Isotopenanalyse der Edelgase in einem Tellurerz von Boliden (Schweden)

Abstract

The stable He-, Ne-, Ar-, Kr- and Xe-isotopes and the Te-, K-, U-, Th- and Se-concentrations in a Precambrian bismuth-tellurium ore have been measured. We have found an age of 0.53 x 10 9 y with K-Ar, 0.55 x 10 9 y with U/Th-He 4 and 1.56 x 10 9 y by the U-Xe 136 method. The Xe 131 -excess and part of the Xe 129 -excess result from n-capture and subsequent decay of Te 128 and Te 130 . The remaining part of the Xe 129 -excess and the high U-Xe 136 age give evidence for a higher neutron flux before the last cooling of the ore. The tellurium was probably associated with much more uranium than now. Before 0.54 x 10 9 y, differentiation of this uranium occured. Moreover, He and Ar but not all of the Xe were removed at this time. After cooling, the Xe 129 excess was formed by the decay of the long-lived I 129 which had been formed from (n,γ) reactions on Te 128 by the higher neutron flux. From the excess in Xe 130 , a half life for the double β-decay of Te 130 of 6.2 x 10 20 y is calculated. Besides fission-Kr, there is an excess of Kr with atmospheric composition and in addition an excess of Kr 82 and Kr 83 also. From the Kr 82 -excess and the selenium content a lower limit for the Se 82 -double β-decay half life of T 1/2 ≥ 10 18 y is calculated. No evidence is given for Ar 38 from U-fission.