MgO-FeO和Fe-Ni-S体系的高温高压研究及其地球物理意义
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摘要
钙钛矿MgSiO_3和镁方铁矿(Mg，Fe)O作为地球下地幔含量最丰富的候选矿物，其高温高压实验数据用来与地震学探测数据进行对比以限定下地幔的真实矿物学组分。本学位论文以冲击波动高压实验技术和多面体压砧静高压实验技术为高压加载手段，测量了下地幔重要候选矿物—镁方铁矿(Mg，Fe)O及MgO在下地幔底部高温高压条件(136GPa，5000K)下的状态方程，分析了可能存在的相变；研究了二元体系MgO-FeO和三元体系Fe-Ni-S的高温高压相图。这些研究，提升了对地球深部组分模型和矿物学模型的认识，并为解释下地幔底部地震波数据的异常现象提供了物理支持。
本文研究主要取得以下一些认识：
(1)由于天然的钙钛矿MgSiO_3和镁方铁矿(Mg，Fe)O很难得到，给这两种矿物的高温高压实验研究工作带来不便。本文分别用活塞圆筒(Piston-cylinder)和大腔体二级压砧(Mutli-anvil)技术在高温高压下合成了大块钙钛矿结构的MgSiO_3和不同Fe／Mg比的镁方铁矿(Mg，Fe)O样品，对所得到样品进行了微区电子探针、拉曼光谱和X射线衍射的分析。为高压实验特别是冲击压缩实验提供了满足实验要求的高压相初始样品。
(2) 测量了单晶MgO在114和192 GPa冲击压缩下的Hugoniot数据。结合前人的冲击波数据，揭示了沿MgO的P—V Hugoniot线在170±10 GPa存在体积不连续的本质。由于这一压力点对应的冲击温度仅为3000±400 K，大大低于熔化温度，从而排除了熔化引起体积变化的可能。因此我们认为Hugoniot线上1.9％的体积增加，是MgO从立方体结构的B1相(NaCl-type)向六角密堆积结构的B8相(NiAs-type)转变所引起。此结果进一步深化了对MgO在高温高压下相转变的认识，有重要的科学意义。
(3) 为了进一步对解释并验证对实验结果的分析，本文还在200 GPa的压力范围内用基于局域密度近似的第一性原理方法模拟考察了MgO在B1相(NaCl)、B2相(CsCl)、B4相(wurtzite)和B8相(NiAs)四种不同结构下的相对稳定性。结果表明，实验测得的MgO Hugoniot线上在170±10 GPa处(对应的温度约3000±400 K)发生的～2％体积跃变可以用MgO从B1相(NaCl)到B8相(NiAs)的结构相变来解释。通过对比计算结果和实验数据得到了在高
MgSiO_3 perovskite and ferropericlase (Mg,Fe)O are generally considered as the most dominant components in the Earth's lower mantle. Experimental measurements of P-V-T properties of Earth-related phases are crucial for developing accurate mineralogical and compositional models of the Earth's interior. Two stage gas-gun and multi-anvil apparatus are used to simulate high pressure and high temperature condition. This study focuses on the P-V-T equation of state and phase transition of (Mg,Fe)O at high pressure and high temperature. We also examine the phase relations of the MgO-FeO and Fe-Ni-S system at high pressure and high temperature and their implications in the composition of the Earth's interior. Our study improves our understanding mineralogical and compositional models of the Earth's interior, and also provides the explanations for the scattering of seismic waves and change in velocity gradient found in the lowermost mantle.
The main achievements in this study are as followings:
(1) We introduce the experimental methods to synthesize large bulk MgSiO_3 perovskite and ferropericlase (Mg,Fe)O using Piston-cylinder and Mutli-anvil apparatuses, and the synthesized samples were analyzed by micro electronic probe, Raman spectrum, and X-Ray diffraction. This makes it possible to do shock wave experiments with the large bulk MgSiO_3 perovskite and ferropericlase (Mg,Fe)O as starting materials.
(2) We report new shock-compression data for single-crystal MgO at 114 and 192 GPa. Our data together with the existing shock-wave data revealed a discontinuity at 170±10 GPa along the MgO Hugoniot. The estimated temperature at the discontinuity is about 3000±400 K, far too low to be melting. The discontinuity gives a volume increase of 1.9%, consistent with a phase transition from a NaCl structure (B1) to a high-temperature phase (most likely a NiAs-type hexagonal B8 phase) along the MgO Hugoniot. Our results add to fundamental understandings of the behavior of MgO, one of the most
本文研究主要取得以下一些认识：
(1)由于天然的钙钛矿MgSiO_3和镁方铁矿(Mg，Fe)O很难得到，给这两种矿物的高温高压实验研究工作带来不便。本文分别用活塞圆筒(Piston-cylinder)和大腔体二级压砧(Mutli-anvil)技术在高温高压下合成了大块钙钛矿结构的MgSiO_3和不同Fe／Mg比的镁方铁矿(Mg，Fe)O样品，对所得到样品进行了微区电子探针、拉曼光谱和X射线衍射的分析。为高压实验特别是冲击压缩实验提供了满足实验要求的高压相初始样品。
(2) 测量了单晶MgO在114和192 GPa冲击压缩下的Hugoniot数据。结合前人的冲击波数据，揭示了沿MgO的P—V Hugoniot线在170±10 GPa存在体积不连续的本质。由于这一压力点对应的冲击温度仅为3000±400 K，大大低于熔化温度，从而排除了熔化引起体积变化的可能。因此我们认为Hugoniot线上1.9％的体积增加，是MgO从立方体结构的B1相(NaCl-type)向六角密堆积结构的B8相(NiAs-type)转变所引起。此结果进一步深化了对MgO在高温高压下相转变的认识，有重要的科学意义。
(3) 为了进一步对解释并验证对实验结果的分析，本文还在200 GPa的压力范围内用基于局域密度近似的第一性原理方法模拟考察了MgO在B1相(NaCl)、B2相(CsCl)、B4相(wurtzite)和B8相(NiAs)四种不同结构下的相对稳定性。结果表明，实验测得的MgO Hugoniot线上在170±10 GPa处(对应的温度约3000±400 K)发生的～2％体积跃变可以用MgO从B1相(NaCl)到B8相(NiAs)的结构相变来解释。通过对比计算结果和实验数据得到了在高
MgSiO_3 perovskite and ferropericlase (Mg,Fe)O are generally considered as the most dominant components in the Earth's lower mantle. Experimental measurements of P-V-T properties of Earth-related phases are crucial for developing accurate mineralogical and compositional models of the Earth's interior. Two stage gas-gun and multi-anvil apparatus are used to simulate high pressure and high temperature condition. This study focuses on the P-V-T equation of state and phase transition of (Mg,Fe)O at high pressure and high temperature. We also examine the phase relations of the MgO-FeO and Fe-Ni-S system at high pressure and high temperature and their implications in the composition of the Earth's interior. Our study improves our understanding mineralogical and compositional models of the Earth's interior, and also provides the explanations for the scattering of seismic waves and change in velocity gradient found in the lowermost mantle.
The main achievements in this study are as followings:
(1) We introduce the experimental methods to synthesize large bulk MgSiO_3 perovskite and ferropericlase (Mg,Fe)O using Piston-cylinder and Mutli-anvil apparatuses, and the synthesized samples were analyzed by micro electronic probe, Raman spectrum, and X-Ray diffraction. This makes it possible to do shock wave experiments with the large bulk MgSiO_3 perovskite and ferropericlase (Mg,Fe)O as starting materials.
(2) We report new shock-compression data for single-crystal MgO at 114 and 192 GPa. Our data together with the existing shock-wave data revealed a discontinuity at 170±10 GPa along the MgO Hugoniot. The estimated temperature at the discontinuity is about 3000±400 K, far too low to be melting. The discontinuity gives a volume increase of 1.9%, consistent with a phase transition from a NaCl structure (B1) to a high-temperature phase (most likely a NiAs-type hexagonal B8 phase) along the MgO Hugoniot. Our results add to fundamental understandings of the behavior of MgO, one of the most
引文
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