Impact-generated hydrothermal systems in terrestrial basalt as analogues for life-supporting environments on Mars

Hydrothermal systems in which warm fluids circulate through rock fractures form a habitat for a wide variety of life on Earth today. These systems, mostly associated with sea-floor spreading, are thought to be continuously active for millions of years. Impacts on a rocky planetary surface can also create hydrothermal systems, of a more transient nature. Impact-generated hydrothermal systems, or IGHS, form a potential, ubiquitous habitat not only on Earth, but throughout our Solar System. These systems are of particular interest for Mars, our nearest neighbour, as the planet is covered with impact craters. However, IGHS are currently poorly understood. Hydrous minerals have been observed in craters, but it is unclear if these formed during hydrothermal activity after impact, or were previously present in the crust and excavated. Limitations of in-situ analyses on other planets largely inhibit a further understanding of these systems. Therefore, analogue impact structures on Earth can provide valuable insight into the development of IGHSs on Mars. Of the roughly 210 impact structures recognized on Earth, only two are formed in basaltic target terrain similar to the Martian crust and are well-exposed: the Vargeão Dome and Vista Alegre impact structures in Brazil. This work represents the systematic analysis of hydrothermal alteration within these two structures, and assesses the implications for similar systems on Mars. Vein-forming alteration within both structures has been analyzed using a combination of optical microscopy, scanning electron microscopy, Raman spectroscopy, Rare-Earth Element analysis, Rb-Sr isotopic analysis, thermodynamic-modelling, and fluid-flow modelling to separate the different alteration stages, relate them to the impact, and determine their thermal and chemical evolution. At Vista Alegre, vein-forming alteration is limited to two stages, represented by zeolite- and calcite-bearing veins, found in only three locations throughout the 9.5 km diameter impact structure. Thermodynamic modelling suggests that the dominant, zeolite-bearing phase was characterized by a prolonged heating- and cooling stage, incoherent with an impact-origin. This suggests that impact-generated hydrothermal activity is extremely limited at most. At Vargeão Dome, vein-forming alteration is also limited to two stages, a zeolite-bearing stage and a fragment-bearing stage – the first being dominant, and the second only occurring in the crater center and inner collar of the 12.8 km diameter impact structure. Incorporation of fragments of the first stage into the latter suggests that the fragment-bearing stage formed through impact-reactivation and cataclasis of the first stage. Furthermore, thermodynamic modelling suggests that iron-oxides within the matrix of this stage formed at low temperatures, <40°C. Fluid-flow modelling suggests that Vargeão Dome experienced sustained fluid flow long after impact – explaining the low formation temperature of the iron oxides. This is not present at Vista Alegre, and suggests that the fluid-flow experienced by Vargeão Dome may be a minimum required for a discernable IGHS. Adaptation of these models to Martian conditions suggests that impacts yielding a crater with a diameter of at least four times that of Vargeão Dome, or about 50 km, are needed to experience similar levels and temperatures of fluid-flow. These results suggest that pre-existing hydrothermal systems can be re-actived by impacts. For Mars, this leads to the hypothesis that impact-generated hydrothermal systems preferably occur in craters that previously experienced hydrothermal alteration, meaning in terrains older than about 3.5 Ga when liquid water covered (parts of) the surface of Mars. Furthermore, the size-limit set by the fluid-flow of Vargeão Dome suggests a minimum crater size of about 50 km. This means that, out of the ~380.000 craters larger than 1 km recognized on this planet, only about 0.5 % may contain fossilized remnants of an impact-generated hydrothermal system, making them of particular interest for future search-for-life missions.