Rheological properties of the lower mantle have strong influence on the dynamics and evolution of Earth. By using the improved methods of quantitative deformation experiments at high pressures and temperatures, we deformed a mixture of bridgmanite and magnesiowüstite under the shallow lower mantle conditions. We conducted experiments up to about 100% strain at a strain rate of about 3 × 10(-5) second(-1). We found that bridgmanite is substantially stronger than magnesiowüstite and that magnesiowüstite largely accommodates the strain. Our results suggest that strain weakening and resultant shear localization likely occur in the lower mantle. This would explain the preservation of long-lived geochemical reservoirs and the lack of seismic anisotropy in the majority of the lower mantle except the boundary layers.

International audienceThe presence of a small fraction of basaltic melt is a potential explanation for mantle electrical conductivity anomalies detected near the top of the oceanic asthenosphere. The interpretation of magnetotelluric profiles in terms of the nature and proportion of melt, however, relies on mathematical models that have not been experimentally tested at realistically low melt fractions (<0.01). In order to address this, we have performed in situ electrical conductivity measurements on partially molten olivine aggregates. The obtained data suggest that the bulk conductivity follows the conventional Archie's law with the melt fraction exponents of 0.75 and 1.37 at melt fractions greater and smaller than 0.5 vol.% respectively at 1350 °C. Our results imply multiple conducting phases in melt-bearing olivine aggregate and a connectedness threshold at ∼0.5 vol.% of melt. The model predicts that the conductive oceanic upper asthenosphere contains 0.5 to 1 vol.% of melt, which is consistent with the durable presence of melt at depths over millions years while the oceanic plates spread apart at the mid-ocean ridge. Beneath ridges a minimum permeability may allow mid-ocean ridge basalts to rise out of the mantle, where our model indicates that melt is present in proportions of up to 4 vol.%

Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle.

For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine-orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere.rheology | two-phase | Zener pinning | deformation mechanisms | electron probe microanalysis P late tectonic style of convection is characterized by regions of localized deformation, such as subduction zones, and the origin of these regions needs to be investigated to understand why plate tectonics operates on Earth. Localized deformation at plate boundaries involves not only brittle fracture at shallow portions but also localized deformation in the ductile shear zones (1-4). Although the basic processes of localized deformation in the brittle regime are well understood (5, 6), mechanisms of localized deformation in the ductile regime remain elusive. The strength profile of the lithosphere in a standard model (7) predicts stresses that are much higher than the critical strength below which plate tectonic style of convection would occur (8,9).Among the possible mechanisms of shear localization (10), grain size reduction is most often recognized in ductile shear zones and has been given particular attention in previous studies (11)(12)(13)(14)(15). Grain size reduction can occur during high-temperature deformation by dynamic recrystallization during dislocation creep (16,17). In many cases, small grains are formed along preexisting grain boundaries (18). If the degree of grain-size reduction is large enough and these fine-grained regions are connected, then regions of small grain size will be deformed by grain-size-sensitive creep processes-diffusion creep or dislocation creep accommodated by grain-boundary sliding (Dis-GBS) introduced by Hirth and Kohlstedt (1995) (19). Deformation in both regimes results in the weakening of a rock, although less so in the Dis-GBS regime. In addition, for this weakening to cause substantial shear localization, the ...