Iridium geochemistry of volcanic ash layers from the early Eocene rifting of the northeastern North Atlantic and some other Phanerozoic events

Abstract

This study shows that some types of explosive volcanism can result in significant Ir anomalies (up to 750 ppt); however, these are readily discerned from impact-related, extraterrestrial Ir anomalies by other chemical and mineralogical criteria. Iridium and major and trace elements were analyzed in 34 of the >180 volcanic ash layers in the Fur Formation of early Eocene age in Denmark. These ashes originate from explosive volcanism during rifting of the Greenland-Eurasia continent and subsequent sea-floor spreading in the young northeastern North Atlantic. In addition, 15 older Phanerozoic ashes, associated with other dramatic global events, have been analyzed for Ir. The Fur Formation ashes are unusually well preserved and have a wide compositional range–for example, rhyolitic, dacitic, tholeiitic and alkaline basaltic, mafic alkaline, and different salic alkaline ashes, and ashes with intermediate or unknown magma origin. The original magma compositions have been determined by immobile-element plots (Nb/Y-Zr/Ti) and other criteria. Iridium shows a bimodal distribution, where all types of salic (alkaline as well as subalkaline) ashes have very low Ir concentrations ( Analyses of 15 of the