Iridium enrichment in volcanic dust from blue ice fields, Antarctica, and possible relevance to the K/T boundary event

Abstract

Blue ice fields in Antarctica, well known as stranding zones of Antarctic meteorites, contain ice that is several tens to hundreds of thousand years old, which records numerous events that have taken place since its formation. Among the most important structures on the ice fields are dust bands which are mostly of volcanic origin and consist of small volcanic glass shards and lithic fragments. Petrographic and chemical studies of a number of dust bands during earlier investigations have shown that significant ash layers are deposited at distances of up to several thousand kilometers from the source volcanoes. Samples of dust layers taken in the Lewis Cliff/Beardmore Glacier area in Antarctica, analyzed in the course of this work for major and trace elements, were identified as consisting of volcanic ash, probably originating from the Pleiades (Melbourne volcanic province, northern Victoria Land). Some of these dust samples were found to contain iridium in concentrations of up to 7.5 ppb. A positive correlation between Ir and Se as well as enrichments in As, Sb, and other volcanogenic elements has been observed. The dust samples investigated here have very small grain size, consistent with the distance from the source volcanoes. The small grain size of the dust suggests that the Ir enrichment is a surface effect, and was probably caused by condensation from a vapor phase. This is the first time that significant Ir enrichments have been found in volcanic dust which demonstrates that Ir (and possibly also other platinum group metals) may be present in some volcanic ash deposits, although this has been doubted by the proponents of the “pure” impact theory for the K/T boundary event. The results of this study suggest that the Ir in the K/T boundary clays is not necessarily of cosmic origin, but alternatively may have originated from mantle reservoirs that were tapped during extensive volcanic eruptions, which may been triggered by impact events.