U-SERIES AND TH-SERIES RADIONUCLIDES IN CO2-RICH GEOTHERMAL SYSTEMS IN THE FRENCH MASSIF-CENTRAL

Uranium-and Th-series radionuclides have been analyzed in CO2-rich geo thermal waters and their associated iron hydroxide and carbonate depos its from the French Massif Central, A geochemical budget for U and Ra in water and solid deposits has been estimated. Most of the U (> 75%) remains in solution, probably because of the presence of soluble U-car bonate complexes. In contrast, a large fraction of Ra (> 50%) is scave nged by both deposit types. The distribution coefficient of U and Ra i n calcite are 0.2 and 0.4 respectively. The DR, value is much higher t han the experimentally determined coefficients and represents a non-eq uilibrium value that might be applicable to natural systems with high CaCO3 precipitation rates.