Evaluation of Intracrystalline Zoning and Grain-scale Intercrystalline Variations in the Oxygen Isotope Composition of Minerals in Metamorphic Rocks by Ion Microprobe

变质岩是地壳深处矿物在高温高压下发生化学反应而形成的，通常与安第斯山脉和喜马拉雅山脉等山脉带的发育有关。 这些反应产生了某些矿床，它们可能会释放出重要的温室气体二氧化碳。 因此，深入了解变质矿物反应具有重要的科学意义和实际意义。 然而，由于变质作用发生在很深的地方，因此不能通过直接观察来理解。 变质作用是如何发生的，必须通过对形成后被挖掘到地球表面的变质岩进行各种化学分析来推断。 最有用的分析之一是氧同位素组成。 由于氧的同位素因不同的反应类型和条件而被分离，氧同位素组成包含了许多关于变质矿物反应的原因和如何进行的信息。 直到最近，氧同位素分析都是对由大量矿物颗粒组成的重约1毫克的样品进行的。 因此，对数据的解释通常假设颗粒的成分是均匀的。 然而，随着新一代离子微探针的出现，现在可以对比其小一百万倍的大量矿物进行原位分析，空间分辨率小于粒度。 该项目的主要目标是使用离子微探针系统地研究变质岩中个别矿物内部和附近颗粒之间氧同位素组成的变化。 如初步研究所示，如果存在重大差异，该项目将产生许多新的信息。 该项目的第二个目标是开发将新信息转化为对变质矿物反应的机制、条件和驱动力的定量理解的方法。该项目将涉及来自加州、佛蒙特和苏格兰的接触和区域变质的泥质岩、砂岩和碳酸盐岩。 样本将在约翰霍普金斯大学使用电子成像技术进行筛选。 在威斯康星州大学将使用Cameca IMS-1280离子微探针在10微米空间尺度上进行原位氧同位素分析。 分析将集中在硅酸盐矿物，其中有关于氧同位素分馏与其他矿物和晶内氧同位素扩散率的数据。 最重要的目标是评估氧同位素组成的晶内和晶粒尺度晶间变化的频率和幅度。 扩散率数据将区分岩石处于最高温度时形成的变化和岩石冷却时形成的变化。 第二个目标是确定测量的矿物对之间的氧同位素分馏是否记录了变质作用的最高温度，记录了冷却温度，或者不包含任何有意义的温度信息。 第三个目标是限制有关变质反应如何从粒度尺度上氧同位素组成变化的幅度和空间分布进行的细节，包括（1）氧同位素如何在矿物反应物和产物中重新分布，（2）反应期间流体流动和扩散的质量输送的相对重要性，（3）反应条件偏离平衡的程度。该项目有望为变质岩的稳定同位素研究奠定更坚实的基础。

Metamorphic rocks are formed by chemical reactions among minerals at elevated pressure and temperature deep in Earth's crust, usually associated with the development of mountain belts like the Andes and Himalayas. The reactions produce certain ore deposits, and they may release the important greenhouse gas, carbon dioxide. A better understanding of metamorphic mineral reactions therefore is of both scientific and practical significance. Because the process of metamorphism occurs at great depth, however, it cannot be understood from direct observation. How metamorphism works must be inferred from various kinds of chemical analysis of metamorphic rocks that are exhumed to the Earth's surface after they form. One of the most useful kinds of analysis is of oxygen isotope composition. Because the isotopes of oxygen are variably separated by different kinds and conditions of reaction, oxygen isotope composition contains much information about the causes of metamorphic mineral reactions and how they proceed. Until recently, oxygen isotope analysis were made of samples weighing about a milligram composed of numerous mineral grains. Interpretation of data therefore typically assumed that the grains are homogeneous in composition. With the new generation of ion microprobes, however, analysis can now be made in situ on a mass of mineral a million times smaller and with a spatial resolution smaller than grain size. The principal goal of the project is to use an ion microprobe in a systematic search for variations in the oxygen isotope composition both within individual minerals and between nearby grains in metamorphic rocks. If significant variations are present, as preliminary studies indicate, the project will yield much new information. A second goal of the project is to develop methods for translating the new information into a quantitative understanding of mechanisms, conditions, and driving forces of metamorphic mineral reactions.The project will involve contact and regionally metamorphosed pelites, psammites, and carbonate rocks from California, Vermont, and Scotland. Samples will be screened using electron imaging techniques at Johns Hopkins University. In situ oxygen isotope analysis will be made at a 10-micron spatial scale using the Cameca ims-1280 ion microprobe at the University of Wisconsin. Analysis will focus on silicate minerals for which there are data about both oxygen isotope fractionation with other minerals and the rate of intracrystalline oxygen isotope diffusion. The most important goal is to evaluate the frequency and magnitude of intracrystalline and grain-scale intercrystalline variations in oxygen isotope composition. Diffusion rate data will distinguish between variations that formed when rocks were at their maximum temperature from variations that formed as rocks cooled. A second goal is to determine whether measured oxygen isotope fractionations between mineral pairs record the maximum temperature of metamorphism, record cooling temperatures, or contain no meaningful information at all about temperature. A third goal is to constrain details about how metamorphic reactions proceed from the magnitude and spatial distribution of variations in oxygen isotope composition at the grain-size scale, including (1) how oxygen isotopes are redistributed among mineral reactants and products, (2) the relative importance of mass transport by fluid flow and diffusion during reactions, and (3) the degree to which conditions of reaction depart from equilibrium. The project promises to put stable isotope investigations of metamorphic rocks on a firmer foundation.