HR: 11:05h
AN: MR52A-04 INVITED    [Abstracts]
TI: Pressure Induced Slip-System Transition in Olivine: Laboratory Experiments and Geophysical Implications
AU: * Raterron, P
EM: Paul.Raterron@univ-lille1.fr
AF: LSPES, CNRS, Bât C6, Université des Sciences et Technologies de Lille, Villeneuve d'Ascq, F-59655, France
AU: Chen, J
EM: Jiuhua.Chen@fiu.edu
AF: Center for the Study of Matter at Extreme Conditions, Florida International University, VH- 140, University Park, Miami, FL 33199, United States
AB: Olivine deforms by dislocation creep in the shallow upper mantle which results in seismic-velocity anisotropies allowing characterization of convective flows in this region. Seismic anisotropy is interpreted from lattice preferred orientations produced experimentally in olivine, which depends on the dominant dislocation slip systems. At pressure P<3 GPa, mantle temperature (T) and in dry conditions, olivine [100] dislocation slip dominates the less active [001] slip. This tends to align crystal fast velocity [100] axis with the principal shear direction. Yet recent deformation experiments carried out at P> 3 GPa (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Miner., 92, 1436), as well as a theoretical study based on first-principle calculations (Durinck et al., 2005, PCM, 32, 646), show that olivine [001] slip may be dominant in the (P,T) range of the deep upper mantle. This would promote a shear-parallel slow-velocity [001] axis which may explain the seismic-velocity attenuation with depth observed in this region (Mainprice et al., 2005, Nature, 433, 731). The [100]-slip/[001]- slip transition has previously been attributed to the presence of water (e.g., Jung et al., 2006, Tectonophysics, 421, 1). A pressure effect with similar consequences would have strong implications on our present understanding of mantle flow. In order to determine the effect of P on both [100] and [001] slip activities, deformation experiments were carried out on dry oriented forsterite crystals at P>6 GPa and T=1400°C, using the Deformation-DIA apparatus at the X17B2 beamline of the NSLS (Upton, NY). Constant applied stress σ <300 MPa and specimen strain rates were monitored in situ using time-resolved x-ray diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of run products reveals that dislocation creep was responsible for sample deformation. The collected data were then compared with those previously obtained by Darot and Gueguen (1981, JGR, 86, 6219) on identical forsterite crystals deformed in comparable T and σ conditions, but at room P. A slip transition with increasing P, from dominant [100]-slip to dominant [001]-slip, will be documented. The extrapolation of crystals rheological laws to upper-mantle conditions, which shows that [001]-slip activity should be dominant in deep upper mantle, as well as the corresponding implication on upper mantle viscosity will also be presented.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 3621 Mantle processes (1038)
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting