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Investigating Smectite Formation and Weathering Processes on Early Mars

This study quantitatively describes the conditions under which smectites formed in low-temperature geochemical systems on early Mars. Utilizing experimental data from synthetic basalt weathering, we explore the precipitation of smectites, the transformation processes involved, and their relationship with other phases such as carbonates and oxides. Our work highlights the significance of Fe-silicates in low-oxygen environments, emphasizing the implications for understanding Mars's habitability and past aqueous chemistry. We seek to integrate findings with mineral weathering processes and clarify the controls on clay and mineral interactions.

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Investigating Smectite Formation and Weathering Processes on Early Mars

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  1. Smectites on Early Mars • Gain a more quantitative description of conditions bracketing smectite formation in low-T martian geochemical systems • From the bottom, up • Begin with experimental data on synthetic basalt weathering • Experimental approach to smectite precipitation • Neoformation • Transformation (?) • Place results in context of weathering processes on early Mars • Describe smectite formation in relation to other phases (e.g., carbonates, oxides, amorphous silicates)

  2. An Experimental Approach to Clay Formation • Synthetic “basaltic weathering” fluids • Chemistry from Tosca et al., [2004] • Addition of SO4/Cl salts for desired solution composition • With constant stirring @ room temperature... • Fluid synthesis by salt addition (no Fe, Si) • pH buffering (TRIS, KH2PO4, etc.) • SiO2(aq) addition with tetraethoxysilane • Fe2+-sulfate salt addition & equilibration • Age solution at 60oC (up to 6 weeks) • “Anoxic” experiments • Deoxygenation with N2(g) saturation • Hydrazine (N2H4) addition as O2(aq) scavenger

  3. An Experimental Approach to Clay Formation • Anoxic & oxic conditions • pH 4-9 (buffered & unbuffered) • Fe-free systems • Effect of salinity (as Mg2+) • Solid characterization • X-ray diffraction • Total X-ray scattering / PDF analysis • XRF • SEM/EDS

  4. Fe-Bearing Systems: Product Summary

  5. Oxic Products: Vis/NIR

  6. Fe3+-SiO2 phases: Unique Precipitates • Fe3+-SiO2 precipitates occur from pH 5-8 • “Amorphous” to powder XRD, but exhibits local structure • Ongoing work to model PDF • Best described as SiO2-bearing (up to ~20 wt. %) Fe3+-(hydr)oxide • Unique spectral features in Vis/NIR (2.21, 2.27 mm) • Similar to materials seen in CRISM (Valles Marineris) • Represents “intermediate” between acid-sulfate and high-pH

  7. Strong hkl and 001 reflections observed after 21 days, pH = 7-9 • Peak position & XRF analysis consistent with: • Trioctahedral Mg-smectite (Stevensite) • Turbostratic ordering Increased Mg2+ expands pH stability & increases crystallization rate

  8. Amorphous SiO2 Mg-smectite precursor

  9. pH Control on Epitaxial Smectite Formation

  10. Inheritance of primary mineral crystal structure e.g., pyroxene to smectite Reaction stoichiometry poorly known Requires transport of some components & rearrangement Velbel & Barker [2008] Opx to Fe/Mg-smectite Fe transport required by stoichiometry How does this process reflect aqueous chemistry? Application to Weathering: Topotaxy

  11. Conclusions • Fe(2+ or 3+)-smectites indicate a low-O2 environment • Nontronite formation through oxidation of Fe2+-phase, not from Fe3+ (aq) • May be able to target more “habitable”, reducing paleo-environments • Increased [Mg] expands pH stability range & increases xtall. rates • Amorphous Fe3+-SiO2 phases reflect transition between acid-sulfate and clay-bearing systems • Need to better understand structural & spectroscopic characteristics (vis/NIR & X-ray scattering studies) • Extend results to mineral weathering processes • Role of Fe in structural inheritance & topotactic crystallization? • Anoxic weathering studies

  12. Application of PDF to crystallization mechanism Integrate mafic minerals into clay formation experiments Quantify relationship between clays & carbonates Most experiments supersaturated wrt carbonates Reproducing experiments with controlled CO2 Ongoing Work

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