Kinetics of Olivine Phase Transformation and the Role of Water

The metastability of olivine is still not very clear as yet. Main reason is that experimental kinetic data on olivine are few. Mostly people employ the kinetic data of analogs to understand olivine's metastability. Among them, more attention has been paid to the results of forsterite. Water has great influence on forsterite's metastabiliy. Recently Hosoya et al. modified the growth equation by introducing a pre-exponential term as power-law function on OH content. Consequently, they refined the kinetic parameters, studied the role of water on forsterite's metastability, and reckoned its existence in earth's deep interior. Here we classify available in-situ growth data of Mg2SiO4 olivine with measured water contents into three categories, i.e. the ¡°dry¡±, ¡°wet¡±, and ¡°saturated¡± in which the average water content are about 800, 3000, and 5000 wt. ppm respectively. Results reveal that the traditional growth theory is useful and the pre-exponential coefficient of the rate equation has little relation with OH content. Refined values are in accordance with the theoretical vision. This fact lends us a robust constraint for determining the kinetic parameters. Accordingly we refine the activation enthalpy of Mg2SiO4 olivine for different OH content, and examine the influence of OH content on metastability of Mg2SiO4 olivine. At last we assess the metastability of the major mantle mineral (Mg0.89Fe0.11)2SiO4 olivine based on a few kinetic data. Results show that there is much difference in metastability between forsterite and olivine and there would exist observable metastable olivine although it might not reach 660-discontinuity.