Platinum group elements and Os-187/Os-188 in a purported impact ejecta layer near the Eifelian-Givetian stage boundary, Middle Devonian

A global faunal crisis close to the Eifelian-Givetian stage boundary in the mid-Devonian has been purported to be related to the impact of one or two major extraterrestrial bodies. This was based on unusual mineralogical and chemical features within a distinct composite marl bed, at a level c. 40 cm below the Global Stratotype Section and Point (GSSP) for the Eifelian-Givetian boundary at Jebel Mech Irdane in Morocco. The impact relation has been challenged based partly on the absence of platinum group element data. We present here detailed Ir, Os, Pd and Pt as well as Os isotopic data across the bed at Mech Irdane and from a correlated 10 cm bed at a nearby site, Rich Haroun. Iridium concentrations of 0.13-0.28 ng/g in the beds represent a small enrichment compared to average shale, but Ir/Al ratios are only a factor 1.5-2 higher than background at respective site. Such small enrichments can readily be explained by terrestrial diagenetic processes, and do not require the presence of an extraterrestrial component. On an Al-normalized basis Pd and Pt show weak enrichments, typically a factor 2-4 higher than background. High Os-187/(OS)-O-188 ratios, 1.1-3.8, reflect ingrowth of radiogenic Os from Re, which is also evident from Os concentrations of up to 3 ng/g. The platinum group inter-element ratios are clearly non-chondritic. The overall platinum group and trace element (e.g. Co, Ni, As, V) patterns indicate that diagenetic processes at redox fronts have played a crucial role in shaping most element enrichments. If an excess siderophile-rich extraterrestrial component exists in these beds it represents less than 0.2%. by weight. (c) 2006 Elsevier B.V. All rights reserved.