Thermodynamic and kinetic analysis of transitions in clay mineral chemistry on Mars

Although abundant evidence exists for past liquid water on Mars, the composition and amount of that liquid water remain unconstrained. The characteristics of liquid water are preserved within weathering profiles, which can be used to interpret past aqueous conditions. Putative weathering profiles consisting of Al-rich units above Fe/Mg-rich units have been documented in numerous locations on Mars. Of these locations, Mawrth Vallis is the largest in area; therefore, understanding the aqueous conditions that could cause such a profile may also help interpret similar transitions in clay mineralogy elsewhere on Mars. The stratigraphy of the Mawrth Vallis region is characterized by an Al-rich unit dominated by kaolinite and/or montmorillonite with Fe-(oxy)hydroxides and/ or oxides, overlying an Fe/Mg-rich unit composed of nontronite and/or saponite. To interpret the implications of the possible formation of Mawrth Vallis by weathering, we used the reactive transport code CrunchFlow to model alteration of parent material consisting of: (1) nontronite; (2) nontronite and montmorillonite; (3) nontronite and saponite, and (4) saponite. Results indicate either the enrichment of Al-rich clay minerals in an upper weathered unit from the weathering of Fe/Mg-rich clay minerals, or weathering of saponite producing a nontronite-rich alteration product which would be anticipated to subsequently alter to an Al-clay mineral rich capping unit. Results indicate that the - profiles formed by weathering that have experienced the most aqueous/acidic alteration would be characterized by a thick Al-rich capping unit composed of kaolinite with Fe-(oxy)hydroxides, overlying nontronite. Weathering profiles containing nontronite overlying saponite would be consistent with less weathering than an Al-capping unit, as an Al-rich capping would likely develop with the continued weathering of nontronite. Al-rich units are observed in the areas around Jezero crater, Mars the landing area of the Mars2020 rover Perseverance. Characteristics of weathering profiles such as those examined here can be used in the examination of locations on Mars to help choose environments that contained liquid water with characteristics most conducive to habitability and biosignature preservation.

Automated summary

Although evidence of past liquid water on Mars exists, the composition and amount of liquid water remains uncertain. A specific weathering profile in the Mawrth Vallis region, characterized by an Al-rich unit overlying an Fe/Mg-rich unit, may provide insight into past aqueous conditions on Mars and similar transitions in clay mineralogy. Reactive transport modeling of alteration processes involving different clay mineral compositions suggests that different weathering profiles can be formed through the alteration of parent materials, with implications for habitability and biosignature preservation on Mars.