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

知识价值：海底扩张中心的热液活动在很大程度上控制了近轴热结构，从而控制了洋中脊的重要物理特征。热液过程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。目前的模式认为近轴热液喷口集中在山脊顶部的一个非常狭窄的区域内。相比之下，在快速扩散中心的地震层析成像数据显示，在山脊轴下的地壳低速体（LVV）在距轴线几公里（2-4）处迅速过渡到更高的速度。高地震速度梯度被解释为地壳的大部分从岩浆附近（熔融或部分熔融）迅速冷却到更冷的侧翼温度的区域，并且与侧翼大断层的开始大致一致。在一个热梯度高的地区，大断层围绕着一个热的部分熔融的地壳体，这表明这些地区是热液喷口的有利地点，一些数值模型和蛇绿岩的地质研究也预测了这一点（这种热液系统的“化石”地质剖面，暴露在陆地上）。然而，到目前为止，还没有调查系统地检查了这些研究表明的热液喷口的横向范围和可能扩大的离轴范围。在这里，我们建议开展一个紧密间隔的近底测量，包括近轴区域和跨越东劳扩张中心的几个部分，以确定热液喷口的分布和下伏海底的地质和地球物理特征。该调查将利用深拖曳DSL120侧扫声纳和磁力计、一系列微型自主羽流记录仪（MAPRs）、现场化学扫描仪和CTD水模。新MAPR上的Eh传感器测量化学还原电位，因此对热液羽流敏感，独立于标准MAPR光学浊度和温度测量。该调查将获得该地区完整的声纳图像，其分辨率足以识别喷口规模的地质结构，绘制周围水体中~500 m垂直范围内的热液羽流，并确定已识别羽流的温度和化学特征。额外的垂直和“拖-yo”水模型将在突出的地点进行，以更好地解析喷口位置，并获得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劳国际空间站的ABE到基洛莫阿纳火山口焦点地点，并且是已知的高羽流发生率区域。它跨越了从轴向高向轴向谷的过渡、岩浆透镜体向无岩浆透镜体的过渡、火山为主向断陷地形的转变、玄武岩向安山岩熔岩沿轴向的过渡。因此，热液活动的性质和分布的任何变化都可以与海底的精细结构和这些较大的脊物理参数变化相关联。这次调查将确定山脊侧面的分布和热液喷口的性质，以及它与海底结构的关系，并在不同的山脊地形上测试不同的假设：a)热液活动高度局限于山脊顶部的一个非常狭窄的区域，或b)显著的喷口从山脊轴线延伸数公里。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。更广泛的影响：选定的调查区域包括主要的ABE焦点和洛洛莫阿纳国际空间站的比较地点，因此调查数据将补充和促进这里的其他研究。该地区也是计划进行地震层析成像实验的地点，旨在确定地壳和上地幔的速度结构。我们提出的调查不仅可以通过提供OBS位置的详细地质背景来帮助优化规划这一重大实验，而且一旦地震工作完成，绘制的羽流分布以及相关的地质和地球物理可以直接与底层地震结构联系起来。重要的离轴喷口的潜在发现将开启一类新的生物喷口环境的研究。离轴喷口可能由大型断层结构承载，因此可能更稳定，寿命更长，因此更有可能容纳大型和经济上重要的块状硫化物矿床，而不是构造不稳定的脊顶位置。该项目也将成为研究生硕士或博士学位论文的主要部分，从而推进国家科学基金会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.