研究地幔熔体中元素的扩散性质有着重要的意义，因其影响着元素的交换和分馏过程。SiO2作为地幔组成的重要组分之一，其物理化学行为对于地幔动力学过程有着重要的意义。本文研究了SiO2熔体中元素的扩散机制和自扩散系数与压力的关系，采用Morse stretch势场对含有4500个原子的熔融SiO2体系进行了分子动力学模拟，计算了硅氧自扩散系数在3000 K温度下随压力的变化。模拟结果显示，在0.0001~40 GPa的压力区间，硅氧元素的自扩散系数均先上升后下降，在17.5 GPa时达到最大值，O原子的扩散速率略高于Si原子。硅氧元素的扩散方式为缺陷控制运移机制，其中硅原子的五配位结构的形成是关键，为导致扩散系数随压力增大而上升的主要原因，扩散系数的最大值意味着SiO2熔体中5配位硅形成机制的改变。本文也计算了单位[SiO2]的平均体积和压力的关系，结果与实验很吻合。%The diffusivity of elements in mineral controls the processes of ion exchange, isotope fractionation, and phase transition. As one of the most important components of silicate melts, SiO2 plays a very important role in the earth mantle structure and the dynamics process. This paper focuses on the ion diffusion mechanism in silica melt and the corresponding pressure when the self-diffusion coefficient reaches its maximum value. We calculated the changing process of Si and O ions, self-diffusion coefficient at temperatures of 3000 K by using the molecular dynamics (MD) simulation which contains 4500 ions in the silica melt with Morse stretch potential. Calculations show that both Si and O self-diffusion coefficients increase at initial compression and reach maximum values at 17.5 GPa. Self-diffusion coefficients for O are slightly greater than that for Si. The Si and O ions , diffusion feature is caused by a defect-controlled transport mechanism and the five-fold coordination structure of Si ion is the main reason for the rise of the diffusion coefficients in accord with the increasing pressure. The fact that the diffusion coefficient reaches its maximum value means the change in the formation of five-fold Si in the liquid silica. This paper also calculated the relationship between the average volume of [SiO2] unit and the pressure, which agrees well with the results of our experiment.
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