PROJECT SUMMARY Overview: Page A A series of experiments is proposed to explore systematically the effects of crystallography and radiation damage on helium diffusion in the minerals monazite, rutile, titanite, xenotime, and zircon. 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, particularly low-temperature thermochronometry based on the production of 4He by U+Sm+Th decay in accessory minerals. The practical utility of these mineral-isotopic systems for thermochronometry is limited by the quality of our knowledge of helium diffusion kinetics. Recent experimental studies of apatite and zircon suggest that radiation damage substantially affects the diffusivity of helium. Recent studies of zircon suggest a strong helium diffusion anisotropy (orders of magnitude greater parallel to the c crystallographic direction). We propose a study aimed at systematically quantifying both of these influences on helium diffusion on a variety of minerals with proven or anticipated value for (U-Th)/He thermochronometry: monazite, rutile, titanite, xenotime, and zircon. We will prepare samples with various levels of radiation damage through thermal annealing and proton irradiation for two kinds of diffusion experiments: 1) the more familiar incremental heating, bulk diffusion approach through which most of the currently available helium diffusion data were obtained; and 2) a newly developed laser ablation depth profiling approach that enables the interrogation of laboratory induced diffusion profiles in predetermined crystallographic orientations. Our goal is to contribute to the development of a schema for assigning appropriately unique diffusion parameters to individual crystals so that themochronologists can more confidently use measured dates for thermal history reconstructions. Broader Impacts : In addition to direct training of a new, young PhD student, a project like this one provides a tremendous opportunity for undergraduate students to become involved in the use of a variety of analytical instrumentation and experimental apparatus. In keeping with the emphasis of ASUs School of Earth and Space Exploration on integrated science and engineering education, the proposed work will give engaged geoscience undergraduates hands-on experience with a state-of-the-art geochemical research laboratory. We particularly hope to provide such opportunities for women students and students of color. Informal discussions at professional meetings in recent years suggest 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 experimental noble gas diffusion in accessory minerals, laser microprobe analysis, and other topics central to the proposed work) will be developed and made widely accessible.
|Effective start/end date||5/15/14 → 4/30/19|
- National Science Foundation (NSF): $273,916.00