Rare gases in separated whitlockite from the St. Severin chondrite: xenon and krypton from fission of extinct 244Pu

Helium, neon, argon, krypton and xenon data are presented from stepwise heating of samples of the mineral whitlockite from the chondritic meteorite St. Severin. The xenon is shown to be a uniform mixture derived from ²⁴⁴Pu fission and rare-earth element spallation. The krypton similarly contains spallation products and ⁸⁶Kr from ²⁴⁴Pu fission. Plutonium-244 fission yields of ⁸⁶Kr/¹²⁹Xe/¹³¹Xe/¹³²Xe/¹³⁴Xe/¹³⁶Xe = 1.9 ± 0.5/4.8 ± 5.5/24.6 ± 2.0/88.5 ±3.0/93.9 ± 0.8/  100 are obtained. The helium, neon and argon are dominated by spallation and radiogenic ⁴He and ⁴⁰Ar. A pile neutron irradiation experiment does not yield a unique Pu/U ratio for this mineral but yields ratios varying from 0.045 down to 0.017. Both the high concentration of ²⁴⁴Pu fission xenon and the high ratio of Pu/U previously reported by Wasserburget al. (J. Oeophys. Res. 74, 4221–4232, 1969) are thus confirmed.