[M-IS17] Aqua planetology

[MIS17-P04] Efficient formation of Al phyllosilicates and weathering profiles through infiltration of acidic surface water on early Mars

*Yasuhito Sekine¹, Mitsuteru Sugiuchi¹ (1.Earth-Life Science Insitute, Tokyo Institute of Technology)

Keywords:Mars, chemical weathering, clay minerals

Remote sensing observations and in-situ measurements in Mars missions have suggested the occurrence of Al-rich clay minerals in the chemical weathering profiles in the low latitude regions. We perform laboratory experiments of chemical weathering using a flow-through type reactor for two years to constrain duration and composition of surface water and chemical gradients near surface. In the experiment using sulfuric acid solution, Al-rich clay mineral (montmorillonite) and sulfate mineral (gypsum/anhydrite) grains can be identified on the primary mineral grains in the top of the reaction vessel. In addition, Mg-rich clay mineral (smectite) can be identified in the middle of the reaction vessel. In the top of the reaction vessel, where sulfate minerals are formed due to supply of Ca from the primary minerals, pH is 2-3 owing to pH buffer by the sulfate minerals. Al, Si and Mg are effectively supplied into the pore water owing to high dissolution of plagioclase as well as olivine and the basaltic glass at pH 2-3. From the top to bottom of the vessel, the solution pH is neutralized due to supply of cations and pH gradient is formed. Upon the neutralization of the pore water including Al, Si and Mg, the concentrations of Al and Si in the pore water decrease. These suggest that Al and Si are consumed due to precipitation of Al-rich clays. In the bottom of the reaction vessel, pH is approximately 6, at which Mg, Ca and Fe are supplied further into the pore water including Mg owing to high dissolution of olivine and the basaltic glass. These cause precipitation of Mg/Fe-rich clays. Our experimental results using sulfuric acid solution suggest that efficient dissolution and neutralization of highly acidic solution including sulfuric acid can effectively generate Al- and Mg/Fe-rich clays. These results suggest that short term reaction by surface water in equilibrium with CO₂ air cause formation of chemical weathering profile mainly consisting of carbonate minerals; whereas, even short term reaction by highly acidic surface water including sulfuric acid could generate Al-rich clay and sulfate minerals in the upper part and Mg/Fe-rich clays in the lower part on early Mars.