| The evolution of life on Earth has been severely punctuated by mass extinctions of species. There is good evidence, in the form of a global ejecta deposit and the Chicxulub crater, that a large impact was responsible for the Cretaceous-Tertiary (K-T)extinction and impacts are also implicated in the Permian-Triassic (P-Tr), the Triassic-Jurassic (Tr-Ju) and the Eocene-Oligocene extinctions, however, the causes of these extinctions is still highly controversial and other killing mechanisms have been proposed. Impact craters and ejecta deposits are identified from the presence of geochemical signatures, melt spherules and shocked minerals. Shock microstructures such as planar deformation features (PDFs) in quartz are very useful robust indicators of extraterrestrial impacts, because PDFs can be easily observed by light microscopy and can be preserved for billions of years. A major limitation, however, is that rigorous distinction of PDFs from tectonic deformation lamellae, usually requires high resolution transmission electron microscopy (TEM) observations. Young PDFs can be identified by etching and SEM observation but TEM is required to identify older annealed relict PDFs. TEM is a time-consuming technique where only small volumes of material can be investigated and furthermore the technique is not readily available to many research groups. As a consequence many discoveries of suspected shocked quartz, such as those at the Triassic-Jurassic boundary and the Australian Permian-Triassic boundary sites, have not yet been confirmed. We propose to develop and apply new methods for the fast identification of shocked quartz using innovative scanning, electron microscopy, based methods, including the new technique of Focused Ion Beam (FIB) tomography using the new dual beam FIB-SEM recently installed at Utrecht University. After an initial stage of technique development and calibration on known examples of shocked quartz, we will apply the new methods to study suspected shocked quartz from i) the Tr-Ju and P-Tr boundaries, ii) from new sites of the K-T global ejecta deposit and iii) the largest known impact structure on the Earth the two billion year old Vredefort structure in South Africa. The development of a fast method to identify shocked minerals will enable large numbers of grains and samples to be investigated providing an improved basis to assess the role of impacts in extinctions in general and an improved basis for estimation of impact shock pressures for particular events. |
