REEVALUATION OF SIDEROPHILE ELEMENT ABUNDANCES AND RATIOS ACROSS THE CRETACEOUS-PALEOGENE (K-PG) BOUNDARY: IMPLICATIONS FOR THE NATURE OF THE PROJECTILE

Since the discovery over 30 years ago at Gubbio (Italy) and Caravaca (Spain) of an enrichment in iridium (Ir) and the other platinum group elements (PGE) up to four orders of magnitude (Ir_max = 0.10-87 ng/g) higher than average continental crustal background levels, similar anomalies have been detected in more than 120 Cretaceous-Paleogene (K-Pg) boundary sites worldwide. Highly elevated Ir and other siderophile element abundances in roughly chondritic ratios are considered strong indicators for the presence of a meteoritic contribution in impact-related lithologies (melt rocks, impact ejecta material, etc.) delivered when an extraterrestrial object strikes Earth. The presented work adds 112 unpublished PGE analyses of 38 K-Pg sections worldwide to the existing literature by combination of a nickel-sulfide fire assay pre-concentration technique with ICP-MS. Through repeated analysis of key siderophile elements (e.g., Cr, Co, Ni, and PGE), the importance of sampling, nugget effects, and analytical methodologies applied becomes more apparent. Even more critically, these analytical effects are superimposed by the local syn- and post-depositional conditions that have affected the pristine meteoritic signature of the K-Pg impactor, including potential fractionation during vaporization and condensation, terrestrial PGE input, sedimentation rate, reworking, diagenesis, bioturbation, and chemical diffusion. While chondrite-normalized PGE patterns of individual sites appear relatively flat (i.e. chondritic), strong variations in siderophile element content and inter-element ratios exist between K-Pg locations, inter-laboratory measurements, and replicate analyses, hampering a precise projectile identification using (highly) siderophile elements. Only when considering improved databases of siderophile element concentrations in meteorites, in combination with linear regression analysis to calculate inter-element ratios from a large suite of ejecta deposit sites, the nature of the K-Pg projectile can be resolved. Application of this methodology to an extensive data set of continental and marine sites, very proximal to distal to the Chicxulub impact structure, supports a carbonaceous chondritic impactor (type CO or CM).