On the possibility of transformational superplasticity in the Earth's mantle

It is well known that metals and some ceramics deform superplastically during polymorphic phase transitions, but this phenomenon has not been considered as a possible deformation mechanism in the earth's mantle. Although phase transitions in the mantle can be expected to move significant distances in response to changing surface loads, the time scale of this motion (of the order of 105 yr) is too long to allow simple applications of laboratory results in the absence of a well understood mechanism. To examine possible mechanisms, an apparatus capable of applying a shear traction parallel to a polymorphic phase boundary has been developed. Preliminary results with the β-α transformation in CsCl show that macroscopic deformation was localized at the phase boundary and that microscopic deformation appears to be restricted to the high-temperature phase, in agreement with the mechanism proposed by Greenwood and Johnson. Seismic evidence suggesting that flow is localized at the 400- and 650-km discontinuities in the earth's mantle is reviewed.