Shock metamorphism in plagioclase

Our life on Earth, how it started and what can affect it, is still debated. On the planetary scale, formation and the evolution of the Solar System is also not completely understood, but we know that impact events played an important role in all the stages of the history of the Solar System. Differentiated bodies, such as our Earth, have developed a crust, which is strongly affected by impact events. In very large events, global implication might affect life. A clear example is the Chicxulub impact event that happened in Mexico 65 million years ago, at the end of the Cretaceous, creating a structure about 200 km in diameter and possibly causing a mass extinction, where a large part of the living species on Earth at that time disappeared. The solidified crust of planetary bodies mostly consists of plagioclase. Plagioclase can be therefore considered an analogue of the whole crust. Impact events generate shock waves that induce permanent modifications in the matter. We propose to investigate impact-induced effects in plagioclase, in order to understand which implications these catastrophic events can have in the formation and evolution of the crust of planetary bodies, in particular the conditions for melting of plagioclase and the chemical and physical processes activated in the shocked plagioclase. As little is known about plagioclase response to the shock metamorphism, our study will fill the gap in knowledge, specially focussing on the control of the chemical composition and crystal lattice on the development of specific shock features. Natural samples, from impact structures on Earth and from shocked meteorites, which are a proxy for the crust of other differentiated bodies, together with experimentally produced samples, under controlled shock pressure, will be analysed, with a combination of traditional and new petrologic- mineralogical approach. Plagioclase has a relatively complex structure and only recently developed analytical techniques enable the investigation and quantification of shock effects in this mineral. Therefore, both traditional instruments, such as optical and electron microscope and microprobe, and instruments that are less commonly applied to this type of studies, such as cathodoluminescence and Raman spectroscopy, as well as numerical approach for quantifying shock effects will be used. This project will require the expertise and the facilities of different departments of the Center for Earth Sciences of the University of Vienna, in collaboration with the Natural History Museum of Vienna. Finally, a scientific team, highly specialised in shock metamorphism, will be permanently established in Vienna, combining mineralogy, petrology, geochemistry, and physics, and the PI will gain the necessary experience for applying for the "Habilitation".

Impacts between planetary bodies have shaped the Solar System and even the Earth. On the surface of the rocky bodies, one of the most common mineral phase is plagioclase, a type of feldspar. Thus, it is important to understand how this mineral responds to shock, which is a deformation process occurring under extreme conditions of pressure, temperature, and speed of deformation. Naturally shocked samples, collected from several impact craters on the Earth, and experimentally shocked samples, produced with dedicated shock experiments, were investigated with multiple analytical techniques, to constrain the damage induced by shock in plagioclase. Minerals are characterized by an ordered arrangement of atoms, called crystal lattice. When this order is destroyed, the crystal transforms into amorphous material, like glass. In detail, such amorphization tends to localize along preexisting planar elements or crystallographic planes that preferentially undergo collapse under shock conditions. Another trigger for the localization of the shock deformation is the presence of internal defects or the difference in the intensity of the shock response between a given grain and the neighbors. Whenever the amorphization is localized, the crystallographic orientation of the shock-induced planar elements within the crystal lattice can be correlated with the maximum shock pressure experienced by the mineral. Within this research project, a method has been developed, to determine this orientation in situations, where the traditional techniques cannot be fully applied. This method is based on a combination between the historic optically microscopy and the modern electron microscopy. In addition, it was proven that a correlation between the propagation direction of the shock waves in the matter and the preferred orientation of the localized effect by shock does exist. This project improved our general understanding of how shock metamorphism affects plagioclase, although dedicated studies and experiments would be required to calibrate in detail the relation between shock effects at the microscopic scale and the shock pressure. Overall, the lesson that was learned is that the local variations in shock degree are not negligible and that are controlled at the local scale by several factors, including the properties of the neighbor phases with respect to the investigated mineral.