Abstract
Grain growth of the dissociation product (MnTiO3+MnO), as an analogue assemblage of (Mg, Fe) SiO3 perovskite and (Mg, Fe) O magnesiowüstite for the lower mantle, was studied at pressures of 0.9 and 1.6GPa and at temperatures of 1000° to 1320°C, for the time duration ranging from 0.125 to 256 hours. By fitting the grain growth data to an equation Gn=K0·t·exp{-(Q*+PV*)/RT}, the grain growth exponent n, the activation energy Q*, and the activation volume V* were determined to be 3.55±0.46, 560±70kJ/mol and 70±10cm3/mol, respectively. These parameters indicate that the mutual intergrowth texture of the dissociation product suppresses grain growth substantially by a pinning effect. The present results thus suggest that a fine-grained mixture of the perovskite plus magnesiowüstite will persist to exhibit superplastic behavior in the slab penetrated into the lower mantle. If the grain growth rate of (Mg, Fe) SiO3+(Mg, Fe) O mixture is similar to that of MnTiO3+MnO, the superplastic slab may cause a dramatic change of the shape and geometry of the subducting plate beneath the 660km discontinuity.
