Intellectual Merit. The last decade has seen significant advances in geochemical techniques that require an understanding of the behavior of helium in minerals. Examples include rapid progress in understanding and applying the (U-Th)/He geo- and thermochronometry method on many minerals including those which host only trace amounts of U and Th, and cosmogenic 3He dating of an increasingly diverse assemblage of phases. For both of these applications and many others it is essential to have a quantitative understanding of He diffusion: is He retained under earth surface conditions? Can the kinetics of He diffusion be quantified sufficiently to permit interpretation of He ages in terms of cooling history? More broadly, how do mineralogical/chemical factors influence helium mobility? A general understanding of what controls He diffusion in minerals is important for answering these questions and ultimately for accurate interpretation of results obtained from these new applications. For example, recent work has shown that alpha-recoil damage acts to impede He diffusion in apatite, with important consequences for (U-Th)/He thermochronometry. One focus of the study proposed here is a carefully designed series of diffusion experiments on apatites which (a) have had a known amount of lattice damage added by irradiation with neutrons in a nuclear reactor, and (b) apatites that have been heated for various combinations of temperature and time to assess if/how the radiation damage trapping phenomenon responds to mineral lattice annealing. The overarching objective of these experiments is to develop a reliable He diffusion kinetic calibration for apatite that incorporates the accumulation and annealing of radiation damage. Additional experiments will be undertaken to evaluate suggestions that He diffusion from zircon is anisotropic, and whether it too is retarded by the accumulation of alpha-recoil lattice damage. Either of these possibilities would have important implications for zircon (U-Th)/He thermochronometry. Unlike most previous diffusion studies that relied on the presence of natural helium in minerals to be investigated, the experiments proposed here will use synthetic 3He. This isotope will be produced by irradiation of samples with 220 MeV protons at a cyclotron. The main advantage of this approach is that a uniform distribution and high concentration of diffusant can be obtained on any material, including synthetic crystals and those that have been degassed by heating and annealing.
Broader Impacts. The proposed project will further explore the potential of the newly developed proton irradiation technique for noble gas studies. Results will have direct bearing on an array of methods now widely applied in the community, especially He thermochronometry. The proposed experiments may also provide new insights to the accumulation of radiation damage in materials, with potential implications for the fission track dating community, and, more speculatively, in materials science. The project promotes scientific education, through support of both a graduate student and an undergraduate student. It will help establish a newly independent investigator (co PI Shuster, PhD 2005). Equally importantly, the project will continue the long standing accessibility of the Caltech laboratory for teaching and disseminating of new techniques, for undertaking reconnaissance investigations, and for providing inter-laboratory calibrations and standards.
This grant supported efforts to characterize noble gas diffusion from minerals with the goal of obtaining a better understanding of how noble gases produced by radioactive decay can be used to determine the cooling path experienced by the mineral, a methodology called thermochronometry. Thermochronometry in turn is useful for understanding a variety of geologic phenomena including faulting, erosion, and mountain range formation. In the mineral apatite, we documented that He diffusion is sensitive to the accumulation of radiation damage from the decay of natural U and Th, as well as to the annealing of that damage at elevated temperatures. We developed a quantitative software tool for interpreting thermochronometry results that incorporates this important phenomenon. This will improve the accuracy of cooling paths determined from apatite helium thermochronometry. Thermochronometry is based on diffusion, and prior to this grant diffusion was always assumed to be isotropic, i.e., occurring at the same rate in every direction in the crystal structure. This assumption is unlikely to be correct but was in part necessitated by a lack of analytical and numerical approaches for handling anisotropic diffusion in relevant situations. We developed simple analytical solutions and a software program to permit the necessary calculations for anisotropic diffusion. We also developed a new analytical technique to map the distribution of U and Th in apatite crystals, using laser ablation inductively coupled plasma mass spectrometry. This distribution is important in helium thermochronometry because it establishes the initial distribution of the He diffusant. This grant supported a portion of the stipend of a Caltech PhD student as well as two summer undergraduate research fellows.
