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

地幔是地球内部最大的组成层，它通过与构造板块的运动，创造和破坏相关的热对流不断混合。从火山岩中可以清楚地看出，这种混合并没有导致均匀的地幔成分，然而，很少有观测限制可以绘制地球地幔内的成分变化。该项目旨在测量地幔成分变化的范围，并绘制其在地球仪上的空间分布图。重点将是矿物橄榄石的分数的变化，因为它是最丰富的地幔矿物和变化，在其本地丰度改变地震反射边界的属性在约400公里的深度。该界面代表橄榄石晶体结构的变化，随着压力和温度的增加，橄榄石转变为硅镁石。将把确定橄榄石和华兹斯利石地震特性的实验室矿物物理实验与密集区域排列的地震数据分析结合起来，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，这是地球深部潜在的地球化学蓄水池。综合研究将为全球地幔混合和地幔产生火山活动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物理学和地震方法对学生科学家进行研究培训和教育。该项目将利用410公里长的不连续面的岩石学简单性，这是上地幔和过渡带之间的边界，以调查地幔体积和挥发性成分的空间变化。橄榄石-华兹斯利石转变是唯一能引起全球观测到的410的相变。水硅镁石的储水能力是橄榄石的5倍，结构水的存在对矿物的声速有很大的影响。因此，410的震级提供了一个很好的估计橄榄石含量和水浓度在410公里深度附近的地球内部。源岩中橄榄石含量高导致岩浆生成能力低，因此橄榄石含量是地幔岩石学的良好指标。此外，橄榄石相对于硅铝石也是强各向异性的，因此取决于橄榄石多晶型物的橄榄石含量和应变诱导的晶格优选取向，410也可以是尖锐的各向异性边界。研究人员还将使用由密集区域地震阵列的全球汇编收集的宽带P到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晶高压温度声速测量进行比较。拟议的研究的最终目标是估计的横向组成变化的橄榄石含量和水浓度410公里深度附近，和各向异性的速度跨越410不连续。大部分项目资金将支持由两名早期职业PI监督的跨学科学生团队。该项目将为研究生和本科生，特别是代表性不足的少数民族学生创造独特的机会，参与校园和最先进的同步加速器设施的科学研究。

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.