CHALCOPHILE ELEMENTS AND IR IN CONTINENTAL CRETACEOUS-TERTIARY BOUNDARY CLAYS FROM THE WESTERN INTERIOR OF THE USA

This study presents data for Ir and the chalcophile elements As, Sb, and Zn in seventeen samples from continental K/T boundary layers in the western interior of the USA. The results show that in contrast to previously studied marine boundary clays, the continental boundary layers show no correlation between Ir and any of the chalcophile elements. Compared with average shale, the continental boundary layers are significantly to strongly enriched in Ir, As, and Sb, but show ordinary Zn concentrations. The Ir enrichments can be attributed to fallout of extraterrestrial matter, whereas As and Sb enrichments can be readily explained by the close spatial association of the boundary layers with coal and the abundance in the layers of jarosite, goethite, and other weathering products of pyrite. If compared to jarosite-rich coal-associated background samples, the boundary layers show much higher Ir concentrations but similar (or possibly slightly enhanced) concentrations of As, Sb, and Zn. The correlation between chalcophile elements and Ir in marine boundary clays, and the absence of such a correlation in continental boundary clays, concurs with the idea that a major fraction of the Ir in extremely Ir-rich (30-460 ppb Ir) marine boundary clays has precipitated from seawater that was enriched in Ir being derived from a vaporized asteroid.