Volcanic crater lake systems: a terrestrial analogue for the sulphate terrains on Mars

Abstract

Sulphate-rich terrains lie scattered over the surface of Mars. Based on spectral data from orbiters and in-situ measurements by rovers these terrains are rich in sulphate and iron minerals. The two main theories of formation, deposition of sulphate minerals in an evaporating sea, or in-situ alteration of host rock by percolating acidic rain, are not able to explain all observations from these sulphate-terrains. Hyperacid, sulphate-rich, high temperature volcanic lakes respond to changing volcanic conditions. Chemical alteration reactions taking place between these waters and the host-rock depend on multiple variables, namely pH, temperature and chemical compositions of both the lake water and the host-rock. The alteration minerals associated with these systems show similarities to the Martian sulphate terrains.These minerals include alunite, anhydrite, Fe-oxides and jarosite, which make hyperacid volcanic lake systems a possible terrestrial analogue for the Martian sulphate terrains. To test the hypothesis, geochemical modelling of the terrestrial systems was performed using the PHREEQC speciation programme (Parkhurst & Appelo, 1999). Saturation indices of various minerals in the crater lake waters were calculated during dilution, evaporation, heating and gas equilibration scenarios. Furthermore, a water-rock interaction model was constructed to mimic the chemical interaction between the lake and its surrounding rock. The basaltic-andesitic Poás volcano (Costa Rica) hosts a crater lake system which was used to construct the models. The models were able to explain the presence of most of the alteration minerals found near Poás. The Copahue system (Argentina), compositionally similar to Poás, was ejected during the December 2012 eruption. The mineral assemblages from the hydrothermally altered ejecta, analysed by XRD, could be compared to the ones in both the heating and water-rock models of Poás. The average Martian rock is basaltic in composition, making the adaption of the water-rock model necessary. Since the volcanic lake compositions depend on the host-rock, a Martian fluid was reproduced. The Martian model was able to explain the presence of most minerals in the sulphate terrains. An exception was the mineral kieserite (MgSO4∙H2O) which did not reach saturation in any of the models. Overall, the terrestrial hyperacid volcanic lake systems seem to be a suitable analogue to the Martian sulphate terrains.