This study presents the abundances of Ru, Ir, Pt, Pd, Rh, and Au (platinum-group elements; PGE) in the ejecta layer at the Cretaceous-Tertiary boundary and melt rocks from East Clearwater Lake, Brent, New Quebec, and Lake Wanapitei impact craters, all believed to have been formed by the impact of chondritic projectiles. The objective was to evaluate the pure application of PGE interelement ratios (e.g., Ru/Ir) for impactor identification, to compare the utility of both melt rocks and ejecta deposits for this purpose, and to expand the PGE database for these craters. The similarity of PGE interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition. Introduction of indigenous PGE from country rock may contaminate the original meteoritic signature. In addition, the extent of PGE fractionation during impact and melt sheet cooling and of post-sedimentary PGE remobilization must be evaluated in each case. For East Clearwater, the PGE chondrite-normalized plot is flat, indicating a chondritic impactor. Based on Ru/Ir and Cr/Ir ratios, a Cl- or L-chondrite is most likely. However, variation of ratios with depth in the core makes any fine classification of the projectile tenuous. For the Brent and New Quebec craters, the flat chondrite-normalized PGE plot indicates a chondritic impactor. but finer classification is precluded by possible post/syn depositional PGE fractionation. At Lake Wanapitei, Ni/Cr and Cr/Ir ratios indicate a LL-chondrite impactor (WOLF et al., 1980) but sparse PGE data for this chondrite-type does not allow further classification of the projectile. A flat chondrite-normalized PGE plot indicates a chondritic rather than an iron impactor at Lake Wanapitei. The global ejecta layer at the Cretaceous-Tertiary boundary has a chondritic PGE signature, but only when the integrated flux of PGE on a global scale is considered. Individual sites may not show chondritic ratios due to differential PGE remobilization resulting from local differences in depositional environment (continental vs. marine), and to vaporization and/or condensation PGE fractionation occurring during impact. The results for melt rocks and the K-T ejecta deposit indicate that PGE can be used for impactor identification but primarily in combination with other siderophile/chalcophile meteoritic elements and their ratios.
