40Ar/39Ar dating of Ordovician K-bentonites in Laurentia and Baltoscandia

Several Ordovician K-bentonites occurring widely in eastern North America and western Europe were dated using the 40Ar/39Ar technique to test previously suggested inter-continental correlations. The three thickest and most widespread bentonites – Deicke, Millbrig (North America) and Kinnekulle (Sweden and Denmark) – were examined. Single-grain analyses of phenocrystic biotites yield 100% concordant plateau ages of 449.8±2.3 Ma (2σ internal error) (Deicke), 448.0±2.0 Ma (Millbrig) and 454.8±2.0 Ma (Kinnekulle in Sweden). The altered biotites from the Denmark Kinnekulle sample show discordant age spectra with highly variable plateau ages being most likely caused by recoil redistribution of Ar isotopes between biotite and interlayer secondary alteration products during neutron irradiation. Such recoil artifacts are likely evident in previously published 40Ar/39Ar data for these units, causing bias towards spuriously old ages. In such cases where intragrain alteration is evident and discordant age spectra are common, geologically meaningless plateau ages may be obtained which are less accurate than integrated ages, hence a 100% concordance criterion for acceptance is useful. The inconsistent 40Ar/39Ar ages and geochemical contrasts between the North American and Swedish bentonites imply distinct volcanic episodes, and preclude a specific correlation between the Millbrig and Kinnekulle K-bentonites. The precise 40Ar/39Ar ages indicate a minimum duration of 7±3 Myr for the Diplograptus multidens graptolite biozone. In comparison with previous U/Pb data, the results are useful for quantifying the bias (ca. 1%) between current calibrations of the 40Ar/39Ar and U/Pb geochronometric systems. This bias is most likely due to errors related to 40K decay constants and neutron fluence monitors used in 40Ar/39Ar dating.