Thermodynamic and Structural Properties of liquid Mg2SiO4 at high temperatures and pressure in the range 0-150 GPa from Molecular Dynamics Simulation

Details Thermodynamic and Structural Properties of liquid Mg2SiO4 at high temperatures and pressure in the range 0-150 GPa from Molecular Dynamics Simulation Thermodynamic and Structural Properties of liquid Mg2SiO4 at high temperatures and pressure in the range 0-150 GPa from Molecular Dynamics Simulation

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr43b1080m&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #MR43B-1080

Statistics

3919 Equations Of State, 3939 Physical Thermodynamics, 5205 Formation Of Stars And Planets

Scientific paper

Growing interest in the dynamics of magma oceans and melting within the terrestrial planets highlights the need for developing equations of state (EOS) and transport properties of molten silicate multicomponent solutions at high temperature and pressure. We report Molecular Dynamics simulations of liquid Mg2SiO4, an important component of the upper mantle. An interatomic effective pair potential that includes Coulomb forces, Born exponential electron repulsion and van der Waals dipolar attractive forces was used with parameters from Matsui (Mineral. Mag, 58A, 571-572, 1994). 50 state points were studied in the NEV microcanonical ensemble with 8001 particles (1143 formula units) each for 50 ps (1 fs timestep). Liquid densities range from 2750 kg/m3 to 4500 kg/m3 with temperature and pressure in the range 2000- 4500 K and 0-150 GPa, respectively. Atom trajectories were post-processed to obtain a comprehensive view of nearest neighbor coordination statistics, internal energy, isochoric heat capacity, and tracer diffusivities of Mg, Si and O at all state points. Computed potential energies scale linearly in T^{3/5} along isochors facilitating EOS development by allowing robust interpolation. First nearest neighbor coordination statistics show a continuous decrease in ^{[4]}Si and increase in ^{[5]}Si and ^{[6]}Si with increasing pressure along an isotherm. In distinction, the abundance of ^{[1]}O (O with one nearest Si neighbor) is roughly constant at about 70%, with ^{[0]}O and ^{[2]}O both at about 15% as pressure increases along an isotherm. Oxygen tracer diffusivity is ~6.7x10-9 m2/s at 9.8 GPa and 3088 K. Mg and Si tracer diffusivities 1.7 and 0.8 that of oxygen, respectively. Using the Stokes-Einstein and Eyring relations between shear viscosity and oxygen tracer diffusivity, a shear viscosity of ~2.1x10-3 Pa s is estimated for molten Mg2SiO4 at 10 GPa and 3100 K. Liquid density computed by MD compares very well with liquid density inferred from experimental Clapeyron slopes along the laboratory fusion curve. We can deduce the likelihood of olivine floatation in a terrestrial magma ocean from these calculations. Please come to our poster to see the result.

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