(228Ra/226Ra) ratios in hydrothermal carbonates and the origin of radium in CO2-rich waters of the Lodève Basin (South of France)

Abstract The present study reports data on CO 2 -rich geothermal waters and their associated carbonate deposits from the SW part of the Permian Lodeve Basin (South of France). Both waters and carbonates are enriched in 226 Ra, with activities up to 2.2 Bq/L and 3.5 Bq/g respectively. A series of carbonate samples precipitated around the studied wells since 1990 were dated through the 226 Ra– 210 Pb and 228 Ra– 228 Th methods, and the results are discussed in a new Concordia diagram. While both methods give comparable dates for many samples, Ra–Pb ages are sometimes much older than Ra–Th ages. We propose that these discrepancies result from the adsorption of 222 Rn daughter nuclides in such a Rn-rich environment. The ( 228 Ra/ 226 Ra) 0 activity ratios calculated at the time of deposition are remarkably constant for about 12 years, suggesting the presence of a steady-state aquifer. The low ( 228 Ra/ 226 Ra) 0 ratios (0.348 ± 0.008) are best explained if most of the Ra derives from the Cambrian carbonate series (mainly dolostone) underlying the Permian Basin, which have low Th/U and similar ( 228 Ra/ 226 Ra) ratios. This study suggests that Ra and other alkaline-earth elements do not necessarily originate from the rocks surrounding the deep geothermal reservoir, but rather may be introduced into the CO 2 -rich water during its transfer towards the surface and interaction with carbonate rocks.