Abstract-Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 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 on 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 53 Mn and 60 Fe, 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 3 He, 21 Ne, and 38 Ar are in agreement. For 4 He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled 36 Ar/ 38 Ar 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 can 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 3,4 21 Ne, and 38 Ar but equal to the 36 Ar 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.

No abstract

Abstract-Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re-determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur-and/or phosphorus-rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (~10 cm) from earlier results (~165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36 Cl data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21 Ne was produced from these minor elements. Using measured 21 Ne concentrations and endmember 22 Ne/ 21 Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21 Ne produced from S and/or P. The thus corrected data are then used to calculate new 41 K-40 K exposure ages-using published K data-which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the 21 Ne concentrations and consequently decreases the 41 K-40 K exposure ages. The discrepancies between 36 Cl-36 Ar and 41 K-40 K ages are accordingly reduced. The existence of a significant long-term variation of the GCR, which is based on a former 30-50% difference between 41 K-40 K and 36 Cl-36 Ar ages, may warrant re-investigation.

No abstract