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

Project: Research project

Description

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. Just as the geochemistry community in the 1970s and 1980s led an explosion in the development of new isotopic tracers as probes of mantle and crustal evolution, our community is now driving a rapid expansion in the range and precision of thermochronology. This proposal contributes to that broader initiative by addressing issues that are important obstacles to (UTh)/ 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 the basis for (U-Th)/He thermochronology also drive the nuclear reaction 18O(,n)21Ne. Empirical evidence suggests that 21Ne lattice diffusion is slower than 4He lattice diffusion 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 (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. We propose to remedy that through 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 A project like this one provides a tremendous 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 of this type (which will include modules on laser microprobe applications) will be developed, using the well-received MIT OpenCourseWare structure as a model.
StatusFinished
Effective start/end date9/15/098/31/13

Funding

  • National Science Foundation (NSF): $198,502.00

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thermochronology
titanite
zircon
accessory mineral
student
laser
geochemistry
probe
engineering
radioactive decay
crustal evolution
noble gas
mineral
geochronology
diffusivity
instrumentation
explosion
irradiation
experiment
temperature