Collaborative Research: Controls on He Diffusion from Minerals

合作研究：控制矿物质 He 扩散

• 批准号: 0738627
• 负责人: Kenneth Farley
• 金额: $ 23.01万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-03-01 至 2011-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0738627&HistoricalAwards=false
• 关键词: Collaborative; Research; Controls; He; Diffusion

Intellectual Merit. The last decade has seen significant advances in geochemical techniques that require an understanding of the behavior of helium in minerals. Examples include rapid progress in understanding and applying the (U-Th)/He geo- and thermochronometry method on many minerals including those which host only trace amounts of U and Th, and cosmogenic 3He dating of an increasingly diverse assemblage of phases. For both of these applications and many others it is essential to have a quantitative understanding of He diffusion: is He retained under earth surface conditions? Can the kinetics of He diffusion be quantified sufficiently to permit interpretation of He ages in terms of cooling history? More broadly, how do mineralogical/chemical factors influence helium mobility? A general understanding of what controls He diffusion in minerals is important for answering these questions and ultimately for accurate interpretation of results obtained from these new applications. For example, recent work has shown that alpha-recoil damage acts to impede He diffusion in apatite, with important consequences for (U-Th)/He thermochronometry. One focus of the study proposed here is a carefully designed series of diffusion experiments on apatites which (a) have had a known amount of lattice damage added by irradiation with neutrons in a nuclear reactor, and (b) apatites that have been heated for various combinations of temperature and time to assess if/how the radiation damage trapping phenomenon responds to mineral lattice annealing. The overarching objective of these experiments is to develop a reliable He diffusion kinetic calibration for apatite that incorporates the accumulation and annealing of radiation damage. Additional experiments will be undertaken to evaluate suggestions that He diffusion from zircon is anisotropic, and whether it too is retarded by the accumulation of alpha-recoil lattice damage. Either of these possibilities would have important implications for zircon (U-Th)/He thermochronometry. Unlike most previous diffusion studies that relied on the presence of natural helium in minerals to be investigated, the experiments proposed here will use synthetic 3He. This isotope will be produced by irradiation of samples with 220 MeV protons at a cyclotron. The main advantage of this approach is that a uniform distribution and high concentration of diffusant can be obtained on any material, including synthetic crystals and those that have been degassed by heating and annealing.Broader Impacts. The proposed project will further explore the potential of the newly developed proton irradiation technique for noble gas studies. Results will have direct bearing on an array of methods now widely applied in the community, especially He thermochronometry. The proposed experiments may also provide new insights to the accumulation of radiation damage in materials, with potential implications for the fission track dating community, and, more speculatively, in materials science. The project promotes scientific education, through support of both a graduate student and an undergraduate student. It will help establish a newly independent investigator (co PI Shuster, PhD 2005). Equally importantly, the project will continue the long standing accessibility of the Caltech laboratory for teaching and disseminating of new techniques, for undertaking reconnaissance investigations, and for providing inter-laboratory calibrations and standards.

智力优点。过去十年，地球化学技术取得了重大进展，需要了解矿物中氦的行为。例如，在许多矿物（包括仅含有微量 U 和 Th 的矿物）上理解和应用 (U-Th)/He 地质测时法和热测时法方面取得了快速进展，以及对日益多样化的相组合进行宇宙成因 3He 测年。对于这些应用和许多其他应用来说，定量了解 He 扩散至关重要：He 是否保留在地球表面条件下？氦扩散的动力学是否可以充分量化，以便根据冷却历史来解释氦年龄？更广泛地说，矿物学/化学因素如何影响氦的流动性？总体了解控制氦在矿物中扩散的因素对于回答这些问题并最终准确解释从这些新应用中获得的结果非常重要。例如，最近的研究表明，α 反冲损伤会阻碍 He 在磷灰石中的扩散，对 (U-Th)/He 热测时法产生重要影响。这里提出的研究的一个重点是一系列精心设计的磷灰石扩散实验，其中（a）通过核反应堆中的中子辐照而增加了已知数量的晶格损伤，以及（b）磷灰石已被加热不同的时间温度和时间的组合来评估辐射损伤捕获现象是否/如何响应矿物晶格退火。这些实验的首要目标是开发一种可靠的磷灰石氦扩散动力学校准方法，其中包括辐射损伤的积累和退火。还将进行额外的实验来评估锆石中 He 扩散具有各向异性的建议，以及它是否也会因 α 反冲晶格损伤的积累而受到阻碍。这些可能性中的任何一种都将对锆石 (U-Th)/He 热测时法产生重要影响。与之前大多数依赖于待研究矿物中天然氦的存在的扩散研究不同，这里提出的实验将使用合成的 3He。这种同位素将通过在回旋加速器中用 220 MeV 质子照射样品来产生。这种方法的主要优点是，可以在任何材料上获得均匀分布和高浓度的扩散剂，包括合成晶体以及通过加热和退火脱气的材料。影响更广泛。拟议的项目将进一步探索新开发的质子辐照技术在稀有气体研究中的潜力。结果将直接影响目前在社区广泛应用的一系列方法，特别是氦热测时法。拟议的实验还可能为材料中辐射损伤的积累提供新的见解，对裂变径迹测年界以及更推测的材料科学具有潜在影响。该项目通过研究生和本科生的支持来促进科学教育。它将帮助建立一个新的独立研究者（联合 PI Shuster，博士，2005 年）。同样重要的是，该项目将延续加州理工学院实验室长期以来的可访问性，用于教学和传播新技术、进行勘察调查以及提供实验室间校准和标准。