Abstract
Calcium carbonate (travertine) deposits precipitated from CO 2-rich thermo-mineral waters of the Auvergne region (French Massif Central) have high 226Ra (up to 2 Bq·g −1) and 228Ra activities. 210Pb and 228Th, produced by decay of 226Ra and 228Ra respectively, are absent in the carbonate at the time of precipitation. It is thus possible to use these nuclides to date deposits in the range 0.1–100 years. In some cases, the decay of 226Ra could be used for dating older travertines (up to 10 000 yr). 226Ra in most thermal waters appears to be correlated with Mg, and both elements can be leached during water flow through formerly hydrothermally altered zones of the crystalline basement, which might have concentrated U in secondary minerals in a reducing environment.
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