Collaborative Research: Behavior of Boron During Prograde Diagenesis and Metamorphism of Pelagic Sediments from the Nankai Trough

俯冲带是一个构造板块滑到另一个板块下面的地方，是水、二氧化碳和其他化学成分在地球表面和内部之间转移的地方。进入俯冲带的一些化学成分被释放回海洋（以水或气体的形式）；另一些则在火山中形成岩浆（在那里它们成为地壳的一部分，成为火成岩或作为火山气体释放到大气中），还有一些则一直被输送到地球的深处。了解地球表面和内部的元素和化合物是如何划分的，可以提供有关大气中二氧化碳水平的调节、地球表面附近放射性元素的浓度以及一些火山猛烈喷发的趋势的重要信息。该项目利用硼元素，与地球内部相比，硼元素在海水和海洋沉积物中浓度很高，通过俯冲带追踪表面成分的循环。如果我们能测量进入海底俯冲带的硼的数量，以及相关火山释放到海洋中的硼的数量，那么我们就能计算出有多少硼被回收到地球深处。该工作计划包括在美国、意大利和日本的合作者之间进行知识交流的机会。研究生和本科生都将参与并受益于合作交流。俯冲带是地壳物质再循环进入地幔的主要场所，对整个地球历史上的地幔和大陆地壳演化具有重要意义。俯冲沉积物尤其向海洋、地幔和弧岩浆贡献了挥发物、微量元素和独特的同位素特征。硼在热驱动解吸和变质脱水过程中具有很强的流体流动性和同位素分馏性，使其成为沉积物和流体的优良示踪剂。相比之下，稀土元素（HREE）和高场强元素（HFSE）一般是不动的，使它们成为沉积物和玄武岩物源的良好示踪剂。通过观察南开边缘一个独特的样品组中逐渐脱水的海相粘土和附属玄武岩中B、d11B和HREE/HFSE浓度的共变，我们提出：1)将板块输入海沟的初始变化限制在单个俯冲带的良好约束段内；2)在进入次弧区之前，沉积物和释放流体成分通过递进解吸脱水演化。该工作计划包括在美国、意大利和日本的合作者之间进行知识交流的机会。研究生和本科生都将参与并受益于国际合作。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。

Subduction zones, where one tectonic plate slides beneath another, are places where water, carbon dioxide and other chemical components are transferred between the Earth’s surface and interior. Some chemical components that enter into subduction zones are released back to the oceans (as water or gas); others contribute to magmas in volcanoes (where they become part of the crust as igneous rocks or are released to the atmosphere as volcanic gases) and still others are transported all the way into the deep interior of the Earth. Understanding how elements and compounds are divided between the surface and the interior of Earth can provide important information about the regulation of carbon dioxide levels in the atmosphere, the concentration of radioactive elements near the surface of the Earth, and the tendency of some volcanoes to erupt violently. This project uses the element boron, which is highly concentrated in seawater and ocean sediments compared to the interior of the Earth, to track the recycling of surface components through subduction zones. If we can measure the amount of boron entering into a subduction zone on the sea floor and how much boron is released by associated volcanoes and into the ocean, then we can calculate how much boron is recycled into the deep Earth. The workplan includes opportunities for intellectual exchange between collaborators in the United States, Italy, and Japan. Students at both the graduate and undergraduate levels will participate in and benefit from the collaborative exchange.Subduction zones are the primary locus for recycling of crustal materials into the Earth’s mantle, with important implications for mantle and continental crustal evolution throughout Earth history. Subducted sediments in particular contribute volatiles, trace elements, and unique isotopic signatures to the oceans, the mantle, and arc magmas. Boron is both highly fluid mobile and isotopically fractionated during thermally-driven desorption and metamorphic dehydration, making it an excellent tracer for both sediment and fluids. In contrast, Rare Earth Elements (HREE) and High Field Strength Elements (HFSE) are generally immobile, making them good tracers of sediment and basalt provenance. By observing the co-variation of B, d11B, and HREE/HFSE concentrations in progressively dehydrated marine clays and subsidiary basalts within a unique sample suite from the Nankai margin, we propose to constrain 1) the initial variability in slab inputs to the trench within a well-constrained segment of a single subduction zone; and 2) the evolution of sediment and released fluid composition through progressive desorption and dehydration prior to entering the sub-arc region. The workplan includes opportunities for intellectual exchange between collaborators in the United States, Italy, and Japan. Students at both the graduate and undergraduate levels will participate in and benefit from the international collaboration.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.