Nanoscale processes and geomaterial’s properties
Jointly run by:
• Freie Universität Berlin
• Technische Universität Berlin
• GeoForschungsZentrum Potsdam
Speaker:
Prof. Dr. Rainer Abart
Freie Universität Berlin
12249 Berlin
E-Mail senden an Prof. Dr. R. Abart
Tel.: 030-83870392
Co-Speaker:
Prof. Dr. Wilhelm Heinrich
GeoForschungsZentrum Potsdam
14773 Potsdam
E-Mail senden an Prof. Dr. Wilhelm Heinrich
Tel.: 0331-288 1410
Abstract
Nanometer scale structures in geological materials such as crystal defects, grain- and phase boundaries and small scale chemical zoning patterns may provide insight into the processes, which occur on the atomic or molecular level and control the kinetics of mineral reactions. The processes involved in breaking and establishing bonds at reaction sites and in material transport in between, govern the development of microstructures and textures and this way coin the geodynamic record contained in the phase assemblage and in the fabric of a rock. In addition, they determine how and at what rate bulk rock properties change in response to external forcing and possibly feed back into geodynamic processes. Both, reading the information stored in rock fabrics as well as a better understanding of the feedback between transient bulk material properties and geodynamics require a thorough understanding of the processes underlying phase and fabric change in geological materi-als.
The intended research will focus on atomic and molecular scale processes at grain- and phase boundaries under pressure and temperature conditions of the Earth’s deeper crust and mantle. Even though they only constitute a negligible volume fraction of polycrystalline materials, grain- and phase boundaries may have a profound influence on the bulk rock properties such as elasticity, strength, electrical conductivity, and diffusive mass transport.
The objective of this proposal is to stimulate coordinated research, which is focused on grain- and phase boundary processes and their implications for reaction kinetics, micro-structure development and bulk material properties. This research builds on well estab-lished concepts from equilibrium thermodynamics and reaction kinetics, and it makes use of recent advances in analytical and experimental techniques as well as of new theoretical and computational capabilities. The proposal aims to integrate expertise from field- and laboratory based researchers in mineralogy, petrology, geochemistry, mineral- and petro-physics as well as from materials science, physics, and applied mathematics to generate new competence in geo-materials science. The expected outcomes of this project will help to answer long standing questions up to the mechanisms and rates of mineral reactions, the size and life times of chemical and isotopic equilibration domains and the feedback between mineral reactions and mechanical stress. They will help to critically evaluate and improve existing petrological, geochemical, and geochronological models, and will contrib-ute to a better understanding of the transient nature of bulk material properties. This will allow refinement of geodynamic models and geophysical tools, and it will foster the under-standing of the links between microscopic processes, bulk material properties and the evo-lution of geological systems in space and time.
Research topics and projects
Topic I:
Physical and chemical properties of grain- and phase boundaries
A first group of projects is concerned with the physical and chemical properties of grain- and phase boundaries. Here the focus is on the investigation of isolated grain and phase boundaries using synthetic bicrystals. Grain boundary structures are investigated using high resolution transmission electron microscopy. Grain boundary structures and the kinet-ics of grain boundary processes are simulated using first principles, molecular dynamics and Monte Carlo simulation techniques.
The electrical conductivity of polycrystals and polyphase materials is grain-size dependent. The influence of polarization effects at solid-melt interfaces in partially molten rocks is in-vestigated by impedance spectroscopy.
The following projects pertain to this topic:
• Project 1:
Structure and properties of grain and phase boundaries in rocks
Proponents: G. Dresen, R. Wirth, S. Jahn
• Project 2:
Polarization effects at grain boundaries
Proponents: F. Schilling, H. Brasse
Topic II:
Diffusion in polycrystals and in polyphase materials
A second group of projects is concerned with diffusion in polycrystals and polyphase mate-rial. We do rim growth experiments in conventional experimental setups, where m-sized grains of incompatible phases are mixed and reacted at high pressure and temperature. There is a wealth of rim growth experiments that has been done on binary systems with simple reactions of the type A+B <=> C. With our experiments we apply the well-established technique to more complex systems, i.e. binary systems that allow for reac-tions of the type A+B <=> C+D and ternary systems. In an alternative approach, rim growth settings are miniaturized using pulsed laser deposition technique to produce multilayer silicate thin films. Such miniaturization is mandatory, if diffusion experiments shall be done at low water fugacity and at moderate temperatures, where diffusion is slow.
The transport properties of grain- and phase boundaries may depend on local stress state We do rim growth experiments under differential stress to counterbalance or reinforce local stress fields, which are internally generated through volume changes during reaction, and, by this technique, quantify reaction induced stress and stress dependence of bulk diffusion in polycrystalline materials.
In view of a generalized diffusion theory close links may be established among the thermo-dynamic system properties, chemical diffusion and microstructure evolution. The latter is governed by the tendency to minimize the free energy at given boundary conditions. We adapt existing mean field theory for application to experimentally produced and natural reaction microstructures. Numerical simulations are calibrated and validated by using labo-ratory experiments on phase separation in silicate systems.
The combined effects of fluid-assisted mass transfer along grain- subgrain- and phase boundaries and reaction induced stress are investigated by studying reaction microstruc-tures generated during the replacement of plagioclase by sodalite. From natural samples it is known that this reaction preferentially occurs along grain and subgrain boundaries. Very likely a positive volume change is associated with this reaction and grain boundaries may be opened or closed forcefully by the growth of reaction products. The replacement phe-nomena observed in natural samples are reproduced under well-controlled conditions in hydrothermal experiments.
In yet another project, we investigate to what extent trace elements, in particular radiogenic and radioactive isotopes may be transferred from a precursor to a product phase during mineral replacement reactions. Systematic investigations of replacement textures in iso-topic carrier minerals are done using natural samples to determine to what extent the iso-topic elements are directly transferred from the precursor to the product accessory mineral and to what extent they are redistributed among all potential carriers.
The following projects pertain to this topic:
• Project 3:
Diffusion-controlled growth of complex reaction rims: bridging the gap be-tween rim growth experiments and metamorphic coronas
Proponents: W. Heinrich, R. Milke, R. Abart
• Project 4:
Effect of water fugacity on grain-boundary diffusion and diffusion driven Li-isotope fractionation: insights from thin film experiments
Proponents: R. Milke, U. Wiechert
• Project 5:
Feedback between transport controlled mineral reactions und differential Stress
Proponents: R. Abart, G. Dresen
• Project 6:
Non-linear diffusion during phase separation and symplectite formation in alkali feldspar: an experimental and modelling approach
Proponents: R. Abart, D. Harlov
• Project 7:
Fluid assisted mineral replacement processes
Proponents: K. Drüppel, G. Franz, R. Romer
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