THE EFFECT OF OXYGEN FUGACITY ON LARGE-STRAIN DEFORMATION AND RECRYSTALLIZATION OF OLIVINE

The rheological properties of olivine, the most abundant phase in the Earth upper mantle, are likely to govern mantle dynamics. In particular, the development of crystallographic preferred orientations (CPOs) in olivine during deformation and recrystallization gives rise to seismic anisotropy in the mantle. Recent experimental studies have shown that different CPOs develop at large strains for dry olivine [1-2], wet olivine [3] and olivine with melt [4]. In this study, we have focused on the effect of oxygen fugacity on the deformation and recrystallization of dry olivine. Large-strain deformation experiments were performed on hot-pressed polycrystalline olivine aggregates under conditions (temperature, pressure, strain rate and grain size) favoring dislocation creep. Cylindrical samples were deformed in torsion in a gasmedium apparatus at 1200-1300oC, a confining pressure of 300 MPa and constant twist rates corresponding to constant shear strain rates of 5·10 to 3·10 s at the samples’ outer diameters. The oxygen fugacity was controlled by placing the samples in an iron or a nickel sleeve, yielding oxygen fugacities near the ironiron wustite (Fe/FeO) or the nickel-nickel oxide (Ni/NiO) phase boundaries, respectively. Simple shear microstructures were observed by light microscopy and CPOs were analysed by orientation mapping using electron backscatter diffraction. The strength of dry polycrystalline olivine is weakly dependent on oxygen fugacity. At low strain, peak stress values (at both Ni/NiO and Fe/FeO) are very consistent with previous rheological data on dislocation creep of olivine obtained in coaxial experiments under dry conditions [5-6]. Microstructures show evidence for dislocation creep and recovery in the form of deformation lamellae and subgrains. A typical deformation texture with [100] axes oblique to the shear direction is present at shear strains of ~ 0.5. After the initial peak stress, significant weakening occurs with increasing shear strain (γ ~ 0.3-3). Weakening becomes less important at larger strains and stress seems to approach a steady value at shear strains of γ ~ 6-8. The total weakening at large strains is approximately 20-40%. The determination of stress exponents of about 3-3.5 at various finite strains and of strong CPOs in all deformed samples suggest that dislocation creep processes remain dominant throughout the experiments. Microstructural observations of samples deformed at Fe/FeO and 1200-1250oC indicate continuous dynamic recrystallization mainly by subgrain rotation and some grain boundary migration. Recrystallization is accompanied by important grain refinement and leads to strongly foliated microstructures. With increasing temperature, grain boundaries become much more mobile. At 1300oC recrystallization occurs mainly by grain boundary migration, producing microstructures with a less pronounced foliation. At all temperatures, a strong recrystallization CPO in highly deformed samples is characterized by alignment of [100] in the shear direction and girdles of [010] and [001] approximately normal to that direction. This texture is interpreted as due to dislocation glide on several {0kl}[100] slip systems, including activation of (010)[100]. At Ni/NiO, recrystallization is more efficient and seems to be dominated by grain boundary migration. Straight and parallel grain boundaries are often aligned across several grains, suggesting some contribution of grain boundary sliding at large strains, possibly assisted by diffusion processes. A strong CPO with [100] aligned in the shear direction and a [010] point maximum perpendicular to the shear plane suggest dislocation creep primarily on the (010)[100] slip system. Our study shows that during large strain deformation and recrystallization of dry olivine in the dislocation creep regime, more oxidizing conditions promote diffusion processes and grain boundary sliding. Although the presence of a strong CPO suggests that dislocation creep remains active at large strains, grain boundary sliding processes may accommodate some of the strain leading to a de-emphasis of stronger slip systems in olivine in favor of dominant slip on the single (010)[100] slip system. References: [1] Zhang S. and Karato S. (1995) Nature 375, 774-777. [2] Bystricky M. et al. (2000) Science 290, 1564-1567. [3] Jung H. and Karato S. (2001) Science 293, 1460-1463. [4] Holtzman B.K. et al. (2003) Science 301, 1227-1230. [5] Karato S. et al. (1986) J. Geophys. Res. 91, 8151-8176. [6] Mei S. and Kohlstedt D.L. (2000) J. Geophys. Res. 105, 2147121481. Oxygen in the Terrestrial Planets (2004) 3059.pdf