Experimental Development and Calibration of (U-Th)/He Thermochronology

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ReinersEAR-0073576Recent resurgence of interest in (U-Th)/He dating has been made possible by interpretational advances combined with careful diffusion studies and apparently successful tectonic applications and calibrations against other thermochronometers. Nevertheless, many exciting aspects of He dating remain unexplored. This work comprises a series of experimental studies to develop, extend, and calibrate new (U-Th)/He thermochronologic methods. One objective is to extend He dating to new phases, including zircon, fluorite, and garnet. This is motivated by the potential benefits of low-T thermal constraints in rocks that lack these phases in abundance such as many sedimentary and hydrothermally mineralized rocks, as well as the unique diffusion properties and closure temperatures (Tc) of other phases. A second objective is to exploit the property that the diffusion domain for He is equivalent to the crystal or grain itself. We will attempt to use this to constrain not only cooling ages, but thermal histories of crystals, by examining the potential uses of 1) core-to-rim He concentration and age zonation within individual crystals using abrasion techniques, and 2) age-grain size correlations that reflect specific thermal histories of partially reset crystals. A final objective is to rigorously compare (U-Th)/He thermochronometry with other methods, primarily multi-domain K-feldspar 40Ar/39Ar dating to provide intermethodcalibrations and unprecedentedly dense time-temperature sampling and thermal constraints. The (U-Th)/He lab at W.S.U. is set to begin measuring ages and performing diffusion experiments in May of 2000, and this funding will provide support for graduate students working on experimental development of these methods, in collaboration with with regional fission-track and 40Ar/39Ar thermochronologic labs, the economic geology programs at W.S.U. and the nearby U.I.