HEUREKA! PROJECTILE MATTER IN THE CRETACEOUS-PALEOGENE (K-PG) BOUNDARY EVENT BED – ON THE BENEFIT OF REE

The discovery of a platinum group element (PGE) anomaly exactly at the K-Pg boundary in Spain (Smit and Hertogen 1980) and Italy (Alvarez et al. 1980) led to the idea of an impact of a ~10km-sized projectile that abruptly ended the dominance of dinosaurs. Consequences of this Chicxulub impact are documented in more than 350 K-Pg boundary sites (Schulte et al. 2010), and the causal connection between impact and mass extinction is well defined. The Cr isotope composition constrains the projectile to a carbonaceous chondrite of type CM2. So far, ~1.5 % of the projectile have been detected in the K-Pg boundary layer; i.e., PGE as well as other siderophiles which always form a spike at the very top of the ejecta layer. Occasionally, contamination of or even a major contribution of extraterrestrial matter to the ejecta layer was proposed, based on Rare Earth Elements (REE) patterns deviating from the typical pattern for the Upper Continental Crust (UCC). According to numerical models, over 500 km³ of the CM2 projectile, mainly Si, Fe, and Mg should have been deposited in the K-Pg event bed outside the continuous ejecta blanket – but were not found up to now. We performed microprobe and LA-ICP-MS analyses of siliceous spherules in thin sections from the K-Pg event bed at Alabama, and NE Mexico. The Fe-Mg-rich as well as Si-K-rich spherules – hydrated glass or altered to sheet silicates, with excellently preserved textures – have very low trace element contents. In detail, REE patterns are flat and well corresponding to the CI-pattern, REE abundances are chondritic or sub-chondritic (= 0.1 x CI), rarely exceeding the CI abundances. Mixing calculations limit the contribution of UCC material to the REE budget to (much) less than 2%. The PGE are mostly below detection limit, the low abundance of Ni, Co, and Cr indicates strong fractionation of the metal-sulfide from the siliceous component of the projectile during the impact process. While the origin of the sub-chondritic REE abundances is currently not understood, the flat REE patterns substantiate that the siliceous host material (= spherules) consists solely of projectile matter. Depending on the sampling site, these spherules amount to between 10 and ~70 vol% of the event bed. The widespread occurrence of condensed projectile matter in the K-Pg event bed reconciles observations with impact models.