Projectile identification in impact craters

In this project we try to characterize the type and origin of the projectiles responsible for the formation of particular impact structures in an attempt to determine whether or not the composition of these projectiles changes through the course of geological time. The identification of the projectile component (or the type of the collided meteorite) in impact crater lithologies is possible by using certain isotopic [chromium (Cr)] and elemental ratios [platinum group elements (PGE)] in 'contaminated' lithologies. To do so with sufficient certainty and resolution, both geochemical techniques will be developed using (new) instrumentation: Multicollector-ICP-MS for the chromium isotopic ratios and sector field ICP-MS for the PGE analyses. The focus of this study is to confirm or deny the existence of an asteroid shower in the late Eocene. During this period of time, the deepwater carbonates of the Massignano section (Italy) were enriched with extraterrestrial 3He, which indicates the increased arrival of interplanetary dust particles (IDPs) on Earth (Farley et al., 1998). These authors attribute this 2.5 million year increased flux of IDPs to the arrival of long-periodical comets to the centre of the solar system, bound to a perturbation of the Oort Cloud. Tagle & Claeys (2004) identified the projectile responsible for the formation of the Popigai impact structure (with a diameter of 100 km, dated 35.7 ± 0.2 Ma) as an L- chondriet, whose source is possibly the type S(IV) asteroids from the inner asteroid belt. Such a composition is difficult to link with a cometary origin. Although the PGE composition of comets is unknown, it is unlikely that a cometary bolide would display PGE ratios similar to that of L-chondrites (Tagle & Claeys, 2004). It is assumed that comets are primitive bodies with a composition comparable to that of carbonaceous chondrites, which clearly differ in PGE concentrations from ordinary chondrites. Because asteroids represent around ~1% of the inner zone of the Oort Cloud objects (Weissman & Levison, 1997), it can not be excluded that a comet shower could contain an L-chondritic body, but that possibility is small. The likeliness that an object with a diameter of 5 km, necessary for the formation of the Popigai crater, strikes the earth is less than once every 26 million years (French, 1998). To complete and/or confirm this hypothesis, the projectiles responsible for the formation of 4 other late Eocene impact structures need to be identified: (1) Chesapeake Bay, offshore Maryland, USA (85 km, 35.5 ± 0.6 Ma), (2) Logoisk in Belarus (17 km, 40 ± 5 Ma), (3) Mistastin in Canada (28 km, 36.4 ± 4 Ma) and (4) Haughton, North-Canada (23 km, 39 ± 2 Ma). The identification of the projectiles in these structures as L-chondrites would confirm the existence of an L-type chondritic asteroid shower in the late Eocene.
In the next phase of this project other important craters and/or ejecta material, formed during other geological times (e.g. the Ordovician and the Late-Devonian), will also be measured to see if the flux of impactors has not changed on a geological scale (e.g. Sudbury and Clearwater in Canada, Boltisch in Ukraine etc.).