269-8 Petrology of Impact Melt Rocks from the Chesapeake Bay Crater

Tuesday, 7 October 2008: 3:40 PM
George R. Brown Convention Center, 342CF
Axel Wittmann1, David A. Kring2, Wolf U. Reimold3, Lutz Hecht4, Ralf T. Schmitt3 and Vera A. Fernandes5, (1)Lunar and Planetary Institute, USRA, Houston, TX
(2)Lunar and Planetary Institute, Houston, TX
(3)Museum of Natural History, Humboldt University, Berlin, Germany
(4)Museum for Natural History, Humboldt University, Berlin, Germany
(5)Berkeley Geochronology Center, Berkeley, CA
The ICDP-USGS Eyreville-B drilling in the inner annular moat, about 9 km from the center of the 85 km diameter Chesapeake Bay impact structure, encountered a section of impactites that was deposited before a marine resurge buried the crater. In these impactites, the first coherent impact melt rocks from Chesapeake Bay were recovered as two 1 and 5.5 m thick bodies. These melts are petrologically distinct from melt fragments in the surrounding suevites in that they crystallized abundant liquidus phase phenocrysts. The melt rocks contain lithic clasts that display all stages of shock metamorphism. Zircon clasts recorded the cooling of the melt from temperatures above 1700 C to below 1500 C within ~5 minutes. Glassy melt is preserved and exhibits a volatile content of ~5 vol.%. It has a peraluminous, rhyolitic composition, although it also contains dark streaks of incompletely assimilated mafic material from the surrounding suevite. The crystallization sequence started with aluminum-rich orthopyroxene and hercynitic spinel followed by plagioclase, titanomagnetite and cordierite, and late sanidine. Biotite occurs sporadically and spherulitic christobalite/tridymite (?) mullite (?) - cordierite aggregates that are characteristic for buchites at temperatures below ~1465 C occur as well. Typical glass transition temperatures of rhyolitic melts are between 775 600 C, but, depending on the speciation of volatiles in the melt, it may have been even lower. Lack of hyaloclastic fragmentation suggests dry emplacement conditions. Alteration is not pervasive and only in part overprints the melt rocks. At temperatures around 300 C, weirakite formed in vugs and cooling cracks, and some glass was altered to chlorite. Later alteration stages, below ~150 C, produced smectite, phillipsite and chalcedony.