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

变质岩是由地壳深处的矿物在高压和高温下发生化学反应形成的，通常与安第斯山脉和喜马拉雅山脉等山脉的发育有关。这些反应产生了某些矿藏，它们可能会释放出重要的温室气体二氧化碳。因此，更好地了解变质矿物反应具有重要的科学意义和现实意义。然而，由于变质过程发生在很深的地方，因此不能通过直接观察来理解。变质作用的原理必须从对变质岩的各种化学分析中推断出来，这些变质岩在形成后被挖掘到地球表面。其中最有用的一种分析是氧同位素组成。由于氧的同位素因反应的种类和条件不同而不同，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重约一毫克的样品进行的，这些样品由大量矿物颗粒组成。因此，对数据的解释通常假定颗粒在成分上是均匀的。然而，有了新一代离子微探测器，现在可以对小一百万倍、空间分辨率小于颗粒尺寸的矿物质量进行现场分析。该项目的主要目标是使用离子微探测器系统地搜索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。如果存在重大差异，正如初步研究表明的那样，该项目将产生许多新的信息。该项目的第二个目标是开发方法，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了解。该项目将涉及加州、佛蒙特州和苏格兰的接触和区域变质泥质岩、沙砾和碳酸盐岩。约翰霍普金斯大学将使用电子成像技术对样品进行筛选。现场氧同位素分析将使用威斯康星大学的Cameca IMS-1280离子微探测器在10微米的空间尺度上进行。分析将侧重于硅酸盐矿物，这些矿物既有与其他矿物的氧同位素分馏数据，也有晶内氧同位素扩散速度的数据。最重要的目标是评估氧同位素组成在晶内和晶间变化的频率和幅度。扩散速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到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，或者根本不包含关于温度的有意义的信息。第三个目标是根据氧同位素组成在颗粒尺度上变化的大小和空间分布限制变质反应的细节，包括(1)氧同位素如何在矿物反应物和产品中重新分配，(2)反应过程中流体流动和扩散的质量传输的相对重要性，以及(3)反应条件偏离平衡的程度。该项目承诺将变质岩的稳定同位素研究建立在更坚实的基础上。

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.