1. ATMOSPHERIC COMPOSITION AND CLIMATE IN THE EARLY PRECAMBRIAN
A comparative analysis of weathering profiles and geochemical simulations
Alexey A. Novoselova, Nadezhda A. Alfimovab, Carlos Roberto de Souza Filhoa
aInstitute of Geosciences, University of Campinas (UNICAMP), Campinas, Brazil
bInstitute of Precambrian Geology and Geochronology, St. Petersburg, Russia
46th Brazilian Geological Congress
Santos 2012

2. RESEARCH OBJECTIVES
Early Precambrian weathering profiles or paleosols were formed under the atmospheric rainfall on the continental surface and their mineral and bulk compositions should reflect the conditions of weathering.
The aim of this study is the reconstruction of main mineralogical and chemical trends involved in the formation of early Precambrian paleosols (older than 2 Ga) derived from basalts by means of geochemical modeling verified by laboratory experiments. This allows us to constrain the environmental conditions (temperature, atmosphere composition and rainfall rates) during this period.
Modern weathering profile formed on the basaltic substratum near Campinas, SP
Introduction

3. ENVIRONMENTAL CONDITIONS IN THE EARLY PRECAMBRIAN
The composition of atmosphere and climatic conditions on the Earth's surface in the early Precambrian is still disputable. There are a few divergent evidences about environmental conditions during that time:
1. Oxygen-isotope studies of mid-Archean cherts ( Ga) provide oceanic temperature estimates as high as 70±15°C (Knauth and Lowe, 2003).
2. Accounting for low luminosity of the Sun during the early Precambrian, the increased pressures of greenhouse gases (CO2 or CH4) in the atmosphere are necessary for avoiding the snowball effect. The carbon dioxide pressure (pCO2) in the Archean atmosphere could be at least present atmospheric levels (PAL, 1 PAL = 410-4 bar), and perhaps as high as 5-10 bars (Kasting and Ackerman, 1986; Lowe and Tice, 2004) or atmosphere could consist from CH4 (Shaw, 2008 ).
3. The paleosols' mineralogy suggests much lower CO2 levels - less than 100 PAL (2.75 Ga, Rye et al., 1995) or PAL (2.2 Ga, Sheldon, 2006).
4. During the Ga timeframe, planetary scale geologic events took place on Earth, such as an increase of oxygen levels in the atmosphere with a possible peak at 2.3 Ga (Rye and Holland, 1998; Holland, 2009) and global glaciations at Ga and at 2.9 Ga (Kasting and Ono, 2006).
Introduction

4. THE MODELING PARAMETERS
MODELING SCHEME OF THE WEATHERING CRUST FORMING
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
with
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
The method description

5. Primary minerals Aqueous solution Secondary minerals
THE CALCULATION PROCEDURE
The thermodynamic calculations with accounting of minerals dissolution were implemented with the use of the program complex GEOCHEQ (Mironenko et al., 2008).
Primary
minerals
Aqueous solution
The kinetic
control dissolution
Secondary
minerals
The thermodynamic control sedimentation
The calculation of chemical equilibrium
[System composition](t+t) = [Solution composition]t + ΔtΣ(RateiSi)
Dissolved matter during Δt
(Zolotov and Mironenko, 2007)
The method description 5

6. Rate = f(pH)·f(T)·f(ΔG/RT) =
ΔtΣ(RateiSi)
THE MINERAL’S DISSOLUTION RATE EQUATION
Rate = f(pH)·f(T)·f(ΔG/RT) =
The Laidler
empirical equation
(Laidler, 1987)
The Arrhenius equation (Xu et al., 1999 )
The Lasaga equation (Lasaga, 1981)
(Zolotov and Mironenko, 2007)
The method description 6

7. THE REACTIONARY SURFACE
ΔtΣ(RateiSi)
THE REACTIONARY SURFACE
The specific surface area (SSA) of the most rocks is m2/g (Brantley et al., 1999).
Si = νi SSASmk, νi – the volume portion of mineral j, Smk - is a sum of primary or secondary minerals' weights.
Index and type of samples
Weight, g
Observed 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
SEM microphotographs illustrate the olivine dissolution (Lazaro and Brouwers, 2010)
The method description

8. THE MODELING SYSTEM The method description
The modeled system: O-H-K-Mg-Ca-Al-C-P-Si-Na-Fe.
As an analog of the Precambrian crust we used the Proterozoic basalt sample collected from the Onega tectonic structure (Nothern Karelia, Russia). It consisted of chlorite - Si3Al2.268Fe2.036Mg2.689Ca0.013Na0.028H8O (31.0 wt. %), albite - Si3Al1.032Fe0.005Ca0.014Na1.004O8.07 (41.4), microcline - Si3Al1.037Fe0.002Na0.041K0.82O7.988 (4.3), augite - Si2Al0.103Fe0.213Mg0.974Ca0.817Na0.016P0.043O6.274 (21.3), quartz - SiO2 (1.0), magnetite – Fe3O4 (1.0).
We used kinetic constants for the next minerals: albite, amorphous silica, augite, brucite, calcite, chrysotile, clinochlore, daphnite, diopside, dolomite, enstitite, fayalite, ferrosilite, forsterite, goethite, greenalite (Fe-serpentine), hematite, hydroxyapatite, laumontite, magnesite, magnetite, microcline, Ca, K, Na-montmorillonites, K, Ca, Mg, Na-nontronites, quartz, siderite, stellerite, stilbite, talc and Fe-talc from (Palandri and Kharaka, 2004). Kinetic data of illite are from Alekseev (2007).
We simulated 14 scenarios of weathering under different temperatures (25, 50 or 75°С), atmospheric compositions (pCO2 = 10-4, 10-2, 1, 10 bar or pCH4 = 1 bar) and rainfall rates (100, 400, 1000, 4000 mm/year).
The method description

9. WEATHERING EXPERIMENT
To calibrate our long-term iterative lixiviation model at the initial stage of basalt leaching we conducted a weathering experiment.
The samples were leached with a initial pH solution of 3 (H2SO4 = mol/l) at normal conditions and W/R ratio of 10. The experiment continued during 100 hours; the observations were done after 10, 60, 600 and 6000 minutes. Thereafter we simulated these experimental interactions of basalt with water solution under the same conditions.
Ab – albite, Ap – apatite, Aug – augite, Chl – clinochlore, Dph – daphnite, Dol – dolomite, Ill – illite, Mag – magnetite, Mc – microcline, Mg, Na-Nnt – Mg, Na – nontrotites, Py – pyrite, Qtz – quartz

10. PRIMARY MINERALS DISSOLUTION
The rates of minerals’ dissolution depend from temperature, pH and saturation of reacting solution.
Ab – albite, Aug – augite, Cal – calcite, Fo – forsterite, Mc – microcline, Mg-Mnt – Mg-montmorillonite, Qtz – quartz
At T = 25°C, pCO2 = 0.01 bar and rainfall rate = 1000 mm per year we received the next sequence of primary minerals dissolution: Mt (20 modeling years)  Aug (460)  Chl (970), Mc (1050)  Alb (2100)  Qt ( ).
Results of calculations

11. Results of calculations
SECONDARY MINERALS
An – analcime, Ap – apatite, Cal – calcite, Chl – clinochlore, Dph – daphnite, Dol – dolomite, Ill – illite, Lmt – laumontite, Ca-Mnt – Ca-montmorillonite, Ca, K, Mg, Na-Nnt – Ca, K, Mg, Na-nontrotites, Qtz – quartz, Tlc – talc, Sdr – siderite, Stb - stilbite
Results of calculations

12. BULK CHEMICAL COMPOSITION CHANGES
Weathering under the most studied environmental conditions suggests a concentration of Si, Al, K and a reduction of Ca, Na, Mg.
Under a CO2-rich atmosphere and at 10 bar, silica accumulates and may reach more than 96 wt.%.
At 75°C, the alteration of basalt is close to isochemical and the residue looses K, P, and Na only.
Scenarios with minimal rainfall rate (100 mm/year) are characterized by reduction of Fe and Na and preservation of other elements.
On the contrary, under high rainfall rate (4000 mm/year) Fe accumulates in the residue.
VOLUME CHANGES
At the initial weathering stage the bulk volume of weathering products increases under all simulated conditions as a result of carbonate minerals crystallization. Later, after about 1000 modeling years, the bulk volume is reduced gradually as a result of carbonate dissolution. Generally, the variation in rock volume is limited to ± 10 % of the initial volume.
Results of calculations

13. NEOARCHEAN AND PALEOPROTEROZOIC WEATHERING PROFILES
Our dataset contains the next early Precambrian paleosols:
Place
Age, Ga
Reference
Mt.Roe, W.Australia
2.76 – 2.78
Yang et al., 2002
Cooper lake, Canada
2.45
Utsunomia et al., 2003
Bolshozero lake, Karelia, Russia
2.4
Alfimova, 2007
Girvas, S.Karelia, Russia
2.3
Heiskanen and Bondar, 1998
Sondali island, Segozero lake, Karelia, Russia
2.2
Gorkovetz et al., 1999
Griqualand West, South Africa
Wiggering and Beukes, 1990
Main features:
Accumulation of K and Al.
Na, Ca and Mg lost.
Silica has a conservative behavior.
Mg is more stable than Ca.
Discussion

14.       CH4 50°C 75°C 75°C 75°C pCO2 = 10 bar
Accumulation of K and Al demonstrated under all modeling scenarios besides weathering at 75°C.
Under CH4-rich atmosphere and at 50 and 75°C Mg wasn’t lost by rock and under pCO2 = 10 bar – Si accumulates too intensive.
Ca was lost by rock at all simulated scenarios besides weathering at 75°C.
75°C
Discussion

15.      100 mm/year Griqualand West pCO2 = 1 bar 100 mm/year
Discussion

16. The Na dissolution shows that:
All analyzed paleosols have been formed within Kyr.
2. Most of analyzed samples correlate to lixiviation under pCO2 = 1 and 25 PAL.
Discussion

17. CONCLUSIONS
The weathering profiles developed over basaltic substrates consist mostly of clay minerals (smectites and illite).
Carbonate phases deposited within the profile at the initial stage of weathering are not resistant and dissolved due to weathering.
The analyzed early Precambrian paleosols were not formed under extreme environmental conditions: elevated temperature or both dense CO2 and CH4 atmosphere. The weathering conditions in Neoarchean and Paleoproterozoic were similar to modern ones and pCO2 was not much higher than 25 PAL.
All analyzed paleosols have been formed within Kyr.
The authors wish to thank Dr. M.V. Mironenko (Vernadsky Institute, Russia) for providing the modeling code GEOCHEQ and consultations.
This investigation was financially supported by FAPESP, grant No. 2011/
Conclusions

18. Thanks a lot
for your attention!!!