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

对最快和最冷的俯冲带中的含水汤加板片进行成像

基本信息

批准号：
1842989
负责人：
Songqiao Wei
金额：
$ 17.94万
依托单位：
Michigan State University
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2019
资助国家：
美国
起止时间：
2019-04-15 至 2022-03-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1842989&HistoricalAwards=false
关键词：
Imaging hydrous Tonga slab fastest

项目摘要

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.

当构造板沿着海洋沟渠降落到地球的地幔中时 - 一种称为俯冲的过程 - 固体岩石和水中存储在主要液态矿物质中的水都被带入深地球。当托管水分解（脱水）时，大多数水将通过火山恢复到地球的表面，因为它们降低了更深的压力和温度。可以将痕量存储在所谓的无水矿物中的水保存在降板（俯冲板）中，以更深入，并参与整个地幔对流。这个过程称为地球的深水周期，它控制着地球内部的水通量。经过数十年的地震观测，数值模拟和岩石学/矿物学实验，人们普遍认为，平板脱水发生在海底几百公里深的深度。但是，一个基本问题仍未解决：俯冲的主要含水矿物如何深？含水矿物可能可以保存在较冷的平板中，因为脱水反应受温度和压力控制。汤加平板是沿着汤加沟渠下降的太平洋板块，被认为是地球上移动最快，最冷的板。因此，作为一个极端案例研究，对含水矿物进行成像并确定其在汤加平板中的最大深度对于理解俯冲带中的水通量和估计地球预算至关重要。该项目将为研究生和本科生提供研究机会。在这项研究中，通过分析当地地震的转化和指导的地震波来对Tonga Slab Crust和最高地幔进行成像，因为这些波浪对平板界面和内部结构最敏感。数据是从2009 - 2010年在汤加湖菲吉地区部署的局部宽带地震阵列中收集的，并对该俯冲带的局部地震性和地幔结构提供了前所未有的约束。汤加平板表面（上界面）位置和中间深度地震的相对位置将被完善。汤加平板内部在50-300 km的深度上的高分辨率地震结构将受到PS和引导P波的约束，从而提供了对这款最冷的平板中俯冲的液压矿物分布的见解。结果将有助于检验以下假设：汤加平板以及地球历史上其他类似的冷板可以将含水矿物载入大于250 km，甚至到地幔过渡区的深度。验证这一假设将对我们对地球深水周期的理解产生重要影响，因为仍然缺乏直接证明大量水被俯冲到地幔过渡区域的证据。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的智力和更广泛影响的评估来通过评估来支持的，这是值得的。