NSFGEO-NERC: Data Mining the Deep: Combining Geochemistry and Imaging Spectroscopy to Quantify the Impact on Ocean Chemistry of Deep Hydrothermal Circulation at Mid-Ocean Ridges

洋壳形成于海中伸展脊，覆盖了地球表面的60%以上。由于连续的板块构造循环和俯冲作用，海洋地壳在地质上是年轻的（2亿年），比构成大陆的岩石年轻得多。加热的海水从形成之初就在整个海洋地壳中循环，直到它在俯冲带被消耗，与岩石反应形成新的矿物质。水、二氧化碳、钾和镁等关键元素和分子在海水和海洋地壳之间交换。这些流体和岩石之间的相互作用是影响海水化学的主要因素，但尚未被量化。通过俯冲作用，影响地球内部的成分。一个关键的挑战是，海洋钻探还没有对海底以下许多公里处最深的海洋地壳进行采样。然而，近年来，国际大陆科学钻探计划的阿曼钻探项目在阿曼钻探了几公里的海洋地壳和最上层地幔，在那里，地壳的这一关键部分被地壳推到了大陆上。在这个项目中，加州理工学院的科学家们将与英国南安普顿大学和普利茅斯大学的同事们合作，调查这些岩心，以计算海水和深海地壳之间的交换程度。该项目采用了一种创新的方法，将对岩心样本的详细但传统的实验室分析与一种新技术——微成像红外光谱技术相结合，该技术利用红外光的反射率来确定整个岩心的矿物学，空间分辨率小于1毫米。这项研究将在理解海洋地壳深部流体循环方面取得重大进展，其尺度和采样完整性在以前是不可能的。这将有助于了解流体-岩石反应对全球地球化学循环和海水化学的贡献，并限制深部流体循环在海洋地壳冷却和蚀变中的作用。将建立一个带有岩心查看器的网站，向更广泛的科学界传播钻孔岩心的矿物图，并在课堂上用于教育用途。该项目支持本科生的培养。最近的估计表明，通过上部地壳熔岩和岩脉的热液通量不足以凝固和冷却靠近洋中脊轴的新地壳，需要在下部地壳中进行主要的、但迄今尚未量化的深部热液循环。迄今为止，这种深层环流的途径和地球化学后果都没有得到详细考虑，但初步估计表明，这些系统对于从下地壳向海洋输出40-60%的可用热量是必要的，并且可能影响海洋化学中的关键元素。在这个由美国国家科学基金会支持的加州理工学院的科学家和英国南安普顿大学和普利茅斯大学的nerc支持的科学家联合开展的项目中，这些来自阿曼钻探项目的岩心将被调查，以检验两种将海洋地壳的形成与更广泛的地球系统联系起来的假设：(A)下洋壳断裂带表现出矿化和热液蚀变，记录了它们作为地壳与海水之间热化学交换的主要管道的作用；(B)下地壳深部热液循环对C、O、S、Li、B、Sr、Mg、Ca和贱金属等特定元素的全球地球化学循环有显著影响。该项目有四个目标：(1)确定蚀变岩石的矿物组合及其随海洋地壳深度的地球化学以及蚀变流体的必要温度和化学成分。(2)量化海水与下洋壳和上地幔之间的元素和同位素交换及其对全球海水成分化学循环的贡献；(3)确定深部热液环流（包括断裂带）在冷却下洋壳中的重要性；(4)建立一个网站，核心查看器托管矿物地图和成像光谱数据集，以供未来的研究人员和学生使用。这些目标将通过以下四项任务来实现：利用成像光谱、岩石学和地球化学联系绘制岩心矿物图、附加的地球化学表征、模拟和综合结果以确定化学和热收支，以及开发岩心矿物图的网络查看器。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。

Ocean crust is formed at mid-ocean spreading ridges and covers more than 60% of the surface of Earth. Because of the continuous plate tectonic cycle and subduction, the ocean crust is geologically young (200 million years), much younger than the rocks that make up continents. Heated seawater circulates throughout the ocean crust from its creation until it gets consumed in subduction zones, reacting with the rock to form new minerals. Key elements and molecules such as water, CO2, K, and Mg are exchanged between seawater and the ocean crust. These interactions between fluids and the rock are a major – but not yet quantified – contributor to seawater chemistry and, through subduction, the composition of the interior of Earth. A key challenge is that ocean drilling has not sampled the deepest ocean crust many kilometers below the seafloor. However, in recent years, the International Continental Scientific Drilling Program’s Oman Drilling Project has drilled a few kilometers of oceanic crust and the uppermost mantle in Oman, where tectonics have pushed this critical portion of Earth’s crust onto the continent. In this project, scientists at the California Institute of Technology will work with colleagues at the universities of Southampton and Plymouth in the UK to investigate these drill cores to work out how much exchange there has been between seawater and the deep ocean crust. The project takes an innovative approach that combines detailed but traditional laboratory analyses of select samples of the core with a novel technique, micro-imaging infrared spectroscopy, which uses the reflectance of infrared light to determine the mineralogy of the entire drill core at a spatial resolution of less than one millimeter. This research will achieve major advances in understanding of deep circulation of fluids within the ocean crust by objective characterization at a scale and sampling completeness previously impossible. It will yield insights into the contribution of fluid-rock reactions to global geochemical cycling and seawater chemistry as well as constrain the role of deep fluid circulation in cooling and alteration of the lower oceanic crust. A website will be built with a core viewer to disseminate mineral maps of the drill cores to the broader scientific community and for educational use in classes. The project supports the training of undergraduate students.Recent estimates suggest that hydrothermal fluid fluxes through upper crustal lavas and dikes are insufficient to solidify and cool new crust close to the mid-ocean ridge axes, requiring major, but hitherto unquantified, deep hydrothermal circulation in the lower crust. To date, neither the pathways nor the geochemical consequences of this deep circulation have been considered in detail, but preliminary estimates indicate that these systems are necessary to export 40-60% of the available heat from the lower crust to the oceans and may influence ocean chemistry for key elements. In this joint project between NSF-supported scientists at Caltech and NERC-supported scientists at the Universities of Southampton and Plymouth in the UK, these cores from the Oman Drilling Project will be investigated to test two hypotheses that connect the formation of the ocean crust to the wider Earth system: (A) that fault zones in the lower ocean crust exhibit mineralization and hydrothermal alteration that record their role as major conduits for thermal and chemical exchange between the crust and ocean waters; and (B) that deep hydrothermal circulation in the lower crust has significant impacts on global geochemical cycles of specific elements including C, O, S, Li, B, Sr, Mg, Ca and base metals. The project has four objectives: (1) Determine the mineral assemblages of altered rocks and their geochemistry with depth in the oceanic crust and the requisite temperatures and chemistries of the alteration fluids. (2) Quantify elemental and isotopic exchanges between seawater and the lower oceanic crust and uppermost mantle and their contribution to global chemical cycles of the composition of seawater; (3) Determine the importance of deep hydrothermal circulation, including in fault zones, in cooling the lower oceanic crust; and (4) Build a website with core viewer hosting mineral maps and imaging spectroscopy datasets to enable use by future researchers and students. The objectives will be achieved through four tasks that map minerals in the core with imaging spectroscopy and petrologic and geochemical tie-ins, additional geochemical characterization, modeling and synthesis of results to determine chemical and thermal budgets, and development of a web viewer for mineral maps of the core.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.