The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.
[1] We use forward models based on recent high-pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite, the dominant mineral in the upper transition zone. These models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition (60% wadsleyite, 40% garnet) at transition zone conditions: $2% for P and $1% for S waves for a shear strain of 1. Both P and S wave anisotropy patterns show an orthorhombic symmetry. P waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane. S wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction. Horizontal shearing results therefore in higher velocities for horizontally propagating P waves (PH ) and horizontally polarized S waves (SH ), as well as in weak azimuthal variation of SV and SH velocities. On the other hand, vertical flow leads to higher velocities for vertically propagating P waves (PV ) and vertically polarized S waves (SV) and to a weak azimuthal variation of SV velocity but to a roughly constant SH velocity. Analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone, in agreement with predictions of geodynamical models that explicitly introduce phase transitions.
We have studied the structural stability of NaBH(4) under pressures up to 17 GPa and temperatures up to 673 K in a diamond anvil cell and formed an extended high P-T phase diagram using combined synchrotron x-ray diffraction and Raman spectroscopy. Even though few reports on phase diagram of NaBH(4) are found in current literature, up to our knowledge this is the first experimental work using diamond anvil cell in a wide pressure/temperature range. Bulk modulus, its temperature dependence, and thermal expansion coefficient for the ambient cubic phase of NaBH(4) are found to be 18.76(1) GPa, -0.0131 GPa K(-1), and 12.5x10(-5)+23.2x10(-8) T/K, respectively. We have also carried out Raman spectroscopic studies at room temperature up to 30 GPa to reinvestigate the phase transitions observed for NaBH(4). A comparative symmetry analysis also has been carried out for different phases of NaBH(4).
Mechanical data have been obtained on silicon single crystal under hydrostatic pressure between room temperature and 450 °C. This temperature domain corresponds to the regime where perfect dislocations control plastic deformation. This was achieved using a D-DIA apparatus in the synchrotron beam of NSLS. Stress strain curves were deduced from X Ray diffraction and sample imaging under 5 GPa and a strain rate of 2.5 10. Yield stresses as a function of temperature exhibit different temperature dependence when deformation is controlled by perfect dislocations.1 Introduction Recent studies of Si single crystals plastic deformation have shown that perfect dislocations are nucleated and control the deformation in very high stress conditions [1,2]. Those deformation microstructures are usually obtained in harsh conditions where the applied stress tensor possesses a hydrostatic component which prevents a brittle behaviour of the material. In these conditions it is usually difficult to derive the applied shear stress during the test. Applied shear stress can be obtained post mortem by TEM investigations. The stress information can then be extracted from the dislocation bends curvature joining rectilinear segments lying in two contiguous Peierls valleys. Those bends are very small which makes difficult measuring the local stress. Nevertheless it was shown that perfect shuffle dislocation configurations are found consistent with local applied shear stresses of the order of 1.5 GPa [3]. In order to get insights in the deformation mechanisms at high stress, it is of importance to be able to get the yield stress as a function of temperature in this range of stress, in deformation conditions which could be comparable to those used in the standard deformation tests performed previously at high temperature. This requires conducting experiments under hydrostatic pressure being able to (i) apply independently the hydrostatic component of the stress tensor and a shear stress, (ii) record during the test the stress tensor and the sample deformation. Getting high stress deformation regime in Si needs to use high hydrostatic pressures of the order of 5 GPa [1,2]. Such pressure range can be only obtained using a solid confining medium whose main drawback is the generation of friction stresses and the impossibility to access stress tensor data by reliable sensors within the confining medium. Such drawbacks can be by-passed by measuring stresses using the shift of X-rays diffraction peaks of the sample or a sensor medium during the deformation experiment. Measuring this way the change in lattice parameter in various crystallographic directions, the stress tensor can be deduced through equation of state and elastic constants of the investigated diffracting medium. Such diffraction data have to be obtained in-situ during deformation under pressure; obviously this requires the use of high intensity X-rays beam as well as an experimental set up providing some transparency to X-rays.
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