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3.4 Transport Properties and Equilibration Processes

Transport processes in minerals, rocks and melts include ionic diffusion, the migration of point defects and electrical conduction. Such processes control the rates of chemical equilibration in materials of the Earth's interior and are also important because of their effects on rheology. In addition, a knowledge of the electrical conductivity of minerals at high pressures and temperatures can be used to interpret geophysical estimates of the electrical conductivity of the Earth's interior in terms of mineralogy, temperature and possibly OH-content. It is therefore important to quantify transport processes in a variety of Earth materials over a wide range of physical and chemical conditions. Because processes in the Earth often occur over much longer time scales than are possible in laboratory experiments, extrapolations of experimental data to significantly lower temperatures are often required. For such extrapolations to be reliable, a detailed understanding of transport mechanisms is required.

The topics of studies reported in this section include hydrogen diffusion and Fe-Mg diffusion in olivine, electrical conductivity of the lower-mantle assemblage of perovskite + magnesiowüstite, multicomponent diffusion in granitic melts, and equilibration processes in high-pressure rocks from China. An understanding of hydrogen diffusion in mantle minerals is important for characterising the nature of point defects and for estimating the OH-content of the upper mantle from peridotite nodules which have been ejected in volcanic eruptions - because hydrogen diffusion is very fast, it is unlikely that the original OH-content is preserved as samples are transported to the Earth's surface. The aim of the study of Fe-Mg diffusion in olivine is to use the TEM to measure very short diffusion profiles which result from experiments performed at relatively low temperatures (thus minimising the extrapolation of data to geological temperatures). A knowledge of multicomponent diffusion in granitic liquids is required for modelling a variety of igneous processes and also for providing information about melt structure; observed correlations between diffusivity and viscosity are also valuable because the number of measurements required to fully characterise transport properties is greatly reduced. Finally, the difficulties of determining the pressure-temperature paths followed by metamorphic rocks during mountain building events is demonstrated and the need to fully understand reaction textures and diffusion rates in the constituent minerals is emphasised.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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