40Ar– 39Ar laser dating of tektites from the Cheb Basin (Czech Republic): Evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact

Abstract

Moldavites (Central European tektites) are genetically related to the impact event that produced the ∼24-km diameter Ries crater in Germany, representing one of the youngest large impact structures on Earth. Although several geochronological studies have been completed, there is still no agreement among 40Ar– 39Ar ages on both moldavites and glasses from Ries suevites. Even recently published data yielded within-sample mean ages with a nominal spread of more than 0.6 Ma (14.24–14.88 Ma). This age spread, which significantly exceeds current internal errors, must be in part ascribed to geological and/or analytical causes. This study reports the results of a detailed geochronological investigation of moldavites from the Cheb area (Czech Republic), which have never been dated before, and, for comparison, of two samples from type localities, one in southern Bohemia and the other in western Moravia. We used 40Ar– 39Ar laser step-heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization. In addition, with the purpose of ascertaining the influence of the dating standards on the age of the Ries impact and making data from this study and literature consistent with the now widely used Fish Canyon sanidine (FCs) standard, we performed a direct calibration of multi-grain splits of the Fish Canyon biotite (FCT-3) with FCs. The intercalibration factors ( R FCs FCT- 3 ), determined for eight stack positions in one of the three performed irradiations, were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1.0086 ± 0.0031 (±2 σ), in agreement with previous works suggesting that biotite from the Fish Canyon Tuff is somewhat older (∼0.8%) than the coexisting sanidine. Laser total fusion analysis of milligram to sub-milligram splits of five tektite samples from the Cheb area yielded mostly concordant intrasample 40Ar– 39Ar ages, and within-sample weighted mean ages of 14.66 ± 0.08–14.75 ± 0.12 Ma (±2 σ internal errors, ages relative to FCs) that overlap within errors. These ages match those obtained for samples from western Moravia (14.66 ± 0.08 Ma) and southern Bohemia (14.68 ± 0.11 Ma), supporting the genetic link between Cheb Basin tektites and moldavites, and, consequently, between Cheb Basin tektites and the Ries impact. In contrast to samples from the Cheb area and Moravia, 40Ar– 39Ar ages from total fusion experiments on the Bohemian specimen ranged widely from ∼14.6 to ∼17.0 Ma. Older apparent ages, however, were systematically obtained from fragments characterized by visible surface alteration. Laser step-heating experiments, although displaying slightly disturbed age profiles, were in line with total fusion analyses and yielded well-defined plateau ages of 14.64 ± 0.11–14.71 ± 0.11 Ma (±2 σ internal errors, ages relative to FCs). A thorough comparison of our and previous 40Ar– 39Ar ages on both moldavites and Ries suevite glasses, recalculated relative to the 40Ar ∗/ 40K ratio recently determined for FCs using intercalibration factors available in or derivable from the literature, reveals some inconsistencies which may be ascribed to either geological or analytical causes. Based on our data, decay constants in current use in geochronology, and ages calculated relative to FCs, we infer that the age of moldavites is 14.68 ± 0.11 Ma (±2 σ, neglecting uncertainties in the 40K decay constants).