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带的形成有关。这些反应会产生某些矿床，并可能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科学和实际意义。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了解。变质作用是如何起作用的，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。最有用的分析方法之一是氧同位素组成分析。由于氧的同位素在不同的反应种类和条件下会有不同的分离，氧的同位素组成包含了很多关于变质矿物反应的原因和过程的信息。直到最近，氧同位素分析都是由重约一毫克的样品组成的，这些样品由许多矿物颗粒组成。因此，对数据的解释通常假定颗粒在组成上是均匀的。然而，有了新一代的离子微探针，现在可以在小一百万倍的矿物上进行原位分析，空间分辨率也比晶粒尺寸小。该项目的主要目标是使用离子微探针系统地搜索单个矿物内部和变质岩中附近颗粒之间氧同位素组成的变化。如初步研究表明的那样，如果存在显著的变化，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理解。该项目将涉及来自加州、佛蒙特州和苏格兰的接触和区域变质的泥岩、沙石和碳酸盐岩。样品将在约翰霍普金斯大学使用电子成像技术进行筛选。现场氧同位素分析将在10微米的空间尺度上进行，使用威斯康星大学的Cameca ims-1280离子微探针。分析将集中在硅酸盐矿物上，其中既有氧同位素与其他矿物分馏的数据，也有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级晶间变化的频率和幅度。扩散速率数据将区分岩石在最高温度时形成的变化和岩石冷却时形成的变化。第二个目标是确定矿物对之间测量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，或者根本没有包含有关温度的有意义的信息。第三个目标是从晶粒尺度上氧同位素组成变化的幅度和空间分布来限制变质反应如何进行的细节，包括(1)氧同位素如何在矿物反应物和产物之间重新分布，(2)反应过程中流体流动和扩散的质量传递的相对重要性，以及(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.