【作者】 许俊闪；

【导师】 吴小平； 桂智男；

【作者基本信息】 中国科学技术大学 ， 固体地球物理学， 2011， 博士

【摘要】 MgSiO₃钙钛矿是地球内部下地幔中最丰富,最重要的矿物。它的蠕变决定着下地幔的流变性质。实验测量元素在MgSiO₃钙钛矿中的扩散系数是理解这些性质最直观最可靠的物理量。由于下地幔的高温高压条件,所以到目前为止MgSiO₃钙钛矿中元素扩散系数研究还很少,尤其是对Si和Mg的扩散研究还存有很大争议。本文利用安装在日本冈山大学地球物质科学研究所的川井型（Kawai-type）多砧高压实验装置,在25GPa和1400-1800oC的条件下,首次同时测得了Si和Mg在MgSiO₃钙钛矿单晶中的晶格扩散系数（体积扩散系数）。结果显示Si和Mg在MgSiO₃钙钛矿中具有相似的扩散系数并且是各向同性的,从而证实了Mg也是MgSiO₃钙钛矿中扩散速率最慢的元素。与大多数其他硅酸盐矿物中Si是扩散速率最慢的元素的情况不同,这一结果表明下地幔的流变性质不仅仅由Si扩散速率控制,而很可能是由Si和Mg的扩散速率共同控制。并结合前人在其他钙钛矿类材料中的扩散研究,给出了Si和Mg在MgSiO₃钙钛矿中的扩散模型。另外,我们在下地幔条件下首次合成了一种烧绿石结构新相MgZrSi₂O₇ ,烧绿石型结构的矿物有很多重要的应用,在材料领域有很广泛的研究,该相可能在极端条件下具有稳定性而在材料领域有潜在应用。本文的主要工作有:1.MgSiO₃钙钛矿单晶的合成:由于高温高压的条件,合成大颗粒钙钛矿单晶是非常困难,实验失败率非常高。但是高质量单晶是研究单晶中扩散系数的基础。我们利用川井型多砧高压装置和本实验室Shatskiy等人的技术经过多次实验在25 GPa和1500 oC条件下合成一批MgSiO₃钙钛矿单晶,并经过了微区X射线衍射分析的确认。2. Si和Mg在MgSiO₃钙钛矿单晶定向和扩散实验利用旋进式X射线照相机选取高质量的MgSiO₃钙钛矿单晶并沿着a,c轴定向,接着经过切割、剖光得到光滑的、与a, c轴垂直的面,利用激光沉淀仪器在晶面进行富含29Si和25Mg同位素镀层。然后利用川井型多砧高压装置在25GPa和1400-1800oC的温度下进行Si和Mg的扩散实验,利用次级离子质谱仪（SIMS）测量扩散数据,并通过拟合扩散数据得到Mg和Si的扩散系数。结果显示Mg和Si在MgSiO₃钙钛矿中具有相同的扩散系数。这是首次在实验中得到该结果。对进一步认识下地幔蠕变机制和理论模拟研究下地幔流变性质打下了很好的实验基础。3.下地幔条件下MgZrSi₂O₇烧绿石新相的合成和标定在25 GPa和15000C的下地幔条件下合成了一个烧绿石新相MgZrSi₂O₇ ,并对该相进行了参数标定。粉晶X射线衍射仪和Rietveld方法的精修的结果显示合成的新相为烧绿石结构（空间群Fd-3m,立方晶系）, a轴长为a = 9.2883（1） A,结构参数x = 0.4295（4）。EPMA分析的结果确认了新相的组成为MgZrSi₂O₇。与其他A3+2 B3+2O7型烧绿石材料不同的是A³+位置上的阳离子被不同的离子（Mg²+和Zr⁴+）离子以1:1的比例占据。A、B半径比值高达2.22,说明新烧绿石相可能在极端条件下保持结构稳定性,在材料中有潜在的应用。4.Si和Mg在MgSiO₃钙钛矿单晶中的扩散模型由于Mg在MgSiO₃钙钛矿中扩散情况与其他矿物不同,并不是比Si快几个量级,而是扩散速率与Si相似。我们从钙钛矿的结构特点出发,首次给出了下地幔条件下Si和Mg在MgSiO₃钙钛矿中的扩散模型。在这个扩散模型中,不同原子在钙钛矿中的扩散是相互联系的,Mg的扩散需要Si和O相连的空位形成的低阻碍。由于要保持系统电荷平衡和缺陷结构的重复,在每个循环中Si和Mg都跃迁了一次,可以解释Si和Mg具有相似扩散系数的结果。而O至少需要跃迁两次,由于O空位是大量存在的,O还可能有其他的跃迁,因此O在钙钛矿中的扩散系数远远高于Si和Mg。该模型解释了下地幔条件下Si和Mg在MgSiO₃钙钛矿扩散相似的实验结果,说明这一模型在高温高压条件钙钛矿结构中同样适用。更多还原

【Abstract】 MgSiO₃ perovskite is the most abundant and important minerals in the Earth’s lower mantle. Its creep determines the rheological properties of lower mantle. The experimental diffusion coefficients of elements in MgSiO₃ perovskite are the most directviewing and reliable parameters for understanding the rheological properties. However, because of the conditions of high pressure and high temperature in the lower mantle, until now there are few experimental studies of diffusion coefficients in MgSiO₃ perovskite. Especially, there is still in controversial of Si and Mg diffusion results. In our study, Si and Mg self-diffusion coefficients were measured simultaneously in the single crystals of MgSiO₃ perovskite under lower mantle conditions using a Kawai-type multi-anvil apparatus at the Institute for Study of the Earth’s Interior, Okayama University. The results showed that Mg has almost identical diffusivity of Si in perovskite. Mg together with Si is the slowest diffusing species in MgSiO₃ perovskite. It is different from the case in most silicates, which is Si. Mg and Si may together control the rheological properties in the lower mantle. Diffusion coefficients obtained in this study for both Si and Mg seemed to be isotropic. Furthermore, based on previous studies of perovskite type materials, we obtained the diffusion model of Si and Mg in MgSiO₃ perovskite. In addition, we synthesized a new pyrochlore type material MgZrSi₂O₇ under lower mantle conditions. The pyrochlore type materials have many important applications and are widely studied in materials science. The new pyrochlore MgZrSi₂O₇ can stabilize in very high pressure and temperature conditions and may have special applications in material science. The mainly contents of my docotoral work are listed as following:1. Synthesis experiments of MgSiO₃ perovskite single crystal: The single crystals of MgSiO₃ perovskite were synthesized at the conditions of 25GPa and 1500oC. It is very difficult to synthesis large grainsize crystals at high pressure and high temperature. However, in order to obtain high quality diffusion coefficients, single crystal were need. We synthesized some high quality single crystals following Shatskiy et al. （2007） and crystals were examined by Microfocused X-ray diffractometer.2. Diffusion experiments of Si and Mg in MgSiO₃ perovskite. Crystals were orientated by procession X-ray camera along a- and c- axes and polished using diamond paste and collidoal silica. The polished surfaces were coated with the 29Si- and 25Mg-enriched MgSiO₃ thin film using the pulsed laser deposition technique （PLD） at Ruhr University of Bochum, Germany. Si and Mg diffusion experiments were conducted at the conditions of 25 GPa and 1400-1800oC. Diffusion profiles were measured by SIMS in Hokkaido University. The results showed that Mg and Si almost have the same diffusivity in perovskite. This is the first time of observation in perovskite simultaneously. This is very important to understand the reological properties of lower mantle. It is helpful to understand perovskite structure properties and diffusion mechanism.3. Synthesis and crystal chemical characterization of pyrochlore type MgZrSi₂O₇ The pyrochlore type of MgZrSi₂O₇ was synthesized at 25 GPa and 1500 oC using a Kawai-type, multi-anvil apparatus. Powder X-ray diffraction and Rietveld analysis revealed that the phase assumed the pyrochlore structure （space group Fd-3m, cubic） with the lattice parameter a= 9.2883（1）? and the structural parameter x = 0.4295（4）. Chemical analysis by the electron probe microanalysis （EPMA） confirmed the stoichiometry of MgZrSi₂O₇ . It was demonstrated that the eight-fold coordinated 16c site is randomly occupied by both Mg2+ and Zr4+ ions in a 1:1 ratio. The high ionic radius ratio RA/RB （where A and B denote Mg+Zr and Si, respectively） of 2.22 necessitates a relatively high pressure to stabilize the pyrochlore structure.4. The model of Si and Mg diffusion in MgSiO₃ perovskite. The situation of Mg diffusion in MgSiO₃ perovskite is different from other minerals （e.g olivine, wadsleyie and ringwoodite）, in which Mg diffusion is several orders of magnitude faster than that of Si. Mg diffusion in perovskite has almost the same diffusivity as that of Si. We explained this result based on the comprehensive of the perovsktie structure. It is the first time of a new model to be used to explain the coupled motion of Si and Mg in perovskite, which is basically derived from the other perovskite-type oxides. In this model, the different atoms are related with each other and Mg diffusion through an unhindered path due to the presence of a Si and an O vacancy. The associated motion would account for similarity of diffusivity of Si and Mg, and the much faster diffusion rate of oxygen in perovskite. It is the first time to apply this model in perovskite structure under lower mantle conditions.更多还原