Earth and Planetary Science Letters 603, 117987 Link to Article [https://doi.org/10.1016/j.epsl.2022.117987]
1Jacobs, NASA Johnson Space Center, Houston, TX 77058, United States of America
2NASA Johnson Space Center, Houston, TX 77058, United States of America
3University of Nevada, Las Vegas, NV 89154, United States of America
Smectites are widespread on Mars, but the water-rich, neutral-to-alkaline pH conditions favorable for smectite formation would be expected to have also produced abundant carbonates on early Mars, which are not observed. Smectite formation from basaltic glass on Mars could occur in acidic environments unfavorable for carbonate formation. Acidic smectite formation has been previously demonstrated in batch experiments (closed hydrologic systems), however, the mechanisms and octahedral composition of smectite forming in acidic flow-through (open hydrologic systems) environments are still not fully understood. We conducted hydrothermal (190 °C) alteration experiments on Stapafell basaltic glass at 0.01 and 0.25 mL min−1 flow rates corresponding to low and high water to rock ratio (W/R) flow conditions, and initial pH (pH0) values of 2, 3, 4 and 6. A batch low W/R experiment was conducted at pH0 2 for comparison to the open system experiment. Kaolinite, montmorillonite and chlorite formed at pH0 2 at low W/R; no phyllosilicates formed at pH0 2 at high W/R; and lizardite formed at pH0 ≥ 3 at both W/R ratios. Lizardite, kaolinite, and montmorillonite in these experiments formed by precipitation from solution and chlorite likely formed through alteration of montmorillonite and/or basalt. Saponite formed at pH0 2 in batch conditions by alteration of basaltic glass. Comparison of experimental data with martian observations of phyllosilicate assemblages indicated that smectite formation on Mars likely occurred under water-limited environmental conditions. Al-rich smectite could form in low W/R open system subsurface environments under a very narrow range of pH (pH < 3) while saponite could form in closed low W/R systems under acidic to alkaline conditions. The combination of open and closed hydrological regimes could be responsible for development of clay mineral stratigraphies observed on Mars. The acidic conditions required for formation of Al-rich smectite montmorillonite were unfavorable for carbonate precipitation, but carbonate precipitation could occur together with Fe/Mg-smectite saponite in closed systems at pH > 4. The lack of widespread carbonates on Mars could not therefore be explained solely by acidic conditions. Phyllosilicate formation under acidic conditions on Mars may affect biosignature stability in martian regolith as preservation capacity is lower in phyllosilicates that formed in or experienced acidic pH.