Microstructural development in olivine aggregates during dislocation creep under hydrous conditions

Wednesday, 17 December 2014
Miki Tasaka, Mark E Zimmerman and David L Kohlstedt, University of Minnesota Twin Cities, Minneapolis, MN, United States
Since hydrogen plays an important role in dynamic processes in the mantle, we conducted high-strain torsion experiments on aggregates of Fe-bearing olivine [(Mg,Fe)2SiO4; Fo50] under hydrous condition. Olivine with a composition of Fo50 was used because of its enhanced grain growth kinetics and low strength relative to Fo90. Two pieces of an oriented San Carlos olivine crystal were embedded in the aggregates to monitor water fugacity both before and after deformation. We deformed samples to high enough strain, γ ≈ 4, to achieve a steady-state microstructure.

A non-linear, least-squares fit to the stress versus strain rate data yielded a stress exponent, n ≈ 3.5, indicative of deformation involving dislocations. The water content determined from Fourier transform infrared (FTIR) spectroscopy analyses of the single crystals demonstrate that the samples are water saturated after deformation. Fabric analyses of the polycrystalline olivine samples, determined using electron backscatter diffraction (EBSD), indicate that the strength of the lattice preferred orientation (LPO) increases with increasing strain. Further, at low strain, γ < 2, two slip systems contribute to deformation: (i) [100] axes parallel to the shear direction with the [001] axes normal to the shear plane, indicative of the (001)[100] slip system, plus (ii) [001] axes are parallel to the shear direction with the [100] axes normal to the shear plane, suggestive of the (100)[001] slip system. With increasing strain, the LPO evolves until (100)[001] becomes the dominant slip system at γ > 3. We interpret the observed fabric evolution to represent the competition between the two easiest slip systems in olivine, (100)[001] and (001)[100]. The evolution of fabric can be applied to investigations of upper mantle seismic anisotropy especially in a mantle wedge or in a shear zone, locations in which hydrous conditions prevail.