MAGNESIOFERRITE SPINEL IN CRETACEOUS TERTIARY BOUNDARY SEDIMENTS OF THE PACIFIC BASIN - REMNANTS OF HOT, EARLY EJECTA FROM THE CHICXULUB IMPACT/

Magnesioferrite spinel in sediments from six Cretaceous/Tertiary (K/T) boundary sites in the Pacific basin are found to have relatively unif orm compositions and textures. They typically have much higher MgO, Al 2O3, and Fe2O3/FeO than spinel from K/T boundaries in western Europe a nd 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 O DP Site 761 (eastern Indian Ocean) and in spherules from a late Plioce ne impact. Pacific spinel commonly exhibit unique textures with large, porous, anhedral grains which we believe reflect a myrmekitic intergr owth with another phase that has been destroyed by diagenetic alterati on. A prime candidate for this other phase is 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 strewnfi eld for the Pacific spinel extends over > 5% of the Earth's surface, b ut it could be a much larger area. These spinel must have crystallized from silicate liquids produced by a large impact event. If their sour ce is the impact that made the Chicxulub structure on the Yucatan Peni nsula, Mexico, then there is a clear compositional asymmetry to the di stribution of this component of the ejecta. We speculate that a possib le cause of this asymmetry could be a low-angle impact. If the project ile came in from the east, the vapor cloud would deposit the earliest, hottest materials over the Pacific basin.