Abstract
The concentrations and isotopic compositions of argon, krypton and xenon have been determined in a grain size suite of zircons separated from pyroxene syenite of the Botnavatn Igneous Complex, southwestern Norway. The U Pb systematics of these zircons has been studied previously. Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of 238U and a component with atmospheric isotopic composition. From correlation diagrams the fissiogenic component is determined to be: 83Kr : 84Kr : 86Kr = (4.6 ± 1.3) : (11.0 ± 2.0) : 100 and 129Xe : 131Xe : 132Xe : 134Xe : 136Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100 . The fissiogenic 136Xe/ 86Kr is 6.0 ± 0.4. The Ar isotopic composition shows radiogenic 40Ar and a small excess of 38Ar. The excess 38Ar of about 1 × 10 −11 cm 3 STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of 238U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed 38Ar concentrations. U Xe ages are (1.19 ± 0.07) Ga, in agreement with U Pb ages. However, if the recoil loss of fissiogenic Xe is considered the U Xe ages of these zircons are about 1.53 Ga, which is comparable with the K Ar ages and some Rb Sr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of 238U prevents to decide which age is the true crystallization age.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call