SHRIMP高空间分辨率磷钇矿定年与微量元素联测方法研究

In-situ micro-analytical techniques using secondary ion mass spectrometry (SIMS) have significantly contributed towards geochronology and isotope geochemistry in the Earth Sciences, and one of the important methods is to determine the depositional age of sedimentary rock using diagenetic xenotime. However, currently there are limitations to the SIMS U-Pb dating of xenotime because (1) highly variable trace element composition of xenotime leads to significant matrix effects; (2) most xenotime crystals are very small and require extremely high spatial resolution; (3) the procedure to determine the origin of xenotime is inefficient. This study aims to develop a new U-Pb dating technique using a SHRIMP IIe instrument and new analytical methods with higher precision and higher spatial resolution and to simultaneously determine the chemical composition of xenotime. In this research, the COMSOL and SIMION software programs will be used to simulate the SHRIMP ion source and ion optical system in order to guide the instrument tuning, to optimize the instrument spatial/mass resolution and to simultaneously determine U, Th, Pb and trace elements. The methods will focus on correcting for isotope fractionation caused by matrix effect, improve the SHRIMP dating precision and to identify xenotime of different origins. This study will provide a high precision/high spatial resolution SIMS xenotime dating system for general use in the earth sciences. SHRIMP dating of xenotime has a great potential to study the history and evolution of sediments.

微区原位分析技术极大的推动了地球科学的发展，用沉积自生磷钇矿微区原位定年厘定沉积岩沉积年龄的方法已应用于沉积岩研究。但目前磷钇矿SHRIMP U-Pb定年仍存在局限性：（1）化学组成变化范围大，有明显的基质效应；（2）空间分辨率无法满足大部分磷钇矿定年，（3）判断磷钇矿成因的效率较低。针对上述问题，提出开发高精度、高空间分辨率的磷钇矿SHRIMP U-Pb定年，并能够同时判断其成因的分析方法与技术。该研究通过COMSOL、SIMION仿真软件模拟SHRIMP离子源和离子光学系统，指导仪器参数设置，提高仪器的空间分辨率与质量分辨率，实现U、Th、Pb、微量元素等化学组成的同时测量，校正基质效应引起的同位素分馏，提高磷钇矿SHRIMP U-Pb定年精度，并能够快速判断磷钇矿成因，为国内外学者提供高精度、高效率、高空间分辨率磷钇矿定年平台，从而为沉积岩的演化历史提供依据。

磷钇矿具有较低的初始Pb，较高的U和Th，封闭温度与锆石、独居石大体相同，是理想的同位素地质年代学定年对象。二次离子质谱（SIMS）是磷钇矿高精度定年最有效的方法之一，可解决碎屑沉积岩定年的难点。但磷钇矿尺寸小，化学组分复杂，具有多种成因，且HREE与Y具有相同化学性质而经常出现HREE替代Y，增加了SIMS磷钇矿高精度定年的难度。. 项目组采用COMSOL、SIMION仿真软件模拟SHRIMP离子源和一次离子光学系统，指导SHRIMP离子源和透镜等参数设置，增大了一次离子流密度，实现了5-10微米小束斑定年。目前O2-离子束直径7μm，离子流强度可达2nA。通过调节双聚焦系统入口和出口狭缝、α和β狭缝、二次离子提取系统的三重四极杆，实现SHRIMP的高质量分辨率和高传输效率，利用谱峰识别软件，寻找合适的稀土元素同位素来计算稀土元素含量，该方法相比能量筛选模式具有离子的传输效率高的优点，当仪器质量分辨率大于10000（1%峰高）时，基本解决轻稀土氧化物对重稀土的同质异位素干扰，实现了U、Th、Pb、稀土元素等的同时测量。研究270UO2、254UO与206Pb的能量分布差别， U、Th、稀土元素含量变化对磷钇矿定年分析的影响，通过多标样基质效应校正，提高了磷钇矿SHRIMP U-Pb定年精度，从而实现磷钇矿SHRIMP小束斑定年和稀土元素联测方法。