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

This study focuses on cosmogenic effects on lithium, boron, and tungsten isotopes in CAIs from carbonaceous chondrites. The results for lithium isotopes have been used to properly correct experimental data obtained by Chaussidon et al. (2006a) for Allende CAI 3529-41 for cosmogenic contributions. After proper correction, the new inferred ratios slightly but systematically differ from the original ones, indicating that the original study slightly overestimated the cosmogenic corrections. A statistical interpretation of the data indicates that there is a non-linear correlation between 7Li/6Li and 9Be/6Li that might be interpreted as a disturbed isochron. For boron isotopes, the data obtained here confirm that cosmogenic corrections on the 10Be–10B dating system are indeed very minor and can in most cases be neglected. However, a statistical interpretation of the data demonstrates that the type of the fitting procedure used can significantly change the results. It is demonstrated that a fitting procedure using the uncertainties as weights gives a slope about 50% shallower compared to the slope obtained using a non-weighted fitting procedure (for the same data). Such a difference can be wrongly interpreted as an age difference of about one half-life. The modeled results for tungsten isotopes clearly demonstrate that cosmic-ray effects might be of importance for samples having high Ta/W ratios due to thermal neutron capture on 181Ta to form 182W. This effect does not increase the scatter of the data around the isochron but increases the slope of the isochron and therefore gives too old formation ages for the studied CAIs. Consequently, a proper handling of cosmic-ray induced shifts is necessary for high precision 182Hf–182W dating studies.