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

洋壳形成于洋中扩张脊，覆盖地球表面的60%以上。由于连续的板块构造循环和俯冲，海洋地壳在地质上是年轻的（2亿年），比组成大陆的岩石年轻得多。加热的海水从形成开始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、CO2、K和Mg等关键元素和分子在海水和洋壳之间进行交换。这些流体和岩石之间的相互作用是海水化学的主要贡献者，但尚未量化，并通过俯冲作用，地球内部的组成。一个关键的挑战是，海洋钻探还没有对海底以下许多公里的最深海洋地壳进行采样。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洋壳和最上层地幔，那里的构造将地壳的这一关键部分推到了大陆上。在这个项目中，加州理工学院的科学家将与英国南安普顿和普利茅斯大学的同事合作，调查这些钻孔岩芯，以计算出海水和深海地壳之间发生了多少交换。 该项目采用了一种创新的方法，将对岩心样品的详细但传统的实验室分析与一种新技术，即显微成像红外光谱法相结合，该技术利用红外光的反射率以小于1毫米的空间分辨率确定整个岩心的矿物学。这项研究将在了解洋壳内深层流体循环方面取得重大进展，以前所未有的规模和采样完整性进行客观表征。它将使人们深入了解流体-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下洋壳冷却和蚀变中的作用。将建立一个网站，配备岩心浏览器，向更广泛的科学界传播岩心矿物图，并用于课堂教学。最近的估计表明，通过上地壳熔岩和岩脉的热液流量不足以使靠近大洋中脊轴线的新地壳凝固和冷却，因此需要在下地壳进行重大但迄今尚未量化的深层热液循环。迄今为止，这种深环流的路径和地球化学后果都没有得到详细考虑，但初步估计表明，这些系统对于将下地壳40-60%的可用热量输出到海洋是必要的，并且可能影响海洋化学关键元素。在加州理工学院NSF支持的科学家和英国南安普顿大学和普利茅斯大学NERC支持的科学家之间的这个联合项目中，这些来自阿曼钻探项目的岩心将被调查，以测试两个假设，将海洋地壳的形成与更广泛的地球系统联系起来：（一）下洋壳的断裂带显示出矿化和热液蚀变，这表明它们是热交换和化学交换的主要通道（2）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具有重要影响。该项目有四个目标：（1）确定蚀变岩的矿物组合及其在洋壳中的深度和蚀变流体的必要温度和化学性质的地球化学。(2)（c）确定海水与下洋壳和上地幔之间的元素和同位素交换及其对海水成分全球化学循环的贡献;（3）确定深层热液循环，包括断层带内的热液循环，在冷却下洋壳方面的重要性;以及（4）建立一个网站，核心浏览器托管矿物地图和成像光谱数据集，以便未来的研究人员和学生使用。这些目标将通过四项任务实现，即利用成像光谱学、岩石学和地球化学联系绘制岩心矿物图、附加地球化学表征、建模和结果综合，以确定化学和热预算，该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识产权进行评估来支持。优点和更广泛的影响审查标准。

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.