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

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.     

40Ar‐39Ar step‐heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites

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% excess ⁴⁰Ar (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.