3D Laser Scanner Techniques: A Novel Application for the Morphological Study of Meteorite Impact Rocks

Introduction: Impact Rock (Karikkoselka Impact Crater)

Numerous studies on terrestrial rocks have shown they have different types of geological features which can be used as micro or macro-markers of large impact events (Koeberl & Martinez-Ruiz, 2003). Certain shock effects have been shown to be uniquely and unequivocally associated with meteorite impact craters. These include, among others, multiple sets of microscopic planar deformation features, mainly in quartz and feldspar grains, high-pressure mineral phases (e.g. stishovite, coesite) and shatter cones (impact textures which form when the shockwave from a meteorite impact event passes through and modifies the target rocks). The study and detailed characterization of these impact-related signatures in terrestrial environments encompasses the use of classical mineralogical and geochemical techniques and the development of theoretical, numerical and experimental models.

The impact rock used in the work (Figure 1) was sampled by one of the authors (JMF) in the Karikkoselka area of Finland and forms part of the lithotheque at the Spanish Centro de Astrobiologia. The well known northern European Fennoscandian Shield has at least 32 impact structures, of which 10 are located in Finland (see Earth Impact Database, Spray, 2009). This shield is extremely important for impact cratering research, since it is well exposed, easily accessible and has been mapped in detail (Plado & Pesonen, 2002). The Karikkoselka structure (62º13’ N; Long. 25º15' E) was proposed as a meteorite impact crater by Lehtinen et al. (1996). The crater is the smallest of those so far identified in Finland, with a diameter of 1.5 km and a depth of 150 m. The geological setting corresponds to the Central Finnish Granite Complex dating from the Paleoproterozoic Age. The target rock is of porphyritic granite from a site where many shatter cones have been found. In general terms, shatter cones can be defined as unusual, striated, horse-tailed conical fractures, measuring from millimetres to meters in length aand produced in different types of rocks by the passage of a shock wave (Sagy et al. 2002), although the mechanism by which they are formed is not well understood (Dawson, 2009). The directional striated surfaces of shatter cones are positive/negative features. An extremely interesting (and useful) feature of shatter cones is that the tips point toward the origin of the shockwave. This means that they can be used to reconstruct the location, size and shape of prehistoric impact craters that have subsequently been modified by later processes. Shatter cones are usually formed at pressures between 2 and 6 GPa, although rocks have been found that had been subjected to pressures around 25 GPa (Milton 1977). Further information on impact textures can be found, for example, in Dietz 1947, Amstutz, 1965, Milton, 1977, Roach et al. 1993, Gibson & Spray, 1998, Baratoux & Melosh, 2003 Sagy et al. 2002, 2004, Lugli et al. 2005, Dawson, 2009, Ferrière L. & Osinski, 2010).

Figure 1.

Meteorite-impact related rock which was used in the project. It comes from the impact crater area of Karikkoselka (Finland). Note its surface textures, which are indicative of an impactogenic origin. The detailed analysis by using 3D laser techniques is extremely important to both determine and characterize the morphological features caused by the shock condition.

The impact rock from the Karikkoselka impact crater (Figure 1) is a red-coloured granitic specimen, measuring 131.10 mm x 82.30 mm x 92.80 mm, with a rough surface marked with incomplete but well-defined shatter cone textural features.