AN L-CHRONDRITE ORIGIN FOR THE POPIGAI CRATER

A set of 22 samples from impact melt originating from outcrops and cores at 3 locations in the Popigai crater (100-km, Late Eocene) were analyzed for this study. The Platinum Group Elements (PGE) were determined using the ICP-MS with NiS fire assay pre-concentration method. This method provides good precision down to concentration around 0.06 ppb Ru, 0.02 ppb Rh, 0.19 ppb Pd, 0.06 ppb Ir, 0.07 ppb Pt and 0.13 ppb Au and is thus ideal to detect even minute meteoritic contamination. All samples analyzed were enriched in PGE, reaching concentrations of Iridium from 1.7 ppb. The PGE concentrations in the impact melt is between 3 and 7 times higher than in the target rock gneisses. Because of the small meteoritic enrichment, the indigenous component was also determined by correlation of the PGE?s with Ir. After subtraction of the indigenous composition, the amount of meteoritic contamination corresponds to 0.2% of nominal CI component. The PGE pattern in the Popigai impact melt appears homogeneous and shows little or no fractionation. The flat PGE-normalized pattern of CI and the high values of the Cr in the melt argue for a chondritic projectile. The Popigai indigenous-corrected PGE inter-elemental ratios are similar to those of ordinary chondrites especially the L type and to a lesser extend to LL or H chondrites. L-chondrites may also have formed the Morokweng (80-km) and Clearwater East (26- km) craters. However, their melt rock contains up to 7 or 10 wt.% meteoritic material. These examples illustrate that the impact mechanisms (angle, speed etc. as attested by computer modeling) can severely influence the total amount of meteoritic material incorporated in the melt. The variability in the total meteoritic component detected in Clearwater, Morokweng and Popigai craters indicates that craters where no meteoritic contamination is detected need not necessarily be formed by impactors depleted in PGE's as frequently assumed.