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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Abundance of fissiongenic and pre-reactor natural rare-earth elements in a uranium ore sample from Oklo

Hiroshi Hidaka, Akimasa Masuda, Isao Fujii, Hiroshi Shimizu
Geochemical Journal, Vol. 22, No. 2, P. 47-54, 1988


The behavior of rare-earth elements (REE) in the Oklo natural nuclear reactors was studied. Isotopic ratios of all REE in the Oklo uranium ore samples were measured with a thermal ionization mass spectrometer, and the REE abundances were determined by the stable isotope dilution method. On the basis of these data, the behavior of REE in the process of formation of ore body and during and/or after the operation of the natural nuclear reactor was investigated. The total amount for each of the REE is allotted to natural and fissiongenic fractions by a calculation based on the nuclear data. The pattern for pre-reactor natural REE explicitly endorses the participation of water in formation of this ore deposit. Meanwhile, a fission yield curve involving Nd, Sm and Gd is found to be generally smooth. But deviations in La and Ce, and shifts among the fissiongenic segments for Nd, Sm and Gd are observed. These features can be accounted for as reflecting the systematic differential partial loss of fissiongenic light REE. The partial loss of progenitors of 140Ce and 142Ce is found to be unlikely.

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