CRC 526 | Projectarea E

Characterization of basic Rheological Parameters of Earth Materials

E1 The dependence of dislocation creep rate of minerals on enviromental variables (P, T and chemistry): III. Deformation behaviour of garnet - a key mineral in the crust and the mantle

Chakraborty, Becker

Summary
The overall goal in this continuing project has been to obtain a microscopic understanding of creep mechanisms in key minerals (rock forming silicates commonly found in both the crust and the mantle) and to exploit this to measure parameters in flow laws that are not easily accessible through direct deformation experiments. We have demonstrated that lattice diffusion of Si+4 is the key step in the dislocation creep of silicates such as olivine and the activation energy for this process is identical to the activation energy for dislocation creep under wet as well as dry conditions. The effect of water on deformation of olivine appears to be more complex than simply "wet" and "dry" and the effect appears to be stronger at high pressures where more water may be dissolved in olivine. Our continuing work on clinopyroxenes indicates that the activation energy is a function of temperature and other intensive variables such as oxygen fugacity at some conditions. Simultaneously, we have been developing a theoretical model to quantitatively calculate creep rates from diffusion coefficients of Si+4 as well as the other cations and anions in the lattice. Existing models are inadequate for this, but our new database of diffusion coefficients provides an excellent framework for the development and testing of alternate formulations.

In continuing this theme, we wish to focus on garnet in this last phase of the project. Garnet is a key mineral in crustal rocks in zones of orogenesis and in the mantle. Although volumetrically subordinate, its behaviour as a rigid body substantially affects the state of deformation and subsequent textural and rheological behaviour of these rocks. In spite of this significance, quantification of the rheological behaviour of garnet remains an unresolved problem, because of the difficulty in performing direct deformation experiments with garnets of natural compositions. Further subtleties, such as the effect of water on the rheological behaviour, remain entirely unexplored in spite of their importance. Yet, the inferences from experiments on analogue systems and natural rocks are often mutually contradictory. While analogue experiments suggest garnets to be extremely rigid at least some observations on natural rocks appear to contradict this. It remains unclear how rheological behaviour of garnets change as a function of environmental variables such as P,T, f(O2) and f(H2O). In this project we propose to measure the diffusion rates of Si in single and polycrystalline garnets of natural compositions to address these issues and to complete the database for the theoretical model.

E3 Elastic properties of selected subduction zone minerals

Schreuer

Summary
This project aims to measure single crystal elastic constants of selected subduction zone minerals as a function of temperature, frequency and crystal structure / chemical composition using a combination of different experimental techniques. The fundamental objectives are (i) to develop simple rules to estimate reliable elastic constants of materials for which direct measurements are unavailable, (ii) to explore how water in hydrous and nominally anhydrous minerals affect their elastic behavior, and (iii) to figure out how grain boundaries and other small-scale discontinuities affect / modify the constitutive relations for calculating elastic behavior of polycrystalline materials from single crystal elastic constants.

E4 The evolution of microstructure in polycrystalline aggregates deforming at high-temperature: Numerical modeling guided by experimental constraints

Hackl, Renner

Summary
The evolution of microstructure in polycrystalline aggregates deforming at high temperature shall be studied by theoretical and experimental approaches. A framework for a variational analysis of the dissipation by microstructure elements was previously developed. In experiments using fine- and coarse-grained aggregates we will study the effect of grain size and dynamic recrystallization. Results of the proposed study will improve the backwards reconstruction of the deformation history of natural rocks and protocols for the production of technical materials.

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