Is There Chemically Fractionated Uranium Fission Xenon in Mantle Rocks, Gases, and the Earth's Atmosphere?

A new concept is proposed on the origin of the atmospheric xenon isotope composition, in which the excess abundance of ¹²⁹Xe is due not to the decay of primordial ¹²⁹I trapped by the earth during its formation but to the decay of ¹²⁹I as a product of the neuron-induced fission of ²³⁵U. The published data on the xenon isotope compositions for rocks and gas of mantle origin show a clear-cut correlation between excess ¹²⁹Xe and elevated abundances of other xenon isotopes, particularly ¹³²Xe and ¹³¹Xe. The isotope composition of the excess xenon corresponds to CFF-Xe, which is due to chemical fractionation of radioactive precursors of xenon in the ß-decay chains from the fission of heavy nuclei. Experimental studies of the xenon in the Oklo natural nuclear reactor and of specimens from roll uranium deposits as well as of various other materials show that chemically fractionated fission xenon can be produced under natural conditions. CFF-Xe in the earth's atmosphere eliminates various difficulties in interpreting the contemporary isotopic composition of atmospheric xenon: there is no need for the hypothetical U-Xe or for searching for the mechanism of selective isotopic fractionation of AVCC-Xe. Balance calculations show that the necessary amount of CFF-Xe can be generated by the neutron-induced fission of ²³⁵U migrating into the crust during differentiation of the earth. The concept casts doubt on the correctness of models for the outgassing of the earth in which it is assumed that ¹²⁹Xe is derived from primordial ¹²⁹I.