Boron Isotope Geochemistry of Metasomatic Minerals in Subcontinental Mantle

目前还不知道地球内部是在变得更湿润、更干燥，还是其含水量基本保持不变。这项工作将利用硼的同位素组成来追踪过去富含水的流体在地幔中的运动，试图阐明这个问题。这个问题很重要，因为这种流体在地球表面环境的演变、地球内部的地球化学和地球物理演变以及具有重要经济意义的矿床的形成中发挥着关键作用。该项目将提高对挥发性元素循环和地幔演化的理解，并将对研究生和高中教师的科学培训以及国际科学合作产生更广泛的影响。硼(B)是一种流体输送元素，其同位素组成可以用来追踪地球表面储层和内部深处储层之间流体的运动。利用连续几代流体穿过大陆下地幔形成的矿物中的B、Sr、Pb和Ar的同位素组成（分别由SIMS、LA- MC-ICPMS和LA-稀有气体质谱测定），本研究将追踪这些流体的起源和时间，确定它们与地表和内部深部地球化学储层的关系，并研究B在地球历史上可能的长期循环变化的证据。需要解决的一个关键问题是，在俯冲过程中，形成不同地幔同位素储层的再生物质在多大程度上耗尽了其地表挥发性成分。通过使用原位微束方法对地幔内形成的矿物进行取样和分析，可以避免地壳和海洋中高浓度B的污染效应，这些污染效应在以前的研究中产生了一些不确定性。研究将集中在南非太古宙地幔和美国西部较年轻地幔的样本上，为了解这些关键地区的构造演化做出广泛贡献。除了培训一名美国女研究生和对当地K-12科学教师进行实地指导外，该项目的更广泛影响还包括与南非和欧洲科学家的合作，这将促进国际科学交流，催化科学能力的发展，并增加未来跨学科科学的机会。

It is at present not known if the Earth's interior is getting wetter, drier, or remaining essentially unchanged in its water content. This work will use the isotopic composition of boron to trace the past movement of water-rich fluids through Earth's mantle in an attempt to shed light on this question. The issue is important because such fluids play a key role in the evolution of Earth's surface environment, the geochemical and geophysical evolution of its interior, and the formation of economically significant ore deposits. This project will lead to an improved understanding of volatile element cycling and mantle evolution and will fulfill broader impacts related to graduate student and high- school teacher training in science, as well as international scientific collaboration.Boron (B) is a fluid-transported element whose isotopic composition can be used to trace the movement of fluids between reservoirs at Earth's surface and those in its deep interior. Using the isotopic compositions of B, Sr, Pb and Ar (measured by SIMS, LA- MC-ICPMS and LA-noble gas mass spectrometry, respectively) in minerals formed from successive generations of fluids passing through the upper mantle beneath continents, this research will trace the origin and timing of these fluids, determine their relationship to surface and deep interior geochemical reservoirs, and examine evidence for possible secular changes in the cycling of B over Earth's history. A key question to be addressed is the degree to which recycled materials giving rise to distinct mantle isotopic reservoirs are depleted in their surface-derived volatile components during subduction. By sampling and analyzing minerals formed within the mantle using in-situ microbeam methods the contaminating effects of high concentrations of B in the crust and oceans that have generated some uncertainty in previous studies may be avoided.Research will focus on samples from the Archean mantle of South Africa and younger mantle from the western US, contributing broadly to understanding the tectonic evolution of those key regions. In addition to training of a US female graduate student and field instruction of local K-12 science teachers, broader impacts of the project include collaboration with South African and European scientists that will promote international scientific exchange, catalyze development of scientific capacity, and enhance opportunities for future transdisciplinary science.