Rare earth elements and stable isotope signatures from hydrothermal alteration in Vargeão dome with implications for impact-generated hydrothermal systems on Mars

Hydrothermal systems can provide a habitat for early life on planetary bodies throughout the Solar System. For Mars, impact-generated hydrothermal systems (IGHSs) are especially interesting, because of its high crater density and the presence of hydrous minerals in Martian craters. However, it is uncertain whether these hydrous minerals formed in an IGHS, or if they formed earlier and were then excavated by the impact. It is also unknown whether conditions in these systems are hospitable for life.
To gain further insight into these open questions, this study investigates two types of vein-forming hydrothermal alteration in the Vargeão Dome impact structure (Brazil) residing in basaltic host rock similar to the Martian surface. Rare Earth Element (REE) patterns of the veins and the surrounding host rock and stable C and O isotopes in calcite were studied using linear regression modelling and thermodynamic modelling to constrain fluid conditions.
REE analysis as proxy for major elements and modelling suggest that elevated amounts of Al, Fe, Mg, and to a lesser extent Na and Ca are needed for the formation of white and red veins. They also suggest that the white veins form under reducing conditions and with limited aquifer influence, whereas the opposite is true for the red veins. Stable isotope signatures indicate that all calcite formed from a meteoric fluid in the same hydrothermal stage as part of the white veins. Furthermore, the thermodynamic modelling suggest that this calcite precipitated from a fluid that underwent gradual heating from 27 to 55 °C combined with degassing of CO2. Together with observed calcite amygdales close to the vein rim and geodes outside of the impact structure, which are both isotopically similar to the calcite in veins, this suggests that the white veins all formed before the impact.
If Martian impact craters are similar to Vargeão Dome, the hydrous minerals are more likely to have been excavated by the impact and did not form as part of an IGHS. However, gradual heating in the Vargeão pre-impact hydrothermal system, as well as the high-nutrient content related to the hydrothermal system, could favour the development of mesophiles in impact-excavated systems on Mars.