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Silicate glasses and sulfide melts in the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure, Virginia, USA

By
Harvey E. Belkin
Harvey E. Belkin
U.S. Geological Survey, 956 National Center, Reston, Virginia 20192, USA
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J. Wright Horton, Jr.
J. Wright Horton, Jr.
U.S. Geological Survey, 926A National Center, Reston, Virginia 20192, USA
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Published:
January 01, 2009

Optical and electron-beam petrography of melt-rich suevite and melt-rock clasts from selected samples from the Eyreville B core, Chesapeake Bay impact structure, reveal a variety of silicate glasses and coexisting sulfur-rich melts, now quenched to various sulfide minerals (±iron). The glasses show a wide variety of textures, flow banding, compositions, devitrification, and hydration states. Electron-microprobe analyses yield a compositional range of glasses from high SiO2 (>90 wt%) through a range of lower SiO2 (55–75 wt%) with no relationship to depth of sample. Some samples show spherical globules of different composition with sharp menisci,suggesting immiscibility at the time of quenching. Isotropic globules of higher interfacial tension glass (64 wt% SiO2) are in sharp contact with lower-surface-tension, high-silica glass (95 wt% SiO2). Immiscible glass-pair composition relationships show that the immiscibility is not stable and probably represents incomplete mixing. Devitrification varies and some low-silica, high-iron glasses appear to have formed Fe-rich smectite; other glass compositions have formed rapid quench textures of corundum, orthopyroxene, clinopy-roxene, magnetite, K-feldspar, plagioclase, chrome-spinel, and hercynite. Hydration (H2O by difference) varies from ~10 wt% to essentially anhydrous; high-SiO2 glasses tend to contain less H2O. Petrographic relationships show decomposition of pyrite and melting of pyrrhotite through the transformation series; pyrite→pyrrhotite→troilite→iron. Spheres (~1 to ~50 μm) of quenched immiscible sulfide melt in silicate glass show a range of compositions and include phases such as pentlandite, chalcopyrite, Ni-As, monosulfide solid solution, troilite, and rare Ni-Fe. Other sulfide spheres contain small blebs of pure iron and exhibit a continuum with increasing iron content to spheres that consist of pure iron with small, remnant blebs of Fe-sulfide. The Ni-rich sulfide phases can be explained by melting and/or concentrating target-derived Ni without requiring an asteroid impactor source component. The presence of locally unaltered glasses in these rocks suggests that in some rock volumes, isolation from postimpact hydrothermal systems was sufficient for glass preservation. Pressure and temperature indicators suggest that, on a thin-section scale, the suevites record rapid mixing and accumulation of particles that sustained widely different peak temperatures, from clasts that never exceeded 300 ± 50 °C, to the bulk of the glasses where melted sulfide and unmelted monazite suggest temperatures of 1500 ± 200 °C. The presence of coesite in some glass-bearing samples suggests that pressures exceeded ~3 GPa.

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GSA Special Papers

The ICDP-USGS Deep Drilling Project in the Chesapeake Bay impact structure: Results from the Eyreville Core Holes

Gregory S. Gohn
Gregory S. Gohn
U.S. Geological Survey, Reston, Virginia, USA
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Christian Koeberl
Christian Koeberl
Department of Earth & Planetary Sciences, Rutgers University, USA
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Kenneth G. Miller
Kenneth G. Miller
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Germany
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Wolf Uwe Reimold
Wolf Uwe Reimold
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Germany
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Geological Society of America
Volume
458
ISBN print:
9780813724584
Publication date:
January 01, 2009

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