1. Geological Institute, University of Bern, Baltzerstrasse 1, 3012 Bern, Switzerland, Environmental Radiometric Research Center, Department of Mathematical Sciences, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England, Mineralogical-Petrological Institute, Isotope Geology, University of Bern, Erlachstrasse 9a, 3012 Bern, Switzerland, and Institute of Geological Sciences, Ukrainian Academy of Sciences, Gonhkar St. 55-b, Kiev 54, 252054 Ukraine