Winfried H. Schwarz^1,2 andHans J. Lippolt²

¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Laboratorium für Geochronologie, Universität Heidelberg, Heidelberg, Germany

Seven impact melts from various places in the Nördlinger Ries were dated by ⁴⁰Ar-³⁹Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2–40% excess⁴⁰Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.

Reference
Schwarz WH and Lippolt HJ  (in press) 40Ar-39Ar step-heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites. Meteoritics & Planetary Science
[doi:10.1111/maps.12309]
Published by arrangement with John Wiley & Sons

Link to Article

C. Alwmark¹, S. Alwmark-Holm¹, J. Ormö² and E. Sturkell³

¹Department of Geology, Lund University, Lund, Sweden
²Centro de Astrobiología (CSIC/INTA), Instituto Nacional de Técnica Aeroespacial, Madrid, Spain
³Department of Earth Sciences, University of Gothenburg, Göteborg, Sweden

The Målingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Målingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Målingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne–Målingen pair).

Reference
Alwmark C, Alwmark-Holm S, Ormö J and Sturkell E  (in press) Shocked quartz grains from the Målingen structure, Sweden—Evidence for a twin crater of the Lockne impact structure. Meteoritics & Planetary Science
[doi:10.1111/maps.12314]
Published by arrangement with John Wiley & Sons

Link to Article