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Uranium Isotopic Fractionation (234U, 238U) in the Formation of Ice Crystals. C. 15–23

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Section: Geosciences

UDC

546.791.027

Authors

Evgeniy Yu. Yakovlev*, Georgiy P. Kiselev*, Sergey V. Druzhinin*, Sergey B. Zykov*
*Federal Center for Integrated Arctic Research, Russian Academy of Sciences (Arkhangelsk, Russian Federation)

Abstract

Since the discovery of the uranium even isotopes fractionation (Cherdyntsev – Chalov effect) some of the researchers have developed a firm conviction that the separations of uranium isotopic pairs 234U and 238U does not occur in mineral systems, and the fractionation is possible only in the transition from the solid phase to the liquid one because of the different isotopic mobility. The observed excess 234U isotope in the total uranium of various minerals and rocks, with the significantly higher age than the period of the secular equilibrium establishment, was explained by the authigenic origin or background whitening of the migrational uranium. Disequilibrium uranium was found later in ore minerals; its origin was difficult to explain by the authigenic nature. However, now the problem of the 234U kinetics and formation of variations of isotopic ratio 234U/238U in natural environments remains unsolved. In this context, we have got the idea of an experiment to monitor the formation of non-equilibrium uranium in the solid phase formation from the solution with a known isotopic composition through the example of water crystallization at freezing, as the most accessible experimental system. The experimental studies of uranium isotopic fractionation in the formation of ice crystals at the partial freezing of trapped fluid are carried out. The forming ice is depleted by isotope 234U, and the residual water is enriched by the radiogenic uranium atoms. A parent nuclide 238U and a daughter product 234U, connected by a single decay chain, in the solid phase formation behave differently, which confirms the existence of uranium in water in two forms: dissolved uranium in the form of individual compounds and uranium, which is in a mineral particle separated by a recoil nucleus 234Th from the rock at entering the water.

Keywords

uranium isotope, alpha decay, ice crystal, isotopic fractionation, 234U/ 238U
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