SHOCK-METAMORPHIC EFFECTS IN LATTICE STRUCTURE OF SPHALERITE (ZNS) FROM POLYMICT IMPACT BRECCIA DIKES, KENTLAND CRATER, INDIANA

The majority of published research regarding shock-metamorphic effects in rocks and minerals primarily describes changes that occur in crystalline target rocks and in the minerals quartz and calcite. The Kentland crater is <13 km in diameter, <97 m.y. old, that consists of highly deformed and shocked Paleozoic sedimentary target rocks. A limestone quarry located within the central uplift (40°45'N, 87°24'W) provides opportunities to study and sample these shocked rocks. Polymict breccia dikes cut steeply dipping beds in the central uplift. The breccias contain abundant carbonate and fewer sphalerite clasts. We are studying the effects of shock-metamorphism on the crystal lattice structure of the shocked sphalerite (ZnS) grains. Sphalerite has a simple cubic atomic structure and is ideal for studying such changes. Eight samples of breccia containing sphalerite were collected from the central uplift. The sphalerite was separated, crushed, sieved, and analyzed using X-ray powder diffraction (XRD). Two unshocked sphalerite samples were also treated in the same way, and, together with published unshocked sphalerite XRD patterns, were used as unshocked standards. We used full width at half maximum peak measurements, Bragg's equation, {hkl} plane geometry, and 3D Pythagorean's theorem to calculate d-spacings and evaluate unit cell lengths, and compared these values for the shocked samples and unshocked standards. Unit cell lengths appear to be unchanged within measurement uncertainties, indicating no mineral polymorphism. All shocked samples however show strong, systematic increases in % d-value variance, demonstrating the high magnitude of random atomic-scale damage and disordering that occurred during shock metamorphism.