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

地幔蛇纹石化和水俯冲：汤加外隆起地震学研究的限制

• 批准号: 0752476
• 负责人: Douglas Wiens
• 金额: $ 19.56万
• 依托单位: Washington University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-04-01 至 2011-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0752476&HistoricalAwards=false
• 关键词: Mantle; Serpentinization; Subduction; Water; Constraints

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.

智力优点：限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、中度和深层地震以及水向更深地幔的输送。也许最大的不确定性是大洋板块进入海沟时上地幔的水合状态。尽管下降板块最上层地幔中的水量可能远大于俯冲沉积物和水合地壳中的水量，但最初无水的大洋地幔在从山脊到海沟的过程中水合的程度尚不清楚。最流行和最有影响力的地幔水化模型涉及沿俯冲带外隆起弯曲期间产生的断层的流体流动导致的蛇纹石化。如果古老的海洋板块被水合到大约 45 公里的深度，正如中深度地震带的宽度和外隆地震的深度所表明的那样，这将对地球的水预算产生重大影响。在这个项目中，LABATTS 实验收集的海底地震仪 (OBS) 数据将用于限制蛇纹石化引起的地幔地震速度的变化。由于地震速度是蛇纹石化程度的线性函数，绘制海沟附近的地震速度变化图将限制以蛇纹石化地幔形式进入汤加俯冲带的水量。 LABATTS 部署是之前收集的唯一一个数据集，它提供了对约 45 公里深度的图像进行成像的机会，了解从旧海洋岩石圈通过海沟轴传入的海洋地幔逐渐蛇纹石化可能产生的影响。来自五个OBS和一个陆地站（纽埃岛）的数据将通过各种技术进行分析，以对最上层洋地幔的速度变化进行成像，这些数据从海沟延伸到汤加外隆起的太平洋板块300公里处。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，地震压缩波和剪切波异常的比率将诊断蛇纹石化。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伸展和压缩地震活动之间的中性面的深度。地震研究的结果将用于限制断层在传入板块的蛇纹石化过程中的作用。更广泛的影响：该项目支持一名年轻研究员（Conder）和一名研究生。该项目还将支持一名本科生研究员两个暑期，为有前途的本科生提供研究经验。 PI（道格拉斯·维恩斯）在指导本科生方面拥有良好的记录，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 数据管理中心正确存档，以便将来向研究人员开放。