Collaborative Research: Controls on He Diffusion from Minerals

智力优势。在过去的十年里，地球化学技术取得了重大进展，需要了解氦在矿物中的行为。例子包括在理解和应用（U-Th）/He地质和热年代学方法对许多矿物，包括那些主机只有微量的U和Th，和宇宙成因的3 He定年的日益多样化的组合相的快速进展。对于这两个应用程序和许多其他的是必不可少的，有一个定量的了解他扩散：他保留在地球表面条件下？氦扩散的动力学是否能被充分量化，以允许根据冷却历史来解释氦的年龄？更广泛地说，矿物学/化学因素如何影响氦的流动性？什么控制他在矿物中的扩散的一般理解是很重要的回答这些问题，并最终从这些新的应用程序获得的结果的准确解释。例如，最近的工作表明，α-反冲损伤的行为，以阻止他在磷灰石中的扩散，与（U-Th）/He热计时的重要后果。这里提出的研究的一个重点是一个精心设计的一系列的扩散实验的磷灰石（a）有一个已知量的晶格损伤增加了中子在核反应堆中的照射，和（B）磷灰石已被加热的各种组合的温度和时间，以评估是否/如何辐射损伤捕获现象响应矿物晶格退火。这些实验的首要目标是开发一个可靠的他的扩散动力学校准磷灰石，结合辐射损伤的积累和退火。将进行额外的实验，以评估的建议，他从锆石扩散是各向异性的，以及它是否也被推迟的α反冲晶格损伤的积累。这些可能性中的任何一种都将对锆石（U-Th）/He热年代学具有重要意义。与大多数以前的扩散研究依赖于矿物中天然氦的存在进行研究不同，这里提出的实验将使用合成3 He。这种同位素将通过在回旋加速器上用220 MeV质子辐照样品来产生。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，包括合成晶体和那些通过加热和退火脱气的材料。拟议项目将进一步探索新开发的质子辐照技术在惰性气体研究中的潜力。结果将直接关系到一系列的方法，现在广泛应用于社会，特别是他热计时。拟议中的实验也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材料科学具有潜在的影响。该项目通过支持一名研究生和一名本科生来促进科学教育。这将有助于建立一个新的独立调查员（合作PI舒斯特，博士2005年）。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.