EVIDENCE FOR A K T IMPACT EVENT IN THE PACIFIC-OCEAN/

THE chemical, mineralogical and isotopic characteristics of deposits a t the Cretaceous/Tertiary (K/T) boundary are suggestive of a large imp act event, the prime candidate1 being the Chicxulub crater in Yucatan, Mexico. Spinel-bearing spherules, which may be associated with such i mpacts, have been reported2 at several K/T boundary sites worldwide, b ut their origin is still uncertain. We have examined the spinel-bearin g material recovered from K/T boundary deposits at site 577 in the Pac ific Ocean3 and find two distinct populations of particles: spherules with dendritic spinel textures dispersed throughout the grains and irr egularly shaped fragments with spinels essentially confined to the rim . The morphology and composition of the particles are characteristic o f melted and partially melted meteoritic ablation debris, but their lo cation is difficult to reconcile with an impact on the Yucatan peninsu la, some 10,000 km away. We suggest instead that the spinel-bearing pa rticles at site 577 are derived from the impact of a 2-km asteroid in the Pacific Ocean, and that several accretionary events of this type a re required to explain the global distribution of spinel-bearing spher ules at the K/T boundary.