Structural state of smectite and mixed layer smectite-illite m inerals as temperature

1. Crater mechanics and structural characteristics NIR–workshop Gardnos/Gol, Norway, June 8 –11 Structural state of smectite and mixed layer smectite-illite minerals as temperature indicator in impact induced hydrothermal systems: example of Ries crater Peeter Somelar, Nele Muttik and Kalle Kirsimäe University of Tartu, Department of Geology, Estonia

2. Clays and clay minerals: # hydrated (alumo)silicates with sheet-like structure # form by alteration of primary silicates (…but not only) # the composition, structure and morphology of clay minerals depends on: ¤ temperature ¤ fluid composition ¤ pH, Eh etc… # clays can be used for reconstruction of environmental variables/conditions

3. <100 ºC 200-100 ºC 300(350)-200 ºC Formation of clay minerals: H+ temperatuere / time K+ Mg2+

4. Results: XRD 17.0 -17.21Å Surficial suevites Montmorillonite type smectite fully expandable in ethylene glycol (EG)

5. Results: XRD 16.48 -16.95 Å Crater fill suevites Mixed-layer illite-smectite Random ordering (R0) ~90%of smectite layers ~10% of illite layers

6. Results: XRD # d values in surficial smectites ranges from 17.0 to 17.21 Å # The crater fill matherial Illite-smectites 16.48 to 16.95 Å (Å)

7. Results: XRD Modeling Surficial suevites measured modelled MLM2C code (Plancon and Drits 2000) R-04 <2μm, EG Depth 385.8m S – 95% I– 5% Crater fill matherial

8. Summary: # mixed-layer smectite-illite mineral with random ordering (R0) # The surficial suevites are without any exception the fully expandable Al-Fe montmorillonite (smectite) minerals # The crater fill material showsup to 5-10% of illitic layers # There is no clear observable tendency of the expandablity changing with the depth of the samples analysed # however, the mixed layering indicates the initial phase of the illitization, probably at elevated temperatures

9. Conclusions: # the heating (lifetime of the hydrothermal activity) must have been relatively short # Illitization of smectite in bentonites (originally volcanic glass) occurs at ~70 °C and the mixed-layer I/S structural ordering transition from R0 to R1 at 35%S at temperatures ~150 °C # The temperatures did not exceeded ~150 °C # The estimated temperatures of 70-150 °C according to illitization are, however, in good accordance with the fluid temperatures(range from ~60 to ~120 °C) according to stable isotope composition

10. Thanks for listening!