石墨Re-Os同位素分析及其在成矿年代学中的初步运用

本文探索性地对江西省铅山县长寿源铅锌共生的晶质石墨进行了Re-Os同位素分析。研究结果显示石墨的Re-Os同位素年龄为424±25Ma(n=7,MSWD=7.8),与该区二云二长花岗岩中锆石的年龄(433±9Ma)在误差范围内相一致,说明石墨的成因与区域上岩浆的侵入存在密切联系。Re的含量介于47×10-9～330×10-9之间,普通Os和187Os的含量分别为0.32×10-9～2.15×10-9和0.27×10-9～1.75×10-9,相对于Re、Os在地壳中的丰度显著富集,Os同位素初始比值187Os/188Os=1.69±0.29,表明石墨来源于富有机质沉积地层。通过测定石墨样品中的有机碳和固定碳,对Re、Os在石墨中的赋存状态进行了研究,表明Re、Os可能是以某种形式赋存在石墨的固定碳中,与其中有机质无关。从原理上解释了石墨的Re-Os同位素年龄反映的是使沉积地层中的炭质变质为石墨,并发生富集迁移的岩浆热变质事件的时期。研究表明石墨的Re-Os同位素分析技术不仅能够为变质作用时间的精确测定提供新的方法,而且对矿床成因的研究也具有重要意义。

Research and preliminary application in metallogenic chronology of Re-Os isotope system in graphite samples.

Re-Os analyses of graphite from the Sinian-Cambrian boundary stratum in Changshouyuan Pb-Zn deposit Jiangxi Province demonstrate the potential utility of the Re-Os system for establishing the absolute timing of metamorphism. The Re-Os age of the graphite is 424±25Ma consistent in error range with the regional magmatic zircon age of 433±9Ma, which indicates that the genesis of the graphite is associated with the regional intrusion event. Compared with the crust abundance, the graphite is significantly rich in Re and Os. The initial ¹⁸⁷Os/¹⁸⁸Os ratio of 1.69±0.29 indicates the graphite deriving from the organic-rich sedimentary rock. The study on the occurrence of Re and Os in the graphite demonstrates that the Re and Os are likely to occur in the fixed carbon rather than organic carbon. Though considerable further research is required to understand the Re-Os isotopic systematic of metamorphism, graphite Re-Os geochronology may provide the possibility of establishing a more comprehensive understanding of metamorphism, as well as important for the research on the metallogeny.