Shock Effects in Meteorites

Abstract

Shock waves have played an important role in the history of virtually all meteorites. All models of the early solar system invoke condensation of mineral grains, aggregation of grains to form small bodies, and aggregation of most of the small bodies to form planets. The remaining small bodies, the asteroids, are accepted as the source of most meteorites. Throughout the history of the solar system, these small bodies have repeatedly collided with one another and with the planets. Since collisions produce shock waves in the colliding bodies, an understanding of shock wave effects is important to unraveling the impact history of the solar system as it is revealed in meteorites. This chapter was originally intended as an update of the chapter by Stoffler et al. (1988). Rather than update that chapter, which relies heavily on laboratory shock-recovery experiments in the interpretation of shock effects in meteorites, we have decided to take a different approach. Here we emphasize the use of static high-pressure data on phase equilibria together with shock wave and thermal physics calculations to interpret observed microstructures of shocked meteorites. A number of papers published during the past ten years have shown that this approach can yield new insights on the impact history of meteorites (Chen et al., 1996, 2004b; Sharp et al., 1997; Langenhorst and Poirier, 2000a; Xie and Sharp, 2000, 2004; Xie et al., 2002, 2005; DeCarli et al., 2004; Beck et al., 2004, 2005; Ohtani et al., 2004). One requirement for use of this approach is that some knowledge of shock wave physics is required. Most general articles on shock wave physics do not present the information in a way that is useful to a reader who has been primarily trained in geology or mineralogy. One of the objectives of this chapter is to present a useful tutorial on shock waves and shock wave calculations, including shock temperature and postshock temperature calculations. Our emphasis is on simple techniques and useful approximations rather than mathematical rigor. We will even attempt to present simple explanations of complicated phenomena, such as the quasichaotic nature of shock propagation in heterogeneous and/or porous materials. We also present examples to illustrate how the principles of shock wave and thermal physics may be used to interpret the history of naturally shocked materials and how the occurrences and formation mechanisms of high-pressure minerals in meteorites can be used to constrain shock pressures. 2. BACKGROUND