New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii front 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based oil our Current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are Mn-53 and Fe-60, possibly due to normalization problems. When an average S content of about 1 +/- 0.5% is assumed for Grant and Carbo samples, which is consistent with Our earlier Study, the model predictions for He-3, Ne-21, and Ar-38 are in agreement. For He-4 the model has to be adjusted by 24%, possibly a result Of our rather crude approximation for the primary galactic alpha particles. For reasons not yet understood the modeled Ar-36/Ar-38 ratio is about 30-40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the Sulphur content of the Studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we call calculate exposure ages that are based on the production rates (cm(3) STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply He-3,He-4, Ne-21, or Ar-36,Ar-38 ages, are calculated assuming the Current GCR flux. Besides calculating noble gas ages we were also able to improve the K-41-K-40-and the Cl-36-Ar-36 dating methods with the new model. Note that we distinguish between Ar-36 ages (calculated via Ar-36 production rates only) and Cl-36-Ar-36 ages. Exposure ages for Grant and Carbo, calculated with the revised K-41-K-40 method, are 628 +/- 30 Ma and 841 +/- 19 Ma, respectively. For Grant this is equal to the ages obtained using He-3, Ne-21, and Ar-38 but higher than the Ar-36- and Cl-36-Ar-36 ages by similar to 30%. For Carbo the K-41-K-40 age is similar to 40% lower than the ages obtained using He-3, Ne-21, and Ar-38 but equal to the Ar-36 age. These differences can either be explained by our still insufficient knowledge of the neutron-induced cross sections or by a long-term variation of the GCR.