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Pressure-induced slip-system transition in forsterite: Single-crystal rheological properties at mantle pressure and temperature

¹ Laboratoire de Structure et Propriétés de l’Etat Solide, Université des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France
² Mineral Physics Institute and Department of Geosciences, Stony Brook University, Stony Brook, New York 11733-2100, U.S.A.

Correspondence: ^* E-mail: Paul.Raterron{at}univ-lille1.fr

Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg₂SiO₄ olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373–1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress () and specimen strain rates () were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the "puzzling" seismic-anisotropy anomalies commonly observed in subduction zones.

Key Words: Upper mantle • olivine • high pressure • slip systems • rheological law • seismic anisotropy • subduction zone