The Cl-36-Ar-36-K-40-K-41 records and cosmic ray production rates in iron meteorites

This is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the la st billion years, as recorded in these fossil detectors. We have performed new Cl-36, Al-26, Be-10, and noble gas measurements, including determinatio n of the shielding parameter, S = He-4/Ne-21, in samples with published K d ata, The K isotopic data, coupled to Ar-36 and Cl-36 concentrations permit selection of meteorites which have only experienced simple (constant geomet ry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. Tn o rder to carry out production rate calibrations based on K-40-K-41 data, we assume constancy of the cosmic flux during the interval 150 to 700 My ago. We note that meteorites with very old potassium ages cannot be included in this calibration, as these meteorites require distinct parameter sets. A ca libration data set representing a total of 13 meteorites was used to comput e long-term (0.5 Gy) average production rates, These average production rat es of Cl-36 from this particular calibration set are significantly (28%) lo wer than those determined for the recent (less than or equal to 10 My) cosm ic ray flux. We also document here the quality of the resulting potassium p roduction rate parameter Mo(S) with a calculated isochron for irons of grou p IVA. (C) 1999 Elsevier Science B.V. All rights reserved.