Abstract
Isotope composition of U in minerals of two mineral associations based on aeschynite and pyrochlore was studied. Although the ratio of U isotopes in the mineral associations as a whole is equilibrium (or close to equilibrium), the distribution of the radiogenic 234U isotope between separate parts of these associations is essentially nonuniform. Two models of the disturbance of the radioactive equilibrium are discussed: dependence of the 234U/238U ratio on the U concentration in the mineral (Adloff-Roessler model) and transfer of 234Th recoil atoms from one phase to another (Sheng-Kuroda model). It is impossible within the framework of any of the models to consistently account for the observed distribution of the radiogenic U between different mineral phases. For the quasi-closed mineral system based on aeschynite, a model of the redistribution of the radiogenic U under the action of natural solutions mainly within the mineral association is suggested; for the open system involving pyrochlore, the effects of natural leaching are explained taking into account the valence and chemical state of uranium in the minerals.
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