4He/3He Thermocronometry and Noble Gas Diffusion Behavior via Proton Irradiation

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Techniques and applications of (U-Th)/He dating have evolved rapidly in the last decade. The method is now in common use to study problems ranging from dating of Quaternary volcanic rocks, to establishing the timing and magnitude of fault motions, to documenting paleotopography. These applications have relied on measurement of the bulk age of the dated grain. By introducing 3He into minerals via proton bombardment followed by step-heat measurements of the 4He/3He ratio, it is possible to produce an "age spectrum" developed as a 4He concentration profile within the grain. The age spectrum carries unique quantitative information on the sample's cooling path that complements the bulk age. 3He measurements also provide a rigorous constraint on helium diffusivity that cannot be obtained from measurements of natural radiogenic 4He alone. Work supported by this grant will field-test the 4He/3He technique in several well-constrained geologic situations including a deep crustal borehole and a normal fault block with a large magnitude of exhumation of known timing. Numerical models will be developed to interpret profiles and to evaluate recognized potential pitfalls with the method.This project will enhance scientific infrastructure by further developing collaboration with physicists at the Northeast Proton Therapy Center's cyclotron facility. Proton beam time supported by this proposal may be shared with other interested users, and if successful the ultimate product will be fundamentally new capabilities in geochronology and geochemistry. Farley's geochronology laboratory at Caltech has been used by many outside investigators in the last several years, and this proposal will support the infrastructure necessary to continue this defacto multi-user facility for the coming years. The proposed work will also support the education of a Caltech PhD student.