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，是现代地质研究中最重要的方面之一。独居石是一种含有钍和铀的稀土磷酸盐，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。利用电子探针微量分析（EPMA）对独居石进行测年在过去十年中变得越来越普遍，并且在限制影响地壳的古代构造事件的年龄方面被证明是不可或缺的，例如变质和变形事件，这些事件是大陆组合和改造的特征。这项技术的强大之处在于，年龄信息是在现场获得的，从同一薄片中获得地质（微观）关系，因此年龄可以直接与特定的地质过程联系起来。然而，微探针测年涉及微量元素的精确分析，电子探针主要用于分析矿物的主要成分。此外，许多不同的仪器和许多不同的程序已经应用于地质年代学，很难比较它们之间的结果或统计数据。在美国国家科学基金会和卡梅卡仪器的支持下，马萨诸塞大学地球科学系已经开始设计和建造一种新的电子探针，用于微量元素分析。这个正在进行的项目的第二个目标是测试和评估微量元素分析的程序、假设和方案，以建立最佳技术并展示可能结果的范围。新的微型探针（Cameca SX-Ultrachron）已经安装在马萨诸塞大学，基本上所有的地质年代学标准产生的年龄都在可接受值的误差范围内。尽管取得了这些成功，但为了使微探针独居石定年得到广泛使用和广泛接受，仍有许多问题必须解决。这些改进包括对复杂光谱干扰的修正，更有效的独居石搜索程序，客观准确的背景测量程序，干扰校准和实验室间比较的微量元素标准，计算最佳计数时间，电压，电流和测量点数量的程序开发，以及评估替代导电涂层。本提案的目的是完成Ultrachron仪器的几个方面的测试和开发，并评估和建立可出口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针来调查程序，表征标准，量化不确定性，并为地球科学社区提供高分辨率研究。电子显微探针为地质年代学研究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，它将发挥最大的作用。而且，由于这项技术与图像密切相关，岩石纹理或结构之间有一种自然的联系，这使我们能够在获得年代结果时解决最基本的问题：“我们在确定什么年代？”由于，至少在一定程度上，仪器的可访问性，EPMA测年继续具有特殊的教育影响。由于微探针可以很容易地整合纹理、成分和年龄信息，学生可以从过程的角度出发，而不是从年龄的获取和制表的角度出发。最终，EPMA定年有助于整合各个领域，并吸引学生进入微观分析和地质年代学的定量方面。微探针测年已经对马萨诸塞大学和许多合作机构（如俄亥俄大学、肯特街大学、麻省理工学院、波士顿学院、新墨西哥大学、锡拉丘兹大学、特tep）的本科生、研究生和教师产生了巨大影响，学生们密切参与了地质年代学分析的规划和实施。随着技术的改进，效率和可用性的提高，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.