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

当一个构造板块沿着海沟向下进入地幔时——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作用回到地球表面，当承载水的矿物质由于下沉更深时遇到的高压和高温而分解（脱水）时。储存在所谓无水矿物中的微量水可能在下降板块（俯冲板块）中保存到更深处，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通量。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，在海底以下几百公里的深度发生了板块脱水。然而，一个基本的问题仍然没有解决：主要的含水矿物能被俯冲到多深？含水矿物可以在较冷的板块中保存到更深的地方，因为脱水反应是由温度和压力控制的。汤加板块是沿汤加海沟向下延伸的太平洋板块，被认为是地球上移动最快、最冷的板块。因此，作为一个极端案例研究，对汤加板块的含水矿物进行成像并确定其最大深度，对于了解俯冲带的水通量和估计地球的水收支至关重要。该项目将为研究生和本科生提供研究机会。在本研究中，汤加板块地壳和上地幔将通过分析当地地震的转换和引导地震波进行成像，因为这些地震波对板块界面和内部结构最为敏感。这些数据是2009-2010年在汤加-劳-斐济地区部署的本地宽带地震阵列收集的，为该俯冲带的本地地震活动性和地幔结构提供了前所未有的约束。汤加板块面（上界面）位置和50-200 km深度的中深度地震的相对位置将得到细化。汤加板块内部深度50-300公里的高分辨率地震结构将受到PS波和引导P波的约束，这将为了解这个最冷板块中俯冲含水矿物的分布提供洞见。这一结果将有助于验证汤加板块和地球历史上其他类似的冷板块可以将含水矿物带到250公里以下的深度，甚至到地幔过渡带的假设。验证这一假设将对我们对地球深水循环的理解产生重要影响，因为仍然缺乏大量水被俯冲到地幔过渡带的直接证据。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。

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.