Strength of (Mg,Fe) 2SiO 4 wadsleyite determined by relaxation of transformation stress

Abstract

The growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change. The strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth. Stress relaxation enables growth to continue, at a rate controlled primarily by the mechanical properties of the rim. We studied this process in high-pressure experiments in order to constrain the rheology of wadsleyite, one of the high-pressure polymorphs of olivine. Single crystal spheres of San Carlos olivine, surrounded by a quasi-hydrostatic pressure medium of NaCl or Au, were partially transformed to wadsleyite at 1100°C and 16–17 GPa to determine the rate of growth as a function of time. Microstructural observations indicate that the relict olivine remains largely undeformed, while the wadsleyite rims deform by dislocation creep. Application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 GPa. A decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples [Kubo, T., Ohtani, E., Kato, T., Shinmei, T., Fujino, K., 1998a. Experimental investigation of the α–β transformation of San Carlos olivine single crystal. Phys. Chem. Miner. 26, 1–6; Kubo, T., Ohtani, E., Shinmei, T., Fujino, K., 1998b. Effects of water on the α–β transformation kinetics in San Carlos olivine. Science 281, 85–87]. Modeling these results gives values for the yield strength of wadsleyite between 2 and 5 GPa under various conditions. A strength difference of ∼1 GPa is indicated by experiments at 1030°C conducted under nominally dry (200 ppm H 2O) and water-added (500 ppm H 2O) conditions, supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite. The calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions, based on an extrapolation of the low-temperature plasticity flow-law for olivine. These results imply a strong increase in the strength of subducting slabs in the mantle transition zone, provided that the deformation mechanism is the same as in the experiments. Furthermore, inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high-pressure polymorphs in subducting slabs.