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

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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.