Determination of atomic diffusivity in silicates at high pressure and temperature

为了研究高温高压下橄榄石中二价阳离子的扩散动力学，进行了以下实验：（a）用FZ和CZ法合成橄榄石单晶和用多次烧结和混合法合成多晶材料;（B）由Mg_2SiO_4和（Mg，M）_2SiO_4 [M= Fe，Co，（c）用电子探针分析回收的橄榄石样品，得到扩散分布，虽然高温高压退火时间短至1-4小时，但已发生了宽达几十μ m的阳离子扩散。通过Boltzmann-Metano方法分析扩散分布以获得阳离子互扩散系数。阳离子扩散系数i被确定为压力、温度和化学组成的函数。我们发现在我们的实验退火样品的压力增强扩散。这种增强被解释为在样品组装或相关的氢和阳离子（镁，铁，钴，镍）在橄榄石扩散的高偏差应力的结果。因此，我们认为橄榄石中的阳离子在高温高压下在上地幔深部的扩散速度比以前提出的要快。此外，为了研究高压下的扩散过程，我们还利用透射电子显微镜（TEM）对实验冲击样品和陨石中硅酸盐的微观结构进行了研究。在许多冲击陨石标本（e。G. ureilites，普通陨石，月球陨石），我们发现扩散诱导相变发生在冲击压缩。

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.