Geochemistry of target rocks, impact-melt particles, and metallic spherules from Meteor Crater, Arizona: Empirical evidence on the impact process
David W. Mittlefehldt, Friedrich Hörz, Thomas H. See, Edward R.D. Scott, Stanley A. Mertzman, 2005. "Geochemistry of target rocks, impact-melt particles, and metallic spherules from Meteor Crater, Arizona: Empirical evidence on the impact process", Large Meteorite Impacts III, Thomas Kenkmann, Friedrich Hörz, Alex Deutsch
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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.