Grain-size-sensitive creep of olivine induced by oxidation of olivine in the Earth's deep upper mantle: Implications for weakening of the subduction interface

Creep behavior of olivine has been studied at pressures of 9-16 GPa and temperatures of 960-1450 K, which is equivalent to the conditions in/around the slab subducted into the deep upper mantle, using a combination of synchrotron radiation and a deformation-DIA apparatus. At pressures above 12 GPa, the creep strength of olivine was primarily controlled by Peierls creep, and nucleation of wadsleyite caused softening by a contribution of grain-size-sensitive creep to deformation. Similarly, oxidation of deforming olivine caused a significant softening due to another grain-size-sensitive creep presumed to be dislocation-accommodated grain boundary sliding (dislGBS) of olivine with a low activation enthalpy (260 kJ/mol) at pressures of 9-12 GPa. Extrapolation of the obtained flow law of dislGBS to mantle conditions predicts a formation of a weak layer along the slabasthenosphere interface in the deep upper mantle. The viscosity contrast between the weak layer and the surrounding asthenosphere is estimated to be up to 3 orders of magnitude, resulting in a partial slab-asthenosphere decoupling in the deeper part of the subduction zones. The weak layer would allow for the occurrence of trench parallel flow above/beneath the slab.