Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints

摘要

Chemical (major and trace elements) and isotopic compositions (deltaD and delta(18)O in waters and delta(13)C in CO2 and He-3/He-4 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 (N-2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N-2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N-2 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 He-3/He-4 ratios, suggesting a contribution of mantle He-3 to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of He-4 in the deeper gases. Paradoxically, there is more relative concentration of He-4 in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the delta(13)C 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-C-13 depleted organic C, have delta(13)C 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 VC 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. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 43]