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Geochemistry of target rocks, impact-melt particles, and metallic spherules from Meteor Crater, Arizona: Empirical evidence on the impact process

By
David W. Mittlefehldt
David W. Mittlefehldt
SR, National Aeronautics and Space Administration Johnson Space Center, Houston, Texas 77058, USACorresponding author: david.w.mittlefehldt@nasa.gov
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Friedrich Hörz
Friedrich Hörz
SR, National Aeronautics and Space Administration Johnson Space Center, Houston, Texas 77058, USA
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Thomas H. See
Thomas H. See
C23, Lockheed Martin SO, 2400 Nasa Road 1, Houston, Texas 77058, USA
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Edward R.D. Scott
Edward R.D. Scott
Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, Honolulu, Hawai'i 96822, USA
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Stanley A. Mertzman
Stanley A. Mertzman
Department of Geosciences, Franklin & Marshall College, Lancaster, Pennsylvania 17604, USA
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Published:
January 01, 2005

We have done lithophile- and siderophile-element analyses of a large suite of target rocks, ballistically dispersed impact-melt particles and ballistically dispersed metallic spherules from Meteor Crater, Arizona. The Moenkopi Formation (topmost unit) has a unique lithophile-element signature that confirms it as a major component of the impact-melt particles. The Kaibab Formation is very heterogeneous, containing dolomite-rich and quartz-rich layers. The lithophile-element compositions of the impact-melt particles can be entirely explained as mixtures of Moenkopi and Kaibab depleted in CO2. The Toroweap and Coconino Formations (lowest units) are not required components, but small contributions from them cannot be excluded. We conclude the impact-melt particles were formed entirely in the upper portion of the section, above the lower two units.

The impact-melt particles average ∼14 wt% projectile material. Most siderophile-element ratios of the impact-melt particles are unchanged from those of the projectile. Many samples are depleted in Au; the most extreme depletions are in impact-melt particles with the highest Kaibab component. Kaibab rocks are highest in Br, and we suggest loss of volatile Au halides may have caused the fractionation.

Ballistically dispersed metallic spherules are enriched in Co, Ni, Ir, and Au compared to Canyon Diablo metal. Element/Ni ratios deviate slightly from projectile ratios, and are inversely correlated with susceptibility to oxidation relative to Ni. We attribute this to partial oxidation of molten metal spherules during flight. Spherule compositions suggest some selective melting of graphite-troilite-schreibersite inclusions of the projectile, consistent with enhanced shock melting of these lower density phases.

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Contents

GSA Special Papers

Large Meteorite Impacts III

Thomas Kenkmann
Thomas Kenkmann
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Friedrich Hörz
Friedrich Hörz
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Alex Deutsch
Alex Deutsch
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Geological Society of America
Volume
384
ISBN print:
9780813723846
Publication date:
January 01, 2005

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