Specchio di Venere alkaline lake (Sicily, Italy): geochemical investigations based on gas and water compositions
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Abstract
The word "volcanic lake" is instinctively linked to a hot and acidic water body although alkaline lakes being not too rare. The abundant geogenic CO2 also contributes to weathering reactions within cold or thermal aquifers that release alkali and bicarbonate to groundwater. Such waters emerging at the surface, together with the right climatic and hydrologic conditions, can possibly lead to the formation of alkaline lakes (Pecoraino and D'Alessandro, in press).
Lago di Venere is a small (1.5 km2) alkaline lake located in an endorheic basin on the northern side of Pantelleria Island (South Italy), a Pleistocene stratovolcano located in the NW-SE trending continental rift developed between North Africa and Sicily. With the aim to discriminate the different contributions (geothermal, meteoric, bubbling gases) to lake waters we have collected free and dissolved gases, major and trace elements in the lake and in the springs and shallow wells on its shores. Three main water components can be inferred from chemical data: a) meteoric water; b) sea water; c) local geothermal water. Mixing between meteoric and local geothermal waters is the most evident process. Lake water shows high Cl- and alkali contents, high electrical conductivity (up to 40 mS/cm2) and pH values up to 9.3. Depletion in Mg2+ and the high concentrations in alkali, mainly Na+, are the result of notable interactions between waters and Na-rich peralkaline rhyolite (Pantellerite). This process is enhanced by large amounts of dissolved CO2 of deep mantle origin. Due to the small volume of water, changes in hydrological and meteorological/rainfall inputs strongly control the composition of lake water which is strongly influenced by evaporation and precipitation of solid phases (mainly carbonates). Thermal spring and well waters have also Cl-SO4 alkaline composition but are characterized by higher temperatures (24-65°C), acidic pH and lower EC values respect the lake. Most of sampled waters show a partial equilibrium with rocks at temperature estimated around 170 °C. Dissolved gases in thermal waters are influenced by hydrothermal/magmatic contribution, CO2 being the predominant species. Finally the trace element content of the lake and the feeding springs is also strongly controlled by the same processes that influences the overall chemical composition (mixing of different end-members, evaporation and precipitation of solid phases).