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 g round natural olivine admixed with 3% enstatite was deformed at high t emperature and high pressure such that the alpha-->beta olivine phase boundary was traversed. Both residual and new phases, their grain size s, their deformation-induced lattice defects, and their compositions w ere then characterized by analytical electron microscopy. Owing to lar ge heterogeneities in temperature, pressure, and deformation condition s, contrasting microstructures were observed. Some areas consist essen tially of untransformed olivine, while the beta polymorph is the main constituent of other areas. Crystals of the beta polymorph are general ly free of dislocations, except at the grain boundaries of the larger grains where slip bands were nucleated (probably induced by stress con centrations). The activated glide systems are {021} [100]. The orthopy roxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic compon ent. Compared to olivine, crystals of the beta phase contain significa ntly higher concentrations of Fe and trivalent cations. All beta cryst als are free of inclusions, confirming the conclusion from previous in frared spectroscopic analysis that the high OH concentration in this s pecimen is dissolved in the beta phase. Coupled with these previously measured OH concentrations, our new measurements suggest that incorpor ation of hydrogen into the beta phase and substitution of trivalent ca tions into the octahedral sites may be coupled. Such increased complex ity of mineral chemistry points to a need for detailed investigation o f the spectrum of compositions stable in the beta structure and coexis ting phases in order to better estimate the modal mineralogy of the tr ansition zone.