Rare earth elements, neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central, France

Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO2-ric h springs are characterized by a neutral pH (6-7) associated with total dis solved solids (TDS) from 1 to 7 g l(-1). The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperat ure of around 200 degrees C with superficial waters. These two mineral wate r pools evidenced by Sr isotopes and dissolved REEs could reflect 2 differe nt stages of water-rock interaction and an equilibrium with different miner al assemblages. The concentrations of individual dissolved REEs and total dissolved REEs (C REE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T degrees C, pH, Total Organic C). The dissolved REE concentrations presented as upp er continental crust normalized patterns show HREE enrichment in most of th e samples. The time evolution of REE patterns does not show significant flu ctuations except in 1 borehole, located in the Limagne d'Allier area, which was sampled on 16 occasions over an 18 month period, Ten samples are HREE- enriched, whereas 6 samples show flat patterns. The aqueous speciation of REEs shows that CO32- complexes dominate (> 80%) over the free metal, F-, SO42- and HCO3- complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO2-rich and pH neutral fluids is due essentially to the predominance of th e CO32- complexes. The Sr isotopic composition of the mineral waters in the Massif Central sho ws different mixing processes, in the Cezallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poo rly mineralized waters with minimal groundwater circulation. Two other mine ralized end-members are identified, although the link between the geographi cal location of spring outflow and the mixing proportion between the 2 end- members is not systematic. The range in epsilon(Nd)(0) for mineralized wate rs in the Massif Central correlates well with that of the known parent rock s except for 4 springs. One way to explain the epsilon(Nd)(0) in these inst ances is a contribution from drainage of volcanic rocks. The isotopic syste matics help to constrain the hydrogeological models for this area. (C) 2000 Elsevier Science Ltd, All rights reserved.