Trace elements in Mirror of Venus lake waters (Pantelleria Island, Italy)

Abstract

Mirror of Venus lake (MV) is a small (1.5 km2) endorheic alkaline lake located in a calderic depression on the northern side of Pantelleria Island (South Italy), a quiescent Pleistocenic stratovolcano located in the NW-SE trending continental rift developed between North Africa and Sicily. The lake, sub-squared in shape (around ca 450 m long and ca 350 m wide and 13 m depth) water input contribution is essentially from the thermal springs situated on its shores (Q = 0.1 l.s-1) and from meteoric waters. A total volume of 580.000 m3 has been estimated in June 2014. CO2 flux measurements made with a floating accumulation chamber in 2014 on the MV surface (both diffusive and bubbling CO2) highlighted a total CO2 flux of 14 t.d-1. MV and spring water chemical compositions are the result of notable interactions between waters and Na-rich peralkaline rhyolite (Pantellerite) enhanced by large amounts of dissolved CO2 of magmatic origin. Due to the small volume of the lake, changes in hydrological and meteorological/rainfall inputs strongly control the composition of MV together with water evaporation and precipitation of solid phases (mainly carbonates). Most of sampled waters show a partial equilibrium with rocks at temperature estimated around 170°C. Vertical profile doesn’t show variation with depth of chemical-physical characteristics and in gas dissolved composition. The frequent overturning of the water lake, together with morphological (small volume and shallow dept) and chemical (high pH) preclude the dangerous accumulation of CO2 gas on the floor. In particular, trace elements like Li, Rb, Cr, Hg, Cs and B in MV are enriched respect springs due to evaporative phenomena, while Mg, Mn, Fe, Sr, Ba, Ti, Ni, Ca and SiO2 are depleted because of their precipitation as solid phases. The influence of rock composition on the concentration of elements in waters is emphasize by their concentration in fumarolic steam. In fact, with the exception of Ni, and partially Al and Cd, the majority of trace elements show concentrations lower than that the MV and springs.