The 36Cl–36Ar–40K–41K 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 last billion years, as recorded in these fossil detectors. We have performed new 36Cl, 26Al, 10Be, and noble gas measurements, including determination of the shielding parameter, S = 4He/21Ne, in samples with published K data. The K isotopic data, coupled to 36Ar and 36Cl concentrations permit selection of meteorites which have only experienced simple (constant geometry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. In order to carry out production rate calibrations based on 40K–41K 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 calibration data set representing a total of 13 meteorites was used to compute long-term (0.5 Gy) average production rates. These average production rates of 36Cl from this particular calibration set are significantly (28%) lower than those determined for the recent (≤10 My) cosmic ray flux. We also document here the quality of the resulting potassium production rate parameter M0(S) with a calculated isochron for irons of group IVA.