Seismic properties of the Earth's upper mantle in various geodynamic settings derived from experiments on olivine-orthopyroxene aggregates deformed up to 8 GPa and 1500 °C

We present here HP and HT deformation experiments on olivine-orthopyroxene aggregates and discuss the effect of pressure, temperature and composition on the development of olivine CPO and the resulting seismic properties. The experiments were prepared, synthesized, deformed and analysed at the Bayerisches Geoinstitut (University of Bayreuth,Germany). The sample consisted of hot-pressed powders composed of an olivine-orthopyroxene mix (87.5-12.5, 75-25 and 50-50 %) that were then placed in a simple-shear geometry assembly and deformed in a multi-anvil press employing 6 independently acting hydraulic rams at a constant strain-rate of 10-4 s-1 and 3 GPa-1300°C, 5 GPa-1400°C and 8 GPa-1500°C. The achieved shear strain ranged between 1 and 2. The sample were then analysed under SEM-EBSD. At 3 GPa, the observed CPO are similar to those predicted in previous studies, while the one developed at 5 and 8 GPa varies according to the orthopyroxene content: 1) Experiments with 12.5% orthopyroxene display annealing textures and random CPO; 2) the ones with 25% orthopyroxene display B-type olivine CPO consistent with high pressure dislocation creep deformation experiments; and 3) the experiments with 50% orthopyroxenes display A-type olivine CPO consistent with olivine-pyroxene aggregates deformed at high temperature in diffusion creep regime. The resulting seismic properties could therefore explain the seismic anisotropy observations in the mid and lower part of the Earth's upper mantle in different geodynamic contexts. For example, the mantle wedge above subduction which has pyroxene content corresponding to our 25% orthopyroxene experiments and often display the olivine fast-axis normal to the expected mantle flow. On the other hand, a more pristine intraplate mantle most often predict olivine fast-axis parallel to the mantle flow and would have an olivine-pyroxene ratio close to our experiment with 50% orthopyroxene.