Fabric Development in Sheared Mantle Rocks: The Source of the ‘a-c’ Switch
Abstract:Researchers often invoke variations in water content, stress state, and melt distribution to account for the observed variety of olivine crystallographic preferred orientations (CPOs). Since the average direction of  axes directly affects seismic anisotropy, there is potential to link observed anisotropy to compositional and thermo-mechanical conditions. It is well established that the (010) is the weakest slip system, and therefore thought to control CPOs, in dry olivine at P < 2 GPa. However, CPOs formed in experiments on olivine plus mid-ocean ridge basalt (MORB) reveal a fabric in which  axes form weak point maxima parallel to the shear direction, and  axes form strong point maxima perpendicular to the shear plane, indicative of (010) as the weak slip system.
To investigate the mechanisms that cause this change in CPO, samples fabricated from fine-grained San Carlos olivine plus MORB were deformed in torsion at T = 1200°C and P = 300 MPa. Samples with starting melt fractions of 0.01, 0.10 and 0.25 were sheared to a maximum strain of γ ≈ 13.
We investigate three hypotheses. 1) The easiest slip direction changes from  to  in partially molten rocks. However, no microstructural evidence for such a change has been found. 2) With the presence of a melt phase, shape preferred orientations (SPOs) play an important role in fabric development. We test this hypothesis by examining the relationship between SPOs and CPOs as a function of strain and melt content. 3) Anisotropy in the melt distribution leads to anisotropy in grain-boundary sliding, thus preferentially favoring grain rotations necessary to produce the observed fabric. We test this hypothesis by detailed analysis of misorientations between neighboring grains. Our results will provide a crucial link between seismic anisotropy and grain-scale deformation processes.