Planar deformation lamellae

Planar deformation lamellae , English planar deformation features or abbreviated PDF , are systematically arranged dislocation planes in crystals that are formed by the shock of an impact event . The structures arise at very high pressures from 8 to 10 gigapascals or 80 to 100 kilobars . Their stability extends up to 35 gigapascals or 350 kilobars, since above 35 GPa quartz and feldspars are only amorphous or as diaplectic glass .

description

Planar deformation lamellae in quartz. Two distinct directions are clearly recognizable.

Planar deformation lamellae are microstructures and can only be seen under the polarization microscope or the transmission electron microscope (TEM). These are very fine, closely-knit, parallel dislocation planes in the crystal lattice, which are mostly covered with amorphous or glassy material about 2 µm thick and enclose a characteristic angle with the c-axis of the silicate mineral concerned . The groups can cross over and the high pressure modification stishovite can grow in them . The lamellae recrystallize after the impact event has subsided and are occupied or "decorated" with small drops of fluid inclusions due to the hydrothermal effect.

The dislocation planes always follow rational crystallographic planes and can therefore be indexed. Since the deformation leaf orientations change with increasing pressure, they can be used as pressure indicators. The resulting orientations also depend on the prevailing ambient temperature in the rock and the impact angle of the impactor, as Langenhorst and Deutsch (1993) were able to show in shock experiments.

Differentiation to similar structures

Tectonically formed groups of deformation lamellae (often just referred to as deformation lamellae ) can look very similar, so an ultimate clear distinction can only be made with the scanning electron microscope or the transmission electron microscope. The most important criterion for differentiating between strictly plane-parallel deformation lamellae of an impact from tectonic lamellae is the slight curvature in the subgrain area of the latter, as they were created on submicroscopic slip planes and height offsets due to exponential creep deformation.

Planar deformation lamellae also have to be distinguished from planar fractures (English planar fractures or PF for short ), which form at the very beginning of the shock metamorphosis. These are planar cracks with an average spacing of 15 to 20 µm, in contrast to the very closely spaced planar deformation lamellae with a spacing of 1, 2, a maximum of 10 µm. Planar breaks also follow rational crystallographic planes, but are limited to very few planes (a maximum of three).

Affected minerals

Histogram of the planar deformation lamella directions in the Azuara impact structure , measured on shocked quartz grains of the polymictic breccia and the Pelarda formation

The resulting by shock metamorphism (or shock wave metamorphosis) planar deformation lamellae, often only planar elements or planar deformation elements or planar deformation features mentioned can be found in the following minerals: quartz , feldspar ( plagioclase and alkali ), mica , amphibole , pyroxene , olivine and zircon .

The deformation lamellae in the quartz are preferably parallel to the planes , and . In the plagioclase the levels (001), (010), (100) and (120) are usually selected. ${\ displaystyle {\ begin {Bmatrix} {\ bar {1013}} \ end {Bmatrix}}}$ ${\ displaystyle {\ begin {Bmatrix} {\ bar {1012}} \ end {Bmatrix}}}$ ${\ displaystyle {\ begin {Bmatrix} {\ bar {1011}} \ end {Bmatrix}}}$

Occurrence

Planar deformation lamellae are not formed in volcanic explosions , they only arise in shock metamorphosis and nuclear weapon explosions . They are therefore an essential prerequisite for the recognition of an impact structure or its ejecta cover. They can be found in meteorite craters such as the Ries crater , the Steinheim basin or the Rochechouart-Chassenon crater .

In the Ries crater, planar deformation lamellae can still be found up to a depth of 667 meters below the crater floor. H. up to 65 meters below the contiguous upper edge of the crystalline basement.

Individual evidence

↑ Stöffler, D .: Deformation and natural transformation of rock-forming minerals by natural and experimental shock processes . In: Fortschr. Miner. tape 49 , 1972, p. 50-113 .
↑ Stöffler, D. and Langenhorst, F .: Shock metamorphism of quartz in nature and experiment: I. Basic observations and theory . In: Meteoritics . tape 29 , 1994, pp. 155-181 .
↑ Vernooij, MGC and Langenhorst, F .: Experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock-induced planar deformation features . In: Meteoritics and Planetary Science . 40, No. 9/10, 2005, p. 1353-1361 .
↑ Kenkmann, Thomas: Asteroid and comet impacts in the history of the earth . In: Z. Geol. Wiss. 2009, p. 1-26 .
↑ Grieve, RAF, Langenhorst, F. and Stöffler, D .: Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience . In: Meteoritics & Planetary Science . tape 31 , 1996, pp. 6-35 .
↑ Huffman, AR and Reimold, WU: Experimental constraints on shock-induced microstructures in naturally deformed silicates . In: Tectonophysics . tape 256 , 1996, pp. 165-217 .
↑ Langenhorst, F. and Deutsch, A .: Orientation of Planar Deformation Features (PDFs) in quartz . In: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference . Part 2, 1993.
↑ Buchner, E. and Kenkmann, T .: Upheaval Dome, Utah, USA: impact origin confirmed . In: Geology . tape 36 , 2008, p. 227-230 .
↑ Ferrière, L., Morrow, JR, Amgaa, T. and Koeberl, C .: Systematic study of universal-stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results . In: Meteoritics and Planetary Science . 44, No. 6, 2009, p. 925-940 .
↑ Engelhardt, W. v. and Bertsch, W .: Shock-induced planar deformation structures in quartz from the Ries crater, Germany . In: Contributions to Mineralogy and Petrology . tape 20 , 1969, p. 203-234 .

[1] Stöffler, D .: Deformation and natural transformation of rock-forming minerals by natural and experimental shock processes . In: Fortschr. Miner. tape 49 , 1972, p. 50-113 .

[2] Stöffler, D. and Langenhorst, F .: Shock metamorphism of quartz in nature and experiment: I. Basic observations and theory . In: Meteoritics . tape 29 , 1994, pp. 155-181 .

[3] Vernooij, MGC and Langenhorst, F .: Experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock-induced planar deformation features . In: Meteoritics and Planetary Science . 40, No. 9/10, 2005, p. 1353-1361 .

[4] Kenkmann, Thomas: Asteroid and comet impacts in the history of the earth . In: Z. Geol. Wiss. 2009, p. 1-26 .

[5] Grieve, RAF, Langenhorst, F. and Stöffler, D .: Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience . In: Meteoritics & Planetary Science . tape 31 , 1996, pp. 6-35 .

[6] Huffman, AR and Reimold, WU: Experimental constraints on shock-induced microstructures in naturally deformed silicates . In: Tectonophysics . tape 256 , 1996, pp. 165-217 .

[7] Langenhorst, F. and Deutsch, A .: Orientation of Planar Deformation Features (PDFs) in quartz . In: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference . Part 2, 1993.

[8] Buchner, E. and Kenkmann, T .: Upheaval Dome, Utah, USA: impact origin confirmed . In: Geology . tape 36 , 2008, p. 227-230 .

[9] Ferrière, L., Morrow, JR, Amgaa, T. and Koeberl, C .: Systematic study of universal-stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results . In: Meteoritics and Planetary Science . 44, No. 6, 2009, p. 925-940 .

[10] Engelhardt, W. v. and Bertsch, W .: Shock-induced planar deformation structures in quartz from the Ries crater, Germany . In: Contributions to Mineralogy and Petrology . tape 20 , 1969, p. 203-234 .