U, Ra and Ba incorporation during precipitation of hydrothermal carbonates:: Implications for 226Ra-Ba dating of impure travertines

We studied U, Ra and Ba incorporation in calcite in a natural CO2-rich hydrothermal area from the French Massif Central. Along the western border of the Limagne graben, several springs are exploited for the petrifaction of various artifacts with calcite. These sites offer the opportunity to sample the water and the calcite layers downflow from the spring, and thus to follow the evolution of their U, Ra and Ba contents as precipitation proceeds. Our results show that the apparent partition coefficients of U, Ra and Ba between water and calcite decrease during precipitation for the three elements. We found no direct relation between this variation and the main factors able to influence the partition coefficient, such as precipitation rate, which suggests that the incorporation of these trace elements could result from a composite process of adsorption and coprecipitation. Ra and Ba have a similar behaviour, with an apparent partition coefficient decreasing from 0.80 to 0.47 for Ra and 0.96 to 0.68 for Ba, resulting in a small (less than or equal to 10%) variation of the Ra/Ba ratio. The apparent partition coefficient of U decreases from 0.38 to 0.20. These apparent coefficients are much higher than equilibrium values but might be applicable to natural systems with high precipitation rates. We also investigated the possibility of using the decay of the Ra-226-excess, or the decrease of the (Ra-226)/Ba ratio to date older deposits. Whereas the Ra-226 initial activity at the time of deposition has not remained constant, and can not be used for dating, the (Ra-226)/Ba method gives better results, when appropriate corrections for detrital contamination in Ba are made. Mixing diagrams using Th as an indicator of contamination allow calculation of the (226Ra)/Ba ratio of the pure carbonate component. The calculated ages of five travertine layers range from 330 to 800 years, suggesting a mean deposition rate of about 1 cm/yr. The relatively good agreement of the calculated ages and stratigraphic positions of the samples suggests that this method could be successfully applied to date impure hydrothermal carbonates in the range 0-10 ky. Copyright (C) 2000 Elsevier Science Ltd.