In this manuscript we review experimental constraints for the viscosity of the upper mantle. We first analyze experimental data to provide a critical review of flow law parameters for olivine aggregates and single crystals deformed in the diffusion creep and dislocation creep regimes under both wet and dry conditions. Using reasonable values for the physical state of the upper mantle, the viscosities predicted by extrapolation of the experimental flow laws compare well with independent estimates for the viscosity of the oceanic mantle, which is approximately 10 19 Pa s at a depth of ∼100 km. The viscosity of the mantle wedge of subduction zones could be even lower if the flux of water through it can result in olivine water contents greater than those estimated for the oceanic asthenosphere and promote the onset of melting. Calculations of the partitioning of water between hydrous melt and mantle peridotite suggest that the water content of the residue of arc melting is similar to that estimated for the asthenosphere. Thus, transport of water from the slab into the mantle wedge can continually replenish the water content of the upper mantle and facilitate the existence of a low viscosity asthenosphere.

Rheology of the Upper Mantle and the Mantle Wedge: A View from the Experimentalists

Seismological models of upper-mantle structure are providing new constraints on the physical and chemical properties that differentiate the lithosphere from the asthenosphere. A wide variety of studies are consistent with an oceanic lithosphere that corresponds to a dry, chemically depleted layer over a hydrated, fertile asthenosphere. At the lithosphere-asthenosphere boundary beneath oceans and many Phanerozoic continental regions, observed seismic velocity gradients require a contrast in mantle hydration, fertility, and/or melt content, perhaps in combination with a vertical gradient in velocity anisotropy. Beneath cratons, evidence is growing for a deeper—but globally ubiquitous—asthenosphere. Some studies conclude that the cratonic lithosphere-asthenosphere boundary is gradual enough to be matched by a purely thermal gradient, whereas others indicate a more rapid transition and a contrast in composition or perhaps melt content.

https://www.eoas.ubc.ca/~mjelline/453website/eosc453/E_prints/newforfall2010/Fisher_lithasthenbndryAREPS10.pdf

The Lithosphere- Asthenosphere Boundary

Terrestrial planet formation is believed to have concluded in our Solar System with about 10 million to 100 million years of giant impacts, where hundreds of Moon- to Mars-sized planetary embryos acquired random velocities through gravitational encounters and resonances with one another and with Jupiter This led to planet-crossing orbits and collisions that produced the four terrestrial planets, the Moon and asteroids But here we show that colliding planets do not simply merge, as is commonly assumed In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects Remnants of these 'hit-and-run' collisions are predicted to be common among remnant planet-forming populations, and thus to be relevant to asteroid formation and meteorite petrogenesis

Hit-and-run planetary collisions

This graduate textbook presents a comprehensive, unified treatment of the materials science of deformation as applied to solid Earth geophysics and geology. The deformation of Earth materials is presented in a systematic way covering elastic, anelastic and viscous deformation. Advanced discussions on relevant debates are also included to bring readers a full picture of science in this interdisciplinary area. This textbook is ideal for graduate courses on the rheology and dynamics of solid Earth, and includes review questions with solutions so readers can monitor their understanding of the material presented. It is also a much-needed reference for geoscientists in many fields including geology, geophysics, geochemistry, materials science, mineralogy and ceramics.

Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth

High-resolution three-dimensional thermomechanical simulations of Earth's lithosphere indicate that mantle plumes could have initiated the first subduction zones, but only in the hotter early Earth for old oceanic plates. Taras Gerya and co-authors use high-resolution three-dimensional numerical thermomechanical models to show that mantle plumes could have initiated the first subduction zones. They find that several factors combine to trigger self-sustained subduction: a strong, negatively buoyant oceanic lithosphere, focused magmatic weakening and thinning of lithosphere above the plume, and lubrication of the slab interface by hydrated crust. They also conclude that plume-induced subduction was only feasible in the hotter early Earth for old oceanic plates, as younger plates would have favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics. Scientific theories of how subduction and plate tectonics began on Earth—and what the tectonic structure of Earth was before this—remain enigmatic and contentious1. Understanding viable scenarios for the onset of subduction and plate tectonics2,3 is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics4. However, plume-induced subduction initiation5,6,7,8,9 could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Plate tectonics on the Earth triggered by plume-induced subduction initiation

Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions

[1] To provide a better understanding of rheological properties of mantle rocks under lithospheric conditions, we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures (∼4–9 GPa), relatively low temperatures (673 ≤ T ≤ 1273 K), and anhydrous conditions, using a deformation-DIA. Differential stress and sample displacement were monitored in situ using synchrotron X-ray diffraction and radiography, respectively. Experimental results were fit to the low-temperature plasticity flow law, . On the basis of this analysis, the low-temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a Peierls stress of σP = 5.9 ± 0.2 GPa, a zero-stress activation energy of Ek(0) = 320 ± 50 kJ mol−1, and AP = 1.4 × 10−7 s−1 MPa−2. Compared with published results for high-temperature creep of olivine, a transition from low-temperature plasticity to high-temperature creep occurs at ∼1300 K for a strain rate of ∼10−5 s−1. For a geological strain rate of 10−14 s−1, extrapolation of our low-temperature flow law to 873 K, the cutoff temperature for earthquakes in the mantle, yields a strength of ∼600 MPa. The low-temperature, high-stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within Earth's lithosphere.

http://web.mit.edu/wbdurham/www1/papers/95-low-T-MSH.pdf

Experimental constraints on the strength of the lithospheric mantle

http://web.mit.edu/wbdurham/www1/papers/90-Durham-2007-Jaoul-vol.pdf

New measurements of activation volume in olivine under anhydrous conditions

Knowledge of the rheological properties of mantle materials is critical in modeling the dynamics of the Earth. The high-temperature flow law of olivine defined at mantle conditions is especially important since the pressure dependence of rheology may affect our estimation of the strength of olivine in the Earth's interior. In this study, steady-state high-temperature (up to 1473 K) deformation experiments of polycrystalline olivine (average grain size ? 10 ?m) at pressure up to 9.6 GPa, were conducted using a Deformation-DIA (D-DIA) high-pressure apparatus and synchrotron X-ray radiation. The oxygen fugacity (fo2) during the runs was in-between the iron-wustite and the Ni/NiO buffers' fo2. The water content of the polycrystalline samples was generally about 150 to 200 wt. ppm but was as low as 35 wt ppm. Typically, 30 % strain was generated during the uniaxial compression. Sample lengths during the deformation process as well as the differential stresses were monitored in situ by X-ray radiography and diffraction, respectively. The strain rate was derived with an accuracy of 10?6 s?1. Differential stress was measured at constant strain rate (?10?5 s?1) using a multi-element solid-state detector combined with a conical slit. Recovered specimens were investigated by optical and transmission electron microscopy (TEM).more » TEM shows that dislocation glide was the dominant deformation mechanism throughout the experiment. Evidence of dislocation climb and cross-slip as active mechanisms are also reported. Deformation data show little or no dependence of the dislocation creep flow with pressure, yielding to an activation volume V* of 0 {+-} 5 cm3/mol. These new data are consistent with the high-temperature rheological laws at lower pressures, as reported previously.« less

/pdf/deformation-of-olivine-at-mantle-pressure-using-the-d-dia-2h6fsi6kaz.pdf

Deformation of olivine at mantle pressure using the D-DIA

[1] We performed triaxial compressive creep tests to study water weakening of clinopyroxenite in the diffusion creep regime. All tests were carried out on fine-grained samples at confining pressures between 100 and 300 MPa at temperatures of 1321 to 1421 K for water-saturated and 1398 to 1508 K for anhydrous conditions. Samples were prepared by hot-pressing ground powder of Sleaford Bay clinopyroxenite. Water was added to the sample during the run by dehydration of a talc sleeve. Water-saturated aggregates crept ∼30 times faster than aggregates under anhydrous conditions at a temperature of 1400 K and a pressure of 300 MPa. The stress exponent in both cases was ∼1, and the activation energies under water-saturated and anhydrous conditions were 340 ± 30 and 760 ± 20 kJ/mol, respectively. Under water-saturated conditions creep rate increased with increasing water fugacity to the power 1.4 ± 0.2 with an activation volume for creep of 14 ± 6 × 10−6 m3/mol. Comparison of the water weakening in aggregates of clinopyroxene, olivine, and anorthite deformed in the diffusion creep regime indicates that the creep strength of clinopyroxene has a stronger dependence on the water fugacity than do the creep strengths of olivine and anorthite. In water-rich environments, clinopyroxene will be the weakest component in polycrystalline rocks composed of these phases.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004JB003414

S. Mei

Papers

Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions

Experimental constraints on the strength of the lithospheric mantle

New measurements of activation volume in olivine under anhydrous conditions

Deformation of olivine at mantle pressure using the D-DIA

Water weakening of clinopyroxenite in diffusion creep