MARS-TYPE MINERAL ASSEMBLAGES IN TERRESTRIAL VOLCANIC LAKE SETTINGS

Remote sensing observations and lander data have ascertained the widespread occurrence of sulphate-rich mineral associations across the Martian surface. These minerals must have formed under acidic and oxidizing conditions in the presence of water. However, there is no consensus on the mechanisms of their formation. Here, we present evidence that active volcanoes hosting hyperacid crater lakes are promising terrestrial analogues where the formation of Mars-type mineral assemblages can be studied in situ. Combined findings at Poás volcano (Costa Rica) and Copahue volcano (Argentina), which include the detection of critical mineral assemblages and results from geochemical modelling, serve as a guide for testing the hypothesis.

In the hot (T>45°C) and hyperacid (pH<2.0) sulphate-chloride lake of Poás, gypsum/anhydrite and amorphous silica are the major precipitating phases, sometimes accompanied by native sulphur with sulphide inclusions. Minor amounts of barite and TiO₂ are commonly produced as well. Alteration minerals in the lake sediments and in rocks that interacted with acid lake or acid rain water include alunite, jarosite, rhomboclase and Fe-oxides. Similar sulphate-rich assemblages mark the secondary mineralogy at Copahue, which usually hosts a hyperacid lake, chemically comparable to Poás. A phreato-magmatic eruption at Copahue, in December 2012, ejected both lake sediments and rock fragments, providing access to the underlying system.

To identify processes and conditions that could create these ‘Martian’ mineral assemblages, geochemical modelling was carried out with PHREEQC. Starting from Poás’ lake, saturation indices of relevant minerals were calculated for a range of scenarios, including dilution, evaporation, heating, and gas-water-rock interactions. Intensive water-rock interaction was the most successful scenario in reproducing the alteration minerals present. Models with increased heating to temperatures higher than the observed ones in the lakes predict the formation of alunite-bearing assemblages, in line with the presence of this mineral in rock fragments ejected from hotter subsurface parts of the magmatic hydrothermal systems. A model involving an artificial Martian fluid and an average Martian basalt could produce most minerals in the sulphate terrains.