CSEDI: Compositional heterogeneity and seismic anisotropy near the 410 km discontinuity

CSEDI：410公里间断面附近的成分异质性和地震各向异性

• 批准号: 1664471
• 负责人: Jin Zhang
• 金额: $ 36.99万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-07-01 至 2022-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1664471&HistoricalAwards=false
• 关键词: CSEDI; Compositional; heterogeneity; seismic; anisotropy; CSEDI; Compositional; heterogeneity; seismic; anisotropy

The mantle is the largest compositional layer within Earth and it is continually mixed by thermal convection associated with the movement, creation, and destruction of tectonic plates. It is clear from volcanic rocks that this mixing has not resulted in a homogeneous mantle composition, however, there are few observational constraints that allow mapping of the compositional variations within Earth's mantle. This project seeks to measure the range of compositional variations in the mantle and map their spatial distribution across the globe. The focus will be variations in the fraction of the mineral olivine because it is the most abundant mantle mineral and variations in its local abundance change the properties of a seismically reflective boundary at about 400 km depth. The interface represents a change in the crystal structure of olivine, which transforms to wadsleyite as pressure and temperature increase with depth. Laboratory mineral physics experiments to determine the seismic properties of olivine and wadsleyite will be combined with analyses of seismic data from dense regional arrays to infer compositional variations. In addition to estimating the fraction of olivine, the project will also estimate the abundance of hydrogen defects in olivine, which represent a potential geochemical reservoir of water in the deep Earth. The combined studies will provide novel insights into global mantle mixing and spatial variations in the ability of the mantle to generate volcanic activity. Grant funding will primarily be used to support research training and education of student scientists using modern mineral physics and seismic methods.This project will take advantage of the petrological simplicity of the 410 km discontinuity, which is the boundary between the upper mantle and the transition zone, to investigate spatial variations in the bulk and volatile composition of the mantle. The olivine-wadsleyite transition is the only phase transition that can cause the globally observed 410. Wadsleyite has about five times greater water storage capacity than olivine and the existence of structural water can profoundly affect sound velocities of minerals. Therefore the magnitude of the 410 provides a good estimation of the olivine content and water concentration near 410 km depth in the Earth's interior. High olivine content in the source rock results in less capability of producing magma, thus olivine content is a good indicator of mantle petrology. In addition, olivine is also strongly anisotropic relative to wadsleyite, so depending on the olivine content and strain-induced lattice-preferred orientation of olivine polymorphs, the 410 can be a sharp anisotropic boundary as well. The investigators will also use broadband P to SV/SH scattering signals collected by a global compilation of dense regional seismic arrays to better assess the isotropic/ anisotropic seismic structures near the 410. The results will be compared with new laboratory single-crystal high pressure-temperature sound velocity measurements. The ultimate goal of the proposed study is to estimate the lateral compositional variations in terms of olivine content and water concentration near 410 km depth, and the anisotropic velocity jumps across the 410 discontinuity. The majority of project funding will support an interdisciplinary team of students supervised by two early career PIs. This project will create unique opportunities for graduate and undergraduate students, especially underrepresented minority students, to participate in scientific research both on campus and at the state-of-the-art synchrotron facilities.

地幔是地球内最大的组成层，它不断被与构造板的运动，创造和破坏相关的热对流混合。从火山岩中可以明显看出，这种混合并未导致均匀的地幔成分，但是，很少有观察性约束允许映射地球披风内的组成变化。该项目旨在衡量地幔中构图变化的范围，并在全球范围内绘制其空间分布。重点将是矿物橄榄石比例的变化，因为它是最丰富的地幔矿物质，其局部丰度的变化改变了地震反射性边界在约400 km深度的性质。该界面代表橄榄石晶体结构的变化，随着压力和温度随着深度的增加而变为wadsleyite。实验室矿物质物理学实验，以确定橄榄石和瓦德斯利岩的地震特性，将与来自密集区域阵列的地震数据的分析相结合，以推断成分变化。除了估计橄榄石的分数外，该项目还将估计橄榄石中氢缺陷的丰度，这代表了深层水中潜在的地球化学水库。合并的研究将为全球地幔混合和地幔产生火山活性的能力的空间变化提供新的见解。赠款资金将主要用于支持使用现代矿物质物理学和地震方法对学生科学家的研究培训和教育。本项目将利用410公里的不连续性的岩石学简单性，这是上层地幔与过渡区之间的边界，以研究大块和旋转的熔炉组成的空间变化。橄榄石 - 岩石晶体过渡是唯一可以引起全球观察到的410的相变。Wadsleyite具有比橄榄石大约五倍的供水能力，而结构水的存在可能会深远影响矿物质的声速。因此，410的大小可很好地估计地球内部410 km深度的橄榄石含量和水浓度。源岩石中的高橄榄石含量会导致产生岩浆的能力，因此橄榄石含量是地幔质学的良好指标。此外，橄榄石相对于wadsleyite也是强烈的各向异性，因此取决于橄榄石含量和应变诱导的橄榄石多晶型晶格优先定位，410也可以是尖锐的各向异性边界。研究人员还将使用宽带P进行通过密集的区域地震阵列的全球汇编收集的SV/ SH散射信号，以更好地评估410附近的各向同性/各向异性地震结构。结果将与新的实验室单个实验室单一晶体高压高压式声音速度测量。拟议研究的最终目标是估计橄榄石含量和水浓度接近410 km深度的横向成分变化，各向异性速度越过410不连续性。大多数项目资金将支持由两个早期职业生涯监督的学生组成的跨学科团队。该项目将为研究生和本科生，尤其是代表性不足的少数民族学生创造独特的机会，以参加校园和最先进的同步设施的科学研究。