Long-term behaviour of continental hydrothermal systems: U-series study ofhydrothermal carbonates from the French Massif Central (Allier Valley)

U-series dating of hydrothermal carbonates, deposited by CO2-rich thermo-mi neral waters of the French Massif Central, provides new insights on the lon g-term behaviour of a continental hydrothermal system. Dating of aragonite veins and travertines impregnating old terrace levels of the Allier river a llowed identification of the main episodes of surface deposition, and thus factors likely to influence these circulations. Fifteen layers from three v eins and four travertines from two separate areas were analysed. Sr isotope compositions were also measured on vein samples. The results show that this region was subjected to at least 3 main episodes of surface or near surface hydrothermal deposition: 253 to 208 ky, 135 to 100 ky and less than 8 ky. Comparison of these ages with a global climatic curve indicates that preferential deposition of carbonates occurs during wa rm periods, suggesting a strong influence of climatic conditions on the hyd rothermal system. It is suggested that this climatic influence does not nec essarily imply the absence of carbonate deposition during cold and dry peri ods, but rather that carbonate precipitation might occur at depth before th e geothermal fluids reach the surface. In addition, the isotope compositions of fluids recorded by the Sr-87/Sr-86 and (U-234/U-238) initial ratios in the aragonite veins from Coudes remain ed remarkably constant over 250 ky, ranging from 0.71360 to 0.71371 and fro m 3.10 to 3.39 respectively. The two samples coming from Saladis show a sli ghtly higher (U-234/U-238) initial ratio around 3.95. The constancy of thes e ratios over such a long period suggests a hydrothermal system in a near s teady state with respect to water-rock interaction. We thus propose a possi ble model allowing a conservative steady state despite variations in the wa ter recharge rates, in response to the climatic variations. The difference between (U-234/U-238) initial ratios measured in the Coudes and Saladis sys tems suggests the existence of two separate reservoirs and constrains their lateral extension to a few km at most. Copyright (C) 2000 Elsevier Science Ltd.