An in situ X ray diffraction study of the kinetics of the Ni₂SiO₄ olivine‐spinel transformation

Abstract

Using the “MAX‐80” cubic‐anvil high‐pressure apparatus with synchrotron radiation, X ray diffraction experiments have been carried out to study the kinetics of the Ni 2 SiO 4 olivine‐spinel transformation at 3.6–4.8 GPa and 740°–980°C. The Ni 2 SiO 4 olivine‐spinel equilibrium boundary has also been redetermined. The starting material for the kinetic experiments was Ni 2 SiO 4 olivine which was hot‐pressed in situ at 980°C and 2.5 GPa for 1.5 hours. During the transformation, X ray diffraction patterns were collected at intervals of either 30 s or 120 s, and estimates of the volume fraction transformed as a function of time have been made from the relative intensities of a total of six diffraction peaks by a least squares procedure. Although the Avrami rate equation can be fitted to the data, it is shown that this equation cannot be used as a basis for extrapolating the results to other temperatures and pressures. Instead, by fitting a more fundamental rate equation for grain boundary nucleation and interface‐controlled growth to the transformation‐time data, rates of nucleation and growth have been estimated. The activation energy for growth at 3.6–3.7 GPa has been estimated as 438 (±199) kJ mol −1 . A rate equation has also been fitted to explain the dependence of the estimated nucleation rates on temperature and pressure. The results suggest that because of the kinetics of nucleation, the olivine‐spinel equilibrium boundary needs to be overstepped by ∼0.4 GPa at relatively high temperature (e.g., 1000°C) for the transformation to start, even on a geological time scale. Although the kinetics cannot be evaluated quantitatively at the low temperatures (500°–600°C) which are predicted to occur within subducting slabs at a depth of 300–400 km, the results are consistent with olivine surviving metastably to very significant depths below the equilibrium phase boundary. The results emphasize the importance of understanding the dependence of the rates of nucleation and growth on pressure and temperature when extrapolating experimental kinetic data for this type of transformation to geological conditions.