Geochemical characteristics of basement target rocks, suevitic glasses from the Eyreville B drill core, Chesapeake Bay impact structure, and three bediasites

Abstract

The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville B core hole, drilled into the 35.5-Ma-old Chesapeake Bay impact crater, Virginia, has recovered postimpact sediments, crater-fill breccias, megablocks of the crystalline basement, and suevites with fresh glass shards. Bulk rock analyses of 2 glass shards, 21 crystalline target rocks, and microchemical analyses of 7 glass shards and 3 bediasites (tektites of the North American strewn field) were performed in order to contribute to the understanding of formation processes and to better constrain the precursor materials of these glasses as well as of the bediasites. Statistical treatment (hierarchical cluster analyses) yielded an assignment of the data for the crystalline basement samples into four groups; two of those (various schists, meta-graywackes, and gneisses) display characteristics similar to the impact glasses in the suevites and the bediasites. However, the suevitic glasses show a broad range in composition at the micrometer scale. These data show the frequent presence of schlieren, and in particular, enhanced TiO2contents that require admixture of an “amphibolitic component” to the melt. Evidence for such a process is provided by the occurrence of relict, in-part thermally corroded grains of rutile and ilmenite, and by formation of Ti-rich tiny mineral aggregates in the glass. The three studied bediasites show only minor inter- and intrasample heterogeneity, and their chemical composition agrees well with previously published data. The new data for the bediasites are compatible with heating of the “tektite melt” to extreme temperatures, followed by quenching.