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

地质年代学是对地球物质进行测年的科学，是现代地质学研究的重要方面之一。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。独居石的电子探针测年（EPMA）在过去的十年中已经变得越来越普遍，并证明是不可或缺的约束的年龄的古代构造事件，影响地壳，如变质和变形事件的特点组装和修改的大陆。该技术的巨大力量来自于这样一个事实，即年龄信息是从获得地质（微观）关系的同一薄切片中原位获得的，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，而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应用于地质年代学，并且很难比较它们之间的结果或统计数据。马萨诸塞州大学地球科学系在国家科学基金会和Cameca仪器公司的支助下，已着手设计和建造一种新的电子微探针，以进行最佳的微量元素分析。这个正在进行的项目的第二个目标是测试和评估程序，假设和协议的微量元素分析，以建立最佳的技术和展示可能的结果的范围。新的微探针（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都在可接受的误差范围内。尽管取得了这些成功，但为了使微探针独居石测年法得到广泛使用和广泛接受，还必须解决一些问题。这些措施包括改进校正复杂的光谱干扰，更有效的独居石搜索程序，一个客观和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，最佳计数时间，电压，电流和测量点的数量计算程序的发展，以及替代导电涂层的评估。本提案的目的是完成Ultrachron仪器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州大学的专用微探针来调查程序，表征标准，量化不确定性，并为地球科学界提供高分辨率的研究。电子微探针为地质年代学提供了一个强大的新工具，当与其他地质年代学技术相结合时，它会达到最大的效果。而且，由于这项技术与图像密切相关，岩石纹理或织物之间存在着天然的联系，这使我们能够解决最根本的问题，“我们在约会什么？”“当我们获得时间顺序的结果。由于，至少部分，仪器的可访问性，电子探针测年继续有特殊的教育影响。由于微探针很容易整合纹理，成分和年龄信息，学生们扎根于过程的角度，而不是年龄的获取和制表。最终，EPMA测年有助于整合领域，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的本科生、研究生和教职员工以及许多合作机构（例如，俄亥俄州大学，肯特街，麻省理工学院、波士顿学院、新墨西哥大学、锡拉丘兹大学、东太平洋大学），学生们密切参与了地质年代学分析的规划和实施。随着技术的改进和变得更加有效和可用，这将增加。

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.