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Coesite, Glass, and Shocked Quartz at Dsdp Site 612: Evidence for Nearby Ipipact in the Late Eocene; B.F

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Coesite, Glass, and Shocked Quartz at Dsdp Site 612: Evidence for Nearby Ipipact in the Late Eocene; B.F 114 LPSC XIX COESITE, GLASS, AND SHOCKED QUARTZ AT DSDP SITE 612: EVIDENCE FOR NEARBY IPIPACT IN THE LATE EOCENE; B.F. Bohor, W.J. Betterton, and E.E. Foord, U .S. Geological Survey, Box 25046, MS 901, DFC, Denver, CO 80225 DSDP Site 612 off New Jersey contains an 8-cm-thick layer of impact debris in core 21, sec. 5, between 111-119 cm. Glass (1) reported coesite in thi s 1 ayer. We found coesite and quartz-coesi te grains throughout the 1ayer, but the largest (>1mm) occur in the basal few centimeters. Fig. 1 shows one of these large grains that appears to be pure coesite by X-ray diffraction, but also contains small areas of lechatelierite (silica glass). Fig. 2 is a closeup view of an area of vesiculated lechatelierite on the coesite grain of Fig. 1. This seems to confirm the conclusion by Kieffer et a1 . (2) that lechatelierite in shocked Coconino Sandstone formed from the melting of coesite. However, the size of coesite grains at 612 is an order of magnitude greater than those observed in the Coconino (3). If the coesite grains at site 612 formed in the same manner as suggested by Kieffer (jetting into collapsed pores), the pore sizes must have been an order of magnitude larger, suggesting that an unconsolidated sediment was part of the target. Shocked quartz grains, first reported by Thein (4), occur throughout the layer but are concentrated in the upper portion. Evidence of shock deformation in quartz is displayed as fractures of various types and planar features. Quartz grains bearing radial fractures (Fig. 3) , termed concussion fractures by Kieffer (3), are prominent and are indicative of low shock pressures ((90 kb). Other quartz grains show multiple directions of both open planar fractures and dense sets of glass-fill ed planar features (Figs. 4-61, and are extremely fragile; some grains are glass mantled. These grains with multiple sets of lamellar features apparently are highly shocked and display almost no coherent extinction. X-ray films show a high degree of asterism or spot streaking and mean indices of refraction are reduced to as low as 1.544 (from 1-5485)--both indicating shock. Some of the shocked grains display small isotropic areas of glass formation. The absorption of index oils by the grains and their fragile nature shows that much of the shock is expressed as open fractures as well as glass-filled planar features, again possibly indicating an unconsol idated target materi a1 . A few K-fel dspar and plagioclase grains were seen, but it is difficult to demonstrate shock effects in these grains because of their pervasive twinning . The presence of fragments of tektite (impact) glass showing flow structures, as large as 7 mm in maximum dimension according to Thein (4), and large grains of coesite and shocked quartz that display a full range of shock pressure effects (<90->250 kb), provide evidence that this debris layer is distal throwout ejecta from a nearby impact. We note that analyses of major and minor elements from the me1 t glass layer (Jansa, 1987, written commun.) in the newly discovered Montagnais impact crater off Nova Scotia (5) almost matches that of tektite glass from the 612 layer and the rest of the North American tektite strewnfield (4, 6), suggesting a possible relationship. REFERENCES: (1) Glass, B.P. (1987) LPSC XVIII, 328-329; (2) Kieffer, S.W., Phakey, P.P. and Christie, J.M. (1916) Contrib. Mineral. Petrol. 59, 41- 93; (3) Kieffer, S.W. (1971) J . Geophys. Res.76, 5449-5473-;.-0--TfieTii,S. (1987) Init. Repts. DSDP, Leg 95: Wash., D.C. (U.S. Gov't. Print. Off.), 565- 579; (51 Jansa, L.F. and Pe-Piper, G. (1987) Nature 327, 612-614; (6) D'Hondt, S.L., Keller, G. and Stallard, R.F. (1987) Meteoritics 22, 61-80. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System COESITE, GLASS, AND StiOCKED QUARTZ (612) Bohor, B.F. et al. LPSC XIX Fig. 1. 2 mm coesite grain with Fig. 2. Closeup of vesiculated areas of vesiculated quartz glass lechatelierite on coesite grain. (lechatelierite) . Fig. 3. Quartz grain showing inten- Fig. 4. Quartz grain showing 3 sets sely fractured rim surrounding re- of shock lamellae (planar features). latively unfractured core. HF etch. Etched with HF. Fig. 5. Quartz grain with multiple sets of planar features and planar (very fine scale) sets of planar fractures. Etched with HF. features in quartz. HF etched. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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