STABLE CARBON ISOTOPE STUDIES OF IMPACT BRECCIA CLASTS FROM THE EYREVILLE CORE, CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA, USA

The Late Eocene Chesapeake Bay impact structure, with a diameter of 85 km, is one of the largest and best preserved impact structures on Earth. It has also been extensively drilled. In 2006, the ICDP-USGS Eyreville drill core reached a depth of 1766 m; the core comprises 1322 m of basement rocks and crater fill (a variety of impactites, sediment, a granitic and an amphibolitic megablock), and 444 m of post-impact sediments. Samples from the Eyreville core provide the opportunity to investigate the carbon abundance and carbon isotope composition in an entire section of crater-fill impactites and sediments in comparison to graphitic basement rocks, as well as the carbon and oxygen isotope ratios of post-impact alteration products (carbonates) within the impactites, with the aim to constrain the composition of fluids involved in their formation. Stable isotope abundances were determined in samples from three of the five major lithologic units. The sediment clast breccia (444 - 1096 m) contains some carbonate clasts, biogenic carbonate, carbonate cement, and melt particles with secondary carbonates, which are, however, too fine-grained for conventional stable isotope analysis by phosphoric acid digestion. Within this breccia (at 762 m depth) one black “coal-like” sample occurs. Additionally, there are numerous clasts of fine-grained carbon-bearing metasediments. Also the suevitic breccia (1393 – ca. 1550 m) has many clasts of fine-grained metasediments, which are, together with some secondary carbonate in melt particles in one suevite sample, analyzed for their carbon isotopic composition. From 1551-1560 m, at the top of the supposed crater floor, breccia with a high amount of graphite, as well as graphite from some basement mica schists, was selected for carbon isotope analysis. We present a detailed carbon stable isotope study of a complete section of impactites in a shallow-marine impact structure, and discuss the isotopic characteristics of the carbon components (e.g., amorphous carbon or graphitic material) in the rocks of the crater floor and in the metasedimentary clasts within the impact breccias. The carbon isotopic analysis is currently in progress.