Shock metamorphism

Another important secondary process affecting many chondrites is shock metamorphism. Sto flier et al. (1991) quantified the shock effects observed in olivine and plagioclase, and... 

ALHA 84001 is a unique Martian meteorite with an ancient age of 4.5 billion years. It consists mostly of orthopyroxene crystals that accumulated in basaltic magma. Its most distinctive feature, however, is the occurrence of small globules of carbonates with unusual compositions and textures. The controversial (now largely discredited) hypothesis that the carbonates contain evidence of extraterrestrial life (McKay et al., 1996) made this the most famous meteorite on Earth. ALHA 84001 has also experienced intense shock metamorphism. 

Rubin, A. E., Scott, E. R. D. and Keil, K. (1997) Shock metamorphism of enstatite chondrites. Geochimica et Cosmochimica Acta, 61, 847-858. 

Once formed, the chondrite parent bodies experience a variety of processes, including thermal metamorphism, aqueous alteration, shock metamorphism due to impacts, and even disruption from large impacts. Several radiochronometers can provide information on the timing of metamorphism and aqueous alteration. The chronology of this processing is summarized in Figure 9.11. 

The degree of shock metamorphism (caused by impacts) recorded in a chondrite is determined from a variety of mineralogical and textural parameters (e.g., Stoffler et al., 1991 Scott et al., 1992). The classihcation scheme by Stoffler et al. (1991) is based on shock effects observed in olivine and plagioclase (Table 4). Since olivine is rare in enstatite chondrites, Rubin et al. (1997) extended this shock classihcation scheme to orthopyroxene (Table 4). 

Classification of Chondritic Meteorites Table 4 Classification scheme for shock metamorphism in chondrites. 

Scott E. R. D., Keil K., and StofUer D. (1992) Shock metamorphism of carbonaceous chondrites. Geochim. Cosmochim. Acta 56, 4281—4293. 

Rubin A. E., Zolensky M. E., and Bodnar R. J. (2002) The halite-bearing Zag and Monahans (1998) meteorite breccias shock metamorphism, thermal metamorphism and aqueous alteration on the H-chondrite parent body. Meteorit. Planet. Sci. 37, 124-141. 

Tomeoka K., Yamahana Y., and SeMne T. (1999) Experimental shock metamorphism of the Murchison CM carbonaceous chondrite. Geochim. Cosmochim. Acta 63, 3683—3703. 

Tomeoka K., Ohnishi I., and Nakamura N. (2001) Silicate darkening in the Kobe CK chondrite evidence for shock metamorphism at high temperatures. Meteorit Planet Sci. 36, 1535-1545. 

Osadchii Eu. G., Baryshnikova G. V., and Novikov G. V. (1981) The Elga meteorite silicate inclusions and shock metamorphism. Proc. 12th Lunar Planet. Sci. Conf. 1049-1068. 

Experimental Techniques Used for the Simulation of Shock Metamorphism... 

A. Deutsch and F. Langenhorst, Mineralogy of Astroblemes—Terrestrial Impact craters. Chapters 1.10.1-1.10.4., Introduetion, Cratering and shock metamorphism, Characteristies of terrestrial impact structures, and Geological formations in and around impact structures, n Advanced Mineralogy, Vol. 3, Mineral matter in space, mantle ocean floor, biosphere, environmental management, and jewelry (ed. 

D. Stoffler and F. Langenhorst (1994), Shock metamorphism of quartz in nature and experiment I. Basic observation and theory. Meteoritics 29, pp. 155 181. 

F. Langenhorst, A. Deutseh, U. Homemann, and D. Stofiler, EfFect of temperature on shock metamorphism of single crystal quartz. Nature 356, pp. 507 509 (1992). 

The cratered surfaces of asteroids and terrestrial planets underscore the importance of impacts for the formation and evolution of the solar system. Early in the history of the solar system such collisions were the mechanism for accretion of planetesimals and finally the planets themselves [1], The effects of these still ongoing collisions are visible from the megascopic down to the submicroscopic length scale, i.e., they range from large impact craters and their ejecta blankets down to shock-metamorphic effects in minerals [2-4]. These effects form as a result of the interaction of strong shock waves with the affected solid matter. 

Over the past several years, we have tested and adapted several novel experimental techniques for simulating shock metamorphism in minerals. To exemplify the progress in the various experimental approaches, this chapter... 

W. Engelhart and D. Stofiler, Stage of shock metamorphism in crystalline rocks of the Ries Basin, Germany, in Shock Metamorphism of Natural Minerals (eds. 

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