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Geochemistry of the impact breccia section (1397–1551 m depth) of the Eyreville drill core, Chesapeake Bay impact structure, USA

By
Katerina Bartosova
Katerina Bartosova
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Dieter Mader
Dieter Mader
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Ralf Thomas Schmitt
Ralf Thomas Schmitt
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany
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Ludovic Ferrière
Ludovic Ferrière
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, and Department of Earth Science, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada
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Christian Koeberl
Christian Koeberl
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Wolf Uwe Reimold
Wolf Uwe Reimold
Museum für Naturkunde–Leibniz Institute at Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany
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Franz Brandstätter
Franz Brandstätter
Natural History Museum, Burgring 7, A-1010 Vienna, Austria
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Published:
January 01, 2009

The Chesapeake Bay impact structure, which is 85 km in diameter and 35.5 Ma old, was drilled and cored in a joint International Continental Scientific Drilling Program (ICDP) and U.S. Geological Survey (USGS) drilling project at Eyreville Farm, Virginia, U.S.A. In the Eyreville drill core, 154 m of impact breccia were recovered from the depth interval 1397–1551 m. Major- and trace-element concentrations were determined in 75 polymict impactite samples, 10 samples of cataclastic gneiss blocks, and 24 clasts from impactites. The chemical composition of the polymict impactites does not vary much in the upper part of the section (above ~1450 m), whereas in the lower part, larger differences occur. Polymict impactites show a decrease of SiO2 content, and slight increases of TiO2, Al2O3, and Fe2O3 abundances, with depth. This is in agreement with an increase of the schist/gneiss component with depth. Concentrations of siderophile elements (Co, Ni) are lower in the polymict impactites than in the basement-derived schists and do not indicate the presence of an extraterrestrial component. The five petrographically determined types of melt particles, i.e., clear glass, altered melt, recrystallized silica melt, melt with microlites, and dark-brown melt, have distinct chemical compositions. Mixing calculations of the proportions of rocks involved in the formation of various polymict impactites and melt particles were carried out using the Harmonic least-squares MiXing (HMX) calculation program. The calculations suggest that the metamorphic basement rocks (i.e., gneiss and schist) constitute the main component of the polymict impactites, together with significant sedimentary and possible minor pegmatite/granite and amphibolite components. The sedimentary component is derived mostly from a sediment characterized by a composition similar to that of the Cretaceous Potomac Formation. Compositions of the melt particles were modeled as mixtures of target rocks or major rock-forming minerals. However, the results of the mixing calculations for the melt particles are not satisfactory, and the composition of the particles could have been modified by hydrothermal alteration. Carbon isotope ratios were determined for 18 samples. The results imply a hydrothermal origin for the carbonate veins from the basement-derived core section; carbon-rich sedimentary clasts from the Exmore breccia and suevite have a δ13C range typical for organic matter in sediments.

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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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