Petrology of impact melt rocks from the Chesapeake Bay Crater, USA

The Eyreville B drill core in the inner annular moat of the 85-km-diameter Chesapeake 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 approximately 40% sedimentary target and approximately 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 degrees C to below 1200 degrees C within the first minutes after formation. Glassy melt with a peraluminous, rhyolitic composition that contains approximately 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-SiO (sub 2) -cordierite aggregates that are comparable to buchites at temperatures below approximately 1465 degrees C complement this assemblage. Lack of hyaloclastic fragmentation suggests dry emplacement conditions. Complete cooling by conductive heat transfer took approximately 7 weeks and approximately 4 years for the 1-m- and the 5.5-m-thick melt bodies, respectively. Alteration stages below approximately 100 degrees C produced smectite, phillipsite, chalcedony, and a rare zeolite phase that is tentatively identified as terranovaite.