DISLOCATION CREEP OF MAGNESIOWUSTITE (MG0.8FE0.2O)

Abstract

Magnesiowustite is likely the second most abundant and probably weakest lower-mantle phase. Its deformation properties will therefore influence the rheological and dynamic behaviour of this region. In this study magnesiowustite aggregates with a composition of Mg0.8Fe0.2O have been deformed under dry conditions in axial compression at a confining pressure of 300 MPa and temperatures from 1200 to 1400 K (0.41<Tm<0.47) with the oxygen fugacity controlled by the iron jacket. In all cases deformation occurred by power law creep, with a stress exponent of n=4.0+/-0.2 and an activation energy for creep of Q=327+/-5 kJ/mol. Deformation in the dislocation creep regime is corroborated by the development of a crystallographic preferred orientation (texture), which is consistent with simultaneous slip on the {100} and {110} slip planes in the <110> direction. Microstructures show the rapid formation of a subgrain boundary network indicating that mobility of dislocations is high and the recovery rate is fast.