EAR-PF: Global Investigation of the Mantle from Shear Velocity Profiles with a Focus on the Water Content and Temperature of the Mantle Transition Zone

Michael Mann博士被授予EAR博士后奖学金，在布朗大学开展研究和教育计划。这个项目的重点是地幔过渡带（MTZ），这是一个在地球表面以下大约410公里和660公里处发现的层。人们对MTZ知之甚少，但认为它含有高浓度的水。由于MTZ边界深度深，确定其深度一直是一项困难的工作。MTZ如此之深的事实阻碍了对其内部和周围地震特性的高分辨率分析，并且在如此深度的观测产生了各种相互竞争的成分假设。曼恩博士将制作精确、高分辨率的地球表层到中带层的模型，这将有助于限制地球的深度（以及温度）、成分和水化状态（含水量）。这些性质对地球形成、固体地球水循环、地幔成分和对流的模型都很重要。拟议的项目将使用互补的数据集来生成地球（从表面到MTZ）的剖面，以更准确地确定温度，并调和不同构造区域下MTZ内部和周围水化成分和状态的相互竞争的假设。曼恩博士还将通过布朗大学本科生研究经验（REU）： 21世纪的动态地球参与指导本科生研究人员，并参加布朗大学谢里登教学与学习中心提供的教学和专业发展研讨会。该项目将利用接收函数的联合反演来确定从地表到MTZ的剪切速度剖面，接收函数对速度的急剧变化很敏感，而表面波相速度对宽深度范围内的速度很敏感。使用这些互补观测数据的标准分析只包含基模瑞利波相速度，它对地球最上层300-350公里的速度很敏感。这里使用的方法还包括表面波泛音，其灵敏度超出了MTZ深度（700公里）。这些速度剖面将限制世界范围内长寿命地震仪台站下的温度、含水量和成分。该项目旨在更好地解决MTZ的地震特性，包括MTZ底部附近的负速度梯度，并了解它们对现今含水量，温度和地幔成分的影响。最近的研究发现，在MTZ内部和周围的深度上，还有一些地震不连续面代表负速度梯度。MTZ底部附近的负速度梯度特别令人感兴趣，因为它被解释为热解地幔低温、俯冲板块亚稳相和MTZ底部的俯冲海洋地壳的结果。这些相互矛盾的假设不可能同时都是正确的。因此，有必要深入研究中波带地震速度和边界的范围和特征，以约束中波带结构。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dr. Michael Mann has been granted an EAR Postdoctoral Fellowship to carry out research and education plans at Brown University. This project focuses on the mantle transition zone (MTZ), a layer found approximately 410-km and 660-km below the Earth’s surface. The MTZ is poorly understood but is thought to incorporate high concentrations of water. Determining the depth to the MTZ boundaries has been difficult because of its deep depth. The fact that the MTZ is so deep has prevented high-resolution analyses of seismic properties within and around it, and observations at such depths have yielded a variety of competing compositional hypotheses. Dr. Mann will produce accurate, high-resolution models of Earth’s layers from the surface through the MTZ, which will help constrain depth (and therefore temperature), composition and state of hydration (water content). These properties are important for models of Earth formation, the solid Earth water cycle, and mantle composition and convection. The proposed project will use complementary datasets to generate profiles of the Earth (from surface to MTZ) to determine temperature more accurately, as well as reconcile competing hypotheses for the composition and state of hydration within and around the MTZ beneath different tectonic regions. Dr. Mann will also participate in mentoring undergraduate researchers through the Brown Research Experience for Undergraduates (REU): Dynamic Earth in the 21st Century and participate in teaching and professional development workshops offered by the Brown University Sheridan Center for Teaching and Learning. This project will determine shear velocity profiles from the surface through the MTZ using a joint inversion of receiver functions, which are sensitive to sharp changes in velocity, and surface wave phase velocities, which are sensitive to velocities over a broad range of depths. Standard analyses using these complementary observables incorporate only fundamental-mode Rayleigh wave phase velocities, which are sensitive to velocities in the uppermost 300-350 km of the Earth. The method used here will also incorporate surface wave overtones, which have sensitivities that extend beyond MTZ depths (700 km). These velocity profiles will constrain temperature, water content, and composition beneath long-lived seismometer stations worldwide. This project aims to better resolve the seismic properties of the MTZ, including negative velocity gradients near the base of the MTZ, and to understand their implications for the present-day water content, temperature, and composition of the mantle. Recent research has revealed additional seismic discontinuities that represent negative velocity gradients with depth both within and surrounding the MTZ. The negative velocity gradient near the bottom of the MTZ is of particular interest, as it has been interpreted as the result of low temperatures in a pyrolytic mantle, metastable phases in subducting slabs, and ponded subducted oceanic crust at the base of the MTZ. These conflicting hypotheses cannot all be true simultaneously. Hence, a deeper investigation into the extent and characteristics of MTZ seismic velocities and boundaries is necessary to constrain MTZ structure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.