Geochemistry of carbonaceous impactites from the Gardnos impact structure, Norway

The Gardnos impact structure in southern Norway is one of only two known impact structures (among 175) whose impactites contain significant amounts (typically 0.2–1.0 wt.%) of carbon, or 5 to 10 times the amount present in the target rocks; Sudbury, Canada is the other. This study extends a previous investigation of the geochemistry and petrology of Gardnos impactites (French et al., 1997) with additional sampling and a detailed investigation of the nature and possible origin of the carbonaceous material present. Two principal carbon components have been identified in Gardnos impactites: (1) impact-produced diamonds, 0.5 to 1 m in size, with a cubic crystal structure, predominantly hexagonal morphologies with platey layers and an estimated concentration of 0.19 ppm in Gardnos suevites and (2) graphitic carbon ranging from poorly ordered to moderately crystalline. Geochemical data suggests that there are no suitable target rocks that could provide a single source for the carbon in Gardnos impactites. However, Raman spectroscopy, stable isotope analysis and transmission electron microscopy of the impact diamonds and graphitic carbon suggests that there were at least two episodes of C emplacement in Gardnos impactites: an impact-related incorporation and shock transformation of graphitic material from target rocks followed by later mobilization of C, possibly during postimpact cooling or later regional metamorphism. Copyright © 2003 Elsevier Ltd