Distinguishing Sediment, Serpentinite, and Altered Oceanic Crust in the Source of Aleutian Volcanic Rocks Using Boron & Molybdenum Isotopes

俯冲是基本的地质过程，一个构造板块下沉到另一个板块之下，进入地球炎热的内部。俯冲有许多重要的人类影响，特别是地震和火山活动带来的危险，这是对世界各地人口中心和商业的持续威胁。俯冲作用在塑造地球表面方面也起着关键作用，对大陆及其蕴藏的矿产资源的形成也很重要。因此，与俯冲有关的过程引起了广泛的公众和科学兴趣。这项工作的关键目标是了解流体(高压过热水)如何驱动俯冲岩浆作用，这在地球表面的火山中表现得很明显。根据该项目，将收集阿留申岛弧火山岩的化学数据。数据收集将侧重于硼(B)和钼(Mo)元素，这两种元素在地球-地表系统中相对水溶，因此，海水丰富，很容易转移到海洋盆地下面的岩石中。当俯冲作用将水合的洋壳推回到地球内部时，水被化学反应从岩石中驱离，产生富含B和Mo的流体。在地球内部极端的压力和温度下，流体降低了它们遇到的岩石的熔点，并触发了导致地球表面火山喷发的熔融过程。反过来，这些火山岩中的硼和钼的浓度揭示了有关流体经过的路径和沿途熔化的岩石的关键信息。这项工作的直接社会效益将是对在该项目下得到支持的研究生进行研究培训。该项目还为南卡罗来纳州哥伦比亚市德雷尔高中的学生启动了一个课后实习计划。学生实习生将协助样本准备、数据整理和数据管理。实习生还将制作自己的小型数据集作为期末项目。该项目的目标是让高中生接触到应用化学领域和NSF资助的地球科学研究。该项目的关键技术目标是利用硼和钼的流体移动行为，以区分俯冲输入(沉积物、蛇纹岩和蚀变玄武岩地壳)在阿留申弧岩浆生成中的竞争作用。我们将收集沿阿留申弧俯冲输入(沉积物、蚀变洋壳、深海橄榄岩)和输出(火山岩)的B和Mo同位素数据，并将化学结果与俯冲速率、沉积物通量和热参数等物理参数联系起来，这些参数沿弧线系统变化。硼同位素被用作流体通道的示踪剂，它也对俯冲板块热状态的变化做出反应。钼同位素的重要性源于它们作为流体和沉积物类型的示踪剂的价值。B和Mo同位素组合示踪剂和阿留申自然实验室创造了区分俯冲沉积物、蛇纹岩和蚀变洋壳来源的机会，特别是在沿弧线AMLIA断裂带进入海沟的点，以及在阿留申山脉最西部，热板效应独一无二地得到了很好的表达。这一奖项反映了NSF的法定使命，并通过使用基金会的智力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 is the fundamental geologic process where one tectonic plate sinks beneath another into the hot interior of the Earth. Subduction has many important human impacts, especially hazards from seismic and volcanic activity that are a constant threat to population centers and commerce worldwide. Subduction also plays a key role in shaping the earth's surface and is important in the formation of continents and the mineral resources they contain. As a result, subduction-related processes are the subject of broad public and scientific interest. The key goal of this work is to understand how fluids (superheated water at high pressures) drive subduction magmatism, which is manifest in volcanoes at the earth’s surface. Under this project, chemical data will be collected on volcanic rocks of the Aleutian Island Arc. Data collection will focus on the elements boron (B) and molybdenum (Mo), which are relatively water-soluble in earth-surface systems, and as a result, are abundant in seawater and are easily transferred to rocks that underlie the ocean basins. When subduction pushes the hydrated oceanic crust back into the Earth’s interior, the water is driven out of the rocks by chemical reactions that produce fluids rich in B and Mo. Under the extreme pressures and temperatures of the Earth’s interior, fluids lower the melting point of the rocks that they encounter and trigger the melting processes that lead to volcanic eruptions at the Earth’s surface. In turn, the concentrations of B and Mo in these volcanic rocks reveal key information about the paths that the fluids follow and the rocks that are melted along the way. Immediate societal benefit of this work will be research training of the graduate students who will be supported under the project. The project also initiates an after-school intern program for students from Dreher High School, in Columbia, South Carolina. The student interns will assist with sample preparation, data reduction, and data management. The interns will also produce a small data set of their own as a final project. The goals of the program are to expose high-school students to areas of applied chemistry and to NSF-funded research in the earth sciences.The key technical goal of the project is to take advantage of the fluid-mobile behavior of boron and molybdenum in order to distinguish the competing roles of subduction inputs (sediments, serpentinite and altered basaltic crust) in the generation of the Aleutian Arc magmas. We will collect B and Mo isotope data for subduction inputs (sediment, altered oceanic crust, abyssal peridotite) and outputs (volcanic rocks) along the Aleutian arc and link the chemical results to physical parameters such as subduction rate, sediment flux, and thermal parameter, which change systematically along the arc. Boron isotopes are used as tracers of fluid pathways, which also respond to changes in the thermal state of the subducting plate. The importance of Mo isotopes stems from their value as a tracer of both fluids and sediment type. The combined B and Mo isotope tracers and Aleutian natural laboratory create opportunities to distinguish sources in subducted sediment, serpentinite, and altered oceanic crust, especially at the point along the arc where the Amlia Fracture Zone enters the trench, and in the western-most Aleutians, where hot-slab effects are uniquely well expressed.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.