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

Abstract

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 slab-asthenosphere 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.