THE GEOCHEMISTRY OF VOLCANIC LAKES

Volcanic lakes on earth can be grouped according to pH and anions: 1. hyperacidic sulfate-chloride brine lakes (crater lakes), 2. acid lakes that are mixtures of meteoric water and volcanic effluents, and 3. carbonate-rich lakes, with pH ranging from 4 to 9. The pH values in hyperacid lakes are ~zero, which upon evaporation can lead to negative pH lakes, and with dilution can reach pH values of 1-2. These lakes are commonly surface expressions of a deeper volcano-hydrothermal system where meteoric waters capture magmatic volatiles (H₂O, HF, HCl and SO₂). Some crater lakes have a direct volcanic gas input. On earth such lakes exist where meteoric water is available: in the wet tropical belt, on high mountains (glaciers), and at high latitudes (glaciation). The acidity is generated during the scrubbing of magmatic SO₂, which disproportionates into HSO₄^-, H⁺, and S^o (usually liquid S^o) or H₂S. These lakes tend to have a floating slick of S^o on the surface or pools of liquid sulfur at their bottom. Abundant liquid sulfur may fill pore spaces in the underlying hydrothermal system. Minerals that commonly form in the subterranean section are anhydrite, hydrothermal silica, and alunite-jarosite, with a suite of metal sulfides. The water-rock reaction at temperatures > 250 ^oC tends to be close to congruent, with the exception of hydrothermal silica precipitating while rock dissolution is ongoing. These lakes carry the volcanic elements S, F, Cl and some volatile traces like As, I and B, whereas rock forming elements are derived from water rock interaction. In the surface environment, these fluids precipitate sulfur, gypsum, hematite and jarosite. The second lake group is transitional into the first and has pH values of 2-3, and may have precipitates of schwertmannite, goethite or ferrihydrite. The iron chemistry may be influenced by photo redox processes. The carbonate-rich lakes are either expressions of underlying geothermal systems rich in carbonate or have a direct CO₂ input. The geothermal fluids tend to be rich in ferrous iron, and siderite may form or Fe-oxides upon oxidation with atmospheric oxygen. Terrestrial examples of these three lake groups will be highlighted and their mineralogy compared to the sulfates and carbonates that have been documented on Mars.