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

智力价值：限制俯冲带的水收支对于了解许多地球过程非常重要，包括岛弧火山作用、中深地震以及水向地幔深处的输送。也许最大的不确定性是大洋板块进入海沟时上地幔的水化状态。尽管下行板块最上部地幔中的水含量可能比俯冲沉积物和水化地壳中的水含量大得多，但最初无水的大洋地幔在从山脊到海沟的过程中水化的程度尚不清楚。最流行和最有影响力的地幔水化模式涉及到俯冲带外部隆起的弯曲过程中产生的沿断层的流体流动产生的蛇纹岩作用。如果旧的大洋板块被水化到45公里以下，如中深地震带的宽度和外部隆起地震的深度所表明的那样，这将对地球的水收支有很大的意义。在这个项目中，由Labatts实验收集的海底地震仪(OBS)数据将被用来约束由于蛇纹岩作用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的部署是之前收集的唯一一个数据集，它提供了在~45公里深处成像来自旧海洋岩石圈的传入大洋地幔通过海槽轴逐渐蛇纹岩化的可能影响。来自汤加外隆起从海沟延伸到太平洋板块300公里的五个OBSS和一个陆地站(纽埃岛)的数据将用各种技术进行分析，以成像最上面的大洋地幔的速度变化。将使用互相关技术来确定地震波延迟时间，以评估由于上地幔速度的变化而引起的总积分延迟时间的变化。与其他走时异常的来源相比，地震纵横波异常的比率将是蛇纹岩化的诊断依据。这项研究还将澄清地震断层的横向和深度程度，以及伸展和挤压地震活动之间可能构成流体渗透障碍的中性面的深度。地震研究的结果将被用来限制断层在即将到来的板块逐渐蛇纹化中的作用。广泛的影响：这个项目支持一名年轻的研究人员(Conder)，以及一名研究生。该项目还将支持一名本科生研究人员两个暑期，为有前途的本科生提供研究经验。PI(道格拉斯·维恩斯)在指导本科生方面有着很好的记录，他实验室的几名过去的本科生研究人员已经进入博士和教职员工的职位。该项目还将确保将Labatts实验海底地震数据妥善地存档在IRIS数据管理中心，以便将来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.