Pyroxene separated from dolerite surfaces, which were exposed to cosmic ray irradiation for millions of years in the Antarctic Dry Valleys, contains considerable amounts of cosmogenic Ar-38, as evident from Ar-38/Ar-36 ratios up to 0.2887 (air: 0.1880). In five out of eight mineral separates the concentrations of K, Ti and CI are tow enough to enable us to deduce the terrestrial production rate of Ar-38 from Ca (the remaining relevant target element) by relating cosmogenic Ar-38 concentrations to He-3 and Ne-21 exposure ages. Correlations of cosmogenic Ar-38 (normalized to Ca) to total He-3 and to cosmogenic Ne-21 normalized to Mg, respectively, are excellent. However, various currently used methods to derive the He-3 and Ne-21 production rates in pyroxene lack consistency. Depending on the method chosen, the Ar-38 production rate derived here ranges between (191 +/- 21) atoms (g Ca)(-1) a(-1) and (254 +/- 28) atoms (g Ca)(-1) a(-1) at sea level (standard atmospheric pressure) and high latitude. A more accurate value may be calculated from our data as soon as the He and Ne production rates from pyroxene are known more reliably. Our production rate determination is largely independent of past geomagnetic field variations and of the scaling method used. (c) 2007 Elsevier B.V. All rights reserved.
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