Re-Os同位素精确厘定长兴“金钉子”灰岩沉积年龄

Direct radiometric dating of Limestone from Changxing Permian-Triassic boundary using the Re-Os geochronometer

Re-Os同位素作为一种强有力的技术工具,在黑色页岩沉积时代的精确定年中得到了广泛应用,但是将Re-Os应用于灰岩沉积时代厘定的研究并不多见.本文采用Re-Os同位素分析技术,对我国浙江长兴二叠-三叠纪界线“金钉子”剖面中灰岩样品进行分析,获得了1~6层Re-Os等时线年龄为257.3±4.0 Ma,与7层锆石U-Pb年龄误差范围内相互一致、18~22层Re-Os等时线年龄为253.3±6.0 Ma,与17层锆石U-Pb年龄相互印证、23~24层Re-Os等时线年龄为251.8±4.7 Ma,与25层锆石U-Pb年龄相互吻合.结果表明Re-Os同位素能够实现对灰岩地层沉积时代进行直接厘定,有利于生物地层学与同位素地质年代学的直接比对研究.根据前人对长兴“金钉子”地层沉积环境的研究,本文还对适合进行Re-Os同位素定年的灰岩样品规律性进行了讨论,结果显示还原环境沉积的灰岩更适合进行Re-Os同位素定年,氧化环境沉积的灰岩,不利于其中Re-Os同位素体系封闭,无法得到准确的Re-Os同位素等时线年龄.

As a powerful geochronometer, the Re-Os isotope system has been widely used for precisely dating depositional age of black shale, however the application of this method on dating limestone is rarely reported. We have systematically dated limestones from the Permian-Triassic boundary interval of the global stratotype section and point （GSSP） at Meishan, Changxing, Zhejiang Province, China. Three sets of samples from layers 1 to 6, 18 to 22, and 23 to 24 yield accurate Re-Os isochron ages of 257.3±4.0 Ma, 253.3± 6.0 Ma and 251.8±4.7 Ma, respectively, consistent with previously reported U-Pb zircon ages. Our results indicate that Re-Os isotope system can provide precise age constraint on the depositional time of limestone, which is significant for the correlation study between biostratigraphy and isotope geochronology. Based on the comparison between our Re-Os isotope age results and previous research of the oxidation state of sedimentary environment, it appears that limestones in reduced environment are more suitable for Re-Os isotope dating because the isotope sytem remains closed, while the Re-Os isotope system of limestone in oxidized environment tends to be disturbed and impossible to give accurate Re-Os isotope age.