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

• Dating of hydrothermal carbonates by 226Ra–210Pb and 228Ra–228Th methods. Data are discussed using a new Concordia diagram.
• (228Ra/226Ra) ratios deduced from dated carbonates trace the origin of radium in the CO2-rich thermal waters.
• The low (228Ra/226Ra) ratio of 0.35 shows that Ra derives from water–rock interaction in Cambrian carbonate rocks.
• 226Ra (and most of the Sr) does not originate from rocks surrounding the deep geothermal reservoir.
• Lower (228Ra/226Ra) ratios away from the thermal plume provide maximum flow rates through fractures in the Lodève Basin.

The present study reports data on CO2-rich geothermal waters and their associated carbonate deposits from the SW part of the Permian Lodève Basin (South of France). Both waters and carbonates are enriched in 226Ra, 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 226Ra–210Pb and 228Ra–228Th 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 222Rn daughter nuclides in such a Rn-rich environment. The (228Ra/226Ra)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 (228Ra/226Ra)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 (228Ra/226Ra) 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 CO2-rich water during its transfer towards the surface and interaction with carbonate rocks.