Kinetics of the olivine-ringwoodite transformation and seismic attenuation in the Earth's mantle transition zone
Résumé
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 = k(0).T.exP[n.X-Fa].exp[-(Delta Ha + PV*)/RT] x [1 - exp(Delta G(r)/RT)], where X-Fa is the fayalite fraction, the exponential factor n = 9.7, Ink(0) = -9.1 m s(-1). X-Fa(-1) and Delta H-a = 199 kJ/mol, assuming V* = 0 cm(3)/mol, Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict Q(K)(-1) and Q(mu)(-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. (C) 2015 Elsevier B.V. All rights reserved.