CHEMICAL CHARACTERISTICS OF THE CRETACEOUS-TERTIARY BOUNDARY-LAYER ATGUBBIO, ITALY

Platinum metals (Pd, Ir, Pt) and Au were determined by radiochemical n eutron activation analysis (RNAA) on Cretaceous-Tertiary (K-T) boundar y samples collected at Gubbio, Italy. In addition, about thirty elemen ts were determined by instrumental NAA (MAA) for both the Cretaceous a nd the Tertiary layer samples, as well as the boundary clay samples. I ridium and other Pt metals were observed to be similarly enriched, rel ative to CI-chondrite, in the boundary layers, i.e., they occur in cho ndritic ratios. On the other hand, correlations among the metals are n ot very good, suggesting that Pt metals were not incorporated into cla y materials in a single phase, but rather, they behaved separately acc ording to their own solution chemistries. In the Gubbio section, the c hemical composition does not change so much across the K-T boundary la yer, except for Ir (and possibly other Pt group elements) and is very similar to that of the ''North American shale composite (NASC)''. Arse nic and Sb were observed not to be anomalously enriched in the Gubbio K-T boundary layers. This implies that their abundances cannot be a cl ue in elucidating an event which must have happened at the K-T boundar y period. NASC-normalized REE abundance patterns for the boundary samp les show characteristic features due to some REE precipitation out of seawater. REE abundances are not variable among the Cretaceous, the Te rtiary, and the boundary samples, suggesting that conditions for preci pitation and sedimentation were similar across the K-T boundary at Gub bio.