Determination of atomic diffusivity in silicates at high pressure and temperature

为了研究高温高压下二价阳离子在橄榄石中的扩散动力学，我们进行了以下实验：(a)利用FZ法和CZ法合成橄榄石单晶，通过多次烧结和混合制备多晶材料；(b)利用ma8型仪器在1400 ~ 1600℃的高温和5 ~ 10 GPa的高压下，对Mg_2SiO_4和（Mg， M）_2SiO_4 [M= Fe, Co， Ni]组成的橄榄石扩散偶进行退火处理；(C)对回收的橄榄石样品进行电子探针分析，得到扩散谱。虽然在高温高压下的退火时间短至1-4小时，但阳离子扩散的宽度已经超过了几十微米。用玻尔兹曼-梅塔诺方法分析了阳离子的扩散曲线，得到了阳离子的相互扩散系数。确定了阳离子扩散系数i作为压力、温度和化学成分的函数。我们在实验退火的样品中发现了压力增强的扩散。这种增强被解释为样品组装中的高偏应力或橄榄石中氢和阳离子（Mg, Fe, Co, Ni）的相关扩散的结果。因此，我们将得出结论，橄榄石中的阳离子在深部上地幔高温高压下的扩散速度比先前提出的要快。此外，利用透射电镜（TEM）研究了实验冲击试样和陨石中硅酸盐的微观结构，探讨了高冲击压力下硅酸盐的扩散过程。在许多冲击陨石标本中(e。G.球粒陨石，普通球粒陨石，月球陨石)，我们发现在激波压缩过程中发生了扩散诱导相变。

To investigate the kinetics of diffusion of divalent cations in olivine at high temperature and pressure, the following experiments were conducted : (a)syntheses of olivine single crystals using the FZ and CZ methods and polycrystalline materials by multiple sintering and mixing, (b)annealing of olivine diffusion couples consisting of Mg_2SiO_4 and(Mg, M)_2SiO_4 [M= Fe, Co, Ni]at high temperatures of 1400-1600 C and high pressures of 5-10 GPa using an MA8-type apparatus, (c)electron microprobe analyses of recovered olivine samples to obtain diffusion profiles.Although the duration of annealing at high temperature and pressure was as short as 1-4 hours, cation diffusion has occurred more than several tens um in width. The diffusion profiles were analyzed by the Boltzmann-Metano's method to obtain the cation interdiffusion coefficients. The cation diffusion coefficients i were determined as a function of pressure, temperature and chemical composition. We found a pressure enhanced diffusion in our experimentally annealed samples. The enhancement is interpreted as a result of high deviatric stress in a sample assembly or associated diffusion of hydrogen and cation(Mg, Fe, Co, Ni)in olivine. We will therefore conclude cation in olivine can diffuse faster at high temperature and pressure in the deep upper mantle than previously proposed.Furthermore, microstructures of silicates in experimentally shocked specimens and meteorites were studied by the transmission electron microscopy(TEM)to investigate the diffusion processes under the high shock pressure. In many shocked meteorite specimens(e. g. ureilites, ordinary chondrites, lunar meteorites), we found diffusion induced phase transformations have occurred during the shock compression.