U‐Pb dating of zircons from an impact melt of the Nördlinger Ries crater

Abstract

In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean 206Pb*/238U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean 206Pb*/238U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published 40Ar/39Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.

Shock deformation in zircon grains from the Mien impact structure, Sweden

“…2019; Schwarz et al. 2020). Furthermore, zircon has the advantage of being able to record both high pressures and temperatures (Timms et al.…”

Shocked quartz in distal ejecta from the Ries impact event (Germany) found at ~ 180 km distance, near Bernhardzell, eastern Switzerland

“…This age overlaps the age estimated for the deposition of the Brockhorizont, ~ 14.9–15.1 Ma, which was achieved using a bracketing method 27 , see also discussion in Refs. 41 , 42 , and indistinguishable within error from the age of the Ries impact of 14.808 ± 0.038 Ma 17 . Additionally, the palynological results also support a Miocene age (pre-late Tortonian) for the Blockhorizont (Supplementary Table S5 ).…”

U‐Pb dating of zircon and monazite from the uplifted Variscan crystalline basement of the Ries impact crater

“…In samples UNT-DG and LAN-GR, some analyses yielded young 207 Pb-corrected 206 Pb/ 238 U dates between approximately 4 and 9 Ma, associated with elevated U abundances >3 wt% (Fig. 11a), a correlation also observed by Schwarz et al (2020) in Polsingen impact melt zircon. These apparent dates younger than 10 Ma correspond to zircon with subareas that have suffered the most from radioactive decay of U, leading to their complete metamictization (Fig.…”

“…2 of Schmieder & Kring, 2020), despite the great potential of zircon to also provide constraints on P – T conditions during impact (e.g., Erickson et al, 2016; Gucsik, Koeberl, et al, 2004; Gucsik, Zhang, et al, 2004; Timms et al, 2017; Wittmann et al, 2006). Zircon (ZrSiO 4 ) U‐Pb dating has been successfully applied to date the 2229 ± 5 Ma Yarrabubba impact structure in Australia (impact‐generated Barlangi granophyre; Erickson et al, 2020), the 2023 ± 4 Ma Vredefort impact crater in South Africa (recrystallized pseudotachylitic breccia; Kamo et al, 1996), the 1850 ± 1 Ma Sudbury impact structure in Canada (impact melt sheet norite; Davis, 2008), the 608 ± 8 Ma Sääksjärvi impact structure in Finland (impact melt and impact melt breccias; Kenny et al, 2020), the 453 ± 3 Ma Brent impact structure in Canada (clast‐bearing impact melt rocks; McGregor et al, 2020), the approximately 385 Ma Nicholson Lake impact structure in Canada (impact melt‐bearing breccia and clast‐laden impact melt rock; McGregor et al, 2018), the approximately 252–255 Ma Araguainha impact structure in Brazil (impact melt in suevite; Hauser et al, 2019), the 215 ± 1 Ma Manicouagan impact structure in Canada (impact melt rock; Hodych & Dunning, 1992), the approximately 207 Ma Rochechouart impact structure in France (impact melt rock; Rasmussen et al, 2020), the 146.1 ± 0.2 Ma Morokweng impact structure in South Africa (impact melt sheet rocks; Hart et al, 1997; Kenny et al, 2021; Koeberl et al, 1997), the 123 ± 1 Ma Vargeão Dome in Brazil (melt vein in impact breccia; Nédélec et al, 2013—although it has been argued that this may not date the impact event itself; Crósta et al, 2019), the 78 ± 1 Ma Lappajärvi impact crater in Finland (impact melt rock; Kenny et al, 2019), the 37.8 ± 0.1 Ma Mistatin Lake impact crater in Canada (impact melt sheet; Sylvester et al, 2013), and the 14.9 ± 0.3 Ma Ries impact crater in Germany (impact melt rocks; Schwarz et al, 2020). In all these cases, dating was carried out on zircon from solidified impact melt and impact melt‐bearing lithologies.…”

“…Formation of the Ries crater has been dated to 14.8 ± 0.1 Ma by 40 Ar/ 39 Ar dating of impact glass spherules in suevite and impact glass in distal ejecta (known as moldavites), and U‐Pb dating of zircon from impact melt samples that are part of the proximal ejecta and crater floor impact melt deposits (e.g., Schmieder et al., 2018; Schwarz et al, 2020; Schwarz & Lippolt, 2014). These samples are all high‐temperature components produced during the impact (Stöffler et al, 2013).…”