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Calibration of cosmogenic noble gas production based on Cl-36-Ar-36 ages. Part 2. The Kr-81-Kr dating technique

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Publication date 2015
Author Leya Ingo, Wieler Rainer, Vogel Nadia, et al.,
Project High energy particle irradiation in the early solar system and stellar nucleosynthesis studied with noble gases in primitive meteorites
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Original article (peer-reviewed)

Journal Meteoritics and Planetary Science
Volume (Issue) 50
Page(s) 1863 - 1879
Title of proceedings Meteoritics and Planetary Science
DOI 10.1111/maps.12515

Open Access

URL http://onlinelibrary.wiley.com/doi/10.1111/maps.12515/full
Type of Open Access Publisher (Gold Open Access)

Abstract

We calibrated the Kr-81-Kr dating system for ordinary chondrites of different sizes using independent shielding-corrected Cl-36-Ar-36 ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q. The thus-determined average cosmogenic Kr-78/Kr-83, Kr-80/Kr-83, Kr-82/Kr-83, and Kr-84/Kr-83 ratios are slightly different from earlier estimates (Lavielle and Marti 1988; Wieler 2002). The cosmogenic Kr-78/Kr-83 ratio is correlated with the cosmogenic Ne-22/Ne-21 ratio, confirming that Kr-78/Kr-83 is a reliable shielding indicator. Previously, Kr-81-Kr ages have been determined by assuming the cosmogenic production rate of Kr-81, P(Kr-81)(c), to be 0.95 times the average of the cosmogenic production rates of Kr-80 and Kr-82; the factor Y=0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However, Y should be regarded as an empirical adjustment. For samples whose Kr-80 and Kr-82 concentrations may be affected by neutron-capture reactions, the shielding-dependent cosmogenic (Kr-78/Kr-83)(c) ratio has been used instead to calculate P(Kr-81)/P(Kr-83), as for some lunar samples, this ratio has been shown to linearly increase with (Kr-78/Kr-83)(c) (Marti and Lugmair 1971). However, the Kr-81-Kr ages of our samples calculated with these methods are on average similar to 30% higher than their Cl-36-Ar-36 ages, indicating that most if not all the Kr-81-Kr ages determined so far are significantly too high. We therefore re-evaluated both methods to determine P(Kr-81)(c)/P(Kr-83)(c). Our new Y value of 0.70 +/- 0.04 is more than 25% lower than the value of 0.95 used so far. Furthermore, together with literature data, our data indicate that for chondrites, P(Kr-81)(c)/P(Kr-83)(c) is rather constant at 0.43 +/- 0.02, at least for the shielding range covered by our samples ([Kr-78/Kr-83](c)=0.119-0.185; [Ne-22/Ne-21](c)=1.083-1.144), in contrast to the observations on lunar samples. As expected considering the method used, Kr-81-Kr ages calculated either directly with this new P(Kr-81)(c)/P(Kr-83)(c) value or with our new Y value both agree with the corresponding Cl-36-Ar-36 ages. However, the average deviation of 2% indicates the accuracy of both new Kr-81-Kr dating methods and the precision of the new dating systems of similar to 10% is demonstrated by the low scatter in the data. Consequently, this study indicates that the Kr-81-Kr ages published so far are up to 30% too high.
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