Chondrite 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... 

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. 

A second interpretation of carbonaceous chondrites is as primary condensates of the solar nebula. By this view, their hydrolytic alteration is due to melting in cometary nuclei during close passes with the Sun, or due to transient heating events by shock waves or collisions (McSween, 1999). Other carbonaceous chondrites show metamorphic alteration with minerals similar to those in Earth formed during deep burial under elevated temperatures and pressures (Brearley, 1999). Like soils and paleosols on Earth and Mars, carbonaceous chondrites demonstrate the great antiquity of hydrolytic weathering in dilute acidic solutions, presumably of carbonic acid derived from water vapor and CO2. These remain the principal gases released from volcanoes, and soils remain important buffers for this environmental acid. 

While most meteorites have solidification ages around 4.56 Ga, there is clear evidence of more recent disturbances of chronometric systems -particularly Pb-Pb and Rb-Sr - in many meteorites. For example, Rb-Sr internal isochrons for E chondrites (believed to have experienced open-system thermal metamorphism) were disturbed 4.3-4.45 Ga ago. Of course, chronometers in heavily shocked L chondrites show clear evidence for late disturbance. 

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