40Ar/39Ar geochronology using high sensitivity mass spectrometry: Examples from middle Miocene horizons of the Central Paratethys

Abstract

40Ar/39Ar radio-isotopic dating of volcanic tuffs intercalated in sediments can provide high accuracy age control on the deposition of sedimentary rocks. State-of-the-art mass spectrometers such as the ARGUS VI+ are able to acquire highly precise ages for relatively small single grains (~90-250 μm for Miocene samples). Single grain measurement can provide insight into the sometimes complex age distributions within volcanic tuffs. The results show that 40Ar/39Ar ages based on multiple grain fusions will not necessarily reflect eruption ages, which can lead to (slight) overestimation of the depositional age. The paper compares multiple and single grain data from different Miocene tuffs in the Central Paratethys, which plays an important role in the establishment of a geological time frame for this area. The examples come from three middle Miocene tuff horizons that span from the Badenian transgression to the Badenian-Sarmatian Extinction Event. The new ages obtained from the Quellgraben section in the Styrian Basin (14.31±0.27 Ma and 14.03±0.04 Ma) are much younger than the previous dating and together with the new data from the Bernhardsthal-4 well, Vienna Basin (15.12±0.19 Ma) indicate, that the Badenian (Langhian) marine flooding did not reach this area before 15.2 Ma. The new weighted mean age of 12.56±0.10 Ma from the Kamenica nad Hronom section in the Danube Basin dates the transition from marine to terrestrial setting, which is possibly connected with a sea level lowstand at the beginning of the Sarmatian.