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 (<50 ppt), similar to values for average continental crust, whereas all types of basaltic ashes show higher concentrations, generally in the range 70–450 ppt. A strongly alkaline, mafic (nephelinitic) ash holds 102 ± 29 ppt Ir. Another ash, with an unusual major-element composition and a composite acid-mafic mineral assemblage, contains 610 ± 26 ppt Ir.

Analyses of 15 of the <140 tholeiitic ash layers in the upper Fur Formation give very homogeneous major-element results. The Ir content, however, varies considerably, from 30 to 380 ppt with a mean of 181 ppt. Iridium does not correlate with any other analyzed elements, and no clues on its phase partitioning are revealed. Neither are there any temporal trends in Ir or any other element distributions indicative of any magma evolution. The older Phanerozoic volcanic ashes analyzed for Ir include: (1) the Upper Ordovician Big Bentonite, a 1- to 2-m-thick bentonite that may represent the largest Phanerozoic volcanic ash fall recorded; (2) volcanic ash layers near the Cretaceous-Tertiary boundary in continental deposits in Montana; (3) bentonites found in Sweden at the Llandovery-Wenlock boundary, where the largest conodont extinction event in the Silurian took place; and (4) two smectitic layers from Upper Cretaceous deposits in Germany, including the marker bed for the lower–upper Maastrichtian boundary. All these layers show very low Ir concentrations (<50 ppt) similar to average continental crust.

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