Identification of projectiles of large terrestrial impact craters and some implications for the interpretation of Ir-rich Cretaceous/Tertiary boundary layers

Impact-generated lunar highland rocks have high Ir and other noble metal concentrations. This Ir enrichment is produced by the admixture of a small amount of an undifferentiated meteoritic component. Similar high Ir concentrations are found in rocks from several large terrestrial impact craters. A compilation of all Ir analyses of terrestrial crater samples shows that there are now nine craters with a significant Ir anomaly (Ir > 1 ppb). Judging from the relative abundances of siderophile elements and the simultaneous enrichment of Cr, four of these craters were produced by chondritic projectiles. The Clearwater East impact melt, with an 8% chondritic component, is a well-characterized example. There is some evidence for achondrites, stony-irons, and iron meteorites being among projectiles of terrestrial impact craters. When present at any given impact site, Ir enrichment is commonly found in melt rocks and tends to occur in all samples from the same melt sheet. Lack of a meteoritic component may be due to a differentiated projectile, low in Ir and/or a high impact velocity. The excellent correlations among all siderophile elements and Cr in 23 samples from the Cretaceous/Tertiary boundary layer at Stevns Klint indicate that all these elements are contained in one component. The chondrite-normalized patterns are distinctly "terrestrial" in having large differences in concentrations of elements with similar cosmo-chemical volatilities, such as Au and Ge (low) and As and Cu (high), and suggest that the Ir-rich component is a mixture of meteorite and ejecta material from the site of the impact. However, it can be shown that a two-component mixture with various types of meteorites and terrestrial rocks cannot explain the observed element pattern. Some fractionation (e.g., volatilization and recondensation) may have modified the original pattern, making it difficult to identify the meteorite and, therefore, introducing large uncertainties on the fraction of meteorite in a given sample from a C/T boundary layer.