Dynamics technique to silicate geoliquids. An understanding of the structure and properties of natural melts is key to geologic problems involving the early Earth and planetary magmatism. A serial MD code that simulated systems of N = 1300 atoms was used to examine anorthite (CaAl2Si2O8) liquid at 4000 K and eight pressures. This preliminary study revealed dramatic changes in atomic structure and associated properties as pressure increases. However, only a single isotherm could be explored due to computational requirements. To address this limitation, a pre-existing parallel code (LAMMPS) was modified by adding new intermolecular potentials and other improvements. This enables modeling larger size systems for longer periods. The role of adjustable parameters in the extraction of shear viscosity and its uncertainty by the Green-Kubo method viscosity was developed and tested. Liquid MgSiO3 was then studied using N = 8,000 atoms at 80 state (P-T) points for equation of state determination. Temperature ranged from 2500 to 5000 K and pressure from 0 to 120 GPa. Self-diffusivity and shear viscosity were found and described by modified Arrhenian expressions featuring a pressure dependent activation volume. Shear viscosity increases by a factor of 75 along the 3000 K isotherm and a factor of 3 along the 3000 K isentrope relevant to magma ocean convection. Molten CaAl2Si2O8 was re-examined using 72 state points spanning ranges in density (2398-4327 kg/m3 ), temperature (3490-6100 K) and pressure (0.84-120 GPa). Dramatic structural changes were found for the pressure interval from 0 to about 20 GPa with the interval biasing toward higher pressure at high temperatures. Changes correlate with variations in the thermodynamic and transport properties of the liquid. Self-diffusivities fit well by a modified Arrhenian relationship, and the Eyring model suggested possible cooperative behavior in liquid CaAl 2Si2O8. The size of the atomic cluster associated with atom mobility and shear viscosity decreased from about 8 atoms to 3 atoms as pressure increases. The MD method enables one to study the structure and properties of a variety of liquid compositions relevant to the geodynamical evolution of Earth.