Sound Wave Velocities and Elasticity of Hydrous Mantle Minerals at High Pressures and Temperatures.

高压和高温下含水地幔矿物的声波速度和弹性。

• 批准号: 1417024
• 负责人: Gabriel Gwanmesia
• 金额: $ 30万
• 依托单位: Delaware State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-07-15 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1417024&HistoricalAwards=false
• 关键词: Sound; Wave; Velocities; Elasticity; Hydrous

The deep interior of the Earth is not directly accessible to study. The most instructive information about its structure is mainly from seismological studies of earthquake waves. The speed at earth quake vibrations or waves travel is different for different types of rocks and strongly depends on the elastic properties of the rocks. The seismological data provides variations of compressional (Vp) and shear (Vs) wave velocities as a function of depth, and data taken for different regions of the Earth have revealed unusual rapid increases in the compressional and shear wave velocities (also called velocity discontinuities or velocity jumps) at depths of 410-km and 660- km in the Earth. The data also show that both seismic velocities increase rapidly with depth in the region between the two discontinuities, or the transition zone. Laboratory petrological studies demonstrate that olivine [α-(Mg,Fe)2SiO4], an iron-magnesium silicate and a major Earth mineral, transforms to denser phases (different crystal structures), wadsleyite [β-(Mg,Fe)2SiO4] and ringwoodite [γ-(Mg,Fe)2SiO4 at high pressures. Moreover, the seismically observed 410-km velocity jump has been ascribed to the change of olivine to the wadsleyite crystal structure, and the transition of wadsleyite to the ringwoodite crystal structure has been attributed to a minor velocity jump at 520-km. Current laboratory velocity measurements on the mantle minerals have mainly utilized dry or anhydrous samples to study and match the seismic data. However, experimental and theoretical studies indicate that wadsleyite and ringwoodite can incorporate up to 2-3 wt. % of H2O as hydroxyl (OH-) in their crystal structures that affect their physical properties such as thermal and electrical conductivities including the speed at which elastic waves travel through the minerals. The proposed study is to fabricate synthetic rock samples of wadsleyite and ringwoodite containing controlled structural water, and to measure the elastic wave velocities of the hydrous specimens, as a function of temperature and pressure similar to the conditions inside the Earth?s transition zone. Data from the study will be compared with the seismic velocity profiles of the Earth?s mantle, to address persistent questions related to the precise depth and magnitude of the seismic velocity jumps, as well as the velocity gradients between the discontinuities. Combined with petrological and geochemical data, the results of the study could significantly enhance our knowledge of the composition and structure of the Earth?s interior. The proposal is to conduct systematic measurements of the elastic wave velocities, to constrain the elastic properties of polycrystalline specimens of hydrous olivine (α-Mg2SiO4) and its high pressure polymorphs, wadsleyite (β-Mg2SiO4) and ringwoodite (γ- Mg2SiO4), as a function of the content of structurally bound water (OH-) in the mineral, pressure (P) and temperature (T), by acoustic ultrasonic interferometry techniques. Experimental and theoretical studies indicate that wadsleyite, and ringwoodite can incorporate up to 2-3 wt. % of H2O as hydroxyl (OH-) in their structures, and thus affecting many physical properties of the phases, including their elastic properties. However, despite their abundance in the Earth?s upper mantle and transition zone (410?660 km depth), there are currently very few data on the elasticity of the hydrated phases of the nominally anhydrous mantle minerals. Two primary activities are proposed: (1.) Hot-pressing of optimum acoustic-quality polycrystalline specimens characterized in detail by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron microprobe analysis, immersion density and bench-top acoustic velocity measurements, and IR spectroscopy and SIMS for quantifying the water content prior-to and after the high P and T ultrasonic studies. (2.) Initial measurement of the elasticity of the materials at high pressure up to 10 GPa, and room T, in a 1000-ton uniaxial split-cylinder apparatus (USCA-1000) of the Kawai-type, to obtain accurate pressure dependences of the elastic bulk (K) and shear (G) moduli for the hydrated phases, followed by measurement of the elastic properties at simultaneous high-pressure to 15 GPa and moderate temperature to 650 K, in conjunction with X-ray diffraction analysis of the sample, at the 13ID beam line of the Advanced Photon Source (APS), Argonne National Laboratory. It is proposed to apply the elastic properties and their variations with pressure (P) and temperature (T) for the Mg end-member hydrated mantle phases, to provide tighter constraints on the depth and sharpness of the 410-km discontinuity, to re-define the velocity jumps associated with the olivine to wadsleyite and the wadsleyite to ringwoodite phase transitions associated with the 520-km discontinuity in the transition zone, to assess the role of water in the lateral inhomogeneity observed from seismic tomographic studies of the Earth?s mantle, and in general to improve our understanding of the Earth?s mineralogical and chemical composition.

地球内部的深处是无法直接进入研究的。关于其结构的最有指导意义的信息主要来自地震学对地震波的研究。地震振动或波传播的速度对于不同类型的岩石是不同的，并且在很大程度上取决于岩石的弹性特性。地震学数据提供了作为深度函数的压缩波（Vp）和剪切波（Vs）速度的变化，对地球不同区域采集的数据显示，在地球410公里和660公里深度处，压缩波和剪切波速度异常快速增加（也称为速度不连续或速度跳跃）。数据还表明，在两个不连续面之间的区域或过渡带，两个地震速度都随深度迅速增加。实验室岩石学研究表明，橄榄石[#945;-（Mg，Fe）2SiO 4]，一种铁镁硅酸盐和一种主要的地球矿物，在高压下转变为致密相（不同的晶体结构），即瓦氏石[#946;-（Mg，Fe）2SiO 4]和林伍德石[#947;-（Mg，Fe）2SiO 4]。此外，地震观测到的410公里的速度跳已被归因于橄榄石的wadsleyite晶体结构的变化，和过渡的wadsleyite的ringwoodite晶体结构已被归因于一个轻微的速度跳在520公里。目前实验室对地幔矿物的速度测量主要是利用干燥或无水样品来研究和匹配地震数据。然而，实验和理论研究表明，硅铝石和硅铝石可以结合高达2-3 wt. %的H2O作为羟基（OH-）在其晶体结构中，影响其物理性质，如导热性和导电性，包括弹性波穿过矿物的速度。建议的研究是制造合成岩石样品的wadsleyite和ringwoodite含有控制结构水，并测量弹性波的含水标本的速度，作为一个函数的温度和压力类似的地球内部的条件？的过渡区。研究数据将与地球的地震速度剖面进行比较。的地幔，以解决有关的精确的深度和地震速度跳跃的大小，以及不连续之间的速度梯度持续存在的问题。结合岩石学和地球化学数据，研究结果可以大大提高我们对地球组成和结构的认识。的内部。 该建议是进行系统的测量的弹性波的速度，以约束的弹性性能的多晶试样的含水橄榄石（#945;-Mg 2SiO 4）和其高压多晶型，wadsleyite（#946;-Mg 2SiO 4）和ringwoodite（#947;-Mg 2SiO 4），作为一个功能的结构结合水（OH-）的含量在矿物，压力（P）和温度（T），通过声学超声干涉技术。实验和理论研究表明，wadsleyite和ringwoodite可以结合高达2-3重量%。%的H2O作为其结构中的羟基（OH-），并因此影响相的许多物理性质，包括其弹性性质。然而，尽管它们在地球上丰富？s上地幔和过渡带（410？660公里深度），目前很少有关于名义上无水地幔矿物水合相弹性的数据。建议开展两项主要活动：（1）热压最佳声学质量的多晶样品，其特征在于详细的X射线衍射，扫描电子显微镜（SEM），透射电子显微镜（TEM），电子探针分析，浸没密度和台式声速测量，红外光谱和西姆斯的定量水含量之前和之后的高P和T超声波研究。（2.）在10 GPa以下的高压和室温下，在1000吨单轴分裂圆柱装置中对材料弹性的初步测量（USCA-1000），以获得水合相的弹性体积模量（K）和剪切模量（G）的精确压力依赖性，然后在高压至15 GPa和中等温度至650 K的同时测量弹性性能，结合样品的X射线衍射分析，在阿贡国家实验室先进光子源（APS）的13 ID光束线上。建议将弹性性质及其随压力（P）和温度（T）的变化应用于Mg端元水合地幔相，以提供对410 km不连续性的深度和锐度的更严格约束，重新定义与过渡区中520 km不连续性相关的橄榄石到wadsleyite和wadsleyite到ringwoodite相变相关的速度跳跃，评估水在地球地震层析成像研究中观察到的横向不均匀性中的作用？的地幔，并在一般情况下，以提高我们对地球的了解？的矿物和化学成分。