Assessment of secondary aqueous processes in sulfate formation on Earth and Mars using geochemical and orbital methods

On Earth, low temperature aqueous processes and chemical weathering of sulfur (S)-rich bedrock often lead to abundant formation of secondary calcium (Ca) and magnesium (Mg)-sulfate minerals, particularly in dry climates. In Gale crater on Mars, secondary Ca- and Mg-sulfates have been observed by the Curiosity rover in veins and sediment cement throughout the central mound called Aeolis Mons (informally named Mount Sharp), but the sulfate source(s) and climate conditions during their formation are poorly understood. Therefore, we investigated the formation of secondary sulfate minerals in an analog environment of the Rio Puerco watershed located in northwestern New Mexico using sulfur, oxygen, and hydrogen isotope compositions. The obtained isotope results suggest that weathering (oxidation) of bedrock sulfide minerals is the main sulfate source for precipitation of sulfate minerals in veins and cement. These formation processes are controlled by multiple events of infiltration and top-down movement of meteoric water occurring in a semi-arid climate and often exhibit distinctive surface expressions of the bright-toned sulfate-rich salt accumulations on the exhumed surfaces at outcrop scale. The latter shows similarities with some of the sulfate occurrences detected by Curiosity in halo-like features and on eroded bedding planes of Aeolis Mons. Comparisons of aerial imagery, mineralogy, and climate data suggest that these surface accumulations of secondary salts result from post-lithification fluid flows involving multiple cycles of redissolution and reprecipitation of already existing sulfate salts in the surficial deposits. Further analyses of orbital hyperspectral and imagery data of hydrated Mg-sulfate minerals from Aeolis Mons in Gale imply they are often bright-toned and present on hillslopes, at local topographic lows, and crossing morphological boundaries. Overall, this suggests that the widespread and abundant presence of secondary Mg-sulfates in the sedimentary strata of Aeolis Mons reflects multiple processes of water-rock interactions that took place during diagenesis and subsequent fluid flows.