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.

地球的深部并不是直接可以研究的。关于其结构的最有指导意义的信息主要来自对地震波的地震学研究。对于不同类型的岩石，地震振动或地震波传播的速度是不同的，并且强烈地依赖于岩石的弹性性质。地震学数据提供了纵波(Vp)和横波(Vs)速度随深度的变化，而针对地球不同区域采集的数据显示，在地球410公里和660公里深处，纵波和横波速度异常迅速增加(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内，这两种地震速度都随着深度的增加而迅速增加。实验室岩石学研究表明，橄榄石[-(mg，Fe)2SiO4]是一种铁-镁硅酸盐，也是一种主要的地球矿物，在高压下会转变为更致密的物相(不同的晶体结构)、瓦兹利石[-(mg，Fe)2SiO4]和环木石[&amp；-(mg，Fe)2SiO4]。此外，地震观测到的410公里速度跃变归因于橄榄石向瓦兹利石晶体结构的转变，而瓦德斯利石向环木结构的转变则归因于520公里处的微小速度跃变。目前对地幔矿物的实验室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，水滑石和环木石可以在其晶体结构中以羟基(OH-)的形式包含高达2-3wt.%的H2O，这会影响它们的物理性质，如热导率和电导率，包括弹性波通过矿物的速度。这项研究拟制作含有受控构造水的水滑石和环木岩合成岩石样品，并测量含水样品的弹性波速，其温度和压力与地球-S过渡带内的条件相似。这项研究的数据将与地球-S地幔的地震速度剖面进行比较，以解决与地震速度跳跃的精确深度和幅度以及不连续面之间的速度梯度有关的长期存在的问题。结合岩石学和地球化学数据，研究结果可以显著提高我们对地球内部成分和结构的认识？S。这项建议是对弹性波速进行系统的测量，通过声波超声干涉技术来限制含水橄榄石(&amp；#945；-Mg2SiO4)及其高压晶型--瓦兹利石(&amp；#946；-Mg_2SiO_4)和环形木材(&amp；#947；-Mg_2SiO_4)的多晶样品的弹性性能，作为矿物中结构结合水(OH-)含量、压力(P)和温度(T)的函数。实验和理论研究表明，水滑石和环杉石可在其结构中以羟基(OH-)的形式掺入高达2-3wt.%的H2O，从而影响相的许多物理性质，包括其弹性性质。然而，尽管在地球-S上地幔和过渡带(410660公里深)中有丰富的矿物，但目前关于名义上无水地幔矿物的水化相弹性的数据很少。建议开展两项主要活动：(1)用X射线衍射仪、扫描电子显微镜、透射电子显微镜、电子探针分析、浸没密度和台式声速测量以及红外光谱和SIMS对高P和T超声研究前后的水分含量进行了详细的表征。(2)为了获得水化相的弹性体积(K)和剪切(G)模数与压力的精确关系，在高达10 Gpa的高压和T室T室中首次测量材料的弹性，然后在Argonne国家实验室的高级光子源(APS)的13ID光束线上同时测量到高达15 Gpa和中等温度至650K下的弹性特性，并结合对样品的X射线衍射分析。建议应用镁端元水化地幔相的弹性性质及其随压力(P)和温度(T)的变化，对410公里不连续面的深度和锐度提供更严格的约束，重新定义与过渡带520公里不连续面有关的与橄榄石-瓦兹利石和瓦兹利石-环木岩相变有关的速度跳跃，评估水在从地球-S地幔的地震层析研究中观察到的横向不均匀性中的作用，并从总体上提高我们对地球-S矿物学和化学成分的认识。