Boron Isotope Geochemistry of Metasomatic Minerals in Subcontinental Mantle

目前尚不清楚地球内部是变得更潮湿、更干燥，还是水分基本保持不变。这项工作将利用硼的同位素组成来追踪过去富水流体在地幔中的运动，试图阐明这个问题。这一问题之所以重要，是因为这种流体在地球表面环境的演化、地球内部的地球化学和地球物理演化以及具有重要经济意义的矿藏的形成中发挥了关键作用。该项目将提高对挥发性元素循环和地幔演化的理解，并将实现与研究生和高中教师科学培训以及国际科学合作有关的更广泛的影响。硼(B)是一种流体输送元素，其同位素组成可用于追踪地球表面和深处储集层之间的流体运动。利用连续几代流经大陆下上地幔的流体所形成的矿物中的B、锶、铅和Ar的同位素组成(分别由SIMS、LA-MC-ICPMS和LA-稀有气体质谱仪测量)，这项研究将追踪这些流体的来源和时间，确定它们与地表和内部地球化学库的关系，并检验B在地球历史上可能发生的循环的长期变化的证据。一个需要解决的关键问题是，在俯冲过程中，产生不同地幔同位素储集层的再循环材料在多大程度上耗尽了其表面衍生的挥发性成分。通过使用原位微束方法对地幔内形成的矿物进行采样和分析，可以避免地壳和海洋中高浓度的B的污染影响，这些影响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年轻地幔的样品上，有助于广泛地了解这些关键地区的构造演化。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包括与南非和欧洲科学家的合作，这将促进国际科学交流，促进科学能力的发展，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.