Enabling the (U-Th)/Ne Thermchronometer: Ne Diffusion in Zircon and Titanite

学术价值：热年代学--使用同位素和核地球化学工具研究地质样品的温度历史--已演变为地球系统研究的基本部分。这项提议通过解决(U-Th)/Ne热年代学的重要障碍问题，促进了这一更广泛的倡议。这种方法是几年前才提出的，源于副矿物中U和Th的放射性衰变产生的α粒子与18O反应生成21Ne。经验证据表明，副矿物中~(21)Ne晶格扩散慢于~4He晶格扩散，因此联合使用(U-Th)/Ne和(U-Th)/He方法可以比单独使用(U-Th)/He方法更深入地探讨样品的温度-时间演化。虽然(U-Th)/Ne热年代学的基本理论已经发展起来，但目前还不可能解释(U-Th)/Ne年龄的地质意义，因为我们没有对矿物中的~(21)Ne扩散系数的定量测量，这可能对热年代学有用。因此，建议在质子辐照的锆石和钛铁矿上进行一系列的成核~(21)Ne扩散实验。除了21Ne外，这些材料的质子轰击也会产生成核的3He。这种核反应构成了另一种强有力的新方法--4He/3He热年代学的基础。这项技术的一个关键假设是，质子辐照在矿物中产生了空间均匀的3He分布。在这项研究的核心扩散实验之前，我们将使用亚利桑那州立大学最近开发的激光显微探头方法绘制3He(以及21Ne)在辐照样品中的空间分布图，以检验这一假设。广泛影响：该项目为本科生提供了一个参与制造和测试新分析仪器的机会。根据亚利桑那州立大学新的地球和空间探索学院对综合科学和工程教育的重视，拟议的工作将涉及为地球科学本科生开发工程实践。近年来在专业会议上的非正式讨论表明，需要为研究生和专业人员开设惰性气体地质年代学短期课程，他们是热年代学数据的使用者，但不是提供者。结合这一项目，将以广受欢迎的麻省理工学院开放式课程结构为模式，开发一个基于网络的短期课程(包括激光微探针应用模块)。

Intellectual Merit: Thermochronology - the study of temperature histories for geologic samples using the tools of isotope and nuclear geochemistry - has evolved into a fundamental part of earth system research. This proposal contributes to that broader initiative by addressing issues that are important obstacles to (U-Th)/Ne thermochronology. Proposed just a few years ago, this method derives from the fact that alpha particles produced by the radioactive decay of U and Th in accessory minerals produce 21Ne via reaction with 18O. Empirical evidence suggests that 21Ne lattice diffusion is slower than that for 4He in accessory minerals, such that the (U-Th)/Ne and (U-Th)/He methods might be used together to probe more deeply into the temperature-time evolution of a sample than by using (U-Th)/He alone. While the basic theory of (U-Th)/Ne thermochronology has been developed, it is currently impossible to interpret the geologic significance of (U-Th)/Ne dates because we have no quantitative measure of 21Ne diffusivity in minerals that might be useful for thermochronometry. Thus, it is proposed to conduct a series of nucleogenic 21Ne diffusion experiments on proton-irradiated zircon and titanite. In addition to 21Ne, proton bombardment of these materials also will produce nucleogenic 3He. This nuclear reaction forms the basis for another powerful new method, 4He/3He thermochronology. A key assumption in that technique is that proton irradiation produces a spatially uniform distribution of 3He in minerals. Prior to the diffusion experiments that are at the core of the proposed study, we will test this assumption by mapping the spatial distribution of 3He (as well as 21Ne) in our irradiated samples using laser microprobe methods recently developed at Arizona State University.Broader Impacts: This project provides an opportunity for undergraduate students to become involved in the fabrication and testing of new analytical instrumentation. In keeping with the emphasis of ASU's new School of Earth and Space Exploration on integrated science and engineering education, the proposed work will involve the development of engineering practica for geoscience undergraduates. Informal discussions at professional meetings in recent years suggests that there is a need for noble gas geochronology short-courses aimed at graduate students and professionals who are users but not providers of thermochronologic data. In conjunction with this project, a web-based short course (including modules on laser microprobe applications) will be developed, using the well-received MIT OpenCourseWare structure as a model.