Magnesioferrite spinel in Cretaceous/Tertiary boundary sediments of the Pacific basin: Remnants of hot, early ejecta from the Chicxulub impact?

Abstract

Magnesioferrite spinel in sediments from six Cretaceous/Tertiary (K/T) boundary sites in the Pacific basin are found to have relatively uniform compositions and textures. They typically have much higher MgO, Al 2O 3, andFe 2O 3/FeO than spinel from K/T boundaries in western Europe and the South Atlantic or in meteor ablation debris. These compositions are similar to those found in a few grains from the K/T boundary at ODP Site 761 (eastern Indian Ocean) and in spherules from a late Pliocene impact. Pacific spinel commonly exhibit unique textures with large, porous, anhedral grains which we believe reflect a myrmekitic intergrowth with another phase that has been destroyed by diagenetic alteration. A prime candidate for this other phase in Ni-rich periclase (Mg,Ni,Fe)O, a mineral that has been observed as trace inclusions in several spinel grains from Pacific Ocean DSDP Sites 577 and 596. The strewnfield for the Pacific spinel extends over > 5% of the Earth's surface, but it could be a much larger area. These spinel must have crystallized from silicate liquids produced by a large impact event. If their source is the impact that made the Chicxulub structure on the Yucata´n Peninsula, Mexico, then there is a clear compositional asymmetry to the distribution of this compoment of the ejecta. We speculate that a possible cause of this asymmetry could be a low-angle impact. If the projectile came in from the east, the vapor cloud would deposit the earliest, hottest materials over the Pacific basin.