Poás volcano in Costa Rica as a hydrothermal analog for Mars

Mars has experienced intensive volcanic and impact activity early in its history, coinciding with a similarly extensive hydrologic activity on a global scale. These activities constitute the main ingredients of hydrothermal activity. Data acquired from the study of Martian meteorites, remote sensing spectral observations, and robotic rovers has shown the surface of Mars to be mineralogically diverse including mineral assemblages that resemble those of analogous hydrothermal systems on Earth. In particular, evidence for extensive acid-sulfate weathering has been observed by the MERs at Gusev and Meridiani, as well as by MSL at Gale crater. Furthermore, there is growing evidence for silicic volcanism on Mars as indicated by the detection of silica-rich mudstone at Gale containing tridymite and cristobalite coupled with spectral observations indicative of felsic rocks in geographically disparate locations on Mars. For that, the Poás volcano in Costa Rica offers a geologic setting that can be analogous to similar environments on Mars. The Poás volcano is a basaltic andesite stratovolcano in central Costa Rica. Its caldera houses a highly acidic lake inside the caldera 130 m below the crater rim. The volcano has been active in recent historical times, and is currently displaying intensive activity since Apr 2017. Unaltered andesitic basalts collected from the 1953-1955 magmatic activity are mainly composed of plagioclase and minor amounts of orthopyroxene and olivine. We collected samples during our fieldwork in March 2017 (few weeks before its eruption) from fumaroles inside the caldera. The fumaroles were emitting gases at 92°C, and the acidic lake < 20 m away had a pH of 1.5. XRD analysis of samples taken from 4 different fumaroles shows high concentrations of elemental sulfur, gypsum, alunite, and cristobalite along with minor abundances of hematite, anatase, and amorphous silica. Most of these minerals have been observed on Mars under potentially similar settings. We plan to continue our investigation by carrying out additional analyses and compare to samples collected from earlier campaigns to gain a better understanding of how the mineralogy changes with ambient conditions and look for short-term changes, which may help constrain further the conditions in which similar assemblages may have formed on Mars.