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

地幔是地球内最大的成分层，与构造板块的运动、创造和破坏有关的热对流不断地混合在一起。从火山岩中可以清楚地看出，这种混合并没有导致均一的地幔成分，然而，几乎没有观测限制来绘制地球地幔内的成分变化图。该项目试图测量地幔成分变化的范围，并绘制它们在全球的空间分布图。焦点将是矿物橄榄石成分的变化，因为它是最丰富的地幔矿物，其当地丰度的变化改变了大约400公里深的地震反射边界的性质。该界面代表了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wadsleyite。确定橄榄石和瓦兹利石地震性质的实验室矿物物理实验将与密集区域阵列的地震数据分析相结合，以推断成分变化。除了估计橄榄石的比例外，该项目还将估计橄榄石中氢缺陷的丰度，这代表着地球深处潜在的水的地球化学库。这些联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和地震方法的学生科学家的研究培训和教育。该项目将利用上地幔和过渡带之间410公里不连续面的岩石学简单性，研究地幔体积和挥发性成分的空间变化。橄榄石-瓦兹利石的转变是唯一可能导致全球观测到的410的相变。水滑石的储水量大约是橄榄石的五倍，结构水的存在可以深刻地影响矿物的声速。因此，410的震级可以很好地估计出地球内部410公里深处的橄榄石含量和水的浓度。源岩中橄榄石含量高，产生岩浆的能力较弱，因此橄榄石含量是地幔岩石学的良好指标。此外，橄榄石相对于黄闪石也具有很强的各向异性，因此，根据橄榄石的含量和应变诱导的晶格择优取向，410也可以是一个尖锐的各向异性边界。调查人员还将使用由全球密集区域地震阵列汇编收集的宽带P至SV/SH散射信号，以更好地评估410附近的各向同性/各向异性地震构造。结果将与新的实验室单晶高压-温度声速测量进行比较。这项研究的最终目标是估计410千米深度附近橄榄石含量和水浓度的横向成分变化，各向异性速度跨越410个不连续面。大部分项目资金将支持一个由两名早期职业指导人员监督的跨学科学生团队。该项目将为研究生和本科生，特别是少数族裔学生，创造独特的机会，在校园和最先进的同步加速器设施中参与科学研究。

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.