Petrology of impact melt rocks from the Chesapeake Bay crater, USA
Axel Wittmann, Ralf T. Schmitt, Lutz Hecht, David A. Kring, W. Uwe Reimold, Harold Povenmire, 2009. "Petrology of impact melt rocks from the Chesapeake Bay crater, USA", The ICDP-USGS Deep Drilling Project in the Chesapeake Bay impact structure: Results from the Eyreville Core Holes, Gregory S. Gohn, Christian Koeberl, Kenneth G. Miller, Wolf Uwe Reimold
Download citation file:
The Eyreville B drill core in the inner annular moat of the 85-km-diameter Ches-apeake Bay impact structure recovered the first coherent impact melt volumes from within the crater as two bodies, 1 and 5.5 m thick. This study focuses on the petrogenesis of these well-preserved rocks. Mixing calculations reveal that the chemical composition of these melts can be modeled as a hybrid of ~40% sedimentary target and ~60% crystalline basement component. The melt rocks contain abundant lithic and mineral clasts that display all stages of shock metamorphism. Zircon clasts record the cooling of the melt from temperatures above 1700 °C to below 1200 °C within the first minutes after formation. Glassy melt with a peraluminous, rhyolitic composition that contains ~5 wt% water is preserved. This melt records a crystallization sequence of aluminum-rich orthopyroxene and hercynitic spinel, followed by plagioclase, titano-magnetite and cordierite, and late sanidine. Spherulitic aluminosilicate-SiO2-cordierite aggregates that are comparable to buchites at temperatures below ~1465 °C complement this assemblage. Lack of hyaloclastic fragmentation suggests dry emplacement conditions. Complete cooling by conductive heat transfer took ~7 weeks and ~4 years for the 1-m- and the 5.5-m-thick melt bodies, respectively. Alteration stages below ~100 °C produced smectite, phillipsite, chalcedony, and a rare zeolite phase that is tentatively identified as terranovaite.