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Smectites on Early Mars

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

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Smectites 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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