Hcl Degassing From Extremely Acidic Crater Lakes: Preliminary Results From Experimental Determinations And Implications For Geochemical Monitoring

Crater 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 degrees 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 10(4) - 10(5) m(3) 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.