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: 0732536
• 负责人: Fernando Martinez
• 金额: $ 41.04万
• 依托单位: University of Hawaii
• 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 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0732536&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.

智力优势：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，从而控制着大洋中脊的重要物理特征。热液过程还维持着多样的生物生态系统，形成矿藏，是固体地球与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，近轴热液喷口集中在脊顶一个非常狭窄的区域内。相反，在快速扩展中心的地震层析成像数据的图像地壳低速体（LVV）的山脊轴下，迅速过渡到更高的速度几公里（2-4）的轴。高地震速度梯度被解释为区域的快速冷却的大部分地壳从附近的岩浆（熔融或部分熔融）到更冷的侧翼温度和大致符合大型侧翼故障的发病。在高热梯度区中，围绕着部分熔融的热地壳体积的大断层非常接近，这表明这些地区是热液喷口的有利场所，一些数值模型和在Ophitheum的地质研究也预测到了这一点（这种热液系统的“化石”地质剖面，暴露在陆地上）。然而，迄今为止，还没有任何调查系统地审查这些研究所提出的热液喷口的横向范围和可能扩大的离轴范围。在这里，我们建议进行一次近底密集调查，包括近轴区域，并跨越东劳扩散中心的几个部分，以确定热液喷口的分布以及下伏海底的地质和地球物理特征。调查将利用深拖DSL 120侧扫声纳和磁力计、一系列微型自主羽流记录仪、一台现场化学扫描仪和CTD水试。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，绘制周围水体内约500米垂直带内的热液羽流图，并确定已确定羽流的温度和化学特性。将在突出地点进行额外的垂直和“拖溜球”水力预测，以更好地解决喷口位置，并获得水柱和悬浮颗粒样品，以进一步检查其化学性质。选定的调查区域包括Lau国际空间站的ABE至Kilo Moana喷口焦点地点，是一个已知羽流发生率高的区域。它跨越了从轴向高到轴向谷的过渡、岩浆透镜到无岩浆透镜的过渡、从火山岩为主到断裂地形的变化以及沿着轴向的玄武质熔岩到安山质熔岩的过渡。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联系起来。这项调查将确定海脊侧面的分布情况和热液喷口的性质及其与海底结构的关系，并在对比鲜明的海脊地形上检验各种不同的假设，即：（a）热液活动高度局限于海脊顶部一个非常狭窄的区域，或（B）重大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的理解热液活动如何控制脊轴thermal structure.Broader影响：该地区选定的调查包括主要ABE焦点和基洛莫阿纳比较网站的刘国际空间站，因此调查数据将补充和促进其他研究在这里。该地区也是计划进行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。我们提出的调查不仅有助于通过为OBS位置提供详细的地质背景来最佳地规划这一重大实验，而且一旦地震工作完成，所绘制的羽流分布和相关的地质和地球物理学可以与底层地震结构直接联系起来。显着离轴喷口的潜在发现将开辟一类新的生物喷口环境供研究。离轴喷口可能位于大型断层结构中，因此可能更稳定，寿命更长，因此与构造不太稳定的脊顶地点相比，更有可能容纳大型和具有经济重要性的块状硫化物矿床。该项目还将成为硕士或博士学位论文的主要部分，从而推进NSF的教育和人力资源开发目标。