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

迈克尔·曼博士获得了EAR博士后奖学金，在布朗大学开展研究和教育计划。该项目的重点是地幔过渡带(MTZ)，这是一个在地球表面以下约410公里和660公里处发现的层。人们对MTZ知之甚少，但人们认为它包含了高浓度的水。确定MTZ边界的深度一直很困难，因为它的深度很深。MTZ如此之深的事实阻碍了对其内部和周围地震性质的高分辨率分析，在这样的深度进行的观测产生了各种相互竞争的成分假设。曼博士将制作从地表到MTZ的精确、高分辨率的地球各层模型，这将有助于限制深度(因此也就是温度)、成分和水合状态(水含量)。这些性质对于地球形成、固体地球水循环、地幔组成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(从地表到MTZ)的轮廓，以更准确地确定温度，并协调关于不同构造区域下MTZ内部和周围的水合成分和状态的相互矛盾的假设。曼博士还将通过布朗大学本科生研究体验(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丹教学中心提供的教学和专业发展研讨会。该项目将利用对速度急剧变化敏感的接收函数和对大范围深度上的速度敏感的面波相速度进行联合反演，确定从地表通过MTZ的剪切速度剖面。使用这些互补观测值的标准分析只包含基模瑞雷波相速度，它对地球最上面300-350公里处的速度很敏感。这里使用的方法还将包括面波泛音，它具有超出MTZ深度(700公里)的灵敏度。这些速度剖面将限制世界各地长期使用的地震台站下的温度、水含量和成分。该项目旨在更好地解决MTZ的地震性质，包括MTZ底部附近的负速度梯度，并了解它们对现今地幔的水含量、温度和组成的影响。最近的研究揭示了更多的地震不连续性，这些不连续性代表了MTZ内部和周围的负速度梯度。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的低温、俯冲板块中的亚稳相以及MTZ底部的滞留洋壳的结果。这些相互矛盾的假设不可能同时都成立。因此，有必要对MTZ地震速度和边界的范围和特征进行更深入的调查，以限制MTZ结构。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.