榍石离子探针U-Pb定年的基体效应研究及微区分析标准物质研制

The geochronology has become one of the unique tools for the research of evaluation of Earth. The quantification of time is fundamental to our understanding of planetary evolution and the geologic processes that that shape our own planet Earth. The precision and accuracy geochronology is integral to testing hypotheses regarding the process in Earth history. Titanite is a common accessory mineral in many types of magmatic and metamorphic rocks. It is also widely occurs in variety of hydrothermal ore deposits. Titanite is widely regarded as the valuable mineral for geochronological research. The high spatial resolution techniques of SIMS allow us to analyze small domains in titanite. However, ion probe data are subject to instrumental effects, including matrix effects, which must be quantified and eliminated before valid ages are obtained. All ion probe element-ratio data are dependent on the composition of the sample being analyzed, because all elements in samples, including the elements being analyzed, have some effect on the ionisation efficiency of all secondary ion species. These effects can only be ignored in minerals of very uniform composition, which are therefore well-matched to reference samples of the same mineral. However, titanite have highly variable minor-element contents, so they are highly susceptible to matrix-induced variations in their measured Pb/U. Consequently, reliable geochronology requires the application of substantial data corrections. In this project, the major and minor composition, texture analysis and the SIMS U-Pb isotopes of titanite test will be analyzed. The matrix effect of will be investigated. More SIMS titanite U-Pb isotopes reference material will be developed. Our work is the foundation for further investigation and application of titanite geochronological research.

精确的同位素地质年代学在反演地质形成过程，探讨矿床成因等方面具有重要的意义。榍石是一种含U–Pb的副矿物，它普遍存在于各种中酸性和碱性侵入岩、变质岩、及各类热液矿床中的，在地质研究中具有非常广泛的应用价值。目前，利用离子探针可以对榍石进行高空间分辨率的测试，但是由于榍石结构的复杂性和成分的多样性，造成榍石在离子探针U-Pb年龄分析中具有很大的基体效应，使得其年龄结果不够精准，难以满足科研的需要。亟需寻找合适的榍石标样和建立校正方法，才能得到准确的定年结果。本项目拟通过对榍石的主微量成分和结构与离子探针年龄进行比较，认识其化学成分及结构变化对离子探针U-Pb年龄的基体效应影响，探明不同榍石的U-Pb同位素分馏差异和其规律，建立和完善离子探针榍石微区年龄分析流程和基体效应校正方法。并系统研发离子探针榍石微区年龄分析标样，为榍石在地质研究中推广应用奠定基础。

精确的同位素地质年代学在反演地质形成过程，探讨矿床成因等方面具有重要的意义。榍石是一种含U–Pb的副矿物，它普遍存在于各种中酸性和碱性侵入岩、变质岩、及各类热液矿床中的，在地质研究中具有非常广泛的应用价值。目前，利用具有高空间分辨率、低样品消耗量、和低检出限的离子探针(SIMS)可以对榍石进行原位微区测试。由于榍石结构的复杂性、成分的多样性、和相对较高的普通铅组成，目前对榍石的离子探针U-Pb定年是否存在基体效应，以及可能的基体效应对年龄结果的精准度有多大影响尚存争议。为了满足更精更准的科研需求，本项目对榍石离子探针U-Pb定年这一科学技术问题进行了多年的系统研究，并获得如下成果。首先是研发了可用于原位微区分析的榍石年龄标样。第二是采用以前的离子探针测试方法，证实了榍石的离子探针U-Pb定年存在明显的基体效应，并经大量的实验和长期统计发现，年龄的基体效应与榍石中的Fe相对含量具有最为显著的相关性。最后是建立了基于Fe-Pb/U-UO/U三者的幂函数关系来校正榍石的Pb-U分馏的方法，完善了离子探针榍石微区年龄分析流程。本项目研究结果说明，离子探针榍石U-Pb定年分析存在严重的Fe相关基体效应，本项目推荐的校正方法可以极大地提高榍石离子探针定年的准确度，为榍石在地质研究中推广应用奠定基础，将为限定岩石的热演化历史提供更准确的信息。