TEM study of meteorite impact glass at New Zealand Cretaceous–Tertiary sites: evidence for multiple impacts or differentiation during global circulation?

Study by transmission electron microscopy of samples from the Cretaceous–Tertiary (K–T) boundary clay at Flaxbourne River and Woodside Creek, New Zealand, has revealed the occurrence of nanometer-sized meteorite impact-derived glass. The average glass composition is exceptionally Ca-rich and is distinct from other glass found on Earth, apart from glass inferred to be of impact origin at Mexican and Haitian K–T sites. The glass shards are partially altered to montmorillonite-like smectite, with the dominant interlayer cation, Ca, reflecting the composition of the parent glass. The data imply a heterogeneous global distribution in composition of K–T boundary impact glass: Si-rich and Ca-rich in Mexico and Haiti, Si-rich in Denmark, and Ca-rich in New Zealand. This heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the Chicxulub impact site. However, recent discovery of an impact crater of K–T boundary age in Ukraine raises the possibility of impact clusters which produce material of heterogeneous composition.