Study of Magmatic and Hydrothermal Processes in an Ultramafic Setting (Rainbow, Mid-Atlantic Ridge) Using Advanced Seismic Modeling and Imaging

洋壳形成于大洋中脊，岩浆从地幔上升并冷却。为了冷却这种融化，海水穿透海洋地壳，导致地壳岩石和海洋发生许多化学变化。因此，“热液”流体流动是地球深处与海洋之间质量和热量传递的主要机制，约占全球每年热损失的三分之一。特别有趣的是，热液系统存在于从更深的地幔中升起的岩石中，由于它们的成分而被称为“超镁铁质”岩石。这是因为：(1)产生这种地幔暴露的机制尚未完全了解；(2)这些超镁质岩石具有封存CO2的潜力；(3)超镁质岩石水化产生的流体可以创造独特的化学环境，这对地球上的生物非常重要。该项目将使用2013年获得的现有数据集，该数据集探测沿大西洋中脊活跃的热液系统下的超镁铁暴露的地下。该项目将通过促进科学进步，特别是通过研究地幔岩石、流体和（地下）地表生态系统之间的联系和相互作用，为国家利益服务。这是地球科学界的一个主要科学目标，其重要性在为科学家、资助机构和决策者提供指导的科学计划中得到了强调，如《2013-2023年IODP科学计划》或《美国国家科学院海洋变化2015-2025年海洋科学十年调查报告》。该项目将支持一名博士后研究员，为独立科学家的早期职业发展提供机会，并为具有先进地震成像技术专长的新一代地球科学家做出贡献。这些专业知识对美国在自然资源勘探和生产领域的工业非常重要。通过利用现有的地震数据集，该项目还将利用NSF之前在海洋地震采集方面的投资。该项目将对2013年收集的地震数据子集进行多尺度先进地震建模和成像研究。这些数据是作为美国国家科学基金会资助的水手号实验的一部分在大西洋中脊的彩虹地块收集的。该数据集包括有源长拖缆地震反射数据、海底地震仪广角数据和微地震活动目录。该方法将包括(1)向下延拓和走时层析成像，产生p波速度结构的中分辨率模型；(2)弹性全波形反演，得到高分辨率的纵波速度结构模型；(3)逆时偏移，生成几何精度高、保真度高的地震反射图像；(4)微地震层析成像，得到千米尺度的区域s波速度结构三维模型。这些模型和图像将用于研究海洋地壳内岩浆岩的物理性质。它们将有助于量化它们的部分熔融或固化状态，以及它们如何与活跃和不活跃的热液场联系起来。此外，模型和图像将阐明流体路径和小规模断层，潜在地将海底热液场与深部岩浆系统联系起来。当将这些结果与现有的海底地质观测、海底采样和以前研究的近底图像联合解释时，将全面了解岩浆活动和其他地质控制对中洋脊地幔暴露区域热液活动的作用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The ocean crust is created at Mid-Ocean Ridges as magmas rise from the mantle and cool. In order to cool this melt, seawater penetrates the ocean crust, resulting in many chemical changes of both the rocks of the crust as well as the oceans. Thus, “hydrothermal” fluid flow is a primary mechanism for mass and heat transfer between the deep Earth and the oceans, accounting for approximately one third of the global yearly heat loss. Of particular interest are hydrothermal systems hosted in rocks that were brought up from the deeper mantle, so-called “ultramafic” rocks due to their composition. This is because: (1) the mechanisms that produce such mantle exposures are not fully understood; (2) there is potential for these ultramafic rocks to sequester CO2 sequestration; and (3) the fluids resulting from hydration of ultramafic rocks can create unique chemical environments that are important for biology on earth. This project will use an existing dataset acquired in 2013 that probes the subsurface of such an ultramafic exposure beneath an active hydrothermal system along the Mid-Atlantic Ridge. This project will serve the national interest by promoting the progress of science, in particular by investigating the linkages and interactions between mantle rocks, fluids, and (sub)surface ecosystems. This is a major scientific goal of the geoscience community and its importance has been highlighted in science plans providing guidance to scientists, funding agencies and policy makers, such as the IODP Science Plan for 2013-2023 or The National Academy of Sciences Sea Change 2015-2025 Decadal Survey of Ocean Sciences Report. This project will support a postdoctoral investigator, providing an opportunity for developing the early career of an independent scientist, and contributing to the new generation of geoscientists with expertise in advanced seismic imaging techniques. Such expertise is of great importance for the US industries in the field of exploration and production of natural resources. By making use of existing seismic datasets, this project will also leverage previous NSF investments in marine seismic acquisition. This project will conduct a multi-scale advanced seismic modeling and imaging study of a subset of a seismic data collected in 2013. These data were collected as part of the NSF-funded MARINER experiment in the Rainbow massif of the Mid-Atlantic Ridge. The dataset includes active-source long-streamer seismic reflection data, ocean bottom seismometer wide-angle data, and a microseismicity catalog. The approach will consist of (1) downward continuation and travel-time tomography, yielding intermediate resolution models of the P-wave velocity structure; (2) elastic full waveform inversion, yielding high-resolution models of the P-wave velocity structure; (3) reverse time migration, yielding geometrically accurate, high fidelity seismic reflection images; and (4) micro-earthquake tomography, yielding a km-scale regional 3-D model of the S-wave velocity structure. These models and images will be used to investigate the physical properties of magmatic rocks within the oceanic crust. They will help to quantify their partially molten or solidified state, and how they are linked to active and inactive hydrothermal fields. Furthermore, the models and images will illuminate fluid pathways and small-scale faulting, potential connecting the seafloor hydrothermal fields to the magmatic system at depth. The results, when jointly interpreted with existing seafloor geology observations, seafloor sampling and near-bottom imagery from previous studies will provide a comprehensive understanding of the role of magmatism and other geological controls on hydrothermal activity in regions of mantle exposures along mid-ocean ridges.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.