Chemical (major and trace elements) and isotopic compositions (d2H and d18O in waters and d13C 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 d13C 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 d13C 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 d13C 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.
Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints.
Minissale A;Magro G;
2002
Abstract
Chemical (major and trace elements) and isotopic compositions (d2H and d18O in waters and d13C 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 d13C 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 d13C 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 d13C 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
