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Results Project 7

Low shock recovery experiments in sandstone

The identification of impact craters formed in porous and wet sedimentary rocks, such as sandstones, on the basis of recognition of shock deformation features is a complex task. Most of the impacted target material is only weakly shocked, especially in the case of eroded remnants of impact structures or in small craters. There still is a lack of diagnostic shock features especially for the low shock pressure range, which is addressed in this project focusing on shock deformation experimentally generated in Seeberger sandstone in the low shock-pressure range (2.5- 17.5 GPa) to establish a classification scheme for porous and/or water-bearing, quartz-bearing rocks

Therefore shock recovery experiments were carried out with a high-explosive set-up generating a plane shock wave and using the shock impedance method

 

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experimental set-up

 

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shocked steel cylinder

 

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shocked steel cylinder including the shocked sandstone sample

 

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Scans of thin sections at different shock pressures

Sandstone cylinders of two different layers (Seeberger sandstone layer 5 and layer 3) with grain sizes of 0.1-0.17 mm, ~96 wt.% SiO2 and porosities of ~19 vol.% (layer 5) and 25-30 vol.% (layer 3), respectively, were shock deformed. Polished thin sections of the recovered shocked materials were then subjected to detailed textural and mineralogical analysis, including optical and scanning electron microscopy, electron microprobe analysis, and Raman spectrometry.

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Optical microscope

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SEM

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Raman Spectroscopy

 

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a. Fracturing starts to be replaced by melt formation at 5 GPa shock pressure. We observed an increasing amount of slightly different, pressure-dependent melt types with increasing shock pressure.

 

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b.
Numerous fractures occur throughout the shocked samples, of both intragranular and intergranular character. Counting of all kinds of fractures demonstrates increasing fracture density up to 10 GPa followed by decreasing fracture densities up to 17.5 GPa.

 

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c.
Fracturing is replaced by melt formation at 5 GPa shock pressure. We observed an increasing amount of slightly different, pressure-dependent melt types with increasing shock pressure.

 

 

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d. Isolated quartz grains shocked to >10 GPa are partly or completely transformed into diaplectic quartz glass. 


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e. We observed the formation of planar deformation features (PDF) in the experiments shocked to 10 GPa and higher although their formation normally begins in quartz single crystals and quartzite at lower pressures of 8-10 GPa.

 

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f.
Additionally, we observed two remarkable structures in quartz that require detailed investigation. Both features have never been described before and, therefore, have potential to be new diagnostic shock features in the low shock pressure range.

 

 

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g. In almost all recovered sandstone cylinders we observed shear zones associated with cataclastic microbreccia, diaplectic quartz glass, and SiO2 melt formation.

 

Especially the formation and distribution of diaplectic quartz glass, which normally requires pressures > 30 GPa but observed at 10 GPa and higher pressures in our experiments, needs further explanation. The strong collaboration with MEMIN subproject 5 provides some important investigations and explanations by the use of mesoscale modeling. The numerical models predict that localized shock amplifications are a result of pore space collapse which can reach four times higher pressures than the initial one.

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Astrid Kowitz

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