Elastic anomalies and anelastic dissipation mechanisms associated with phase transitions in minerals.

The elastic properties of minerals have long been investigated for their relevance to the seismic properties of the earth. Interest has largely focussed on discontinuities associated with changes in mineral assemblages and these have led to our present understanding of the layered nature of the mantle and core. It has been recognised, however, that almost any change in the structure of a mineral can give rise to changes in elastic constants and that, for structural transitions driven by displacements of atoms, magnetic ordering and electronic effects, changes in elastic constants can give be as much as tens of percent. Substantial improvements in geophysical methods have also allowed attenuation effects to be investigated more systematically, and a number of models of the variation in attenuation with depth have recently been published. The present project will contribute to this new direction by providing fundamental information on the diversity of mechanisms by which attenuation can occur. The specific focus is on elastic and dissipative processes associated with phase transitions in minerals and their synthetic analogues. The objectives of the project are (a) to measure elastic constants and attenuation parameters for selected oxides and silicates in situ at high and low temperatures using Resonant Ultrasound Spectroscopy (RUS), (b) to test the influence of frequency and applied stress on dissipation phenomena, and (c) to develop further the RUS facility in Cambridge for measurements of highly dissipating samples and for iron bearing minerals at high temperatures.