Fractionation factor of 238U and 235U isotopes in the process of hydrothermal pitchblende formation: A numerical estimate

Owing to the rapid increase in available data on the natural variations of the ²³⁸U/²³⁵U ratio, new isotopic geochemical mark of redox processes are beginning to emerge. In this connection, numerical estimates of the ²³⁸U and ²³⁵U fractionation factor (α(U^IV?U^VI)) accompanying the reduction U^VI → U^IV are needed. Such an estimate has been obtained for hydrothermal pitchblende formation based on results of high-precision (±0.06‰) measurements of the ²³⁸U/²³⁵U ratio in local microsamples of coarse spherulitic pitchblende from carbonate-pitchblende veins at the Oktyabr’sky deposit (Strel’tsovsky uranium ore field, eastern Transbaikal region). For this purpose, we used the formation temperature of hydrothermal pitchblende and a maximum estimate of the fractionation factor for ²³⁸U and ²³⁵U isotopes in the solution-solid phase system under normal (25°C) conditions (Murphy et al., 2014). The most probable isotopic fractionation factor accompanying pitchblende crystallization from hydrothermal solution at T = 320?250°C falls into the interval α(U^IV?U^VI) = 1.00020?1.00023.