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3.1 p. Extent of olivine metastability in subducting lithospheric slabs (J.L. Mosenfelder and D.C. Rubie, in collaboration with L. Kerschhofer/Münster, F. Marton/Washington and C.R. Ross II/Memphis)

The olivine-wadsleyite equilibrium boundary is elevated to shallower levels within subducting slabs relative to its depth of 410 km in the surrounding mantle. This elevation, by 50 km or more, is caused by low temperatures in the slab combined with the positive Clapeyron slope of the phase transition. However, it has also been proposed that the transformation is kinetically inhibited in cold subducting slabs, with the result that olivine persists metastably to depths far below the equilibrium boundary. The extent of this metastability can be explored by combining a kinetic model, based on experimental data, and a 2-dimensional heat conduction model. Early thermo-kinetic models of this type (e.g. Kirby et al. 1996, Rev. Geophys. 34, 261-306) were predominantly based on kinetic data obtained on analogue compositions (e.g. Mg2GeO4 and Ni2SiO4) and corrected empirically for composition. The results of such models suggested that metastable olivine can persist to depths as great as 700 km in fast-subducting (and therefore relatively cold) slabs, such as the Tonga slab. The extent of such metastability is consistent with the hypothesis that deep-focus earthquakes are caused by the transformation of metastable olivine to wadsleyite or ringwoodite (e.g. by "transformational faulting"). Moreover, the existence of a metastable wedge has important implications for the buoyancy forces that drive subduction.

Recent experimental work on realistic mantle compositions (Mg2SiO4 and Mg1.8Fe0.2SiO4) allows us to define better the kinetics of the olivine-wadsleyite-ringwoodite transformations under subduction zones. We fit the new experimental data on growth kinetics of wadsleyite and ringwoodite to a rate equation for interface-controlled growth:

where is the growth rate, k0 is a constant, T is absolute temperature, is the activation enthalpy for growth, V* is the activation volume for growth, is the free energy change of reaction, and R is the Gas Constant. The fit yields an activation enthalpy, critical for extrapolation to low temperatures, of 303± 80 kJ mol-1. The new parameters have been incorporated in 2-dimensional thermo-kinetic models of subducting lithosphere. In contrast to earlier models, the effects of the latent heat of transformation on both thermal structure and kinetics are included. Results suggest that the extent of olivine metastability is likely to be less than thought previously. For example, for the coldest slabs, such as Tonga, olivine is transformed completely by a depth of 550 km. In this case, the deepest earthquakes cannot be explained by transformational faulting.

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