Seismological and experimental constraints on metastable phase transformations and rheology of the Mariana slab

A wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the Mariana slab. Based on these seismic observations and numerical calculation of thermal structures of the slab, we suggest that the phase transformations from olivine to ringwoodite, and from ringwoodite to perovskite + ferropericlase (the post-spinel transformation) occur at the depth of 630 km and 550 degrees C, and at 690 km and 700 degrees C, respectively, in the central coldest part of the Mariana slab. Combining this information with recent experimental kinetic data, we constrain the details of non-equilibrium phase transformations and rheology of the Mariana slab. The observed depth of the metastable olivine inside the Mariana slab can be explained by growth-controlled olivine-ringwoodite transformation under relatively dry condition such as the water content of about 150 wt. ppm H2O. On the other hand, nucleation process controls the depth of the post-spinel transformation in the Mariana slab, and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative Clapeyron slope for the transformation. When the overpressure for the nucleation is 0.5 GPa, the observed depressions can be explained by the Clapeyron slope of -0.7 MPa/K. Grain-size evolution and viscosity structures in the Mariana slab are estimated based on these metastable phase transformations. The slab weakening due to the grain-size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km, whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post-spinel transformation. These viscosity structures are consistent with the behaviors of the Mariana slab estimated from seismic tomography: the slab vertically descends to the bottom of the transition zone, and substantially deforms and thickens at the top of the lower mantle. (C) 2009 Elsevier B.V. All rights reserved.