TSK 11 Göttingen 2006 Trepmann & Spray Microstructural evidence of impact-induced crystal- plastic deformation and post- shock annealing of quartz Vortrag Claudia A. Trepmann1 John G. Spray2 Introduction During impact, rocks at the surface and accessible depths encounter ex- treme conditions. The hydrostatic com- ponent of the shock wave-associated stress, the so-called shock pressure, can reach several tens of GPa in the cen- tral part of the structure. The shock pressure can cause the transformation of target minerals into their high pres- sure modifications or amorphous phases. The role of the deviatoric component of the shock wave-associated stress dur- ing shock-metamorphism is only poorly understood. Shock effects in quartz are particularly useful for providing in- formation on the conditions during de- formation, given the widespread occur- rence of this mineral in the Earth’s crust and its comprehensive experimen- tal calibration. Two distinct types of quartz microstructure in charnockitic target rocks and quartz veins of the Charlevoix impact structure are com- pared and contrasted in order to dis- tinguish shock-induced microstructures that indicate a high hydrostatic stress component of the shock wave-associated stress from those that indicate a high deviatoric component, as well as asso- 1 Institut für Geologie, Mineralogie und Geo- physik, Ruhr-Universität Bochum, Germany, Collaborative Research Center 526 2 Plane- tary and Space Science Centre, Department of Geology, University of New Brunswick, Canada Figure 1: Bright field TEM micrograph showing rhombohedral PDFs that comprise dislocations and fluid inclusions. ciated microstructures that were gener- ated during post-shock relaxation. Type 1 microstructure The dominant shock effects in the type 1 microstructure in charnockites at ca. 2–4 km from the centre of the structure are planar deformation features (PDFs) parallel to rhombohe- dral planes of quartz. The rhombo- hedral PDFs comprise a high density of dislocations and fluid inclusions, as revealed by transmission electron mi- croscopy (TEM) (Fig. 1). They have been interpreted as the result of water- assisted, post-shock crystallisation of the amorphous phase along rhombohe- dral planes, initially generated during shock compression (e.g. Goltrant et al. 1992, Leroux et al. 1994, Leroux & Doukhan 1996). The abundance of different sets of these PDFs indicates a high hydrostatic component of the shock wave-associated stress (ca. 10– 15GPa). Evidence of crystal-plastic de- 1 Trepmann & Spray TSK 11 Göttingen 2006 Figure 2: Bright field TEM micrographs showing inclined Brazil twin boundaries in (0001) representing basal PDFs. Note par- tial dislocations within the boundaries. formation due to high deviatoric stresses is absent. Post-shock recovery is in- dicated by the actual microstructure of rhombohedral PDFs, dislocations in climb configuration and well-ordered low angle grain boundaries. Type 2 microstructure In contrast, PDFs parallel to the basal plane are predominant in the type 2 mi- crostructure developed in rocks at ca. 4–9 km from the centre of the structure, whereas rhombohedral PDFs are rare. This indicates a lower hydrostatic stress component (ca. 7–8GPa) compared to the type 1 microstructure, which corre- lates with a radial decrease in recorded peak shock pressure. The basal PDFs are revealed by TEM to represent me- chanical Brazil twins (Fig. 2), which give evidence for crystal-plastic defor- mation at high deviatoric stresses. In the type 2 microstructure, numerous de- formation bands, strong undulose ex- tinction and cataclastic zones at the op- tical scale, as well as glide-dislocations and microcracks at the TEM scale, oc- cur in association with basal PDFs, and are therefore also interpreted to be shock-induced. Post-shock recovery is indicated by the occurrence of small elongate subgrains with low angle grain boundaries paralleling low-index planes. Conclusions Although mechanical Brazil twins in the basal plane are common in naturally- shocked quartz (e.g. Goltrant et al. 1991, Leroux et al. 1994), shock- induced crystal-plastic deformation of quartz is generally considered to be in- effective due to the high rates of loading during shock (e.g. Langenhorst 1994). However, the type 2 microstructure records highly heterogeneous and lo- calised glide-controlled deformation ac- companied by twinning and microcrack- ing. Glide-controlled deformation of quartz is characteristic of high stress and strain rate conditions, e.g. during co-seismic loading (Trepmann & Stöck- hert 2003). Therefore, this crystal- plastic deformation is interpreted to be due to high deviatoric stresses and high loading rates during shock. The dominant occurrence of rhombohe- dral PDFs in the type 1 microstructure, in contrast to their rare occurrence in the type 2 microstructure in combina- tion with the abundance of mechani- cal Brazil twins, indicates that the de- viatoric component of the shock wave- associated stress increases relative to the hydrostatic component with increas- ing distance from the centre of the im- pact structure. This relationship has also been reported from other impact structures (Leroux et al. 1994; Leroux & Doukhan 1996). 2 TSK 11 Göttingen 2006 Trepmann & Spray Post-shock recovery is indicated in the type 1 microstructure by the actual microstructure of rhombohedral PDFs, dislocations in climb configuration and well-ordered low angle grain boundaries, as well as in the type 2 microstruc- ture by the occurrence of small elongate subgrains with low angle grain bound- aries paralleling low-index planes. This has probably taken place during an- nealing shortly after the impact event at quasi-static conditions and still suf- ficiently high post-shock temperatures, rather than during a separate regional thermal event. References Goltrant O, Cordier P & Doukhan J-C (1991) Planar deformation features in shocked quartz; a transmission electron microscopy investigation: Earth Planet. Sci. Lett. 106, 103–115 Goltrant O, Leroux H, Doukhan J-C & Cordier P (1992) Formation mechanisms of planar deformation features in naturally shocked quartz. Phys. Earth Planet. Inter. 74, 219– 240 Langenhorst F (1994) Shock experiments on pre-heated quartz: II. X-ray and TEM in- vestigations. Earth Planet. Sci. Let. 128, 683–698 Leroux H & Doukhan J-C (1996) A transmis- sion electron microscope study of shocked quartz from the Manson impact structure. in ‘The Manson Impact Structure, Iowa: Anatomy of an Impact Crater’, C Koe- berl, RR Anderson, (eds) Boulder, Colorado, Geol. Soc. Am. Spec. Paper 302, 267–274 Leroux H, Reimold WU & Doukhan J-C (1994) A TEM investigation of shock metamor- phism in quartz from the Vredefort dome, South Africa. Tectonophysics 230, 223–239 Trepmann CA & Stöckhert B (2003) Quartz microstructures developed during non- steady state plastic flow at rapidly decaying stress and strain rate. J. Struct. Geol. 25: 2035–2051 3