The effect of strain energy on growth rates during the olivine‐spinel transformation and implications for olivine metastability in subducting slabs

Abstract

We have studied growth kinetics during the transformation of Mg1.8Fe0.2SiO4 San Carlos olivine to its high‐pressure polymorphs wadsleyite (β phase) and ringwoodite (γ phase) at 800°–1200°C and nominal pressures of 16–20 GPa. In experiments in which a large (500–600 μm) olivine single crystal (contained in a matrix of either fine‐grained olivine or NaCl) was transformed, reaction rims of wadsleyite/ringwoodite form on the margins of the single crystal by incoherent grain‐boundary nucleation and interface‐controlled growth. Contrary to theoretical expectations, the growth rate of these reaction rims decreases sharply as a function of time; for instance, at 1100°C and 18 GPa, growth ceases on an experimental timescale after the rim width reaches 20–25 μm. In order to explain this observation, we develop an elastic model based on the theory of a misfitting inclusion. Comparing the results of this model with the experimental data suggests that elastic strain energy, which develops because of the large volume decrease associated with the transformation, is responsible for the decreasing growth rates. On the other hand, experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form; this is the case when the nucleation rate is low, the growth rate is fast, and the reactant olivine is fine grained. On a geological timescale in subducting lithosphere, where the grain size of the olivine is large, the growth rates of grain‐boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation. Therefore current kinetic models of olivine metastability in subducting slabs, which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure, need to be reevaluated.