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.
The element Helium is a daughter product of naturally occurring radioactive decay of Uranium and Thorium that accumulates in minerals located near earth’s surface. Because Helium is a relatively small atom, it diffuses through minerals over geologic time at the temperatures of rock located within two kilometers depth beneath earth’s surface (i.e., below the boiling point of water). By understanding the rate at which He diffuses through certain minerals as a function of temperature, an observed abundance and spatial distribution of Helium within minerals constrains how that rock cooled through geologic time as it approached the surface, for example by bedrock erosion. The purpose of this study was to perform laboratory experiments designed to quantify the rate of Helium diffusion through minerals that commonly contain Uranium and Thorium, and to develop our understanding of what are the primary chemical and physical controls on Helium diffusion in these minerals, as well as noble gas diffusion in general. We unambiguously demonstrated that the accumulation of naturally occurring radiation damage (also associated with Uranium and Thorium decay) has an important control on how Helium diffused through minerals. This knowledge was developed into a quantitative model that is now widely used to interpret Helium observations commonly made in many research groups across the world. These results have important and broad implications for the study of how mountain topography develops and, more generally, how tectonic and erosion processes have shaped earth’s surface through geologic time. The work of this project also promoted the educational development of graduate and undergraduate students, as well as early career scientists.
