HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring

Abstract

AbstractCrater lakes are monitored to detect volcanic unrest starting from the assumption that they behave as condensers for magmatic gases. A further assumption is that acidic gases such as HCl are conservative once dissolved in water. This is not true for extremely acidic crater lakes, whose H+ activity is high enough to induce Cl− hydrolysis and consequently HCl degassing. This study presents the results of experimental determinations at 40–45°C demonstrating that HCl degassing from acidic water depends on pH and Cl− concentration. HCl degassing starts at pH values c. 0.05–0.1 with a rate of 5–10 mg min−1 l−1, increasing up to c. 70 mg min−1 l−1 at pH<−0.2. This implies that the rate of HCl removal from a crater lake with a volume of 104–105 m3 and a seawater-like Cl− concentration ranges from 5 to 50 t h−1. The estimated HCl/H2O ratio in the separated vapour phase (0.01–0.2) is coherent with HCl/H2O ratios of fumaroles. Our experiments imply that: (i) the presence of very acidic gas species in fumaroles can be associated with a liquid-dominated feeding system, and (ii) dissolved in extremely acidic crater lakes, Cl− behaves as a non-conservative component.