title: Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle.
abstract: Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan.
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Ohuchi
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan. ; Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Takaaki
authors.initials: T
authors.lastname: Kawazoe
authors.affiliation: Japan Synchrotron Radiation Institute, Sayo, Hyogo 679-5198, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Higo
authors.affiliation: Japan Synchrotron Radiation Institute, Sayo, Hyogo 679-5198, Japan. ; Research Center for Neutron Science and Technology, Ibaraki 319-1906, Japan.
authors.forename: Ken-Ichi
authors.initials: K
authors.lastname: Funakoshi
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
authors.forename: Akio
authors.initials: A
authors.lastname: Suzuki
authors.affiliation: Photon Factory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan.
authors.forename: Takumi
authors.initials: T
authors.lastname: Kikegawa
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan. ; Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan.
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Irifune
keywords: dislocation-accommodated grain boundary sliding; olivine; pressure; upper mantle; viscosity; water