Collaborative Research: Controls on He Diffusion from Minerals

智力上的功绩。在过去的十年里，在需要了解矿物中氦的行为的地球化学技术方面取得了重大进展。例如，在理解和应用(U-Th)/He地质和热计时方法方面取得了快速进展，对许多矿物，包括那些只含有微量U和Th的矿物，以及对越来越多样化的相组合的宇宙成因3He测年。对于这两种应用和许多其他应用来说，对氦扩散有一个定量的了解是至关重要的：他是在地球表面条件下保留的吗？氦扩散的动力学能被充分量化以允许根据冷却历史来解释氦年龄吗？更广泛地说，矿物学/化学因素如何影响氦的流动性？全面了解是什么控制了He在矿物中的扩散，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是很重要的。例如，最近的工作表明，α-反冲损伤阻碍了He在磷灰石中的扩散，这对(U-Th)/He热计时具有重要的影响。这项研究的重点之一是对磷灰石进行精心设计的一系列扩散实验，这些磷灰石(A)在核反应堆中经中子辐射增加了已知数量的晶格损伤，以及(B)已加热不同温度和时间组合的磷灰石，以评估辐射损伤陷阱现象是否/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定标，该定标结合了辐射损伤的累积和退火。将进行更多的实验，以评估从锆石中扩散是各向异性的说法，以及它是否也被阿尔法反冲晶格损伤的积累所阻碍。这两种可能性对锆石(U-Th)/He热计时都有重要意义。与大多数以前的扩散研究依赖于待研究矿物中天然氦的存在不同，这里提出的实验将使用合成的3He。这种同位素将通过用220 MeV质子在回旋加速器上照射样品而产生。这种方法的主要优点是可以在任何材料上获得均匀的分布和高浓度的扩散剂，包括合成晶体和经过加热和热处理脱气的材料。拟议的项目将进一步探索新开发的质子辐照技术用于惰性气体研究的潜力。结果将直接影响到目前在社会上广泛应用的一系列方法，特别是他的热计时。拟议中的实验还可能为辐射损伤在材料中的积累提供新的见解，对裂变径迹测年领域有潜在的影响，更有可能在材料科学中产生影响。该项目通过对研究生和本科生的支持来促进科学教育。它将帮助建立一名新的独立调查员(co Pi Shuster，2005年博士)。同样重要的是，该项目将继续保持加州理工学院实验室的长期便利，用于教授和传播新技术、进行侦察调查以及提供实验室间校准和标准。

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.