Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Smectites on Early Mars Gain a more...
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Transcript of Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Smectites on Early Mars Gain a more...
![Page 1: Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Smectites on Early Mars Gain a more quantitative description of conditions bracketing smectite.](https://reader036.fdocuments.us/reader036/viewer/2022062714/56649d625503460f94a44a4e/html5/thumbnails/1.jpg)
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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)
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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...1. Fluid synthesis by salt addition (no Fe, Si)
2. pH buffering (TRIS, KH2PO4, etc.)
3. SiO2(aq) addition with tetraethoxysilane
4. Fe2+-sulfate salt addition & equilibration
5. Age solution at 60oC (up to 6 weeks)
• “Anoxic” experiments– Deoxygenation with N2(g) saturation
– Hydrazine (N2H4) addition as O2(aq) scavenger
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Fe-Bearing Systems: Product Summary
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Oxic Products: Vis/NIR
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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 m)
– Similar to materials seen in CRISM (Valles Marineris)
• Represents “intermediate” between acid-sulfate and high-pH
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
• 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
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Mg-smectite precursor
Amorphous SiO2
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
pH Control on Epitaxial Smectite Formation
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Application to Weathering: Topotaxy
• 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?
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
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Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Ongoing Work
• 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