INTERPHASE SEPARATION OF EVEN URANIUM ISOTOPES IN DICTYONEMIC SHALES FROM THE BALTIC BASIN

The content and isotopic composition of uranium extracted under variou s leaching conditions from the dictyonemic shale from Estonian deposit s are determined by a-spectrometry. The following features characteris tic of the natural nuclear-chemical processes in black shales are eluc idated: (i) the preferential oxidation of uranium-234 to the hexavalen t state despite the presence of 16% of organic matter, which is respon sible for the occurrence of mobile radiogenic uranium; (ii) ultrasmall size of uranium phase particles (ca. 250 Angstrom), which is responsi ble for the loss of 80-82% of recoil atoms; and (iii) incorporation of uranium recoil atoms into surface layers of relatively soluble minera ls, which is responsible for dispersion of uranium decay products in t he environment. The secular disequilibrium in various components of th e rock enables us to advance a concept of the secular radioactive disb alance among various phases of the multiphase mineral system. A quanti tative model is proposed of the kinetics of redistribution of uranium- 234 between the source of uranium recoil atoms and collector phases.