【作者】 邓力维；

【导师】 龚自正； 费英伟；

【作者基本信息】 西南交通大学 ， 固体力学， 2008， 博士

【摘要】 (Mg，Fe)SiO₃钙钛矿作为下地幔最主要的组成矿物，其高压物性（如状态方程，热力学稳定性等）的研究对于准确限定地幔矿物组成，了解地幔动力学过程（如地幔对流模式，地震火山形成机理等）具有十分重要的意义。本学位论文采用静高压与冲击压缩技术相结合的手段，并辅以计算机模拟技术，对（Mg，Fe）SiO₃钙钛矿开展了高温高压状态方程及相稳定性的理论、实验研究。这些研究为建立准确、合理的下地幔矿物学组成模型提供了确凿的数据。提升了我们对于地球深部物理的了解与认识。本学位论文还利用静高压大腔体技术对地核的可能组成元素体系Fe-S-C开展了一系列高温高压熔化实验研究。基于对淬火样品的定量定性分析，初步掌握该三元合金体系熔化关系随压力、温度、初始组份的变化规律。这对于推测星体演化过程中核元素的分布及建立行星核矿物组成模型有极为重要的意义。本文的主要研究内容和认识如下：（1）通过对大腔体压机样品装配腔的设计与改进，利用活塞圆筒压机和大压机成功合成了适合冲击波实验尺寸要求的大块钙钛矿MgSiO₃样品。通过微区电子探针、拉曼光谱和X射线衍射分析证实合成材料确为钙钛矿MgSiO₃，为下一步的冲击波高压实验研究提供了所必须的初始样品，使得在高压下对钙钛矿MgSiO₃开展直接冲击压缩研究成为可能。（2）在47-107 GPa冲击压力（估算温度为600-3300K）范围内进行了初始样品为钙钛矿MgSiO₃的冲击压缩状态方程测量实验。这是国际上首次开展的以钙钛矿MgSiO₃为冲击样品的动高压实验，是本文的创新点之一。得到冲击波速度（Us）与粒子速度（u_p）关系：U_s=6.42+1.48u_P。通过Rankin-Hugoniot方程拟合得到了Grüneisen参数γ₀=1.33（q=1）。利用三阶欧拉有限应变方程拟合得到的绝热体积模量K_0S及其对压力的一阶偏导数K_0S’分别是254（±10）GPa和3.9（±0.17）。通过与以顽火辉石为初始样品的冲击数据的对比，发现：MgSiO₃钙钛矿在下地幔温压条件下无化学分解反应发生，并且90GPa冲击压力以上时MgSiO₃顽火辉石才彻底转变为钙钛矿相。本文的数据完全排除了低压相顽火辉石的影响，首次提供了通过直接冲击压缩钙钛矿所得到的状态方程参数，为今后构建地幔矿物学组成模型提供了更准确的热力学约束。也为进一步利用冲击手段研究地幔高压物性做了有益的探索。（3）在3.5-20GPa，1100-1700℃范围内开展了Fe-C-S三元体系的高温高压熔化实验研究，通过对淬火样品纹理的仔细观察，并结合扫描电镜、电子探针等测试分析手段，考察了压力、温度、组份对该体系高压相关系的影响。低压下，该体系将出现富S熔体与富C熔体的不互溶现象。随着压力的升高，不互溶液体渐渐融合，并最终在4.9GPa成为一个化学均匀的熔体。压力的改变还促使了一系列复杂中间相的生成，共熔温度随之变化。在等压线上随着温度的降低，Fe-C合金首先结晶，继而是熔点较低的Fe-S合金结晶出溶。使用两种不同的初始样品导致固相线下形成的矿物相迥异。我们的实验结果对于了解在行星演化过程中，（富含C,S）行星核的分化历史及对核元素组成的限定具有非常重要的意义：当行星核分化过程中核压小于5GPa时，由于熔体的不互融，将出现分层的核，即富S的液体外核及富C的液体内核。随着核的冷却，Fe-C合金将慢慢析出形成固体内核。而对于核压大于5GPa的行星体，在其核内将无分层现象。特别地，基于本文的研究推论，我们提出了含C的固体内地核及富含S（贫C）的液态外地核模型。（4）采用密度泛函理论和平面波赝势方法针对MgSiO₃钙钛矿开展了第一性原理计算研究。模拟得到了0-120GPa范围内MgSiO₃钙钛矿零温晶格常数、密度、声速等弹性数据，模拟得到的状态方程参数（K₀=264GPa，K₀’=3.83）与实验数据较好符合，证实了本文所采用的计算方法的可靠性。考虑到真实下地幔的高温环境，我们利用热力学方法对模拟得到的声速、密度、体模量进行了温度修正，并将修正后的数据与地球初步参考模型的相应剖面比较，对比结果很好的支持了以MgSiO₃钙钛矿为主要组成矿物的下地幔矿物组成模型。（5）针对MgSiO₃和(Mg_x，Fe_1-x)SiO₃钙钛矿在高压下可能存在的微结构相变，我们采用密度泛函理论和平面波赝势方法分别对MgSiO₃（Pbnm-P4mbm-Pm（?）m）及(Mg_x，Fe_1-x)SiO₃ （Pm-Pmmm-P4mmm）的可能相变序列在0-150GPa内开展了第一性原理计算模拟研究。基于在同一压力下不同相的焓差的比较，钙钛矿MgSiO₃在高压下的相稳定序列（由稳定到次稳定）是：Pbnm（orthorhombic）-P4mbm（tetragonal）-Pm（?）m（cubic）.而对于(Mg_0.75，Fe_0.25)SiO₃来说，相应的相稳定序列是pm（orthorhombic）-pmmm（tetragonal）-p4mmm（cubic）。我们的计算结果支持具有低对称性的正交MgSiO₃较其他相结构在下地幔压力条件下更为稳定，此外，在MgSiO₃加入少量Fe将使得其稳定性降低。更多还原

【Abstract】 (Mg,Fe) SiO₃ perovskite is the most abundant mineral in the Lower mantle.Its high pressure physical properties,such as equation of state,phase stability,are very crucial for constraining the mineralogy composition and describing dynamics process （such like mantle convection,earthquake and volcano mechanism） of the Earth interior.In my work,the eqation of state,crystal structure and thermodynamic stability of （Mg,Fe）SiO₃ perovskite have been investigated at lower mantle high pressure and temperature condition by combination of shock wave loading,static compression and computer simulation.Our results have significant implications for modeling lower mantle mineral composition model accurately.Hence it will improve our further understanding about the interior of the Earth.This dissertation is also devoted to the study of melting behavior of Fe-C-S ternary system under high pressure.Based on the analysis of quenched sample texture,we demonstrate how the melting relations,subsolidus formations and element partition in this system change along with pressure,temperature and starting component.Our results have important implications for planetary differentiation and core composition stratification of planetary body.The main achievements in this study are as followings:（1） Large MgSiO₃-perovskite samples were successfully synthesized by using modified multi-anvil sample assembly.The recovered samples are confirmed to be MgSiO₃ perovskite by electron microscopy analysis and Raman spectrum.Successful syntheses provide essential starting sample for further shock loading experiments.（2） Shock wave data on the pre-synthesized perovskite samples up to 107 GPa yielded a linear relationship between the shock wave velocity U_s and particle velocity u_p described by U_s=6.47（±0.63）+1.56（±0.31） u_p.Fitting experiment data to the Rankin-Hugoniot equation,we obtained the Grüneisen parameterγ₀= 1.33 with q=1.The best fitted values for the adiabatic bulk modulus K_0S and its pressure derivative K_0S’ are 254（±10） GPa and 3.9（±0.17）,respectively,which are in general agreement with values derived from static compression data.By direct comparison with dynamic compression data using enstatite as starting material,we observed that MgSiO₃ enstatite completely transformed into perovskite phase above 90GPa shock pressure.Improvement of the precision in determining the Hugoniot relationship by additional shock wave data is needed to further constrain the thermoelastic properties of perovskite.However,this is the first demonstration that direct shock wave loading of the pre-synthesized perovskite samples can provide a new way to determining the thermal equation of state of silicate perovskite without the complication of phase transformations along the Hugoniot path,ultimately leading to a better constrained thermoelastic parameters for this important mantle mineral.（3） High pressure melting experiments for Fe-S-C ternary system have been conducted on a piston-cylinder and multi-anvil press by using starting material with different Fe/（C+S） ratio from 3.5 to 20GPa and up to 1700K.For Fe（90wt%）-S（5 wt%）-C（5 wt%）,two immiscible C-rich and S-rich Fe-C-S liquids were observed at 3.5GPa,1873 K.At 5GPa,only one homogeneous liquid is quenched,which indicates the miscibility gap close between 3.5GPa and 5GPa.Pressure changes the subsolidus phase relations fundamentally.At 5GPa,Fe₃Ccrystallize first with temperature decrease and coexist with FeS.However,Fe₇C₃appear instead of Fe₃C between 10-20GPa,which means Fe₃C melt incongruently above 5GPa.Meanwhile,FeS melts incongruently into Fe₃S₂+liquid above 10GPa.In the contrast group of Fe（90w%）-C（w2%）-S（8w%）,Fe₃C,FeS and Fe are the major crystallized phases under solidus up to 10GPa.Given probable Earth’s core element composition of Fe-C-S,our results document no composition stratification is expected in the planetary core with P>5GPa.Based on the crystallization sequence a C-rich solid inner core and S-rich liquid outer core can be deduced for the Earth.（4） High-pressure behavior of orthorhombic MgSiO₃ perovskite crystal has been simulated using density functional theory and plane-wave pseudopotentials approach up to 120 GPa pressure at zero-temperature.The lattice constants and mass density of the MgSiO₃ crystal as functions of pressure were computed and the corresponding bulk modulus and bulk velocity were evaluated.Our theoretical results agree well with high-pressure experiments data.A thermodynamic method was introduced to correct the temperature effect on the 0 K first-principles results of bulk wave velocity,bulk modulus and mass density to lower mantle P/T range. Taking into account the temperature corrections,the corrected mass density,bulk modulus and bulk wave velocity of MgSiO₃-perovskite estimated from the first-principles results is 2%,4%,and 1% lower than Preliminary Reference Earth Model （PREM） profile,respectively,supporting MgSiO₃-perovskite primarily composed lower mantle model.（5） Relative stability of different phases for MgSiO₃ and (Mg_0.75,Fe_0.25)SiO₃ within 0-120GPa are investigated using first-principles method.For MgSiO₃,the computed equation of state for orthorhombic phase of Pbnm space group agrees well with experimental results.The relative stability reduces from observed Pbnm orthorhombic phase to intermediated tetragonal P4mbm phase,and then to hypothetical cubic Pm（?）m phase.For (Mg_0.75,Fe_0.25)SiO₃ ,same sequence of relative phase stability is observed.Thus,our work suggests the low-symmetric orthorhombic MgSiO₃ should be favored in the lower mantle condition.However,adding Fe into MgSiO₃ will make it less stable at the same depth.更多还原