Radiogenic, fissiogenic and nucleogenic noble gases in zircons

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 UPb systematics of these zircons has been studied previously.Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of²³⁸U 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 and¹²⁹Xe :¹³¹Xe :¹³²Xe :¹³⁴Xe :¹³⁶Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100. The fissiogenic¹³⁶Xe/⁸⁶Kr is 6.0 ± 0.4.The Ar isotopic composition shows radiogenic⁴⁰Ar and a small excess of³⁸Ar. The excess³⁸Ar of about 1 × 10⁻¹¹ cm³ STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of²³⁸U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed³⁸Ar concentrations.UXe ages are (1.19 ± 0.07) Ga, in agreement with UPb ages. However, if the recoil loss of fissiogenic Xe is considered the UXe ages of these zircons are about 1.53 Ga, which is comparable with the KAr ages and some RbSr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of²³⁸U prevents to decide which age is the true crystallization age.