CHARACTERIZATION OF THE CHESAPEAKE BAY IMPACTOR BY DETERMINATION OF THE PLATINUM GROUP ELEMENT PGE RATIOS IN THE IMPACTITE SAMPLES OF THE 2005-2006 ICDP-USGS DEEP DRILLING PROJECT CORES

The late Eocene epoch is characterized by both a 2.5 Ma long increase of extraterrestrial 3He, first detected in the marine carbonates of the Massignano section (Italy) and the formation of possibly 5 or more impact structures (Popigai, Chesapeake Bay, Wanapitei, Haughton, and Mistastin). One scenario attributes the high impact rate and enhanced flux of interplanetary dust particles to the arrival of long-period comets in the inner solar system caused by a perturbation of the Oort cloud. Platinum group element (PGE) concentrations form one of the three methods (next to Os and Cr isotopes) capable of identifying these sometimes minute (~ 0.2 wt.%) proportions of extraterrestrial projectile. A comparison of the specific inter-elemental ratios of an impactor with the same ratios in the different types of meteorites (e.g., chondrites), renders possible a precise projectile identification. Both the Popigai and Wanapitei impactors proved to be L-type ordinary chondrites using this method. The presence of two of this same type of impact structures indicates a major collision in the asteroid belt resulting in an asteroid shower on Earth. To confirm the nature of the shower at the end of the late Eocene, PGE ratios were measured in 15 impactite samples (80 g) of the 2005–2006 ICDP-USGS Deep Drilling Project cores at Chesapeake Bay. This crater in south-eastern Virginia, United States, is with 85 km the seventh largest impact crater documented on Earth, and is very similar to the Popigai impact structure both in age and in size. Previous studies were unable to constrain the exact nature of the projectile responsible for its formation due to the PGE target material signal and a possible fractionation during syn- or post-impact events. Inductively coupled plasma mass spectrometry (ICP-MS) is used to determine the major and trace elements (including indicative Ni and Cr), as well as PGEs and Au (after preconcentration using a NiS fire assay). In this session, we present the measured results and try to address the question whether or not the late Eocene Chesapeake Bay impact event is related to an asteroid shower.