Dating of Impact Events
This chapter presents the different methods used or potentially useful to determine the age of an impact event on terrestrial planets or meteorite parent bodies. Two approaches exist: (1) geological methods such as Stratigraphy, which is the study of succeeding geological events, and (2) methods based on natural radioactivity (radiometric dating), as well as the measurement of isotopes produced by cosmic rays in space or on the Earth’s surface. The nuclear methods are preferentially discussed because they yield absolute ages, whereas geological methods define relative ages only. Shock metamorphic effects on minerals and rocks are presented in the frame of an impact crater. The different phase transformations induced by shock wave passage are typical for high-pressure regimes (20–60 GPa) which cannot be produced by any classical geological process like regional metamorphism or magmatism. In consequence, impacts produce rocks called “impactites” that are very distinct from lithologies of continental or oceanic crust. Their composition varies between molten glass (tektites and spheruls), diaplectic glasses (amorphous phases), breccias with either entirely or partially glassy matrix, and rock fragments enclosed in a fine grained fragmental matrix. To determine an age, isotopic equilibrium must be achieved among the newly produced phases. Such conditions are potentially present in glasses that are produced by residual heat after decompression (> 1000° C). The best candidates on the large scale are: samples taken in melt layers of craters, in the glassy matrix of breccias in the crater area, as well as tektites and spheruls found in distant ejecta. The presence of such types of ejecta allows to date the impact event through the analyses of layers preserved in sediments. An excellent example for this scenario is the Cretaceous-Tertiary boundary where a large series of impact produced minerals have been preserved. In certain cases, it is possible to trace events even if the crater remains unknown. The majority of shock-wave produced rocks are not in isotopic equilibrium, and often the values measured represent mixed ages or disturbed systems, not giving the impact age. Only the combination of different dating methods, and very selective sample selection allows impact dating.
KeywordsImpact Event Fission Track Impact Crater Terrestrial Planet Cosmogenic Nuclide
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