东天山梅岭铜矿床黄铁矿Re-Os等时线年龄：Os同位素不均一的结果  

Re-Os Isochron Age of Pyrites from Meiling Cu Deposit in the Eastern Tianshan:A Case Study for the Os Isotopic Heterogeneity

作　　者：赵冰爽 李杰[1] 龙晓平 袁超[1] Zhao Bingshuang;Li Jie;Long Xiaoping;Yuan Chao(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi＇an 710069,China)

机构地区：[1]中国科学院广州地球化学研究所同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049 [3]西北大学地质系,大陆动力学国家重点实验室,陕西西安710069

出　　处：《地球科学》2018年第9期2966-2979,共14页Earth Science

基　　金：国家重点基础研究发展计划(973计划)项目(No.2014CB440801); 国家自然科学基金项目(Nos.41522202,41373034)

摘　　要：金属硫化物Re-Os等时线年龄的合理解释是揭示金属矿床成矿时代的关键.通过对新疆东天山梅岭铜矿床开展Re-Os同位素定年研究,结果表明浸染状和脉状矿石中黄铁矿样品在Os浓度和Os同位素比值方面都有很大的变化,这两类样品定义了很好的Re-Os等时线年龄,分别为523±59Ma和707±99Ma.由于得到的等时线年龄明显老于它们的实际地质成矿年龄,且^(187) Os/^(188) Os与普通Os的倒数(以1/^(192) Os为例)之间存在着很好的相关性(R^2分别为0.997 3和0.994 5),因此这些样品存在着初始Os同位素组成不均一的现象,这些Re-Os等时线是混合等时线,没有地质意义.理论和数学公式推导显示观测到的Re-Os等时相关性是在形成时期没有达到完全的同位素平衡的二元混合的结果,这种同位素扩散不平衡产生的原因主要是在矿床形成时期Os同位素在金属硫化物与硅酸盐矿物之间的同位素扩散存在限制性.回归得到的Os的初始值更偏向于地壳值,表明矿床形成时期的地壳混染可能造成金属硫化物体系中的这种初始Os同位素不均一.因此,将Re-Os同位素体系应用于金属硫化物样品定年时,^(187) Os/^(188) Os与1/^(192) Os之间是否存在相关关系可以作为Re-Os等时线年龄是否具有地质意义的判断标准.Interpretations of the Re-Os isochron age of metallic sulfides are crucial to the reconstruction of ore-formation age of metallic deposits.Re-Os dating of pyrites from Meiling Cu deposit in eastern Tianshan,NW China indicates that the disseminated and massive ores show large variations in both Os concentrations and isotopic ratios.Our isotopic data of the two kinds of ores have defined good Re-Os correlations and the isochron ages are 523±59 Ma and 707±99 Ma,respectively.The two isochron ages are both significantly older than the true formation age of the Meiling Cu deposit.Together with the good correlations between ^187 Os/^188 Os ratio and the reciprocal of the common Os（i.e.,1/192 Os,R^2 are 0.997 3 and 0.994 5 for the disseminatedand massive ores,respectively）,we suggest that the initial Os isotopic compositions of these samples are heterogeneous and the two ages are mixed isochron ages.The mathematical deduction demonstrates that the observed Re-Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation,primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and silicate minerals.The initial Os isotopic compositions may approach the value of continental crust.Crustal contamination during the formation of the deposit was responsible for the initial Os isotopic heterogeneity in the metallic sulfide system.It is concluded that whether there is a good Re-Os isochron correlation for samples can be used to constrain the geological meaning of Re-Os isochron ages when the Re-Os isotopic system of metallic sulfides is used to date the formation age of metallic deposit.

关 键 词：金属硫化物 RE-OS同位素 初始Os同位素 等时线年龄 混合等时线 年代学 

分 类 号：P611[天文地球—矿床学]