Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden

Abstract

A distinct Ir anomaly with an Ir content of up to 4.5 ppb is found in resurge deposits tied to the Middle Ordovician Lockne impact structure, Jämtland, Sweden. The impact took place in a marine environment and the sea water played an important role in the impact pro-cess and in the aftermath. The samples yielding the Ir anomaly also contain abundant shocked quartz, which proves that the structure was formed by the impact of an extraterrestrial body. The mid-Ordovician volcanism close to Scandinavia did not generate any distinct Ir content. About 20% of the meteorite Ir remained in the resurge deposit around the crater. The Ir from the impact-related deposits in Lockne correlates well with Cr. There is a weaker correlation between Ir and chalcophile and siderophile (e.g., Fe, Sb, Zn, and Ni, Co) elements. The low-temperature hydrothermal system, which acted after the impact event, mobilised some of the chalcophile and siderophile elements and, thus, resulted in their weaker correlation with Ir in some samples.

Keywords:

• Impact
• iridum
• chromium
• gold
• chalcophile elements
• Middle Ordovician
• Lockne
• Jämtland
• Sweden