Calibration of Cosmogenic Argon Production Rates in Antarctica

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0125194RenneThis award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for research to establish the systematics of cosmogenic argon production necessary to establish its measurement as a routine surface exposure dating tool analogous to existing methods based on 3He, 10Be, 14C, 21Ne, and 26Al. Cosmogenic argon offers advantages over existing cosmogenic chronometers in that it is stable (hence applicable to long-term or ancient exposure dating) and less prone to diffusive loss than He or Ne. Production of 38Ar is principally by spallation of Ca and (probably) K, and is most easily measured using neutron-irradiated samples as has been performed routinely on extraterrestrial samples for several decades. Initial measurements on Antarctic samples by the principal investigators demonstrate the viability of this method for terrestrial samples, and suggest an average production rate 100 atoms/gm-Ca/year. Existing data suggest that 38Ar/Ca exposure ages younger than 10^5 years may be determined by this method with useful accuracy. Further work on calcic minerals (apatite, sphene, clinopyroxene, plagioclase, calcite) whose exposure histories are constrained by 3He and 21Ne concentration data, will be used to determine the Ca-derived production rate. Analogous work on K-rich minerals (K-feldspars, micas) will be used to constrain the production of 38Ar from K, which should theoretically be comparable to that from Ca, using the same neutron-activation method. The analytical work will use existing samples plus new samples to be collected from the Dry Valleys of Antarctica in order to maximize cosmic radiation dosage for calibration purposes. Laboratory studies of 38Ar retentivity in appropriate minerals will be used to help evaluate our results and guide future applications. The results of this work will be important to a wide array of investigations that require knowledge of surface exposure ages.