GEOCHEMICAL AND STRUCTURAL PROPERTIES OF IMPACT GLASSES –EXAMPLES FROM SUEVITES AT THE LAKE BOSUMTWI STRUCTURE, GHANA, AND IVORY COAST TEKTITES

The formation of the different melt lithologies (i.e., glass shards and bombs in suevite, tektites, microtektites and microkrystites) in impact processes is still not understood in detail. Open questions include proper identification of the respective precursor rocks (to be tackled by analysis of radiogenic isotopes), degree of glass homogenization (microprobe, LA-ICP-MS with excimer laser, TEM-EDX), as well as high-temperature chemistry and redox conditions in the vapor plume (by applying the EELS technique), and finally the various cooling paths of specific melt volumes. The combined effort of the various analytical approaches allows characterization of the respective material from the mm- over the µm- down to the nano-meter scale. The 1.07 Ma complex L. Bosumtwi impact structure is excellently suited to study these problems for the following reasons: recent target of ICDP drilling, presence of tektites and microtektites, and extraordinary fresh glass lithologies in some breccias. The investigated samples include suevites collected between Nyameani and Nkowinkwanta and Ivory Coast tektites (IVC spheres with the typical pitted surface). The suevite contains various glasses ranging from diaplectic quartz crystals with coesite over mineral glasses with fluidal texture to shards of vesicular “mixed” glasses with schlieren. First LA-ICP-MS data for the IVC sample indicate rather uniform distribution of the measured elements (V, Co, Ni, REE, Zr, Nb, Hf, and Ta) at the scale of the 60-µm-diameter spots.