POLYMORPHIC TRANSFORMATIONS BETWEEN OLIVINE, WADSLEYITE AND RINGWOODITE - MECHANISMS OF INTRACRYSTALLINE NUCLEATION AND THE ROLE OF ELASTICSTRAIN

Kinetic models and rate equations for polymorphic reconstructive phase transformations in polycrystalline aggregates are usually based on th e assumptions that (a) the product phase nucleates on grain boundaries in the reactant phase and (b) growth rates of the product phase remai n constant with time at fixed P-T. Recent observations of experimental ly-induced transformations between (Mg,Fe)(2)SiO4 olivine (alpha) and its high pressure polymorphs, wadsleyite (beta) and ringwoodite (gamma ), demonstrate that both these assumptions can be invalid, thus compli cating the extrapolation of experimental kinetic data. Incoherent grai n boundary nucleation appears to have dominated in most previous exper imental studies of the alpha-beta-gamma transformations because of the use of starting materials with small (<10-20 mu m) grain sizes. In co ntrast, when large (0.6 mm) olivine single crystals are reacted, intra crystalline nucleation of both beta and gamma becomes the dominant mec hanism, particularly when the P-T conditions significantly overstep th e equilibrium boundary. At pressures of 18-20 GPa intracrystalline nuc leation involves (i) the formation of stacking faults in the olivine, (ii) coherent nucleation of gamma-lamellae on these faults and (iii) n ucleation of beta an gamma. In other experiments, intracrystalline nuc leation is also observed during the beta-gamma transformation. In this case coherent nucleation of gamma appears to occur at the intersectio ns of dislocations with (010) stacking faults in beta, which suggests that the nucleation rate is stress dependent. Reaction rims of beta/ga mma form at the margins of the olivine single crystals by grain bounda ry nucleation. Measurements of growth distance as a function of time i ndicate that the growth rate of these rims decreases towards zero as t ransformation progresses. The growth rate slows because of the decreas e in the magnitude of the Gibbs free energy (stored elastic strain ene rgy) that develops as a consequence of the large volume change of tran sformation. On a Longer time scale, growth kinetics may be controlled by viscoelastic relaxation.