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The transition of olivine to its high pressure polymorphs in the Earth's interior is believed to have an important effect on the dynamics of subduction zones. Under conditions of an undisturbed mantle geotherm these phase transitions occur at depths between 400 km and 660 km. However, it is likely that in subducting slabs phase transformations are kinetically inhibited and that olivine persists metastably to depths far below 400 km (see also Annual Report 1994).The metastable olivine hypothesis can be tested by obtaining kinetic data for olivine phase transformations and extrapolating these data to subduction zone conditions, using an appropriate kinetic model.

To investigate the transformation kinetics of San Carlos olivine (Fo₉₀) to its high pressure polymorphs wadsleyite (ß-phase) and ringwoodite (γ-phase) two series of multianvil experiments have been performed at P-T-conditions within the stability field of γ-phase (18 to 20 GPa; 900° to 1100 °C) for various reaction times, using two different starting materials. Type 1 samples consisted of hot-pressed San Carlos olivine powder with an average grain size of about 20 µm. After reaction, these samples were investigated using transmission electron microscopy (TEM) and optical microscopy in order to determine the size, shape, and orientation of the product phases. In all reacted samples γ-phase and ß-phase as well as some relict olivine grains were found. Although the product grains nucleate preferably at the olivine grain boundaries (Fig. 3.1-1), some nuclei also seem to develop within olivine grains that have high dislocation densities. The grain sizes of the product phases were used to calculate the growth rates of the product phases. These preliminary growth rate data range from 1.6 x 10^-10 m s^-1 at 900 °C to 3.3 x 10^-8 m s^-1 at 1100 °C, but have large uncertainties. Since precise growth rates are crucial for calculations of the activation energy of the transformation, a second starting material was used for further experiments (type 2 samples). A single oriented olivine crystal (700 x 600 x 500 µm³) was embedded in a fine-grained mixture of hot-pressed olivine and pyroxene powder (96 % olivine + 4 % enstatite). After reaction the olivine cubes showed a sharply defined rim of the product phase. The widths of the reaction rim, which varied slightly with the crystallographic orientation of the olivine crystal, can be used to determine growth rates more precisely.

The experimental results together with data from further experiments will be used to determine rates of nucleation, growth and overall reaction as a function of temperature, pressure, oxygen fugacity, and OH-content. The kinetic results will then be combined with thermal models of subduction zones thus allowing us to model the mineralogical structure of subducting slabs.
 

Fig. 3.1-1: TEM image of 400 and 500 nm grains of γ-phase that have nucleated at an olivine-olivine grain boundary.