Impact metamorphosis

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Impaktmetamorphose (including shock waves - Metamorphosis ) refers to the transformation of rocks and minerals by the impact ( impact ) such as a meteorite on Earth or another planet or asteroid .

When impacted, the shock waves propagate through the rock at supersonic speed . Temperatures of several thousand degrees and pressures between 10 5 and 10 6 bar can occur. These conditions lead to melting and evaporation and large-scale fragmentation of rocks. Due to the extremely high pressure and the very high temperature that arise during the impact, the minerals are also transformed into a phase that is stable for the current conditions, for example quartz into coesite .

The effects of such an event can be studied particularly well using the changes in the quartz and in feldspars . The effects in other minerals are varied. Biotite crystals z. B. have kink bands. The shock waves can also cause also characteristic structures in the rock so-called cone of rays ( Shatter cones ).

This type of rock metamorphosis can e.g. B. at the Barringer Crater in Arizona and in the Nördlinger Ries can be traced. The phenomenon is more common on the moon and planets with solid surfaces and no atmosphere ; many meteorites have been changed by impact metamorphosis (s) on their flight through space . Shock veins with molten troilite and isotropic feldspar ( maskelynite ) are found in chondrites . Martian meteorites in particular have been altered by the impact that led to their detachment from the Martian mass .

Scaling

The intensity of the metamorphic change in the parent rock depends not only on the pressure or temperature but also on their mineralogical composition, which makes a direct comparison difficult. The following scale was developed on the basis of petrographic examinations of rocks from the Nördlinger Ries :

step pressure

[GPa]

temperature

[° C]

Changes in the mineral phases
0 10 100 Mica: appearance of kink bands
I. 10-35 300 planar elements in quartz and feldspar

first appearance of stishovite

II 35-45 900 Formation of diaplectic glasses from quartz and feldspar

first appearance of Coesit

III 45-60 1500 Feldspar melts, forms bubble-rich glass
IV 60-100 2000-5000 Rock melts completely with the formation of inhomogeneous glass
V > 100 up to over 10000 Rock evaporates

Often several of these changes are made in one handpiece and therefore only one range specification is useful.

A similar scale was presented for the impact metamorphic phenomena in chondrites (Stöffler-Keil-Scott scale), with particular emphasis on the changes in the important mineral phases olivine and plagioclase .

step pressure

[GPa]

Temperature increase

[° C]

Changes in the mineral phases
S1 <5 <10 irregular breaks
S2 5-10 10-20 undiluted extinction in olivine and plagioclase
S3 10-20 20-100 first appearance of opaque shock veins

planar breaks in olivine

S4 20-35 100-300 first appearance of mosaicism in olivine

Formation of melting pockets

S5 35-55 300-600 Formation of maskelynite from plagioclase

planar elements in olivine

S6 55-90 600-1500 Recrystallization of olivine

Plagioclase melts

Formation of ringwoodite

Shock melt > 90 > 1500 Formation of impact melts and impact melt breccia

Differential diagnosis

The following diagnostic criteria are generally recognized as reliable indicators for an impact metamorphic influence on a rock:

  1. the presence of so-called cone of rays (engl. shatter cones );
  2. the detection of remains (fragments) of the impactor;
  3. chemical or isotope signatures in the rock that indicate extraterrestrial influence;
  4. the presence of high pressure mineral phases;
  5. the presence of diaplectic glasses ;
  6. the presence of solidified high-temperature melts;
  7. planar openings (. Engl planar fractures , abbreviation .: PFs) in quartz grains;
  8. so-called planar deformation features (English. planar deformation features , abbreviation PDFs) in quartz grains.

Of these features, only the presence of cones of rays can be determined macroscopically; all other characteristics require extensive laboratory testing. Since there are also uncertainties here (for example, the high-pressure mineral coesite has also been detected in rocks of terrestrial origin) and the possibility of confusion (e.g. between planar deformation elements in quartz and deformation lamellae created by terrestrial tectonics ), a careful diagnosis is necessary.

Numerous other features of rocks and geomorphological structures (such as the formation of a ring-shaped or crater-like structure in the terrain) are associated with impact metamorphosis, but cannot, on their own, justify the impact metamorphic nature with certainty.

See also

literature

  • B. French, N. Short (Eds.): Shock Metamorphism of Natural Materials . Mono Book Corp., Baltimore 1968.
  • Wolfhard Wimmenauer : Petrography of igneous and metamorphic rocks . Stuttgart (Enke) 1985, ISBN 3-432-94671-6

Individual evidence

  1. ↑ R. Hüttner, H. Schmidt-Kaler: The geological map of the Ries 1: 50,000 . In: Geologica Bavarica . tape 104 . Bavarian Geological State Office, Munich 1999, p. 7-76 .
  2. ↑ W. v. Engelhardt, D. Stöffler, W. Schneider: Petrological investigations in the Ries . In: Geologica Bavarica . tape 61 . Bavarian Geological State Office, Munich 1969, p. 229-295 .
  3. ^ D. Stöffler, K. Keil, E. Scott: Shock metamorphism of ordinary chondrites . In: Geochimica et Cosmochimica Acta . tape 55 , 1991, pp. 3845-3867 .
  4. ^ OR Norton: The Cambridge Encyclopedia of Meteorites . Cambridge University Press, Cambridge 2002, ISBN 0-521-62143-7 , pp. 93-95 .
  5. ^ A b B. M. French, C. Koeberl: The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why . In: Earth Science Reviews . tape 98 , 2010, p. 123-170 .