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Early martian surface conditions from thermodynamics of phyllosilicates

Early martian surface conditions from thermodynamics of phyllosilicates. Workshop on Martian Phyllosilicates : Recorders of Aqueous Processes ? Paris, October 21-23, 2008. Vincent F. Chevrier. Why?. Access to surface / subsurface conditions

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Early martian surface conditions from thermodynamics of phyllosilicates

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  1. Early martian surface conditions from thermodynamics of phyllosilicates Workshop onMartian Phyllosilicates:Recorders ofAqueous Processes?Paris, October 21-23, 2008 Vincent F. Chevrier

  2. Why? • Access to surface / subsurface conditions • Environmental conditions: acidity, oxidation, temperature • Bedrock composition • Atmosphere • Evolution of the surface • Feedback for spectral data analysis • Spectroscopy gives what there is • Equilibrium give what are the parageneses • Especially what should NOT be there

  3. Plan 1 – Geochemical modeling 2 – Stability conditions of smectites 3 – CO2 in the Noachian atmopshere 4 – Phyllosilicates and sulfates 5 – Effect of temperature 6 – Conclusions

  4. 1. Geochemical modeling

  5. Phyllosilicate diversity Mustard et al., 2008 Montmorillonite Hyd. silica Mg/Fe Smectite Illite / Chlorite Kaolinite Muscovite Wavelength (mm)

  6. Geochemical modeling Total rock composition (no transport) Primary phases (Cpx, Pg, ol) Dissolution model T, pH, pe, atm Secondary phases Smectite, kaol, silica… Solution composition Precipitation model T, pH, pe, atm

  7. Starting composition

  8. Equilibrium simulations • Geochemical workbench software package • thermo_phrqpitz basic database • Updated with ferric species and Pitzer coefficients (for high concentrations) • Updated with ~300 silicate phases (thermo.com.v8.r6+) • Total number of phases for equilibrium calculations: 395

  9. 2. Stability conditions of smectites

  10. Nontronite stability Slightly acidic to high pH High oxidation levels Weak effect of temperature (from 298 to 373K) Chevrier et al., 2007, Nature

  11. Mineral control • Noachian terrains (OMEGA) • Primary phases • Hedenbergite = Fe2+ • Diopside = Mg2+ • Anorthite = Al3+ • High pH (> 6) • Formation of ripidolite (var. clinochlore) at low pH Clinochlore (Mg,Fe2+)5Al2Si3O10(OH)8 Fe2+2Al2SiO5(OH)4

  12. Aluminum phases Log aK+ = -4.2 Log aK+ = -2 Transition of saponite to montmorillonite to kaolinite with decreasing pH Muscovite can form at neutral pH if K+ increases

  13. Smectites formation • High water to rock ratio • Weakly acidic to alkaline pH (6 to 12) • High oxidation (for nontronite) • High silica activity (log SiO2 = -4 to -5) • Variations depend on activity of other cations Fe, Mg, Ca, Al, K, Na

  14. 3. CO2 in the Noachian atmosphere

  15. Evaporation simulationsStandard conditions pCO2 = 6 mbar pH ~ 6-7 Cl- = 120 mg/kg pe = 13.05 (Fe2+/Fe3+)

  16. Evaporation simulationsHigh pCO2 pCO2 = 1 bar pH ~ 5 to 7 Cl = 23 mg/kg

  17. Evaporation simulationsAl – system - High pCO2 pCO2 = 1 bar pH ~ 5 to 7 Cl = 23 mg/kg Fe = 0.45 kg/kg Al = 10 mg/kg

  18. CO2 in the Noachian atmosphere CO2 pulse pe = 5

  19. Carbonates on Mars • May have formed on Mars • Same pH conditions as for phyllosilicates • Mainly dolomite and magnesite • Need some evaporation process

  20. 4. Phyllosilicates and sulfates

  21. Presence of sulfates

  22. Evaporation simulationStandard solution pCO2 = 6 mbar Cl- = 120 mg/kg SO42- = 17.3 mg/kg pe = 13.05 pH ~ 6

  23. Evaporation simulationsulfur rich conditions pCO2 = 6 mbar pH = 2.5 to 1 Cl- = 120 mg/kg SO42- = 173 mg/kg pe = 13.05

  24. Evaporation processConcentrated solutions pCO2 = 6 mbar pH ~ 1 SO42- = 5000 mg/kg All other concentration x10 pe = 13.05

  25. Impact of sulfate • Strong decrease of the pH (7 to 1) • Inhibition of smectite formation • First phases to disappear: carbonates • Precipitation of sulfates

  26. 5. Effect of temperature

  27. Stability diagrams pe = 13.05 pH = 7 Clinochlore Mg-chlorite Fe2+-chlorite Nontronite stable at low T At higher temperature: chlorite in more reducing environments, ferrihydrite (hematite) at lower pH

  28. Temperature effectOxidant conditions pCO2 = 6 mbar pe = 13.05 pH = 7

  29. Temperature effectReducing conditions pCO2 = 6 mbar pe = 0 pH = 7

  30. Effect of temperature • Nontronite destabilization • Formation of Fe2+-Mg-phyllosilicates (minnesotaite, chlorite) • Formation of Fe2+-Mg serpentine minerals (talc, antigorite)

  31. 6. Conclusions

  32. Phyllosilicate stratification • Al-rich phyllosilicate • Kaolinite and montmorillonite • Fe2+ + hydrated silica • Fe3+/Mg2+ smectites Bishop et al., 2008 Temperature changes Aqueous chemistry change (pH, oxidation) Atmosphere evolution Bedrock variation

  33. Mineralogical relationship Fe2+ phyllosilicates Temperature pH decrease Fe-Mg smectites Montmorillonite Kaolinite Sulfates CO2 Al / Fe activity Carbonates

  34. Phyllosilicate stratification • Acidity change • Kaolinite records transition to acidic conditions? • Compatible with the “varnish” aspect of the deposits • Compatible with • Temperature increase • Locally possible • Problem with absence of serpentine minerals

  35. Problems • Pure phases in calculations • Clays are “geochemical trashcans” • Some thermodynamic properties are not known • Does not take into account kinetics • Necessity for clear identification of what is present

  36. Some solutions= Future work • Determination of water compositions • Equilibrium with primary rocks • Kinetics of the processes • Necessity for experiments • Kinetic constants • Transitory metastable phases • Verification of models

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