【作者】 刘雷；

【导师】 杜建国；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2008， 博士

【摘要】 通过第一性原理结合准简谐德拜模型研究了α和β镁橄榄石的高温高压结构及相变特征;计算了α和β镁橄榄石的弹性模量、波速、电子带结构、带隙等随压力的变化规律,并探讨了相变对这些性质的影响;研究了水对α镁橄榄石弹性模量、波速和带隙的影响;探讨了水对地幔中不连续层的作用以及矿物相变对过渡带波速不连续的作用。本文研究主要取得以下一些陈果:1.镁橄榄石晶格中存在水(H-O)会使镁橄榄石弹性模量及波速降低。在0-14GPa的压力下,含3.2wt%水的镁橄榄石比不含水镁橄榄石的Vp和vs值分别降低了3.1-7.1%和3.6-9.7%。在常温常压下,2个氢原子置换1个镁原子会使镁橄榄石的密度降低3.0%,是波速及弹性模量减小的原因,同时使镁橄榄石体系的基态总能增加,降低矿物的稳定性和键之间的强度。这可能是壳幔内形成低速层的重要原因。2.带结构和态密度研究表明随着压力的增加,α和β镁橄榄石带隙增加,绝缘性增强。这表明镁橄榄石的导电可能不是由镁离子位置上的质子移动引起的。自然界中橄榄石导电是因为其中掺杂了铁、氢或者别的杂质粒子引起,镁离子占据全部阳离子位置时,橄榄石不会导电。3.水减小了镁橄榄石带隙。两个氢原子替代了一个镁原子后,增强了晶体结构中的电子运移,但是含水镁橄榄石仍然是不导电的。氢替代橄榄石晶格中的二价离子可能对镁橄榄石导电性影响较小。4.第一性原理结合准简谐德拜模型可以在不耗费太多机时的情况比较快捷的计算物质的高压高温特性,计算的α和β橄榄石晶格参数随温度压力的变化与实验结果一致。同时发现,LDA比GGA近似更适合模拟橄榄石体系特征。5.计算的α-β相边界在0-1000K范围内与实验结果符合的较好,而且实验结果正好落在LDA和GGA范围内。6.利用LDA和GGA计算镁橄榄石α-β相变引起矿物Vs平均值分别增加11.6%和10.4%;Vp均值分别增加9.2%和9.0%。更多还原

【Abstract】 In this thesis, the crystal structure and phase transition behaviors of a andβforsterite (Mg2SiO4) at high temperature and high pressure were investigated by the first principle calculations and the quasi-harmonic Debye model. Variations of elastic properties, band structure, density of state, band gap ofαandβforsterite with pressure were calculated, and the effect ofα/βphase transition on these characters was analyzed. The effects of water on elastic modulus, sound velocity and band gap of forsterite were also simulated. Finally, the effect ofα/βphase transition and water on seismic discontinuity in earth was discussed.The main achievements are as follows:1. Adding water (H-O) in forsterite crystal cell decreases bulk modulus (K), shear modulus (G), and sound velocities (Vp, Vs) of forsterite. The sound velocities of hydrous forsterite reduce by 3.1-7.1% (Vp) and 3.6-9.7% (Vs) respectively at 0-14 GPa with 3.2wt% water. At the same time, the K, G, Vp, and Vs of forsterite decreased too because of density decrease 3.0% caused by H atoms replacing Mg atoms in the crystal. Water also increases the total energy of the forsterite system, i. e. the crystal stability and the power of bond were reduced at high pressure and temperature. This may be the main reason for formation the low velocity layers in the earth.2. The study results of band structure and density of state (DOS) indicate that band gaps and insulativity of a andβphases forsterite increase with increasing pressure. It indicates that electrical conduction ofα- andβ-phases is not caused by transport of protons bounded at Mg tetrahedral position. Conduction of natural olivine is induced by mixing of other particles like Fe, H. When Mg atom holds all cation positions, olivine is not conductive.3. Adding water in the forsterite crystal reduces the bag gap of the crystal. To replace one magnesium atom with two hydrogen atoms increases the electron transport of forsterite crystal, and the hydrous forsterite dose not become a conductor. The substitution of hydrogen with cation of lattice in crystal structure of forsterite has little effect on forsterite conduction.4. Combination of first-principles and the quasi-harmonic Debye model is an efficient approach to simulate the behavior of minerals at high pressure and/or high temperature. Variations of the calculated forsterite properties with temperature and pressure agree well with the experimental data.5. The calculated results are compatible with the previous experimental work at 0-1000 K. The experimental data just fall between LDA and GGA results. The results also show that LDA is more suitable to simulate characters of the olivine mineral system than GGA.6. The average increases of velocities caused by a-|3 phase transition are 11.6% (LDA) and 10.4%(GGA) for Vs and 9.2%(LDA) and 9.0%(GGA) for Vp respectively.更多还原