Evidence for a second L chondrite impact in the Late Eocene: Preliminary results from the Wanapitei crater, Canada

Introduction: The Wanapitei impact structure (46°45 ́N; 80°45 ́W) is located 40 km NE of the city of Sudbury, Ontario, Canada. The 7 to 8 km in diameter crater is not exposed and lies completely within Lake Wanapitei [1]. The Wanapitei impact melt rocks contains about 1% of an extraterrestrial component and, based on Ni/Ir, Ni/Cr and Co/Cr ratios an L or LL chondrite projectile is advocated [2, 3, 4]. Two samples of impact-melt glass provide a K-Ar age of 37 ± 2 Ma [5]. Wanapitei crater is formed in the late Eocene, along with the two largest structures in the Cenozoic, the 100-km Popigai (35.7 ± 0.2 Ma) in northern Siberia and the 85-km Chesapeake Bay (35.5 ± 0.6 Ma) offshore Virginia. Enrichments in He, detected in marine carbonates from the Massignano section in Italy, also characterizes the Late Eocene [6]. The possible higher impact rate and enhanced flux of interplanetary dust particles during that time are explained by either a comet shower caused by a perturbation of the Oort cloud [7] or an asteroid shower triggered by a major collision in the asteroid belt [8]. The latter interpretation is supported the identification of an L chondrite projectile for the Popigai crater [8]. Samples: To confirm the origin of the Wanapitei projectile, 10 samples of impact melt rock were analyzed for PGE by nickel sulfide fire assay combined with ICP-MS according to the procedure described in [8]. Siderophile elements (Ni, Co, Cr) were measured by ICP-MS. The samples were collected from glacial till, south of the Lake Wanapitei. Results: In agreement with [2, 3, 4], the new data demonstrate the presence of an extraterrestrial component in the Wanapitei impact melt rocks (Fig. 1).