Computer Simulation of the Physical and Chemical Properties of the Seismic Discontinuities in the Mantle

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Molecular dynamics (MD) simulation is used to predict the structure and elasticity of the olivine(α), modified-spinel(β), and spinel(γ) forms of Mg_2SiO_4, MgO, and MgSiO_3 perovskite at high temperatures and high pressures. The interionic potentials are taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In order to take account of non-central forces in crystals, the breathing shell model (BSM) is used for simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal. The MD simulation with BSM is found to be very successful in reproducing accurately the observed values for these five phases, including the structural parameters and individual elastic constants at ambient conditions, the temperature and pressure derivatives of bulk and shear moduli, and the volume thermal expansivity and volume compression over wide temperature and pressure ranges. We apply MD sim … More ulation to predict the density and bulk sound velocity jumps in the phase transformation from Mg_2SiO_4 spinel to MgSiO_3 perovskite plus MgO periclase at high-temperature and high-pressure conditions corresponding to the 660 km seismic discontinuity in the mantle. The simulated density and bulk sound velocity jumps are then compared with seismological models reported for the 660 km discontinuity. The MD simulation for a pyrolite composition (60 vol% spinel) is found to be compatible with a recent model SF99 [Shearer and Flanagan, Science 285 (1999) 1545-1548] within the 1σ limit for both density and bulk sound velocity jumps. The MD simulated bulk sound velocity jump for a piclogite composition (40 vol% spinel) is also consistent with SF99 within 1σ, while the MD density jump for the piclogite lies outside the SF99 data. Our MD computed density and bulk sound velocity jumps, either for the pyrolite or the piclogite composition, differ substantially from the values by the PREM or the ak135 model. Less