9305184 Brearley The principal objective of this project is to study in detail the mechanisms of the high pressure mantle phase transformations of Mg2SiO4 and (Mg, Fe)2SiO4 under a wider range of PT conditions relevant to the model mantle geotherm and subducting oceanic lithosphere. The study will continue our efforts to delineate the pressure-temperature regimes over which different transformation mechanisms may operate in the Earth's mantle and identify what factors may cause changes in the transformation mechanism (P,T, degree of overstepping etc.). In particular we will concentrate on the mechanisms of transformations involving the polymorph of Mg2SiO4 and (Mg,Fe)2SiO4. We will examine the role that water may play in transformations involving -phase and examine in detail the nature of order-disorder transformations in -phase and how they may influence the stability of -phase in the mantle. The results have important geological implications for upper mantle dynamics, especially the rheological behavior of subducting oceanic slabs and the mechanisms of deep focus earthquakes. The project will involve high pressure kinetic experiments using a multi anvil high pressure apparatus coupled with detailed microstructural studies using optical and electron microscopy. Special emphasis will be placed on the use of transmission electron microscope techniques, particularly electron diffraction and high resolution electron microscopy to elucidate the transformation mechanisms.
