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.
Outcome of the primary project The oxygen isotope compositions (δ18O) of minerals in metamorphic rocks have numerous applications to the study of metamorphism, including geothermometry, determination of the source(s) of coexisting fluid, and estimation of the duration of metamorphic processes. All applications, however, normally involve untested assumptions about the grain-scale variability of the δ18O of minerals in metamorphic rocks. The primary project provides the first systematic evaluation of the grain-scale oxygen isotope geochemistry of minerals in metamorphic rocks. Results unexpectedly reveal common, significant grain-scale intercrystalline and intracrystalline variations in δ18O. Except for reconnaissance studies, conventional bulk analysis of δ18O of minerals in metamorphic rocks should be considered suspicious unless proven otherwise by ion microprobe analysis. Taking into account analytical precision, spatial resolution, preservation of petrographic context, and speed of analysis, the project establishes ion microprobe analysis as the new standard for measurements of the oxygen isotope compositions of minerals in metamorphic rocks. Outcome of secondary project A Fluid flow during regional metamorphism plays a vital role in a variety of processes, including heat transport in the crust, the origin of the some of the world's largest ore deposits, and the global CO2 budget. Secondary project A produced new transport models for coupled fluid flow and mineral-fluid reaction during regional metamorphism that resolved a decades-old controversy over the understanding of fluid flow in the crust based on hydrodynamics and on the petrologic record of fluid flow in exhumed metamorphic terrains. The new models, for the first time, explicitly consider mineral solid solution and implicitly consider diffusion perpendicular to the flow direction. They prove that vertical, upward directed fluid flow predicted by hydrodynamic models of regional metamorphism is perfectly consistent with the observed spatial distribution of reactants and products of metamorphic mineral-fluid reactions observed in the field. More generally, results demonstrate that transport models for coupled fluid flow and mineral reaction must explicitly incorporate the effects of solid solution if mineral reactants and/or products are solid solutions. Outcome of secondary project B Diffusion chronometry is a burgeoning new field that uses stranded diffusion profiles within mineral crystals to estimate the duration of geological processes. Secondary project B documents an important, widespread, new example: ankerite grains with a dolomite core in regionally metamorphosed clastic sedimentary rocks. Application of diffusion chronometry to these features, however, reveals unrecognized problems. The likliest sources of the problems are either poorly calibrated rates of intracrystalline diffusion and/or unrecognized differences in processes of mass transport by diffusion in nature compared to in laboratory experiments. Outcome of secondary project C Carbonate clumped isotope thermometry is a revolutionary new method for determining the temperature of formation of natural carbonate minerals from the concentrations of 13C-18O bonds in the mineral. The range of geological conditions in which carbonate clumped isotope thermometry may be safely applied, however, has been unknown. In secondary project C, new laboratory measurements of the rates of modification of the carbonate clumped isotope thermometer as a function of time and temperature have identified a window in temperature (<100 °C) in which carbonate clumped isotope thermometry faithfully records the temperature of formation of a calcite grain over geologic time. This has critically important relevance to studies in paleoclimate, paleoecology, and paleoenvironment that use carbonate clumped isotope termometry.
