Kinetics of the olivine-ringwoodite transformation and seismic attenuation in the Earth's mantle transition zone

Perrillat J., Chollet M., Durand S., van de Moortèle B., Chambat F., Mezouar M., Daniel I.

Abstract

In regions of the mantle where multi-phases coexist like at the olivine-wadsleyite-ringwoodite transitions, the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases, a possible source of dissipation. In such a situation, the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave. Here we report synchrotron-based measurements of the kinetics of the olivine to ringwoodite transformation at pressure-temperature conditions of the co-stability loop, for iron-rich olivine compositions. Both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries. Transformation-time data show an increase of reaction rates with temperature and iron content, and have been fitted to a rate equation for interface-controlled transformation: G=k0·T·exp [n·XFa]·exp [-(δHa+PV*)/RT]×[1-exp (δGr/RT)], where XFa is the fayalite fraction, the exponential factor n=9.7, lnk0=-9.1 ms-1. XFa-1 and δHa=199 kJ/mol, assuming V*=0 cm3/mol. Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict QK-1 and Qμ-1 significantly higher than the PREM values for both body waves and normal modes. This attests that the olivine-wadsleyite transition can significantly contribute to the attenuation of the Earth's mantle transition zone.