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

用分子动力学(MD)模拟方法预测了高温高压下橄榄石(α)、改性尖晶石(β)和尖晶石(γ)的结构和弹性。离子间相互作用势被认为是两两相加的库仑、范德华吸引和排斥相互作用的总和。为了考虑晶体中的非中心力，采用呼吸壳模型(BSM)进行模拟，允许O离子的排斥半径在晶体中其他离子的影响下各向同性地变形。用BSM进行的MD模拟非常成功地再现了这五个相的观测值，包括环境条件下的结构参数和单个弹性常数，体积和剪切模数的温度和压力导数，以及在较宽的温度和压力范围内的体积热膨胀和体积压缩。我们应用MD SIM…进一步预测了与地幔660公里地震不连续相对应的高温高压条件下，镁硅尖晶石向镁硅钙钛矿+镁方镁石相变过程中的密度和体积声速跳跃。然后将模拟的密度和体积声速跳跃与报道的660公里不连续面的地震学模型进行了比较。分子动力学模拟发现，在密度和体积声速跳跃的1σ限制范围内，对热解石成分(60vol%尖晶石)的MD模拟与最新的SF99型[Sheeller and Flanagan，Science 285(1999)1545-1548]兼容。在1σ范围内，40%尖晶石的分子动力学模拟的体积声速跃迁也与SF99一致，而苦橄岩的密度跃迁不在SF99数据范围内。我们的MD计算的密度和体积声速跳跃，无论是对于地榴石还是对于苦橄岩成分，都与Prem或ak135模型的值有很大的不同。较少

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