锆石SIMS原位微区U-Th不平衡定年:以云南腾冲火山岩为例  

In-situ U-Th dating of zircons by secondary ion mass spectrometry: A case study of the volcanic rocks from Tengchong, Yunnan Province

作　　者：李玲[1,2] 夏小平[1] 杨晴[1] 李如操[1,2] 

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

出　　处：《地球化学》2016年第4期398-406,共9页Geochimica

基　　金：广州市科技计划项目(二次离子质谱原位微区分析技术开发与应用)(201607020029);国家自然科学基金(41173007)

摘　　要：报道了利用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的CAMECA IMS1280-HR型二次离子质谱仪(SIMS)建立的锆石原位微区U-Th不平衡定年方法。一次离子为^(16)O~–,束斑大小约25μm,质量分辨率设定为约6000,使用单接收系统EM跳峰方式接收各相关质量峰。每个点经35次循环测量,共耗时约27 min。使用Plesovice为外标,对处于久期平衡状态的U-Pb定年标准锆石91500和Qinghu进行(^(230)Th)/(^(238)U)分析测试,得到两个标准锆石(^(230)Th)/(^(238)U)重现性分别为2.3%和1.3%,准确度分别为98.1%和98.6%,测试精密度和准确度都达到了国际上其他实验室的水平。对云南腾冲地区已知U-Th年龄的打鹰山、马鞍山第四纪火山岩的年轻锆石进行了U-Th定年,得到马鞍山锆石样品U-Th等时线年龄为(84.1±9.2)ka(2σ,MSWD=2.9);打鹰山锆石样品U-Th等时线年龄为(85.6±7.6)ka(2σ,MSWD=2.1),在误差范围内与前人报道一致,表明我们建立的SIMS原位微区U-Th定年方法准确可靠。This paper reports a method of in-situ U-Th dating of Quaternary zircons by a CAMECA IMS 1280-HR Secondary Ion Mass Spectrometer installed at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Analytical settings included mass-filtered 160- beam, -25 μm diameter spot size, and -6000 mass resolution. A single electron multiplier collector was used to collect the related mass peaks with peak-jumping mode. Each analysis was consisted of 35 cycles and a total analytical time was about 27 minutes. Using zircon Plesovice as an external standard, the measured ratios of （^230Th）/（^238U） for standard zircon 91500 and Qinghu in this study showed a reproducibility of 2.3% and 1.3% and an accuracy of 98.1% and 98.6%, respectively, which stand at a level of what was reported previously. Quaternary zircon crystals separated from the Dayingshan and Maanshan volcanic rocks at Tengchong, Southwest China, were analyzed in this study and yielded the isochron ages of （84.1±9.2） ka （20, MSWD = 2.9） and （85.6±7.6） ka （20, MSWD = 2.1）, respectively. These results show an excellent agreement with previous data, and indicate that the method is successfully established.

关 键 词：二次离子质谱 U—Th不平衡定年 原位微区 腾冲火山岩 

分 类 号：P597[天文地球—地球化学]