Instrument and Technique Development: Trace Element Analysis and Geochronology by Electron Microprobe

地质年代学，即地球物质的年代测定，是现代地质学研究的最重要方面之一。独居石是一种稀土磷酸盐，含有钍和铀，存在于许多岩石中，现在被认为是一种提供丰富地质年代学信息的矿物。在过去的十年里，使用电子探针显微分析(EPMA)测定独居石的年龄已经变得越来越普遍，并被证明在限制影响地壳的古代构造事件的年龄方面是不可或缺的，例如表征大陆组装和改造的变质和变形事件。这项技术的强大之处在于，年龄信息是在现场获得的，从获得地质(微观)关系的相同薄片中获得，因此年龄可以直接与特定的地质过程联系起来。然而，微探针测年涉及到对微量元素的精确分析，而电子探针主要是为分析矿物的主要成分而设计的。此外，许多不同的仪器和许多不同的程序被应用于地质年代学，很难比较它们之间的结果或统计数据。马萨诸塞大学地球科学系在美国国家科学基金会和Cameca仪器公司的支持下，已着手设计和建造一种新的电子微探针，该探针针对微量元素分析进行了优化。这一正在进行的项目的第二个目标是测试和评价痕量元素分析的程序、假设和协议，以便确定最佳技术并展示可能结果的范围。新的微型探头(Cameca SX-Ultrachron)已经安装在美国马萨诸塞州大学，基本上所有的地质年代学标准得出的年龄都在接受值的误差范围内。尽管取得了这些成功，但仍有许多问题需要解决，才能使微探针独居石测年得到广泛应用和广泛接受。这些措施包括改进对复杂光谱干扰的校正、更高效的独居石搜索程序、客观和准确的背景测量程序、用于干扰校准和实验室间比较的微量元素标准、制定计算最佳计数时间、电压、电流和测量点数量的程序，以及评估替代导电涂层。这项建议的目的是完成对Ultraachron仪器的几个方面的测试和开发，并评估和建立可以输出到其他实验室的程序。最终目标是使用马萨诸塞大学的专用微探头来调查程序、表征标准、量化不确定性，并为地球科学界提供高分辨率研究。电子微探头为地质年代学提供了一种强大的新工具，当与其他地质年代学技术相结合时，它将获得最大的效率。而且，由于这项技术与图像密切相关，岩石纹理或面料之间存在着天然的联系，这让我们能够解决最基本的问题：“我们在约会什么？”当我们得到年表结果的时候。至少在一定程度上，由于仪器的可获得性，EPMA测年继续具有特殊的教育影响。由于微探头很容易集成质地、成分和年龄信息，所以学生的基础是过程的观点，而不是年龄的获取和制表。归根结底，EPMA测年有助于整合各个领域，并将学生吸引到微观分析和地质年代学的量化方面。微探针测年已经对马萨诸塞州大学的本科生、研究生和教职员工以及许多合作机构(如大学)产生了巨大的影响。俄亥俄州、肯特圣州、麻省理工学院、波士顿学院、北卡罗来纳大学、锡拉丘兹大学、UTEP大学)，学生们一直密切参与地质年代学分析的规划和实施。随着技术的改进和变得更加高效和可用，这一数字将会增加。

Geochronology, the dating of Earth materials, is one of the most important aspects of modern geological research. Monazite is a rare-earth phosphate containing thorium and uranium that occurs in many rocks, and is now recognized as a mineral that provides a wealth of geochronologic information. Dating monazite using electron probe microanalysis (EPMA) has become increasingly common over the last decade, and is proving to be indispensable in constraining the age of ancient tectonic events that have affected Earth's crust such as the metamorphic and deformational events which characterize the assembly and modification of continents. The great power of the technique comes from the fact that age information is acquired in-situ, from the same thin sections from which geologic (microscopic) relationships are obtained, and thus ages can be directly linked to specific geologic processes. However, microprobe dating involves the precise analysis of trace elements, and the electron probe was mainly designed for analysis of the major components of minerals. Also, many different instruments and many different procedures have been applied to geochronology, and it is difficult to compare results or statistics between them. The Department of Geosciences at the University of Massachusetts has undertaken to design and build, with support from NSF and Cameca Instruments, a new electron microprobe, optimized for trace element analysis. A second goal of this on-going project is to test and evaluate procedures, assumptions, and protocols for trace element analysis in order to establish optimal techniques and to demonstrate the scope of possible results. The new microprobe (Cameca SX-Ultrachron) has been installed at UMass, and essentially all geochronologic standards yield ages within error of accepted values. These successes notwithstanding, there are a number of issues that must be addressed in order for microprobe monazite dating to be widely used and widely accepted. These include improved corrections for complex spectral interferences, more efficient monazite search procedures, an objective and accurate background measurement procedure, trace element standards for interference calibration and inter-lab comparison, development of procedures for calculation of optimal count time, voltage, current, and number of measurement spots, and evaluation of alternative conductive coatings. The purpose of this proposal is to complete testing and development of several aspects of the Ultrachron instrument, and to evaluate and establish procedures that can be exported to other laboratories. The ultimate goal is to use the dedicated microprobe at the University of Massachusetts to investigate procedures, characterize standards, quantify uncertainties, and to be available to the geoscience community for high-resolution studies.The electron microprobe provides a powerful new tool for geochronology, one which attains its' greatest effectiveness when integrated with other geochronologic techniques. And, because the technique is intimately connected to images, there is a natural linkage between rock textures or fabrics that allows us to address the most fundamental question, "What are we dating?" when we obtain chronologic results. Due, at least in part, to the accessibility of the instrumentation, EPMA dating continues to have exceptional educational impact. As the microprobe easily integrates texture, composition, and age information, students are grounded in the perspective of processes, rather than the acquisition and tabulation of ages. Ultimately, EPMA dating helps to integrate fields and draw students into quantitative aspects of microanalysis and geochronology. Microprobe dating has already had a great impact on undergraduate students, graduate students, and faculty at UMass and many collaborating institutions (e.g., Univ. of Ohio, Kent St., MIT, Boston College, UNM, Syracuse, UTEP), where student have been intimately involved with the planning and implementation of the geochronologic analyses. This will increase as techniques are refined and become more efficient and available.