Petrology of Impact Melt Rocks from the Chesapeake Bay Crater

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 ºC to below 1500 ºC 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 ºC occur as well. Typical glass transition temperatures of rhyolitic melts are between 775 – 600 ºC, 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 ºC, weirakite formed in vugs and cooling cracks, and some glass was altered to chlorite. Later alteration stages, below ~150 ºC, produced smectite, phillipsite and chalcedony.