Imaging the hydrous Tonga slab in the fastest and coldest subduction zone

当一个构造板块沿着沿着一条海沟下降到地球的地幔中时--这一过程被称为俯冲--固体岩石和储存在主要含水矿物中的水都被带入地球深处。大部分的水将通过火山活动返回地球表面，因为当它们下降到更深处时，由于高压和高温，承载水的矿物质分解（脱水）。储存在所谓无水矿物中的微量水可能保存在下降板块（俯冲板片）中更深的地方，并参与整个地幔对流。这个过程被称为地球的深水循环，它控制着地球内部的水流量。经过几十年的地震观测、数值模拟和岩石学/矿物学实验，人们普遍认为，板块脱水发生在海底以下几百公里深处。然而，一个基本问题仍然没有解决：主要的含水矿物可以俯冲到多深？含水矿物大概可以在较冷的板块中保存到更深的地方，因为脱水反应受温度和压力控制。汤加板块是太平洋板块，沿着汤加海沟沿着移动，被认为是地球上移动最快和最冷的板块。因此，成像含水矿物和确定其最大深度在汤加板块，作为一个极端的案例研究，是至关重要的了解俯冲带的水通量和估计地球的水预算。该项目将为研究生和本科生提供研究机会。在这项研究中，汤加板块地壳和上地幔将通过分析转换和引导地震波从当地地震成像，因为这些波是最敏感的板块界面和内部结构。这些数据是从2009-2010年在汤加-劳-斐济地区部署的当地宽带地震阵列收集的，并对该俯冲带的当地地震活动和地幔结构提供了前所未有的限制。汤加板块表面（上界面）的位置和50-200公里深处中深度地震的相对位置将得到改进。深度为50-300公里的汤加板块内部的高分辨率地震结构将受到PS和导波的约束，从而深入了解这一最冷板块中俯冲含水矿物的分布。这些结果将有助于检验这样一种假设，即汤加板块，沿着与地球历史上其他类似的冷板块，可以携带含水矿物到250公里以上的深度，甚至到地幔过渡带。这一假设将对我们理解地球的深水循环产生重要影响，因为仍然缺乏直接证据证明大量的水被俯冲到地幔过渡带。这一奖项反映了NSF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。

When a tectonic plate descends into Earth's mantle along an oceanic trench - a process called subduction - both solid rocks and water stored in major hydrous minerals are carried into the deep earth. Most of the water will return to the surface of Earth through volcanism when the minerals hosting the water break down (dehydration) because of the high pressure and temperature encountered as they descend deeper. Trace amounts of water stored in so called anhydrous minerals may be preserved in the descending plate (subducting slab) to greater depths and get involved in whole mantle convection. This process is called Earth's deep-water cycle, which controls the water flux in Earth's interior. After decades of seismic observations, numerical simulations, and petrological/mineralogical experiments, it is generally agreed that slab dehydration takes place at a few hundred kilometers depth below the seafloor. However, a fundamental question remains unsolved: How deep can major hydrous minerals be subducted? Hydrous minerals presumably can be preserved to greater depths in a colder slab because dehydration reactions are controlled by temperature and pressure. The Tonga slab, which is the Pacific Plate that descends along the Tonga Trench, is believed to the fastest moving and coldest slab on Earth. Therefore, imaging hydrous minerals and determining their maximum depths in the Tonga slab, as an extreme case study, are critical to understanding the water flux in subduction zones and estimating Earth's water budget. This project will provide research opportunities for both graduate and undergraduate students.In this study, the Tonga slab crust and uppermost mantle will be imaged by analyzing converted and guided seismic waves from local earthquakes because these waves are most sensitive to slab interface and internal structure. The data were collected from a local broadband seismic array deployed in 2009-2010 in the Tonga-Lau-Fiji region, and provide unprecedented constraints on local seismicity and mantle structure of this subduction zone. The Tonga slab surface (upper interface) position and relative locations of intermediate-depth earthquakes at depths of 50-200 km will be refined. The high-resolution seismic structure of the Tonga slab interior at depths of 50-300 km will be constrained by PS and guided P waves, providing insights into the distribution of the subducted hydrous minerals in this coldest slab. The results will help to test the hypothesis that the Tonga slab, along with other similarly cold slabs in the history of Earth, can carry hydrous minerals to depths greater than 250 km, or even to the mantle transition zone. Verifying this hypothesis will have important impacts on our understanding of the Earth's deep-water cycle because direct evidence of a significant amount of water being subducted into the mantle transition zone is still lacking.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.