Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints |
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| Institution: | 1. Department of Biological, Geological and Environmental Sciences, University of Bologna, Via Zamboni 67, 40126 Bologna, Italy;2. Department of Geological and Environmental Sciences, University of Bari Aldo Moro, Via E. Orabona 4, 70125 Bari, Italy;3. Department of Earth and Marine Sciences, University of Palermo, Via Archirafi n. 20, 90123 Palermo, Italy;4. Jesi, Italy |
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| Abstract: | Chemical (major and trace elements) and isotopic compositions (δD and δ18O in waters and δ13C in CO2 and 3He/4He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO3, B-depleted groundwaters and rising convective Ca-SO4(HCO3), CO2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3He/4He ratios, suggesting a contribution of mantle 3He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4He in the deeper gases. Paradoxically, there is more relative concentration of 4He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ13C of CO2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-13C depleted organic C, have δ13C in CO2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO2 should generate CO2 with more positive values of δ13C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed. |
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