Highly siderophile elements (Re, Os, Ir, Ru, Rh, Pd, Au) in impact melts from three European impact craters (Sääksjärvi, Mien, and Dellen): Clues to the nature of the impacting bodies

Abstract

Twenty-two large (10 g) impact melt samples from three Scandinavian craters (i.e., Sääksjärvi, Finland; and Mien and Dellen, Sweden) were analyzed for highly siderophile elements (HSE: platinum group elements, Rh, and Au) by the nickel sulfide technique in combination with neutron activation. The ten impact melt samples from Sääksjärvi are enriched in Ir and other highly siderophile elements (Ir = 2.48 ± 0.73 ng/g) relative to average upper crust concentrations (0.03 ± 0.02 ng/g Ir). The twelve Dellen and Mien samples are marginally enriched in Ir (0.48 ± 0.23 ng/g for Dellen, and 0.37 ± 0.23 ng/g for Mien). The amount of meteoritic component corresponds to 0.5% of a nominal CI component for Sääksjärvi, and about 0.1% for Mien and Dellen. The Sääksjärvi pattern is fractionated relative to CI-chondrites. Normalized abundances increase from the refractory to the more volatile siderophile elements (Ir < Ni). The trend is qualitatively similar to magmatic iron meteorites and corresponds to 0.4% of a nominal magmatic iron meteorite component (Tamarugal, IIIAB). For Mien and Dellen no projectile assignment can be made. All samples from the three impact craters have low Os Ir ratios compared to chondritic ratios. Either the projectile had low Os Ir ratios or Os was lost during the impact as volatile OsO 4. Based on the results of this study and on a compilation of literature data, average upper crustal abundances of the highly siderophile elements in the target area (Baltic shield) are estimated as 0.03 ng/g for Ir and Os, 1.1 ng/g for Ru, 0.38 ng/g for Rh, 2.0 ng/g for Pd, 8 μg/g for Co, and 37 μg/g for Cr. These data are representative of the upper crust. They allow, for the first time, reliable estimates of crustal abundances for Rh and Ru. Upper crustal abundances also show that Ir and Os are the most favourable elements for the identification of meteoritic signatures. Fractionated Os Ir ratios in magmatic iron meteorites or potential losses of Os during the impact leaves Ir as the best indicator element for meteoritic contamination. With Ir, extraterrestrial components as low as 2 × 10 4 × CI may be identified.