Collaborative Research: Controls on He Diffusion from Minerals

知识价值。在过去的十年里，地球化学技术取得了重大进展，这需要了解氦在矿物中的行为。例如，对许多矿物（包括那些只含有微量U和Th的矿物）的(U-Th)/He地质和热时测定法的理解和应用取得了迅速进展，以及对越来越多样化的相组合的宇宙成因3He定年。对于这两种应用和许多其他应用来说，对He扩散有一个定量的理解是至关重要的：He是否保留在地球表面条件下？氦扩散动力学能否被充分量化，以允许根据冷却历史来解释氦的年龄？更广泛地说，矿物/化学因素如何影响氦的迁移？了解是什么控制了矿物中的He扩散，对于回答这些问题以及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，这对(U-Th)/He热时学有重要影响。本文提出的研究重点之一是对磷灰石进行一系列精心设计的扩散实验，这些磷灰石(a)在核反应堆中通过中子辐照增加了已知数量的晶格损伤，以及(b)经过不同温度和时间组合加热的磷灰石，以评估辐射损伤捕获现象是否/如何响应矿物晶格退火。这些实验的总体目标是开发一个可靠的磷灰石扩散动力学校准，包括辐射损伤的积累和退火。进一步的实验将评估锆石中He扩散的各向异性，以及它是否也被α -反冲晶格损伤的积累所延缓。这两种可能性中的任何一种都将对锆石(U-Th)/He热时学产生重要影响。与之前大多数依赖于矿物中天然氦存在的扩散研究不同，这里提出的实验将使用合成氦。这种同位素将通过在回旋加速器中用220兆电子伏特的质子照射样品而产生。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，包括合成晶体和通过加热和退火脱气的材料。更广泛的影响。该计划将进一步探索新发展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广泛应用于社会的一系列方法，特别是He热时计。提出的实验也可能为材料的辐射损伤积累提供新的见解，对裂变径迹测年界有潜在的影响，更有可能对材料科学产生影响。该项目通过一名研究生和一名本科生的支持来促进科学教育。它将有助于建立一个新的独立调查员（co PI Shuster, PhD 2005）。同样重要的是，该项目将继续加州理工学院实验室长期以来的教学和传播新技术，进行侦察调查，并提供实验室间的校准和标准。

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.