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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >~(40)Ar/~(39)Ar dating of Ordovician K-bentonites in Laurentia and Baltoscandia
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~(40)Ar/~(39)Ar dating of Ordovician K-bentonites in Laurentia and Baltoscandia

机译：Laurentia和Baltoscandia中奥陶纪钾膨润土的〜（40）Ar /〜（39）Ar年代

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摘要

Several Ordovician K-bentonites occurring widely in eastern North America and western Europe were dated using the ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar and U/Pb geochronometric systems. This bias is most likely due to errors related to ~(40)K decay constants and neutron fluence monitors used in ~(40)Ar/~(39)Ar dating.
机译：使用〜（40）Ar /〜（39）Ar技术测算了在北美东部和西欧广泛分布的几种奥陶纪钾膨润土，以检验先前提出的洲际相关性。考察了三个最厚，分布最广的膨润土-Deicke，Millbrig（北美）和Kinnekulle（瑞典和丹麦）。单晶黑云母的单颗粒分析可得出100％一致的高原年龄449.8±2.3 Ma（2σ内部误差）（Deicke），448.0±2.0 Ma（Millbrig）和454.8±2.0 Ma（瑞典的Kinnekulle）。来自丹麦Kinnekulle样品的黑云母蚀变显示出不一致的年龄谱，高原年龄变化很大，这很可能是由于中子辐照期间黑云母与层间次生蚀变产物之间的Ar同位素反冲重新分布所致。此类反冲伪影可能在先前发布的这些单元的〜（40）Ar /〜（39）Ar数据中很明显，从而导致了对虚假年龄的偏见。在这种情况下，晶粒内的变化很明显并且年龄谱不统一是常见的，可以获得没有地质意义的高原年龄，其准确性不如综合年龄，因此100％一致的接受标准是有用的。北美和瑞典膨润土之间不一致的〜（40）Ar /〜（39）Ar年龄和地球化学对比暗示着火山爆发的不同，并且排除了Millbrig和Kinnekulle K-膨润土之间的特定相关性。精确的〜（40）Ar /〜（39）Ar年龄表明Diplograptus multiden graptolite生物区的最小持续时间为7±3 Myr。与以前的U / Pb数据相比，该结果对于量化〜（40）Ar /〜（39）Ar和U / Pb测年系统的当前校准之间的偏差（约1％）很有用。这种偏差很可能是由于与〜（40）Ar /〜（39）Ar测年中使用的〜（40）K衰减常数和中子注量监测仪有关的误差。

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