石灰岩铼-锇同位素分析方法研究及应用初探
Research and Preliminary Application of Re-Os Isotope System for Limestone Samples
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摘要: 针对石灰岩样品Re-Os同位素分析,在选样和溶样方法上进行了改进,在Carius管封闭前加入HCl与石灰岩反应释放出大量CO2,然后加入氧化剂和稀释剂封闭Carius管溶解样品,大大增加了样品取样量。利用改进的方法对采自青海玉树地区二叠世九十道班组底部的灰黑色微细晶灰岩的Re-Os同位素体系进行了分析测定,得到了精确的沉积年龄(283.1±7.1) Ma(MSWD=0.61,Model1,n=7)。187Os/188Os同位素初始值为0.56±0.12,与二叠纪时海水的187Os/188Os值相一致,反映了石灰岩沉积时海水的187Os/188Os比值。所得石灰岩年龄与其中的生物化石年龄相吻合,并且与区域上岩浆岩锆石年龄相互印证,表明Re-Os同位素体系在该石灰岩中的封闭性较好。通过石灰岩中有机碳含量以及其中Re、Os含量关系研究,得出了Re、Os在灰岩中主要赋存于有机质中的结论。从原理上解释了Re-Os同位素体系在灰岩中的应用具有十分广泛的前景。Abstract: Sampling and digestion methods for Re-Os isotope analysis are improved for limestone samples. Hydrochloric acid was added to Carius tube to react with limestone in order to release CO2. Then the Carius tube was sealed after adding oxidant reagent and spikes. Re-Os isotope system of the limestone, hosted on the bottom of Jiushidao formation of the Permian stratum at Yushu, Qinghai Province, was firstly analyzed worldwide by using this method. The obtained accurate age of (283.1±7.1) Ma (MSWD=0.61, Model 1, n=7) is consistent with the geological setting of the Schwagerina bearing stratum and especially the age U-Pb zircon dates for magmatic rocks. 187Os/188Os initial ratio obtained is 0.56±0.12, in agreement with the 187Os/188Os value of Permian seawater. Reasonable Re-Os isotopic age and initial Os value are obtained showing the closure of Re-Os isotope system in Limestone is perfect. Concentrations of Re and Os are positively correlated with organic carbon, which indicates Re and Os occur in the organic matter of limestone. This application provides a new tool for the stratigraphic classification and correlation.
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