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

合作研究：调查刘综合研究场的热液活动分布及其地质和地球物理控制

• 批准号: 0732372
• 负责人: Joseph Resing
• 金额: --
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-09-01 至 2011-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0732372&HistoricalAwards=false
• 关键词: Collaborative; Research; Investigating; Distribution; Hydrothermal

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.

智力优点：海底扩张中心的热液活动在很大程度上控制着近轴热结构，从而控制着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，并且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顶部的一个非常狭窄的区域内。相比之下，快速传播中心的地震层析成像数据对脊轴下方的地壳低速体积 (LVV) 进行成像，该低速体积在距轴几公里 (2-4) 处迅速转变为较高速度。高地震速度梯度被解释为地壳主体从接近岩浆（熔化或部分熔化）到冷得多的侧翼温度的快速冷却区域，并且与大侧翼断层的出现大致一致。在高热梯度区域中，围绕热的部分熔融地壳体积的大断层非常接近，这表明这些区域是热液喷发的有利场所，正如一些数值模型和蛇绿岩地质研究（“化石”地质交叉）所预测的那样-暴露在陆地上的此类热液系统的部分）。然而迄今为止，还没有任何调查系统地检查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，我们建议进行一次密集的近海底调查，涵盖近轴区域并跨越东刘扩散中心的几个部分，以确定热液喷口的分布以及底层海底的地质和地球物理特征。该勘测将利用深拖式 DSL120 侧扫声纳和磁力计、一系列微型自主羽流记录仪 (MAPR)、现场化学扫描仪和 CTD 水力铸件。新型 MAPR 上的 Eh 传感器测量化学还原电位，因此对热液羽流敏感，与标准 MAPR 光学浊度和温度测量无关。该调查将以足够的分辨率获得该区域的完整声纳图像，以识别喷口尺度的地质结构，绘制周围水体中约 500 m 垂直范围内的热液羽流图，并确定已识别羽流的温度和化学特征。将在显着地点进行额外的垂直和“tow-yo”水力铸造，以更好地确定通风口位置并获取水柱和悬浮颗粒样本以进一步检查其化学成分。选定的调查区域包括劳国际空间站的 ABE 至基洛莫阿纳喷口焦点地点，并且是已知羽流发生率较高的区域。它跨越了从轴高到轴谷的过渡、岩浆透镜体到无岩浆透镜体的过渡、从火山为主到断层地形的变化、以及沿轴的玄武岩到安山岩熔岩的过渡。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以及海脊物理参数的这些较大变化相关。该调查将确定山脊侧面的分布和热液喷发的性质、其与海底结构的关联，并测试对比山脊地形的替代假设：a）热液活动高度限制在山脊顶部的一个非常狭窄的区域，或b）显着通风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：为调查选择的区域包括主要的 ABE 焦点和 Lau ISS 的基洛莫阿纳比较地点，因此调查数据将补充和方便这里的其他研究。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为 OBS 位置提供详细的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学可以直接与底层地震结构联系起来。重要的离轴通风的潜在发现将为研究开辟一类新的生物通风环境。离轴喷口可能由大型断层结构托管，因此可能更稳定、寿命更长，因此与构造不太稳定的山脊-顶部地点相比，更有可能托管大型且经济上重要的块状硫化物矿床。该项目还将构成研究生硕士或博士论文的主要部分，从而推进 NSF 教育和人力资源开发目标。