The Impact History of Chondrites as Revealed by High-Pressure Minerals

A record of impact processes on meteorite parent bodies is recorded as shock-metamorphic effects in meteorites. The key to reading this record is to use the shock effects to estimate the pressure and duration of shock events, which can constrain velocities and sizes of the impacting bodies. Shock pressures have been estimated in natural samples by calibrating the pressures needed to generate specific deformation and transformation effects in shock-recovery experiments. However, this approach is limited by the large activation energies for reconstructive phase transitions combined with relatively low shock temperatures and microsecond durations of shock-recovery experiments. As a result, the calibrated pressures for highly shocked rocks are too high. For example, olivine and pyroxene have never been transformed in a shock recovery experiment, but highly shocked (S6) meteorites, where olivine is transformed to ringwoodite are inferred to have reached pressures from 45 to 90 GPa [1]. One alternative approach is to use the results of static high-pressure experiments to interpret phase transformations in meteorites. However, these experiments generally involve highly reactive crushed starting materials and timescales much longer than those of natural shock events. The resulting transformations occur near equilibrium and are not directly relevant to shock-induced transformations. Static kinetic experiments provide useful kinetic data for specific transformation mechanisms, but these are not generally the same mechanisms that are active during shock. An alternative to calibrating the pressures of phase transitions is to use the mineralogy of shock-induced melt that crystallizes at high pressure [2]. Pressure estimates based on crystallization mineralogy indicate that many meteorites classified as highly shocked (S6) were exposed to equilibrium shock pressures in the range of 18-25 GPa, having durations in excess of 0.1 s. The relatively low shock pressures of chondrites suggests that either the parent body impacts had low velocities or the samples that we have came from depth in the parent body. [1] Stoffler D. et al. (1991) GCA,55, 3845-3867. [2] Chen M. et al. (1996) Science 271, 1570-1573.