Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events

Abstract

In earlier studies, the 65-75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by KAr and Ar-Ar chronology at 342-368 Ma, with the commonly quoted age being 362.7 +/- 2.2 Ma (2 sigma, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 +/- 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 +/- 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% 39Ar release of 377.5 +/- 2.4 Ma (2 sigma). Our conservative estimate for the age of Siljan is 377 +/- 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 +/- 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earths biostratigraphic record.