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

摘要: Abstract U-series dating of hydrothermal carbonates, deposited by CO2-rich thermo-mineral waters the French Massif Central, provides new insights on long-term behaviour a continental system. Dating aragonite veins and travertines impregnating old terrace levels Allier river allowed identification main episodes surface deposition, thus factors likely to influence these circulations. Fifteen layers from three four two separate areas were analysed. Sr isotope compositions also measured vein samples. The results show that this region was subjected at least 3 or near deposition: 253 208 ky, 135 100 ky less than 8 ky. Comparison ages with global climatic curve indicates preferential deposition carbonates occurs during warm periods, suggesting strong conditions It is suggested does not necessarily imply absence carbonate cold dry but rather precipitation might occur depth before geothermal fluids reach surface. In addition, recorded 87Sr/86Sr (234U/238U) initial ratios in Coudes remained remarkably constant over 250 ranging 0.71360 0.71371 3.10 3.39 respectively. samples coming Saladis slightly higher ratio around 3.95. constancy such long period suggests system steady state respect water-rock interaction. We propose possible model allowing conservative despite variations water recharge rates, response variations. difference between systems existence reservoirs constrains their lateral extension few km most.