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

Johns Hopkins University, Baltimore MD

Investigators

Abstract

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.