科研之友
微信
新浪微博
Facebook
分享链接
摘要
Moldavites (Central European tektites) are genetically related to the impact event that produced the similar to 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 Ar-40-Ar-39 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 Ar-40-Ar-39 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 sigma), in agreement with previous works suggesting that biotite from the Fish Canyon Tuff is somewhat older (similar to 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 Ar-40-Ar-39 ages, and within-sample weighted mean ages of 14.66 +/- 0.08-14.75 +/- 0.12 Ma (+/- 2 sigma 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, Ar-40-Ar-39 ages from total fusion experiments on the Bohemian specimen ranged widely from similar to 14.6 to similar 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 sigma internal errors, ages relative to FCs). A thorough comparison of our and previous Ar-40-Ar-39 ages on both moldavites and Ries suevite glasses, recalculated relative to the Ar-40K-40 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 sigma, neglecting uncertainties in the K-40 decay constants).
- 出版日期2009-1-15
