Untitled

More documents

Recommendations

Info

The recognition of terrestrial impact structures Melosh H. J. (1981): Atmosphere breakup of terrestrial impactors. In: Schultz P. H. and Merrill P. B. (eds) Multi-Ring Basins. New York, pp. 29–35. Melosh H. J. (1989): Impact Cratering: A Geologic Process. Oxford University Press, New York. Milton D. J. (1977): Shattercones - an outstanding problem in shock mechanics. In: Roddy D. J. et al. (eds) Impact and Explosion Cratering. New York, pp. 703–714. Milton D. J., Glikson A. Y., Brett R. (1996): Gosses Bluff - a latest Jurassic impact structure central Australia. Part 1: geological structure, stratigraphy and origin. AGSO J. Austr. Geol. Geophys. 16, 453–486. Morgan J., Warner M., Grieve R. (2002): Geophysical constraints on the size and structure of the Chicxulub impact crater. In: Koeberl C. and MacLeod K. G. (eds) Catastrophic Events and Mass Extinctions: Impacts and Beyond. Boulder, Colorado. Geol. Soc. Amer. Spec. Pap. 356, pp. 39–46. Müller W. F., Hornemann U. (1969): Shock-induced planar deformation structures in experimentally shock-loaded olivines and in olivines from chondritic meteorites. Earth Planet. Sci. Lett. 7, 251–264. Ormö J., Lindström M. (2000): When a cosmic impact strikes the seabed. Geol. Mag. 137, 67–80. Ormö J., Sturkell E. F. F., Blomqvist G., Törnberg R. (1999): Mutually constrained geophysical data for the evaluation of a proposed impact structure: Lake Hummeln, Sweden. Tectonophysics 311, 155–177. Ostermann M. (1996): Die Geochemie der Impaktschmelzdecke (Sudbury Igneous Complex) im Multiring-Becken Sudbury. PhD thesis, Univ. Münster. Palme H., Goebel E., Grieve R. A. F. (1979): The distribution of volatile and siderophile elements in the impact melt of East Clearwater (Quebec). Proc. 10 th Lunar Planet. Sci. Conf., 2465–2492. Palme H., Grieve R. A. F., Wolf R. (1981): Identification of the projectile at Brent crater, and further considerations of projectile types at terrestrial craters. Geochim. Cosmochim. Acta 45, 2417–2424. Pike R. J. (1980): Formation of complex impact craters: Evidence from Mars and other planets. Icarus 43, 1–19. Pilkington M, Grieve R. A. F. (1992): The geophysical signature of terrestrial impact craters. Rev. Geophys. 30, 161–181. Pohl J. (1971): On the origin of the magnetization of impact breccias on Earth. Z. Geophys. 37, 549–555. Robertson P. B. (1968): La Malbaie Structure, Quebec – A Palaeozoic Meteorite Impact Site. Meteoritics 4, 1–24. Robertson P. B. (1973): Shock metamorphism of potassic feldspars. PhD thesis, Univ. of Durham. Robertson P. B., Sweeney J. F. (1983): Haughton impact structure: Structural and morphological aspects. Can. J. Earth Sci. 20, 1134–1151. Sagy A., Reches Z., Fineberg J. (2002): Dynamic fracture by large extraterrestrial impacts as the origin of shatter cones. Nature 418, 310–313. Sharpton V. L., Burke K., Camargo-Zanoguera A., Hall S. A., Lee D. S., Marin L. E., Suarez-Reynoso G., Quezaela-Muneton J. M., Spudis P. D., Urrita-Fucugauchi J. (1993): Chicxulub multiring impact basin: Size and other characteristics derived from gravity analysis. Science 261, 1564–1567. Spray J. G., Thompson L. M. (1995): Friction melt distribution in terrestrial multi-ring impact basins. Nature 373, 130–132. Spudis P. D. (1993): The Geology of Multi-ring Impact Basins. Cambridge University Press, Cambridge. Stöffler D. (1971): Progressive metamorphism and classification of shocked and brecciated crystalline rocks in impact craters. J. Geophys. Res. 76, 5541–5551. Stöffler D. (1972): Deformation and transformation of rock-forming minerals by natural and experimental shock processes. I. Behavior of minerals under shock compression. Fortschr. Mineral. 49, 50–113. Stöffler D. (1974): Deformation and transformation of rock-forming minerals by natural and experimental shock processes. II. Physical properties of shocked minerals. Fortschr. Mineral. 51, 256–289. Stöffler D., Hornemann U. (1972): Quartz and feldspar glasses produced by natural and experimental shock. Meteoritics 7, 371–394. Stöffler D., Langenhorst F. (1994): Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. Meteoritics 29, 155–181. Stöffler D., Deutsch A., Avermann M., Bischoff L., Brockmeyer P., Buhl D., Lakomy R., Müller-Mohr V. (1994): The formation of the Sudbury Structure, Canada: Towards a unified impact model. Geol. Soc. Amer. Spec. Pap. 293, 303–318. Therriault A. M., Fowler A. D., Grieve R. A. F. (2002): The Sudbury Igneous Complex: A differentiated impact melt sheet. Econ. Geol. 97, in press. Therriault A. M., Grieve R. A. F., Reimold W. U. (1997): Original size of the Vredefort Structure: Implications for the geological evolution of the Witwatersrand Basin. Meteoritics Planet. Sci. 32, 71–77. Tsikalas F., Gudlaugsson S. T., Faleide J. I. (1998): The anatomy of a buried complex impact structure: The Mjølnir Structure, Barents Sea. J. Geophys. Res. 103, 30,469–30,483. Tsikalas F., Gudlaugsson S. T., Faleide J. I., Eldholm O. (1999): Mjølnir Structure, Barents Sea: A marine impact crater laboratory. In: Dressler B. O. and Sharpton V. L. (eds) Large Meteorite Impacts and Planetary Evolution II. Boulder, Colorado, Geol. Soc. Amer. Spec. Pap. 339, pp.193–204. Twiss R. J., Moores E. M. (1992): Structural Geology. W.H. Freeman and Company, New York. Vishnevsky S. A., Lagutenko V. N. (1986): The Ragozinka astrobleme: An Eocene crater in the central Ural. Akad. Nauk. SSSR 14, 1–42. (in Russian) von Engelhardt W. (1984): Melt products from terrestrial impact structures. Proc. 27 th Intern. Geol. Congr. 19, 149–163. von Engelhardt W. (1990): Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany – A review. Tectonophysics 171, 259–273. von Engelhardt W., Stöffler D. (1968): Stages of Shock Metamorphism in the Crystalline Rocks of the Ries Basin, Germany. In: French B. M. and Short N. M. (eds) Shock Metamorphism of Natural Materials. Mono Book Corp., Baltimore, pp. 159–168. Wood C. A., Head J. W. (1976): Comparison of impact basins on Mercury, Mars and the Moon. Proc. 7 th Lunar Sci. Conf., 3629–3651. Zhang P., Rasmussen T. M., Pedersen L. B. (1988): Electric resistivity structure of the Siljan impact region. J. Geophys. Res. 93, 6486–6501. Handling editor: Roman Skála 263
Bulletin of the Czech Geological Survey, Vol. 77, No. 4, 265–282, 2002 © Czech Geological Survey, ISSN 1210-3527 Shock metamorphism of some minerals: Basic introduction and microstructural observations FALKO LANGENHORST Bayerisches Geoinstitut, University of Bayreuth, Universitätsstr. 30, D-95447 Bayreuth, Germany; e-mail: Falko.Langenhorst@uni-bayreuth.de Abstract. Minerals show a unique behaviour when subjected to shock waves. The ultradynamic loading to high pressures and temperatures causes deformation, transformation and decomposition phenomena in minerals that are unequivocal indicators of impact events. This paper introduces into the basics of shock compression, required to understand the formation and experimental calibration of these shock effects in minerals, and particularly focuses on the recent advances in the field of shock metamorphism achieved by the application of transmission electron microscopy (TEM). TEM studies underline that the way minerals respond to shock compression largely depends on their crystal structures and chemical compositions, as is illustrated here on the basis of four minerals: quartz, olivine, graphite and calcite. The crystal structure of a mineral exerts an important control on the shock-induced deformation phenomena, comprising one- to two-dimensional lattice defects, such as dislocations, mechanical twins, planar fractures, and amorphous planar deformation lamellae. For example, dislocations cannot be activated in quartz due to the strong covalent bonding, whereas the island silicate olivine easily deforms by dislocation glide. Transformation phenomena include phase transitions to (diaplectic) glass and/or high-pressure polymorphs. TEM studies reveal that high-pressure polymorphs such as coesite, stishovite and ringwoodite are liquidus phases, which form upon decompression by crystallization from high-pressure melts. The graphite-to-diamond transition is however a rare example for a solid-state transformation, taking place by a martensitic shear mechanism. Shock-induced decomposition reactions are typical of volatile-bearing minerals and liberate toxic gases that, in case of large impacts, may affect Earth’s climate. Shock experiments show that degassing of calcite does not take place under high pressure but can massively occur after decompression if the post-shock temperature is sufficiently high. A recombination reaction happens however if CaO and CO 2 are not physically separated. Key words: impact features, shock metamorphism, shock waves, minerals, crystal structure, defects, experimental study Introduction Number of craters (>1 km) per km 2 0.04 0.03 0.02 0.01 0.00 4 3 2 1 0 Age (Ga) Apollo Luna Earth Fig. 1. Cratering statistics of the Moon-Earth system for the last 4 Ga years [data from Stöffler and Ryder (2001) and regression curve after Neukum et al. (2001)]. Collisions of solid bodies played a crucial role in the formation of Earth and its subsequent evolution. The Proto-Earth accreted by collisions of planetesimals (Wetherill 1984) and underwent, in Archean times, a very heavy bombardment (Melosh 1989). The high collision rate at the beginning of Earth was simply the consequence of the chaotic states in the orbital movements of early solid bodies. Subsequently, this early collision rate distinctly declined until about 2.5 Ga, and since then we have an essentially constant flux of bodies colliding with Earth (Fig. 1). This knowledge of the collision history of Earth is however not exclusively based on observations of terrestrial impact craters, as one would possibly expect, but it largely relies on the cratering history of the Moon (Chapman and Morrison 1994, Neukum et al. 2001, Stöffler and Ryder 2001). Due to the absence of an atmosphere (and hence weathering) and the early cessation of volcanic activity on Moon, impact craters accumulated, in nearly undisturbed fashion, particularly in the lunar highlands, where crater densities are highest. On the geologically very active Earth, the morphologic landforms of impact craters easily disappear beyond recognition due to erosion, plate tectonics and other exogenic and endogenic processes. Although four Archean impact layers have been reported (Byerly et al. 2002), we have no knowledge of an Archean crater (Fig. 2). According to the cratering statistics (Fig. 1), one would however expect that most craters formed during this early episode of Earth (Grieve and Shoemaker 1994). 265
Page 2 and 3: Bulletin of the Czech Geological Su
Page 4 and 5: Asteroids: Their composition and im
Page 12 and 13: Bulletin of the Czech Geological Su
Page 14 and 15: The recognition of terrestrial impa
Page 24 and 25: Falko Langenhorst Identified crater
Page 26 and 27: Falko Langenhorst a b booster laser
Page 28 and 29: Falko Langenhorst (101 — 1) a 50
Page 30 and 31: Falko Langenhorst 1979). These effe
Page 32 and 33: Falko Langenhorst a Graphite b Diam
Page 34 and 35: Falko Langenhorst 3 µm 1 µm a b F
Page 36 and 37: Falko Langenhorst Acknowledgements.
Page 38 and 39: Falko Langenhorst Langenhorst F., S
Page 40: Falko Langenhorst M A West K H K N

Untitled

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?