Collaborative Research: Investigating the Distribution of Hydrothermal Activity and its Geologic and Geophysical Controls at the Lau Integrated Studies Site

智力优势：海底扩张中心的热液活动在很大程度上控制着近轴的热结构，从而控制着大洋中脊的重要物理特征。热液作用还维持着多样化的生物生态系统，形成矿藏，是固体地球和海洋之间化学和能量流动的重要组成部分。目前的模型认为，近轴热液喷发集中在山脊的一个非常狭窄的区域。相反，在快速扩展中心的地震层析成像数据显示了山脊轴线下方的地壳低速体积(LVV)，它在距离山脊轴线几公里(2-4)的地方迅速过渡到更高的速度。高地震速度梯度被解释为地壳主体从接近岩浆(熔融或部分熔融)到更低的侧向温度的快速冷却区，与大型侧向断层的开始大致一致。一些数值模型和蛇绿岩(这种热液系统的“化石”地质横截面，暴露在陆地上)的地质研究也预测，在高温度梯度带中，围绕热部分熔融地壳体积的大断层非常接近，这表明这些地区是热液喷发的有利地点。然而，到目前为止，还没有一项调查系统地检查这些研究表明的热液喷发的横向范围和可能扩大的离轴范围。在这里，我们建议进行一项围绕近轴线地区的近海底调查，跨越东部刘扩张中心的几个区段，以确定热液喷发的分布以及海底的地质和地球物理特征。调查将利用深拖曳DSL120侧扫声纳和磁强计、一组微型自主羽流记录器(MAPR)、一台现场化学扫描仪和CTD水铸。新型MAPR上的EH传感器测量化学还原电位，因此独立于标准MAPR光学浊度和温度测量，对热液羽流敏感。调查将获得该地区的完整声纳图像，其分辨率足以确定喷口规模的地质结构，绘制周围水域范围内约500米垂直条带内的热液羽流图，并确定已确定的羽流的温度和化学特征。为了更好地确定喷口的位置，将在显眼的地点进行更多的垂直和“拖溜溜”水铸，并获取水柱和悬浮颗粒样本，以进一步检查它们的化学成分。选定的调查区域包括刘国际空间站的Abe至Kilo Moana喷口焦点地点，是已知的烟羽高发区。它跨越了从轴向高地到轴谷的过渡、岩浆透镜向无岩浆透镜的过渡、火山为主向断裂地体的过渡、玄武岩向安山质熔岩的轴向过渡。因此，热液活动性质和分布的任何变化都可以与海底的细微结构以及海脊物理参数的这些较大变化相关联。这项调查将确定海脊侧面的分布和热液喷发的性质及其与海底结构的关系，并检验不同海脊地形之间的替代假设，即a)热液活动高度局限于海脊顶部的一个非常狭窄的区域，或b)显著的喷口从海脊轴线延伸数公里。解决这些替代假设将从根本上影响我们对热液活动如何控制脊轴热结构的理解。广泛的影响：选定的调查区域包括LAU ISS的主要Abe Focus和Kilo Moana比较地点，因此调查数据将补充和促进这里的其他研究。该地区也是一个计划中的地震层析成像实验的地点，该实验旨在确定地壳和上地幔的速度结构。我们拟议的调查不仅将通过为OBS位置提供详细的地质背景来帮助优化计划这一重大实验，而且一旦地震工作完成，绘制的烟柱分布以及相关的地质和地球物理可以直接与潜在的地震结构联系起来。重要的离轴喷口的潜在发现将为研究开辟一类新的生物喷口环境。离轴喷口可能由大型断层构造托管，因此可能更稳定、寿命更长，因此与构造不太稳定的山脊-山顶地点相比，更有可能拥有大型和经济上重要的块状硫化物矿床。该项目还将成为研究生硕士或博士学位论文的主要部分，从而促进NSF的教育和人力资源发展目标。

Intellectual Merit: Hydrothermal activity at seafloor spreading centers largely controls the near-axis thermal structure and thus the important physical characteristics of the mid-ocean ridge. Hydrothermal processes also sustain a diverse biological ecosystem, form mineral deposits, and are important components of chemical and energy flux between the solid Earth and the oceans. Current models view near-axis hydrothermal venting as concentrated within a very narrow region at the ridge crest. In contrast, seismic tomographic data at fast spreading centers image a crustal low velocity volume (LVV) underlying the ridge axis which rapidly transitions to higher velocities a few km (2-4) from the axis. The high seismic velocity gradients are interpreted as zones of rapid cooling of the bulk of the crust from near magmatic (melted or partially-melted) to much cooler flanking temperatures and broadly coincide with the onset of large flanking faults. The close proximity of large faults surrounding a hot partly molten crustal volume in a zone of high thermal gradients suggests that these areas are favorable sites for hydrothermal venting, as is also predicted by some numerical models and geologic studies at ophiolites ("fossil" geological cross-sections of such hydrothermal systems, exposed on land). Yet to-date no survey has systematically examined the lateral extent and possibly expanded off-axis range of hydrothermal venting suggested by these studies. Here we propose to carry out a closely-spaced near-bottom survey encompassing the near axis region and spanning several segments of the Eastern Lau Spreading Center to determine the distribution of hydrothermal venting and the geologic and geophysical characteristics of the underlying seafloor. The survey will utilize the deep-towed DSL120 side-scan sonar and magnetometer, an array of miniature autonomous plume recorders (MAPRs), an in-situ chemical scanner and CTD hydrocasts. Eh sensors on the new MAPRs measure chemical reduction potential and are thus sensitive to hydrothermal plumes independently of standard MAPR optical turbidity and temperature measurements. The survey will achieve complete sonar imagery of the area at a resolution sufficient to identify geologic structures at the vent scale, map hydrothermal plumes within a ~500 m vertical swath within the surrounding water volume, and determine temperature and chemical characteristics of identified plumes. Additional vertical and "tow-yo" hydrocasts will be made at prominent sites to better resolve vent locations and obtain water column and suspended particulate samples to further examine their chemistry. The selected survey area encompasses the ABE to Kilo Moana vent focus sites of the Lau ISS and is an area of known high plume incidence. It spans the transition from axial high to axial valley, the magma lens to no magma lens transition, the change from volcanically dominated to faulted terrain, and the basaltic to andesitic lava transition along axis. Thus any changes in the nature and distribution of hydrothermal activity can be correlated with the fine-scale structure of the seafloor and with these larger changes in ridge physical parameters. The survey will determine the ridge flank distribution and nature of hydrothermal venting, its association with seafloor structures and test the alternative hypotheses across contrasting ridge terrains that a) hydrothermal activity is highly confined to a very narrow region of the ridge crest or b) that significant venting extends several km from the ridge axis. Resolving these alternative hypotheses will fundamentally affect our understanding of how hydrothermal activity controls ridge axis thermal structure.Broader Impacts: The area selected for the survey encompasses the primary ABE focus and Kilo Moana comparison sites of the Lau ISS and thus the survey data will compliment and facilitate other studies here. This area is also the site of a planned seismic tomography experiment that aims to determine the crustal and upper mantle velocity structure. Not only will our proposed survey help to optimally plan this major experiment by providing detailed geologic context for OBS locations but once the seismic work is completed the mapped plume distribution and correlated geology and geophysics can be directly linked with the underlying seismic structure. The potential discovery of significant off axis venting will open a new class of biological vent environments for study. Off-axis vents would likely be hosted by large fault structures and therefore potentially be more stable and longer-lived and thus more likely to host large and economically important massive sulfide deposits than less tectonically stable ridge-crest sites. The project will also form a major part of a Master's or PhD level thesis for a graduate student and thus advance NSF educational and human resource development goals.