ANALYTICAL ELECTRON-MICROSCOPY OF A SYNTHETIC PERIDOTITE EXPERIMENTALLY DEFORMED IN THE BETA-OLIVINE STABILITY FIELD

A synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3% enstatite was deformed at high temperature and high pressure such that the alpha-->beta olivine phase boundary was traversed. Both residual and new phases, their grain sizes, their deformation-induced lattice defects, and their compositions were then characterized by analytical electron microscopy. Owing to large heterogeneities in temperature, pressure, and deformation conditions, contrasting microstructures were observed. Some areas consist essentially of untransformed olivine, while the beta polymorph is the main constituent of other areas. Crystals of the beta polymorph are generally free of dislocations, except at the grain boundaries of the larger grains where slip bands were nucleated (probably induced by stress concentrations). The activated glide systems are {021} [100]. The orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component. Compared to olivine, crystals of the beta phase contain significantly higher concentrations of Fe and trivalent cations. All beta crystals are free of inclusions, confirming the conclusion from previous infrared spectroscopic analysis that the high OH concentration in this specimen is dissolved in the beta phase. Coupled with these previously measured OH concentrations, our new measurements suggest that incorporation of hydrogen into the beta phase and substitution of trivalent cations into the octahedral sites may be coupled. Such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the beta structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone.