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

俯冲带，即一个构造板块在另一个板块之下滑动的地方，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)；另一些成分在火山中形成岩浆(在那里它们作为火成岩的一部分成为地壳的一部分，或者作为火山气体释放到大气中)，还有一些成分被一路输送到地球深处。了解元素和化合物在地球表面和内部是如何分配的，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。该项目使用元素硼，与地球内部相比，它在海水和海洋沉积物中高度集中，以跟踪俯冲带表面成分的循环。如果我们可以测量进入海底俯冲带的硼的数量，以及相关火山释放出多少硼进入海洋，那么我们就可以计算出有多少硼循环进入地球深处。该工作计划包括美国、意大利和日本的合作者之间进行智力交流的机会。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。尤其是俯冲沉积为海洋、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。在热驱动解吸和变质脱水过程中，硼具有高度的流动性和同位素分馏作用，使其成为沉积物和流体的良好示踪剂。相比之下，稀土元素(HREE)和高场强元素(HFSE)通常是不可移动的，使它们成为沉积物和玄武岩来源的良好示踪剂。通过观察南开边缘独特样品组中逐渐脱水的海相粘土和次玄武岩中B、d11B和HREE/HFSE浓度的协同变化，我们建议限制1)在单个俯冲带的一个良好约束段内输入海沟的板岩的初始变异性；以及2)在进入弧下区之前通过逐渐解吸和脱水作用来演化沉积物和释放的流体成分。该工作计划包括美国、意大利和日本的合作者之间进行智力交流的机会。研究生和本科生都将参与并受益于国际合作。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.