Effective formation of Al-rich surface clays through highly acidic water activity on early Mars

Abstract:

Remote sensing observations have revealed a widespread occurrence of chemical weathering profiles in the low-latitude regions of Mars (Murchie et al., 2009). The weathering profiles consist of Al-rich clays and sulfate minerals in the upper part and Mg/Fe-rich clay minerals in the lower part (Bishop et al., 2008). These secondary minerals would have been formed through water-rock reactions between basement rocks and infused surface water in early Hesperian (Bishop et al., 2018).

Here, we perform laboratory experiments of chemical weathering using a flow-through type reactor to investigate the effect of pH and composition of infused surface water on the formation of the chemical weathering profile. To this end, we used two types of solution: One is CO₂-rich water in dissolution equilibrium with 1 bar of CO₂ (pH ~ 4), and the other is sulfuric acid solution in dissolution equilibrium with 1 bar of CO₂ (pH ~ 1).

Our experimental results show that an infusion of sulfuric acid solution can effectively generate a sequence of Al-rich and Mg-rich clay minerals, which is similar to the weathering profile on Mars. In the sulfuric acid experiment, solution pH at the top of the reaction vessel becomes 2-3 owing to the formation of sulfate mineral. At the low pH, dissolution of plagioclase proceeds to provide dissolved Al and Si in the porewater. In the lower part of the vessel, pH of the infused porewater increases to 5–6. Upon the downward transport of Al- and Si-containing porewater, Al-rich clay mineral precipitates at pH 5-6 in the reaction vessel. After consumption of Al and Si in the porewater, Mg-rich clay mineral forms using dissolved Mg in the lower part of the vessel. In the CO₂-rich water experiment, on the other hand, supply of Al is restricted due to a low dissolution rate of plagioclase at pH 4-5. Thus, no Al-rich clay mineral formation occurred in the experimental time; whereas, abundance of carbonate mineral precipitation proceeded.

Based on the consumption rate of Al in our sulfuric acid experiment, together with a suggested precipitation rate on early Mars, we suggest that the observed Al-rich clay deposits on Mars can be generated in a relatively short-term (~10³ years) through infusion of sulfuric acid surface water. The weathering profiles may have formed in a short time at locations where sulfuric acids accumulated.