USE OF PLATINUM-GROUP ELEMENTS FOR IMPACTOR IDENTIFICATION - TERRESTRIAL IMPACT CRATERS AND CRETACEOUS-TERTIARY BOUNDARY

This study presents the abundances of Ru, Ir, Pt, Pd, Rh, and Au (plat inum-group elements; PGE) in the ejecta layer at the Cretaceous-Tertia ry boundary and melt rocks from East Clearwater Lake, Brent, New Quebe c, and Lake Wanapitei impact craters, all believed to have been formed by the impact of chondritic projectiles. The objective was to evaluat e the pure application of PGE interelement ratios (e.g., Ru/Ir) for im pactor identification, to compare the utility of both melt rocks and e jecta deposits for this purpose, and to expand the PGE database for th ese craters. The similarity of PGE interelement ratios within the chon drite and iron meteorite groups precludes their use in the fine classi fication of impactors of this composition. Introduction of indigenous PGE from country rock may contaminate the original meteoritic signatur e. In addition, the extent of PGE fractionation during impact and melt sheet cooling and of post-sedimentary PGE remobilization must be eval uated 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 pro jectile tenuous. For the Brent and New Quebec craters, the flat chondr ite-normalized PGE plot indicates a chondritic impactor. but finer cla ssification is precluded by possible post/syn depositional PGE fractio nation. At Lake Wanapitei, Ni/Cr and Cr/Ir ratios indicate a LL-chondr ite impactor (WOLF et al., 1980) but sparse PGE data for this chondrit e-type does not allow further classification of the projectile. A flat chondrite-normalized PGE plot indicates a chondritic rather than an i ron impactor at Lake Wanapitei. The global ejecta layer at the Cretace ous-Tertiary boundary has a chondritic PGE signature, but only when th e integrated flux of PGE on a global scale is considered. Individual s ites may not show chondritic ratios due to differential PGE remobiliza tion resulting from local differences in depositional environment (con tinental vs. marine), and to vaporization and/or condensation PGE frac tionation occurring during impact. The results for melt rocks and the K-T ejecta deposit indicate that PGE can be used for impactor identifi cation but primarily in combination with other siderophile/chalcophile meteoritic elements and their ratios.