Weathering and mineral equilibria. Seminar at NGU 23 May 2016 Håkon Rueslåtten

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1 Weathering and mineral equilibria Seminar at NGU 23 May 2016 Håkon Rueslåtten

2 Weathering is the breakdown of rocks and minerals that are exposed to surface processes (climatically controlled). Water is a necessary prerequisite for geochemical weathering, which starts with dissolution of primary rock minerals. Intrusion of water in faults, cracks, and fissures of the bedrock; i.e. the initial water/rock interaction surface Oxidation of divalent iron, manganese, sulfide, etc. These processes can extend to considerable depths in the bedrock (hundreds of meters).

3 Organic and carbonic acids in the water are crucial weathering agents. Organic acids are humic and fulvic acids, and small amounts of stronger acids (e.g. formic, oxalic, and sulfuric acids). Climatically controlled. Grain sizes (texture): coarse grained granite is more vulnerable to weathering than a fine grained one. This is caused by different thermal expansion of the minerals upon cooling, and water get access routes between the crystal grains. This mechanism causes arenization of rocks. Topography, vegetation and soil controls water run-off and infiltration, e.g.. leaching of soluble elements (e.g. sodium, potassium and calcium), to prevent equilibrium between the minerals and pore water. Erosion of dissolved material influences on water infiltration and exposure of rock surfaces. Time: dissolution of rocks and formation of new minerals are slow processes.

4 Case studies: Podzol weathering; Dagali, Southern Norway Deep weathering; Stora Silevatten, Southern Sweden Deep weathering; Trondheim area

5 Podzol profile developed in till (# 37) at 1040 m.a.s.l., Dagali, Southern Norway.

6 Non-weathered (left) and weathered (right) till fractions 4-8 micron. The weathered fraction is separated by a heavy liquid at 2.59 g/cc, and shows the quartz and plagioclase fraction. Plagioclase grains are strongly weathered.

7 Weathered till fractions 4-8 micron < 2.59 g/cc; dominantly K-feldspar.

8 Weathered quartz particles in 4-8 micron fraction (>2.59 g/cc fraction).

9 Weathered feldspar particle containing sericite: demonstrate that sericite (muscovite) is more resistant to weathering than feldspar.

10 2 micron Fine fraction from B-horizon before (left) and after (right) citrate-bicarbonatedithionite (CBD) cleaning, to remove amorphous Fe + Al compounds.

11 Carbon (wt%) Height a.s.l. (m) CBD+NaOH extracted (wt% <2mm) R² = 0, Ign. loss (wt% <2 mm) 5 R² = 0, Ign. loss (wt% <2 mm) Fe+AL CBD (wt% <2mm)

12 Difference between C and A2 horizons in podzol profiles. Till fractions 4-8 micron: Plagioclase K-feldspar Quartz Dioctahedral illite Chl Tri. Phy. V A Plag. K-fsp. Qtz. Di.Ill Di. ML Sm Si, Al, Na, Ca Si, Al, K Si K K Si, Ti, Al, Fe, Mg, Ca, Na, K Element gram Weight % SiO TiO Al 2 O Fe 2 O FeO MnO MgO CaO Na 2 O K 2 O Ign.loss Sum Results of podzol weathering: Disintegration and dissolution of primary minerals No new minerals are crystallized Surface area of 4-8 micron fraction: 12%. Total dissolved during 9000 years: 1.43 kg100/12= 11.9 kg; i.e. More than 1.3 ton per km 2 per year Input of acids: From the soil: more than 85% From precipitation: less than 15%

13 Sampling Depths: A0: 0-15 cm A1: cm A2: cm B: cm C1: 130 cm C2: 190 cm RII+RIII: 250 cm RI: 300 cm ph: Location: Stora Silevatten, Southern Sweden

14 Metamorphic Precambrian gneiss: quartz, plagioclase, K-feldspar, biotite, and epidote. Retrograde metamorphism seen by chloritization of biotite. Weathering attacks along fissures, plagioclase boundaries and micaceous laminae.

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16 eth. glycol sat. quartz Plagioclase? K-feldspar Chl+Ka Chl. 4.7Å chlorite + kaolinite mica smectite ML (12Å)

17 Kaol

18 Smectite Kaolinite Verm + ML Dioct. Verm. Dioct. Illite Chlorite

19 20-63 micron fractions

20 A stability diagram (from Helgeson et al., 1969) with analytical results from various water samples. Precipitation falls in the kaolinite field due to low ph and low contents of dissolved components. Ground water (sample 6 to 10) is close to calcite saturation line, while surface water (sample 2 to 5) is closer to the Ca-smectite field. If the water analysis plots in the leonhardite field, it does not mean that leonhardite is formed.

21 Liaåsen 08 <6 Liaåsen 10 <6 Styggdalen12 <6 Lia Pukkverk21 < Mineral composition of the fraction <6 micron from four weathered greenstone samples. Two of the samples (Styggdalen and Lia Pukkverk) contain also smectite; saponite (Ca 0.25 (Mg,Fe) 3 ((Si,Al) 4 O 10 )(OH) 2 n(h 2 O)) and not montmorillonite ((Na,Ca) 0.33 (Al,Mg) 2 (Si 4 O 10 )(OH) 2 nh 2 O).

22 Rock quarry in greenstone outside Trondheim (Lia Pukkverk). Deep weathering along fractures.

23 Smectite <20 micron (weight %) 120 R² = 0, Na-bentonite Ca-bentonite K-bentonite ,5 1 1,5 2 2,5 Swelling pressure (MPa) Swelling pressure (in MPa) versus smectite content in fraction <20 micron from the Tertiary section of a North Sea well. Ten grams dried sample material packed in oedometer and preconsolidated at 2 MPa for 12 hours, followed by 2 hours decompression. The steel piston rested gently on the sample material when distilled water was added. Three bentonite samples (100% smectite) are included, saturated with Na, Ca, and K, respectively. (Augedal, Klungsøyr, Rueslåtten, 1985)

24 Glacial transported block in till, Styggdalen, Trondheim: is this a core stone from a saprolite?

25 Thank you for your attention