【摘要】 离子探针得益于高空间分辨率的特点,在复杂变质锆石的微区原位年代学研究中,其常被作为首选技术手段。对具有复杂环带结构的锆石进行原位分析,通常可以得到岩石多期次地质演化信息,因此在一颗有核(幔)边结构的锆石上进行多点分析是常见现象。为了定量地评估多点分析时测点的相对位置对离子探针测量结果的影响,本研究采用目前报道最为均一的锆石标样M257为测试对象,分为四种情况,即同一测点重复测试年龄、同一测点先氧同位素分析后年龄测试、在首个测点的上/下/左/右方且预剥蚀区域有部分重叠的范围内测试、距首个测点有显著距离测试等,进行了系统性U-Pb定年对比实验。结果显示,相对一个颗粒上的首个测点,第二个测点的测试深度和预剥蚀区域的变化会给测试结果带来-5%到+11%的系统偏差,尤以与首个测点的预剥蚀区域有重叠且位于一次离子入射方向后侧的分析点会产生最为显著的系统偏差。因此,在采用离子探针进行副矿物U-Pb测试的时候,当需要原位重复分析或在临近区域多点分析时,需要特别注意多个测点的相对位置和分析顺序。本实验结果指示,以往通过离子探针方法对变质锆石进行过核边多点分析,通过变质锆石边部所获得的年龄结果值得重新审视,有将变质时代持续时间扩大化的隐忧。更多还原

【Abstract】 The Secondary Ion Mass Spectrometry(SIMS)is normally considered as the first choice to reveal the age of complicate metamorphic zircon due to its high spatial resolution.It is very common to test the rim and the core of a metamorphic zircon grain at the same analytical session to investigate the metamorphic and protolith age,respectively.In order to evaluate the effect of relative measured position on SIMS U-Pb dating quantitatively,the most homogenous zircon reference material M257 was tested under four different situations:1)testing U-Pb age of the zircon twice on the same spot,2)analyzing O isotopes firstly and U-Pb dating secondly in the same pit,3)bombing the up/down/left/right region relative to the first measured position with overlapped pre-sputter area successively,4)detecting two spots on the same zircon grain with visible distance.The results showed that the depths of the bombardment and the pre-sputter area relative to the first measurement can cause obvious U-Pb data bias ranging from-5%to 11%.The maximum bias is recorded in the rear region relative to the direction of primary ion of the first measured spot with overlapped presputter area the rear.Cautions are suggested when setting the relative measured position on the same area.In addition,previous SIMS dating results on metamorphic zircon rims after measuring the zircon core should be re-evaluated.The time span of metamorphism could be extended without evaluation of the relative measured position effect.更多还原