Since hydrogen plays an important role in dynamic processes in Earth's mantle, we conducted torsion experiments to shear strains of 0.6 to 5.0 on Fe-bearing olivine aggregates [(Mg0.5Fe0.5)2SiO4: Fo50] under hydrous conditions at T=1200°C and P=300MPa. We deformed samples to high enough strains that a steady state microstructures were achieved, which allowed us to investigate the evolution of both the rheological and microstructural properties. The stress exponent of n≈5.0 and the grain size exponent of p≈0 determined by fitting the strain rate, stress, and grain size data indicate that our samples deformed by dislocation creep. Fourier transform infrared spectroscopy measurements on embedded olivine single crystals demonstrated that our samples were saturated with hydrogen during the deformation experiments. The lattice preferred orientation (LPO) of olivine changes as a function of strain due to competition among three slip systems: (010), (100), and (001). Observed strain weakening can be attributed to geometrical softening associated with development of LPO, which reduces the stress by ~1/3 from its peak value in constant strain rate experiments. The geometrical softening coefficient determined in this study is an important constraint for modeling and understanding dynamical processes in the upper mantle under hydrous conditions.
Bibliographical noteFunding Information:
We thank our lab members for helpful discussions and technical assistance. We thank T. Mizukami and H. Stünitz for their discussions, and L. Hansen for providing the original EBSD data in Hansen et al. [2012b]. The manuscript was significantly improved by insightful comments from J. Renner, an anonymous reviewer, and the associate editor. This study was supported by a JSPS Research Fellowship for Young Scientists (26–4879) to M.T., a NSF grant (EAR-1214876) and a NASA (NNX15AL53G) to D.L.K., and a NSF grant (EAR-1345060) and a NASA grant (NNX11AF58G) to M.E.Z. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which is a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. Electron microprobe analyses were carried out at the Electron Microprobe Laboratory, Department of Earth Sciences, University of Minnesota-Twin Cities. Data used in this paper are available by request from the corresponding author.
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- high-strain deformation