U- and Th-series radionuclides in CO₂-rich geothermal systems in the French Massif Central

Abstract

Uranium- and Th-series radionuclides have been analyzed in CO₂-rich geothermal waters and their associated iron hydroxide and carbonate deposits 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-carbonate complexes. In contrast, a large fraction of Ra (>50%) is scavenged by both deposit types. The distribution coefficient of U and Ra in calcite are 0.2 and 0.4 respectively. The D_Ra value is much higher than the experimentally determined coefficients and represents a non-equilibrium value that might be applicable to natural systems with high CaCO₃ precipitation rates.