电子探针分析锆石Hf和Ti含量的结果意义与技术优势  

Electron Probe Microanalysis of Hf and Ti in Zircon:Significance and Advantage

作　　者：李小犁 LI Xiaoli(Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871

出　　处：《岩矿测试》2023年第1期89-101,共13页Rock and Mineral Analysis

基　　金：国家自然科学基金项目(41872190)。

摘　　要：锆石是一种重要的定年矿物,其微量元素地球化学行为是解释锆石年龄地质意义的重要依据。锆石微量元素分析一般采用的是在大束斑条件下(10~50μm)的质谱仪测试方法,其结果反映的是在该束斑范围内,元素的平均含量信息。相比之下,电子探针显微分析可以在极小微区范围内(<5μm)进行元素的定量分析,更能有效地揭示元素的地球化学行为,可作为研究锆石微量元素的重要技术补充。本文以锆石定年中常用标准锆石(TEMORA、Qinghu和Ple2ovice)作为研究对象,在20kV加速电压、50~300nA轰击电流以及2~5μm束斑条件下,对其中的Hf和Ti进行了定量分析,包括常规的点和线分析(Point/Line Analysis)以及网格分析(Grid Analysis),并以此为基础进行了线性拟合并建模,对元素的面分布情况进行了探讨。在较长的计数时间条件下(300s),本文得到了20μg/g(1σ)的Ti含量检测限。实验结果表明:锆石中的Zr与Hf之间具有负相关性,反映了两者的类质同象替代;其次,在概率统计方面,电子探针Hf和Ti的定量分析结果明显高于LA-ICP-MS方法,反映了其在更小微区下的含量信息以及地球化学行为,须在诸如(锆石)Ti温度计的应用中特别注意。再者,测试所用标准锆石样品的Hf含量(和Zr/Hf比值)在微区下具有环带分布特征,表现为从核部到边部具有升高(降低)的趋势,这与锆石结晶生长过程中的熔体分异程度有关。另一方面,标准锆石样品中的Ti并未表现出类似Hf的分布特征,且Hf(和Zr/Hf比值)与Ti含量之间也没有发现明显的相关性。因此,对于前人提出的Hf具有类似Ti一样的温度计指示功能的观点仍有待进一步探讨。BACKGROUND:Zircon is an important refractory mineral that is ubiquitous in most magmatic,metamorphic and sedimentary rocks.It can provide robust U-Pb age(s)for most geologic processes.The trace elements in zircon are of great importance in the interpretation of zircon U-Pb age and relevant geological significance.Commonly,the trace elements abundance in zircon is analyzed by mass spectrometry methods,such as laser ablation inductively coupled plasma-mass spectrometry(LA-ICP-MS),sensitive high-resolution ion microprobe(SHRIMP)and others,which usually require a large spot diameter(10-50μm).Electron probe micro-analyzer(EPMA)is capable of conducting elemental quantitative analyses at micro scales(<5μm)with relatively high spatial resolution and analytical accuracy/precision.Under appropriate analytical condition settings,the detection limit(D.L.)and analytical uncertainty(standard deviation)of EPMA can be comparable to other high-sensitive methods(e.g.LA-ICP-MS)with only a small difference of one order of magnitude.Besides,EPMA is an accessible instrument in most facilities and is much cheaper and efficient than other techniques.Hafnium is an important trace element in zircon that often substitutes the zirconium ion,since they possess almost the same effective ionic radius in the eightfold coordination,and the zircon has a complete solid solution with the hafnon(HfSiO4).The Hf abundance in zircon is of importance in the zircon U-Pb age interpretation,and so is the Zr/Hf ratio—it commonly relates with magmatic differentiation:the higher Hf or lower Zr/Hf ratio domain is indicative of zircon growth from a more fractionated melt.Besides,the Hf-related geochemistry is also informative of certain tectonic settings,in which zircon formed.Titanium is an important temperature sensor in zircon that can be utilized in the crystallization temperature estimate.The Ti cation prefers to substitute Si in the four-fold coordination instead of replacing the eightfold Zr in the zircon lattice.Currently,the Ti-in-zircon thermomete

关 键 词：锆石 微量元素Hf和Ti Zr/Hf比值 电子探针 定量分析 

分 类 号：P597.3[天文地球—地球化学]