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

知识价值：控制俯冲带的水预算对于理解许多地球过程非常重要，包括岛弧火山活动、中深地震和水向更深地幔的输送。也许最大的不确定性是当大洋板块进入海沟时上地幔的水化状态。虽然下行板块的最上层地幔所含的水量可能比俯冲的沉积物和水合地壳大得多，但在从洋脊到海沟的过程中，最初无水的海洋地幔的水合程度是未知的。地幔水化作用最流行和最具影响力的模型是由俯冲带外隆起弯曲过程中产生的断裂引起的流体流动引起的蛇纹石化。如果古大洋板块的水合作用深度达到45公里，这是由中深地震带的宽度和外隆起地震的深度所表明的，这将对地球的水预算产生重大影响。在本项目中，LABATTS实验收集的海底地震仪（OBS）数据将用于约束由蛇纹岩化引起的地幔地震速度变化。由于地震速度是蛇纹岩化程度的线性函数，因此在海沟附近绘制地震速度变化图将限制以蛇纹岩化地幔形式进入汤加俯冲带的水量。LABATTS的部署是唯一一个先前收集的数据集，它提供了在~ 45公里深处对来自旧海洋岩石圈通过海沟轴进入的海洋地幔的渐进蛇纹石化可能产生的影响进行成像的机会。从海沟延伸到汤加外隆起的太平洋板块300公里处的5个海底地震仪和1个陆地监测站（纽埃岛）的数据将用各种技术进行分析，以成像最上层海洋地幔的速度变化。地震波延迟时间将使用相互关联技术来确定，以评估由于上地幔速度变化而导致的总积分延迟时间的变化。与其他走时异常源相比，地震纵波和横波异常的比值将是蛇纹岩化的诊断。这项研究还将澄清地震断层的横向和深度程度，以及可能对流体渗透构成障碍的伸展和挤压地震活动之间的中性面深度。地震研究的结果将用于限制断层在进入板块的渐进式蛇纹石化中的作用。更广泛的影响：该项目支持一名年轻的研究员（Conder），以及一名研究生。该项目还将资助一名本科生研究2个暑假，为有前途的本科生提供研究经验。PI （Douglas Wiens）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生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，以便将来向研究人员开放。

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.