Pressure and temperature dependence of dislocation mobility in the [100](010) and [001](010) slip systems in olivine

Abstract:

Seismic anisotropy in the upper mantle is considered to be caused by crystallographic preferred orientation of olivine. The seismic anisotropy rapidly decreases below 200 km depth, which is attributed to the fabric transition from A-type to B-type with pressure demonstrated by deformation experiments. However, stress and strain-rate conditions in the deformation experiments are by orders of magnitude higher than in the upper mantle, which may cause misinterpretation. Hence the observed fabric transition has to be examined by an independent technique.

A- and B-type fabrics should be produced by the dominant slip systems of [100](010) (a-slip) and [001](010) (c-slip), respectively. Therefore, the fabric transition may indicate a larger decrease in dislocation mobility with pressure in a-slip system than in c-slip system. To examine this hypothesis, we have determined mobility of [100](010) edge (a-dislocation) and [001](010) screw (c-dislocation) dislocations at pressures of 0 to 12 GPa and temperatures of 1470 to 1770 K by the dislocation recovery technique, in which dislocation mobility is determined from dislocation annihilation rates under quasi-hydrostatic conditions. The a- and c-dislocations were produced in [100](010) and [001](010) simple shear geometries. TEM observations showed that the dislocations produced in these geometries are by 79-90 % a- and c-dislocations, respectively. Dislocation density was measured on (001) plane by the oxidation decoration technique.

The followings are a summary of the experimental results. (1) The mobility of a-dislocations is almost identical to or up to 0.5 orders of magnitude lower than that of c-dislocations at ambient pressure. (2) The activation energies of both dislocations are comparable, 400(100) kJ/mol. (3) The activation volumes of both dislocations are also comparable, about 3.5(9) cm3/mol.

The comparable activation energies and volumes suggest that a- and c-dislocations move in the same mechanisms. Olivine creep in a- and c-slips will be driven by movement of a- and c-dislocations in the [100] direction. The fabric transition of A-type to B-type simply by pressure and/or temperature is unlikely. The rapid decrease in seismic anisotropy below 200 km will be due to decrease in flow rate in this depth.