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

- We report in situ kinetics measurements across the olivine-ringwoodite loop transition.
- Reaction rates increase with temperature and the iron content of olivine.
- Kinetic results are included in a mechanical model of a two-phase loop to calculate attenuation.
- Olivine transitions significantly contribute to the attenuation of the Earth's mantle transition zone.

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, ln⁡k0=−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.

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