Early martian surface conditions Early martian surface conditions from thermodynamics of from thermodynamics of

phyllosilicatesphyllosilicates

Vincent F. ChevrierVincent F. Chevrier

Workshop on Martian Phyllosilicates:Recorders of Aqueous Processes?

Paris, October 21-23, 2008

Why?Why?

Access to surface / subsurface conditionsAccess to surface / subsurface conditions Environmental conditions: acidity, oxidation, Environmental conditions: acidity, oxidation,

temperaturetemperature Bedrock compositionBedrock composition AtmosphereAtmosphere

Evolution of the surfaceEvolution of the surface

Feedback for spectral data analysisFeedback for spectral data analysis Spectroscopy gives what there isSpectroscopy gives what there is Equilibrium give what are the paragenesesEquilibrium give what are the parageneses Especially what should Especially what should NOTNOT be there be there

PlanPlan

1 – Geochemical modeling1 – Geochemical modeling

2 – Stability conditions of smectites2 – Stability conditions of smectites

3 – CO3 – CO22 in the Noachian atmopshere in the Noachian atmopshere

4 – Phyllosilicates and sulfates4 – Phyllosilicates and sulfates

5 – Effect of temperature5 – Effect of temperature

6 – Conclusions6 – Conclusions

1. Geochemical 1. Geochemical modelingmodeling

Phyllosilicate diversityPhyllosilicate diversity

Wavelength (m)

Montmorillonite

Hyd. silica

Mg/Fe Smectite

Illite / Chlorite

Kaolinite

Muscovite

Mustard et al., 2008

Geochemical modelingGeochemical 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 compositionStarting compositionSpecie Conc. (mg L-1) Log (activity)

SiO2 60.1 -4.5

Al3+ 1 -4.4

Fe2+/3+ 44.7 -3.1

Mg2+ 24.3 -3.0

Ca2+ 20 -3.3

Na+ 18.4 -3.1

K+ 2.7 -4.2

Cl- 23 -3.2

SO42- 17.3 -3.7

Equilibrium simulationsEquilibrium simulations

Geochemical workbench software Geochemical workbench software packagepackage

thermo_phrqpitzthermo_phrqpitz basic database basic database

Updated with ferric species and Pitzer Updated with ferric species and Pitzer coefficients (for high concentrations)coefficients (for high concentrations)

Updated with ~300 silicate phases Updated with ~300 silicate phases ((thermo.com.v8.r6+thermo.com.v8.r6+))

Total number of phases for equilibrium Total number of phases for equilibrium calculations: 395calculations: 395

2. Stability conditions 2. Stability conditions of smectitesof smectites

Nontronite stabilityNontronite stability

•Slightly acidic to Slightly acidic to high pHhigh pH

•High oxidation High oxidation levelslevels

•Weak effect of Weak effect of temperature (from temperature (from 298 to 373K)298 to 373K)

Chevrier et al., 2007, Nature

Mineral controlMineral control

Noachian terrains Noachian terrains (OMEGA)(OMEGA)

Primary phasesPrimary phases Hedenbergite = FeHedenbergite = Fe2+2+

Diopside = MgDiopside = Mg2+2+

Anorthite = AlAnorthite = Al3+3+

High pH (> 6)High pH (> 6)

Formation of ripidolite Formation of ripidolite (var. clinochlore) at (var. clinochlore) at low pHlow pH

Fe2+2Al2SiO5(OH)4

Clinochlore

(Mg,Fe2+)5Al2Si3O10(OH)8

Aluminum phasesAluminum phases

• Transition of saponite to montmorillonite to Transition of saponite to montmorillonite to kaolinite with decreasing pHkaolinite with decreasing pH

• Muscovite can form at neutral pH if KMuscovite can form at neutral pH if K++ increases increases

Log aLog aK+K+ = -4.2 = -4.2 Log aLog aK+K+ = -2 = -2

Smectites formationSmectites formation

High water to rock ratioHigh water to rock ratio

Weakly acidic to alkaline pH (6 to 12)Weakly acidic to alkaline pH (6 to 12)

High oxidation (for nontronite)High oxidation (for nontronite)

High silica activity (log SiOHigh silica activity (log SiO22 = -4 to -5) = -4 to -5)

Variations depend on activity of other Variations depend on activity of other cations Fe, Mg, Ca, Al, K, Nacations Fe, Mg, Ca, Al, K, Na

3. CO3. CO22 in the in the

Noachian atmosphereNoachian atmosphere

Evaporation simulationsEvaporation simulationsStandard conditionsStandard conditions

•ppCO2CO2 = 6 mbar = 6 mbar

•pH ~ 6-7pH ~ 6-7

•ClCl-- = 120 mg/kg = 120 mg/kg

•pe = 13.05 pe = 13.05 (Fe(Fe2+2+/Fe/Fe3+3+))

Evaporation simulationsEvaporation simulationsHigh pHigh pCO2CO2

•ppCO2CO2 = 1 bar = 1 bar

•pH ~ 5 to 7pH ~ 5 to 7

•Cl = 23 mg/kgCl = 23 mg/kg

Evaporation simulationsEvaporation simulationsAl – system - High pAl – system - High pCO2CO2

•ppCO2CO2 = 1 bar = 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

COCO22 in the Noachian in the Noachian atmosphereatmosphere

pe = 5

CO2 pulse

Carbonates on MarsCarbonates on Mars

May have formed on MarsMay have formed on Mars

Same pH conditions as for Same pH conditions as for phyllosilicatesphyllosilicates

Mainly dolomite and magnesiteMainly dolomite and magnesite

Need some evaporation processNeed some evaporation process

4. Phyllosilicates and 4. Phyllosilicates and sulfatessulfates

Presence of sulfatesPresence of sulfates

Evaporation simulationEvaporation simulationStandard solutionStandard solution

•ppCO2CO2 = 6 mbar = 6 mbar

•ClCl-- = 120 mg/kg = 120 mg/kg

•SOSO442-2- = 17.3 = 17.3

mg/kgmg/kg

•pe = 13.05pe = 13.05

•pH ~ 6pH ~ 6

Evaporation simulationEvaporation simulationsulfur rich conditionssulfur rich conditions

•ppCO2CO2 = 6 mbar = 6 mbar

•pH = 2.5 to 1pH = 2.5 to 1

•ClCl-- = 120 mg/kg = 120 mg/kg

•SOSO442-2- = 173 = 173

mg/kgmg/kg

•pe = 13.05pe = 13.05

Evaporation processEvaporation processConcentrated solutionsConcentrated solutions

•ppCO2CO2 = 6 mbar = 6 mbar

•pH ~ 1pH ~ 1

•SOSO442-2- = 5000 = 5000

mg/kgmg/kg

•All other All other concentration concentration x10x10

•pe = 13.05pe = 13.05

Impact of sulfateImpact of sulfate

Strong decrease of the pH (7 to 1)Strong decrease of the pH (7 to 1)

Inhibition of smectite formationInhibition of smectite formation

First phases to disappear: First phases to disappear: carbonatescarbonates

Precipitation of sulfatesPrecipitation of sulfates

5. Effect of 5. Effect of temperaturetemperature

Stability diagramsStability diagrams

• Nontronite stable at low TNontronite stable at low T

• At higher temperature: chlorite in more reducing At higher temperature: chlorite in more reducing environments, ferrihydrite (hematite) at lower pHenvironments, ferrihydrite (hematite) at lower pH

pe = 13.05 pe = 13.05 pH = 7pH = 7

Clinochlore

Mg-chlorite

Fe2+-chlorite

Temperature effectTemperature effectOxidant conditionsOxidant conditions

•ppCO2CO2 = 6 mbar = 6 mbar

•pe = 13.05pe = 13.05

•pH = 7pH = 7

Temperature effectTemperature effectReducing conditionsReducing conditions

•ppCO2CO2 = 6 mbar = 6 mbar

•pe = 0pe = 0

•pH = 7pH = 7

Effect of temperatureEffect of temperature

Nontronite destabilizationNontronite destabilization

Formation of FeFormation of Fe2+2+-Mg-phyllosilicates -Mg-phyllosilicates (minnesotaite, chlorite)(minnesotaite, chlorite)

Formation of FeFormation of Fe2+2+-Mg serpentine -Mg serpentine minerals (talc, antigorite)minerals (talc, antigorite)

6. Conclusions6. Conclusions

Phyllosilicate Phyllosilicate stratificationstratification

• Al-rich phyllosilicateAl-rich phyllosilicate

• Kaolinite and Kaolinite and montmorillonitemontmorillonite

• FeFe2+2+ + hydrated silica + hydrated silica

• FeFe3+3+/Mg/Mg2+2+ smectites smectites

• Temperature changesTemperature changes• Aqueous chemistry change (pH, oxidation)Aqueous chemistry change (pH, oxidation)• Atmosphere evolutionAtmosphere evolution• Bedrock variationBedrock variation

Bishop et al., 2008

Mineralogical Mineralogical relationshiprelationship

Fe-Mg smectitesFe-Mg smectites

CarbonatesCarbonates

MontmorilloniteMontmorillonite

KaoliniteKaolinite

FeFe2+2+ phyllosilicatesphyllosilicates

SulfatesSulfates

pH decreasepH decrease

COCO22

TemperatureTemperature

Al / Fe activityAl / Fe activity

Phyllosilicate Phyllosilicate stratificationstratification

Acidity changeAcidity change Kaolinite records transition to acidic Kaolinite records transition to acidic

conditions?conditions? Compatible with the “varnish” aspect of the Compatible with the “varnish” aspect of the

depositsdeposits Compatible withCompatible with

Temperature increaseTemperature increase Locally possibleLocally possible Problem withProblem with absence of serpentine absence of serpentine

mineralsminerals

ProblemsProblems

Pure phases in calculationsPure phases in calculations Clays are “geochemical trashcans”Clays are “geochemical trashcans” Some thermodynamic properties are not Some thermodynamic properties are not

knownknown

Does not take into account kineticsDoes not take into account kinetics

Necessity for clear identification of what is Necessity for clear identification of what is presentpresent

Some solutionsSome solutions= Future work= Future work

Determination of water compositionsDetermination of water compositions Equilibrium with primary rocksEquilibrium with primary rocks

Kinetics of the processesKinetics of the processes

Necessity for experimentsNecessity for experiments Kinetic constantsKinetic constants Transitory metastable phasesTransitory metastable phases Verification of modelsVerification of models