Production Rates and Production Rate Ratios for Cosmogenic Kr Isotopes in H- Chondrites Based on Chlorine-36/argon-36 Ages

We present the Kr-Kr derived exposure ages for H-chondrites which were recently investigated for their Cl-Ar ages. After demonstrating that our Kr-Kr ages are reliable and reproducible we find hints that Kr/Kr and Kr/Kr might depend on the relative concentrations on the target elements Rb, Sr, Y, Zr, and Nb. In addition, the Kr-Kr ages for the large L-chondrite Gold Basin indicate that the Kr-Krmethod might fail for large meteoroids. Introduction: The Kr-Kr method of cosmic-ray exposure dating [1] is based on the assumption that the production rate ratio Kr/Kr can be determined from measured Kr-Kr-Kr concentrations by using one of the following equations: P81/P83 = 0.95(Kr/Kr+ Kr/Kr)/2 (1) P81/P83 = 1.262(Kr/Kr) + 0.381 (2) These relations allow the determination of shielding corrected cosmic-ray exposure ages based on a single Kr analysis. Eqn. 2 is insensitive to neutron capture production of Kr and Kr by the reactions on Br, which might compromise the analyses of spallogenic Kr in large Br-rich meteorites. Eqn. 1 and 2 were deduced from Apollo 12 lunar samples. They are also widely used to date meteorites, which may have widely different concentrations of the major target elements Rb, Sr, Y, Zr, and Nb. However, so far the reliability of these equations has been tested on two meteorites only [2]. In addition, the above equations might also be valid within a limited range of shielding conditions only. The goal of the present study therefore is to test the dependence of eqn. 1 and 2 on the relative concentrations of the main target elements (Rb, Sr, Y, Zr, Nb) and on the shielding conditions of the meteorites. For this we analysed He, Ne, Ar, Kr, and Xe so far in 9 H chondrites which were recently investigated for their Cl-Ar cosmic-ray exposure ages [3] and light noble gas production rates [4]. We also analysed 7 samples from the very large L4 chondrite Gold Basin [5]. In addition, a sample of the well studied L/LL5 chondrite Knyahinya was analysed. Experimental: The meteorites studies are listed in Table 1. The bulk samples were prepared by crushing about 1 g of bulk meteorite in an agate mortar. The samples were then wrapped in ∼35 mg of commercial Al foil and loaded into the storage positions of an allmetal rare gas extraction system. In order to release atmospheric gas contaminations all samples were preheated at ∼80°C for ∼20 h. Gases were released in a Mo crucible in two temperature steps. The first step at 600 °C mainly released residual atmospheric gas contamination and small amounts of sample He and Ne (less than 10%). Total gas extraction was performed in the second step at 1700 °C. The 600 °C fractions were not analysed routinely because of the large concentrations of H2O, CH4, and CO2, which would increase the memory in the mass spectrometer and therefore compromise further measurements. Due to the nearly complete release of atmospheric Kr and Xe in the 600 °C step the remaining non-cosmogenic Kr and Xe in the total extraction step is nearly of primordial meteoritic composition, i.e. Kr/Xe usually is less than 1. This allows to accurately correct the measured Kr isotopic composition for the non-cosmogenic component. Table 1: Meteorites, their Cl-Ar ages [3] and the Kr-Kr ages deduced in this work. Name T36-36 [Myrs] T81 [Myrs] Bath H4 7.62 ± 0.24 11.8 ± 2.5 Canellas H4 7.79 ± 0.25 10.7 ± 1.1 Nassirah H4 8.77 ± 0.28 7.2 ± 0.8 Ochansk H4 7.18 ± 0.23 8.4 ± 1.7 Uberaba H5 6.45 ± 0.21 28.8 ± 5.7 Cereseto H5 6.72 ± 0.21 15.9 ± 1.9 Kerilis H5 8.19 ± 0.27 10.6 ± 2.0 Epinal H5 10.20 ± 0.32 9.8 ± 1.6 Allegan H5 4.41 ± 0.15 6.1 ± 0.6 Gold Basin, UA1217 L4 122 ± 51 Gold Basin, UA682 L4 46 ± 5 Gold Basin, UA682 L4 43 ± 5 Gold Basin, ZH1 L4 16 ± 2 Gold Basin, ZH2 L4 9.2± 1.9 Gold Basin, ZH4 L4 7.6± 2.6 Gold Basin, UA426 L4 29 ± 7 Knyahinya L/LL5 41 ± 4 Results: Two aliquots of the Gold Basin meteorite (UA 682) yield Kr-Kr ages in good agreement. This confirms that the method used here produces reproducible K1-Kr exposure ages in meteoritic samples of ∼ 1 gram. A second test was performed using the L/LL5-chondrite Knyahinya. The Kr-Kr age of 41 ± 4 Myrs is in very good agreement with the ages of 40.5 Myrs given by [6] and 39.5 ± 1.0 Myrs by [7]. A comparison of the exposure ages deduced via Cl-Ar [3] with the Kr-Kr ages determined here is shown in Fig. 1 for the meteorites Bath, Canellas, Nas Lunar and Planetary Science XXXIV (2003) 1219.pdf