Mantle Serpentinization and Subduction of Water: Constraints from Seismological studies of the Tonga Outer Rise

智力优势：在俯冲带限制水预算是非常重要的了解许多地球过程，包括岛弧火山作用，中深地震，水的运输到更深的地幔。也许最大的不确定性是当海洋板块进入海沟时上地幔的水合状态。尽管下行板块最上层地幔中的含水量可能比俯冲沉积物和水合地壳中的含水量大得多，但最初无水的海洋地幔在从洋脊到海沟的过程中水合的程度尚不清楚。地幔水化作用的最流行和最有影响力的模型涉及流体流动的蛇纹石化沿着断层在弯曲期间产生的俯冲带的外部上升。如果古老的海洋板块被水化到45 km的深度，这将对地球的水收支产生重大影响。本项目将利用LABATTS实验收集的海底地震仪（OBS）数据来约束由于蛇纹石化而引起的地幔地震速度的变化。由于地震速度是蛇纹化程度的线性函数，绘制海沟附近的地震速度变化图将限制以蛇纹化地幔形式进入汤加俯冲带的水量。LABATTS部署是以前收集的唯一数据集，它提供了对~ 45公里深度进行成像的机会，这些深度可能是从旧海洋岩石圈通过海沟轴进入海洋地幔的渐进蛇纹石化作用的影响。将利用各种技术分析从海沟延伸到汤加外隆太平洋板块300公里处的五个海洋观测站和一个陆地站（纽埃岛）的数据，以便对最上层海洋地幔的速度变化进行成像。将使用互相关技术确定地震波延迟时间，以评估由于上地幔速度变化而引起的总综合延迟时间的变化。与其他走时异常源相比，地震压缩波和剪切波异常的比值将是蛇纹石化的诊断依据。这项研究还将阐明地震断层的横向和深度范围，以及可能对流体渗透构成障碍的拉伸和压缩地震活动之间的中性面的深度。地震研究的结果将被用来限制断层作用的进步serpentinization的传入plate.Broader影响：这个项目支持一个年轻的研究员（Conder），以及一个研究生。该项目还将支持一名本科生研究员2个夏天，为有前途的本科生提供研究经验。PI（道格拉斯威恩斯）在指导本科生方面有着良好的记录，他实验室的几位过去的本科研究人员已经获得了博士学位和教职。该项目还将确保LABATTS实验海底地震数据在IRIS数据管理中心妥善存档，以便今后向研究人员公开提供。

Intellectual Merit: Constraining the water budget at subduction zones is of great importance for understanding many Earth processes, including island arc volcanism, intermediate and deep earthquakes, and the transport of water to the deeper mantle. Perhaps the largest uncertainty is the hydration state of the upper mantle as the oceanic plate enters the trench. Although the amount of water contained in the uppermost mantle of the down-going plate is potentially much larger than in subducting sediments and hydrated crust, the degree to which initially anhydrous oceanic mantle is hydrated during its passage from ridge to trench is unknown. The most popular and influential model for mantle hydration involves serpentinization by fluid flow along faults produced during bending at the outer rise of subduction zones. If old oceanic plates are hydrated to depths of ~45 km as suggested by the width of intermediate-depth seismic zones and the depths of outer rise earthquakes it will have great implications for the earth's water budget.In this project ocean bottom seismograph (OBS) data collected by the LABATTS experiment will be used to constrain changes in mantle seismic velocity resulting from serpentinization. Since seismic velocity is a linear function of the degree of serpentinization, mapping the seismic velocity change near the trench will constrain the amount of water entering the Tonga subduction zone in the form of serpentinized mantle. The LABATTS deployment is the only previously collected dataset that offers the opportunity to image to depths of ~ 45 km the possible effects of progressive serpentinization of incoming oceanic mantle from old ocean lithosphere through the trench axis. Data from five OBSs and one land station (Niue Island) extending from the trench to 300 km onto the Pacific plate in the Tonga outer rise will be analyzed with various techniques to image velocity changes in the uppermost oceanic mantle. Seismic wave delay times will be determined using cross-correlation techniques to assess changes in total integrated delay time due to changes in upper mantle velocity. The ratio of seismic compressional and shear wave anomalies will be diagnostic of serpentinization as compared to other sources of travel time anomalies. This study will also clarify the lateral and depth extent of earthquake faulting, as well as the depth of the neutral surface between extensional and compressional seismicity that may pose a barrier to fluid penetration. The results of the earthquake study will be used to constrain the role of faulting in the progressive serpentinization of the incoming plate.Broader Impacts: This project supports a young researcher (Conder), as well as a graduate student. The project will also support an undergraduate researcher for 2 summers, providing research experiences for promising undergraduates. The PI (Douglas Wiens) has a strong record of mentoring undergraduate students, and several past undergrad researchers from his lab have gone on to PhDs and faculty positions. This project will also ensure that the LABATTS experiment ocean bottom seismic data are archived properly at the IRIS Data Management Center, to be made available openly to researchers in future.