IGCP-SIDA 599 Project Launching Meeting

Mekrijärvi 2011

Modern weathering crust derived from

the serpentinite substratum, BC, Canada

Weathering crusts form as a result of long-term interactions with rain- or seawater at low temperature and pressure. As a consequence they are characterized by incompleteness of the chemical reactions passing.

WEATHERING CRUSTS

Introduction

The thermodynamic calculations with accounting of minerals dissolution were implemented with the use of the program complex GEOCHEQ [Mironenko et al., 2008].

CARBONE DIOXIDE OR METHANE?

CO2CH4

1. Absence of the early Earth’s carbonate rock’s remnants [Shaw, 2008]

2. Inconvenience of none-highly reduced conditions for origin of life [e.g. Natochin et al., 2008]

1. Carbonate minerals (dolomite) described in the Isua sediments (3.8 Ga) [Myers, 2001]

2. The methane atmosphere could be provided in the case of a very low oxygen fugacity of upper mantle (less than 10-

40-10-60 bar) [Holland, 1984]

Liquid water might exist on the early Earth’s surface as early as 4.4 Ga [e.g. Mojzsis et al., 2001; Cavosie et al., 2005; Watson and Harrison, 2005]. Thus, according to the faint young Sun paradox, a greenhouse gas is necessary

OR

Introduction

THE MODELING PARAMETERS(t, W/R, T, P)

t – the general weathering duration (n – the quantity of solution waves, ΣΔtτ – the duration of one solution wave percolation)

W/R – the ration of one water portion to the weathered rock weight

Т and Р correspond to the conditions on the weathered substratum surface

t n t

if ΣΔtτ = 1 day, a quantity of precipitation is 1000 mm/year, and a weathering crust thickness is 1 m, W/R would be 0.001

n with

0t

THE WEATHERING CRUST FORMING MODELING SCHEME

The method description

The method description

THE CALCULATION PROCEDURE

Primaryminerals

Secondaryminerals

[System composition](t+t) = [Solution composition]t + ΔtΣ(RateiSi)

Aqueous solution

The kineticcontrol dissolution

The thermodynamic control sedimentation

The calculation of chemical equilibrium

Dissolved matter during Δt

[Zolotov and Mironenko, 2007]

Rate = f1°(pH)·f2(T-T0)·f3(ΔG/RT) =

0( )0 0 2 0

0

( ) ( / )

1 1exp[ ] 1 exp ( )

b T T n mH H O OH WH H

qa

k a k k K a

E Gk p

R T T RT

ΔtΣ(RateiSi)

[Zolotov and Mironenko, 2007]

The Arrhenius

equation [Xu et al., 1999 ]

The Lasaga equation [Lasaga, 1981]

The method description

THE MINERAL’S DISSOLUTION RATE EQUATION

The Laidlerempirical equation

[Laidler, 1987]

THE INFLUENCE OF DIFFERENT FACTORS TO THE OLIVINE

DISSOLUTION RATE

[Olsen, 2007]The method description

THE REACTIONARY SURFACE

SEM microphotographs illustrate the

olivine dissolution [Lazaro and Brouwers,

2010]

ΔtΣ(RateiSi)

Si = νi SSA, νi – the volume portion of mineral j

The specific surface area (SSA) of the most rocks is 10-2-103 m2/g [Brantley et al, 1999].

The method description

Index and type of samples

Weight, gObserved surface, m2/g

6005 granite 1.241 0.451 ± 0.064

22105 basalt 1.272 1.432 ± 0.005

2906 granite 1.102 0.264 ± 0.031

Кс-1 clay 0.857 53.04 ± 2.06

THE EXPERIMENTAL AND CALCULATED DATA

The quartz dissolution at 23°С and atmospheric pressure

SSAquartz = 0.0219 – 0.0230 m2/g [Worley, 1994]

Time, days

THE CALCULATION RESULTS AS AGAINST THE EXPERIMENTAL DATA.

THE SOLUTIONS COMPOSITION

THE MODELING SYSTEM

The method description

The modeled system: O-H-K-Mg-Ca-Al-C-Si-Na-Fe.

As an analog of the early Earth’s protocrust we used the next basaltic komatiite compound from the Archean greenstone belt Munro Township (Canada) [Arndt, Nesbitt, 1982], wt. %: SiO2 = 48.76, Al203 = 9.36, Fe2O3 = 3.07, FeO = 8.04, MgO =21.65, CaO = 8.05, Na2O = 0.90, К2O = 0.16.

We used kinetic constants for the next minerals: albite, amorphous silica, brucite, calcite, chrysotile, clinochlore, daphnite, diopside, dolomite, enstitite, fayalite, ferrosilite, forsterite, goethite, greenalite (Fe-serpentine), illite, magnesite, magnetite, Ca, K, Na,Fe-montmorillonites, siderite, talc.

Temperature was 15°С and pressure 1 bar. The system was open by CO2 or CH4.

Results of calculations

THE WEATHERING CRUSTThe CO2 atmosphere (PCO2 = 1 bar)

The primary minerals dissolution sequence: Opx (32 model years) Ol, Cpx (54) Mag (60) Pl (1900).

The resulted weathering crust consisted from amorphous silica (61.8 vol. %), Fe-montmorillonite (nontronite) (35.3 vol. %), goethite (2.8 vol. %) and illite (0.06 vol. %).

Initial composition

58 model years

100 model years

800 model years

24 000 model years

SiO2

Al2O3

FeOK2O

CaOMgONa2O

СO2

H2O

48.769.36

10.800.168.05

21.650.900.000.00

36.877.118.800.095.78

16.590.41

24.340.51

37.407.118.950.105.86

16.210.30

24.070.60

50.207.19

12.450.137.725.90

0.002416.42

1.40

83.069.835.57

0.00360.001.530.000.002.54

V/Vinitial*100% 100 166 164 129 86

Results of calculations

THE BULK COMPOSITION OF WEATHERING CRUSTThe CO2 containing atmosphere (PCO2 = 1 bar)

The resulted weathering crust lost Mg, Ca, Na, and on the final stage Fe and K. It accumulated Si and Al.

Results of calculations

THE WEATHERING CRUSTThe CH4 atmosphere (PCH4 = 1 bar)

The primary minerals dissolution sequence: Opx (0.3 model years) Cpx (100) Mag (615) Ol (5200) Pl (6000).

The resulted weathering crust consisted from amorphous deweylite (58 vol. %) and chlorite (42 vol. %).

Initial composition

17 model years

68 model years

1200 model years

79 000 model years

SiO2

Al2O3

FeOK2O

CaOMgONa2O

CO2

H2O

48.769.36

10.800.168.05

21.650.900.000.00

46.9310.0211.05

0.075.46

24.170.020.022.26

46.349.98

11.020.014.70

24.080.000.063.81

45.299.75

10.830.004.36

23.550.000.006.22

45.979.02

11.760.000.00

23.620.000.009.62

V/Vinitial*100% 100 95 99 105 103

Results of calculations

THE BULK COMPOSITION OF WEATHERING CRUSTThe CH4 containing atmosphere (PCH4 = 1 bar)

The resulted weathering crust lost Na, K and Ca. It accumulated Si, Fe, Al and Mg.

Discussion

DISCUSSION

Conclusions

CONCLUSIONS• The carbonate minerals deposit effectively at the CO2 atmosphere. Carbonates are the most stable at low quantity of atmospheric precipitates. During the consecutive weathering crust evolution they can be dissolved completely and removed from the substratum.

• The weathering crust formed at the CO2 atmosphere conditions consists from amorphous silica, iron oxides and clay minerals. At the CH4 atmosphere conditions – from deweylite and chlorite.

• The methane presence in the carbon dioxide atmosphere (CO2/CH4>1) doesn’t influence on the weathering crust composition.

•A developed weathering crust may be formed during first thousand years.

We thank M.V. Mironenko (Vernadsky Institute) for providing programs and consultations.

This investigation was financially supported by program no. 25 of the Presidium of the Russian Academy of Sciences, subprogram 1, theme "Reconstruction of the Formation Conditions of the Protocrust of the Early Earth and Its Role in the Evolution of the Composition of the Primary Atmosphere and Hydrosphere"

Thanks a lotfor your attention!!!