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

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

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

1. Geochemical modeling

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

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

Starting composition SpecieConc. (mg L -1 )Log (activity) SiO Al Fe 2+/ Mg Ca Na K+K Cl SO

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

2. Stability conditions of smectites

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

Mineral control Noachian terrains (OMEGA) Primary phases Hedenbergite = Fe 2+ Hedenbergite = Fe 2+ Diopside = Mg 2+ Diopside = Mg 2+ Anorthite = Al 3+ Anorthite = Al 3+ High pH (> 6) Formation of ripidolite (var. clinochlore) at low pH Fe 2+ 2 Al 2 SiO 5 (OH) 4 Clinochlore (Mg,Fe 2+ ) 5 Al 2 Si 3 O 10 (OH) 8

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

Smectites formation High water to rock ratio Weakly acidic to alkaline pH (6 to 12) High oxidation (for nontronite) High silica activity (log SiO 2 = -4 to -5) Variations depend on activity of other cations Fe, Mg, Ca, Al, K, Na

3. CO 2 in the Noachian atmosphere

Evaporation simulations Standard conditions p CO2 = 6 mbarp CO2 = 6 mbar pH ~ 6-7pH ~ 6-7 Cl - = 120 mg/kgCl - = 120 mg/kg pe = (Fe 2+ /Fe 3+ )pe = (Fe 2+ /Fe 3+ )

Evaporation simulations High p CO2 p CO2 = 1 barp CO2 = 1 bar pH ~ 5 to 7pH ~ 5 to 7 Cl = 23 mg/kgCl = 23 mg/kg

Evaporation simulations Al – system - High p CO2 p CO2 = 1 barp CO2 = 1 bar pH ~ 5 to 7pH ~ 5 to 7 Cl = 23 mg/kgCl = 23 mg/kg Fe = 0.45 kg/kgFe = 0.45 kg/kg Al = 10 mg/kgAl = 10 mg/kg

CO 2 in the Noachian atmosphere pe = 5 CO 2 pulse

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

4. Phyllosilicates and sulfates

Presence of sulfates

Evaporation simulation Standard solution p CO2 = 6 mbarp CO2 = 6 mbar Cl - = 120 mg/kgCl - = 120 mg/kg SO 4 2- = 17.3 mg/kgSO 4 2- = 17.3 mg/kg pe = 13.05pe = pH ~ 6pH ~ 6

Evaporation simulation sulfur rich conditions p CO2 = 6 mbarp CO2 = 6 mbar pH = 2.5 to 1pH = 2.5 to 1 Cl - = 120 mg/kgCl - = 120 mg/kg SO 4 2- = 173 mg/kgSO 4 2- = 173 mg/kg pe = 13.05pe = 13.05

Evaporation process Concentrated solutions p CO2 = 6 mbarp CO2 = 6 mbar pH ~ 1pH ~ 1 SO 4 2- = 5000 mg/kgSO 4 2- = 5000 mg/kg All other concentration x10All other concentration x10 pe = 13.05pe = 13.05

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

5. Effect of temperature

Stability diagrams Nontronite stable at low T Nontronite stable at low T At higher temperature: chlorite in more reducing environments, ferrihydrite (hematite) at lower pH At higher temperature: chlorite in more reducing environments, ferrihydrite (hematite) at lower pH pe = pH = 7 Clinochlore Mg-chlorite Fe 2+ -chlorite

Temperature effect Oxidant conditions p CO2 = 6 mbarp CO2 = 6 mbar pe = 13.05pe = pH = 7pH = 7

Temperature effect Reducing conditions p CO2 = 6 mbarp CO2 = 6 mbar pe = 0pe = 0 pH = 7pH = 7

Effect of temperature Nontronite destabilization Nontronite destabilization Formation of Fe 2+ -Mg-phyllosilicates (minnesotaite, chlorite) Formation of Fe 2+ -Mg-phyllosilicates (minnesotaite, chlorite) Formation of Fe 2+ -Mg serpentine minerals (talc, antigorite) Formation of Fe 2+ -Mg serpentine minerals (talc, antigorite)

6. Conclusions

Phyllosilicate stratification Al-rich phyllosilicate Al-rich phyllosilicate Kaolinite and montmorillonite Kaolinite and montmorillonite Fe 2+ + hydrated silica Fe 2+ + hydrated silica Fe 3+ /Mg 2+ smectites Fe 3+ /Mg 2+ smectites Temperature changes Temperature changes Aqueous chemistry change (pH, oxidation) Aqueous chemistry change (pH, oxidation) Atmosphere evolution Atmosphere evolution Bedrock variation Bedrock variation Bishop et al., 2008

Mineralogical relationship Fe-Mg smectites Carbonates MontmorilloniteKaolinite Fe 2+ phyllosilicates Sulfates pH decrease CO 2 Temperature Al / Fe activity

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

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

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