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

Abstract

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.

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

“…Randomly selecting 40 meteorites (corresponding to the actual independent iron meteorite data base; see below) yielded apparent exposure age histograms (not shown) roughly reflecting the presumed GCR flux variation. A variable flux, if it existed, should therefore indeed be reflected in the iron meteorite data, contrary to the assertion by Ammon et al (2009) that 40 K-K exposure ages are independent of the GCR flux. As noted above, a crucial assumption is that the actual infall rate of the iron meteorites selected has remained constant over the past ∼10 9 years.…”

Cosmic-Ray Exposure Ages of Meteorites

“…The values of N found from these equations range from $1.45 to $1.57. Ammon et al (2009) updated this approach with limited new cross-section data and with modern calculations that model missing cross sections and the propagation of nuclearactive particles. They recommend the equation…”

“…Because the half-life of 40 K is so long, because the 40 K concentration never reaches saturation (see the succeeding text), and because the absolute concentrations of 40 K are usually unknown, we cannot get directly at 40 K production rates from the available data as we 'can' do for shorter-lived nuclides, such as 26 Al and 81 Kr. Although there has been some progress (Ammon et al, 2009), the lack of measured cross sections for the relevant reactions (e.g., Fe(p, X) 40 K) poses a problem for obtaining the production rates from modeling. To make matters still more complicated, N varies with irradiation hardness or shielding, so that it is not enough to evaluate N once and apply that result to numerous meteorites, as is done for 36 Cl/ 36 Ar dating.…”

“…Ammon et al (2009) updated the production rate calculations that underlie the relationships between the production rate ratios of noble gases and of potassium isotopes (eqn [35a]-[35c]). They concluded that 40 K/K ages may not, after all, disagree systematically with ages based on shorter-lived nuclides.…”

Cosmogenic effects on 7Li/6Li, 10B/11B, and 182W/184W in CAIs from carbonaceous chondrites

“…The physical model is the same as used earlier for calculating production rates of cosmogenic nuclides in iron meteorites (Ammon et al, 2008) and in ordinary and carbonaceous chondrites (Leya and Masarik, 2009). Briefly, the production rate of a cosmogenic nuclide is calculated by folding the flux densities of the projectiles with the excitation functions of the relevant nuclear reactions.…”