Shock metamorphism as a fundamental process in the evolution of planetary bodies; information from meteorites
Shock metamorphism as a fundamental process in the evolution of planetary bodies; information from meteorites
European Journal of Mineralogy (August 1992) 4 (4): 707-755
Shock effects in meteorites comprise two major phenomena: 1) shock metamorphism due to mechanical deformation and transformation of rocks below or above the solidus by shock compression, and 2) breccia formation involving ballistic or non-ballistic transport and the relative movement of rock fragments and melts by displacement from the primary location in the target bodies. Various scenarios are reviewed involving specific combinations of shock metamorphism and breccia formation if the relative sizes and velocities of the colliding bodies and the specific impact energy are freely available above a certain threshold value of the impact velocity. Progressive stages of shock metamorphism in the range approx 5-80 GPa have been observed in ordinary chondrites; localized melting (veins, melt pockets) is a feature in the 10-80 GPa range. Ordinary chondrite breccias are formed by shock lithification of clastic asteroidal surface debris or by accretion of disrupted parent body material. Fragmented and regolith breccias are known for the enstatite and carbonaceous chondrite groups. Iron meteorites range from unshocked to shock melted and display variable degrees of static thermal annealing. It appears that collision-induced hypervelocity impacts took place prior to, simultaneously with, and subsequent to the end of accretion and early differentiation of the parent bodies to which ordinary chondrites and differentiated meteorites are related.