Collaborative Research: Impact of crystal defects on helium diffusion in apatite crystals in (Uranium-Thorium)/Helium isotopic dating for the Earth sciences

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Understanding how the Earth's surface changes in response to mountain building and other tectonic processes requires accurate methods to determine the state and history of the crust and of sedimentary basins in which gas and oil deposits are developed. One approach involves measuring the temperature history of rocks, which allows geologists to track the rate and timing at which rocks are moved from depth to the surface. One of the most widely used of such tools is (Uranium-Thorium)/Helium isotopic dating of the mineral apatite. In apatite, the helium produced from decay of uranium and thorium will remain in a grain only at very low temperatures and in general the diffusion of helium is very sensitive to temperature, such that dates on apatite are a record of thermal history. Useful as the technique has been, in some instances isotopic dates obtained from even a single sample are scattered, even taking into account complicating factors such as grain size, radiation damage, and grain chemistry. The proposed study will test the hypothesis that the excess scatter that is sometimes seen in apatite dates originates in crystal imperfections that change helium diffusion behavior within apatite crystal grains. This project benefits society and advances desired societal outcomes by its emphasis on engaging K-12 underrepresented students in STEM (science, technology, engineering and math) earth science education through outreach efforts particularly in Bethlehem, PA, and by contributing to undergraduate and graduate student training and involvement in active research, thus contributing to a strong and globally competitive scientific workforce. The research is proving support for an early career researcher who is part of an underrepresented group in STEM as the professional level. The project is bridging the fields of geology and experimental research such that more accurate models of geologic and geochemical processes can be developed. It is also further the application of the apatite (Uranium-Thorium)/Helium dating as a robust tool in both research and commercial applications, which may have implication for resource development. Using various analytical techniques applied over different spatial scales, we will characterize a variety of apatite grains from samples known to yield dispersed apatite (Uranium-Thorium)/Helium ages. Characterization will include chemical composition by electron-probe analysis, quantification of crystal defects using etching experiments and optical microscopy, and electron-beam techniques. Diffusion experiments on natural samples using continuous ramped heating experiments will provide kinetic parameters for diffusive helium loss and characterize the temperature dependence of Helium release associated with various types of defects. To study the specific role of lattice-scale defects associated with deformation, we will deform Durango apatite single crystals to create samples with varying dislocation density. Diffusion experiments on these samples of known dislocation density will be conducted by Helium-3 ion implantation and nuclear reaction analysis. In addition, at the whole-grain level, proton-irradiated samples will be analyzed by step-heating experiments. Changes in helium diffusion kinetics will then be modeled as a combination of lattice and fast pathway diffusion to obtain a relationship between dislocation density and diffusivity. Data from deformation experiments will also be used to construct a flow law and determine a stress/dislocation-density relationship. To test these relationships, samples from four shear zones of known deformation history will be dated using an improved protocol that incorporates the new relationship between dislocation density and diffusivity.The project is a share between the Division of Earth Science's Tectonics and Petrology and Geochemistry programs.