Uranium and thorium series disequilibrium in quaternary carbonate depositsfrom the Serra da Bodoquena and Pantanal do Miranda, Mate Grosso do Sul State, central Brazil

Activities of gamma-ray emitting members of the uranium (U-238) and thorium (Th-232) series were measured in a quaternary limestone deposit that outcr ops in the southeastern Pantanal Matogrossense Basin and in quaternary tufa s deposited at the drainage of the Serra da Bodoquena. It is a first step i n a study of the mobilization of uranium and thorium series and its: relati on to surface hydrology, in a region vr here carbonate deposits are bring c ontinuously dissolved and reprecipitated. The obtained results show that al l these deposits are characterized by very low concentrations of uranium an d thorium, The U-238/Ra-226 and Th-228/Ra-228 activity ratios are significa ntly different than 1.0, indicating that both series are in radioactive dis equilibrium. Although the Serra da Bodoquena deposits seem to be very recen t, their very fine granulation and high porosity suggest that they behave a s open systems for geochemical exchanges of uranium and thorium series memb ers, The Pantanal do Miranda limestone has a radiocarbon age of 3900 yr BF. Since the thorium series is in disequilibrium it is also concluded that th is deposit behaves as an open system for geochemical exchanges. (C) 2000 El sevier Science Ltd, All rights reserved.