Shear deformation of bridgmanite and magnesiowüstite aggregates at lower mantle conditions

Deformation of two-phase aggregates with in situ X-ray tomography in rotating Paris-Edinburgh cell at GPa pressures and high temperature

High-pressure (>1 GPa) torsion apparatus can be coupled with in situ X-ray tomography (XRT) to study microstructures in materials associated with large shear strains. Here, deformation experiments were carried out on multi-phase aggregates at ∼3-5 GPa and ∼300-500°C, using a rotational tomography Paris-Edinburgh press (RoToPEc) with in situ absorption contrast XRT on the PSICHE beamline at Synchrotron SOLEIL. The actual shear strain reached in the samples was quantified with respect to the anvil twisting angles, which is γ ≤ 1 at 90° anvil twist and reaches γ ≃ 5 at 225° anvil twist. 2D and 3D quantifications based on XRT that can be used to study in situ the deformation microfabrics of two-phase aggregates at high shear strain are explored. The current limitations for investigation in real time of deformation microstructures using coupled synchrotron XRT with the RoToPEc are outlined.

Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800-1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction3, accelerates plume ascent4 and inhibits chemical mixing5. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection5-7. The high Mg:Si ratio of the upper mantle relative to chondrites8, the anomalous 142Nd:144Nd, 182W:184W and 3He:4He isotopic ratios in hot-spot magmas9,10, the plume deflection4 and slab stagnation in the mid-mantle3 as well as the sparse observations of seismic anisotropy11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.

Periclase deforms more slowly than bridgmanite under mantle conditions

Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO₃ and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite¹. Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions^2,3. However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.

Viscosity of bridgmanite determined by in situ stress and strain measurements in uniaxial deformation experiments

To understand mantle dynamics, it is important to determine the rheological properties of bridgmanite, the dominant mineral in Earth's mantle. Nevertheless, experimental data on the viscosity of bridgmanite are quite limited due to experimental difficulties. Here, we report viscosity and deformation mechanism maps of bridgmanite at the uppermost lower mantle conditions obtained through in situ stress-strain measurements of bridgmanite using deformation apparatuses with the Kawai-type cell. Bridgmanite would be the hardest among mantle constituent minerals even under nominally dry conditions in the dislocation creep region, consistent with the observation that the lower mantle is the hardest layer. Deformation mechanism maps of bridgmanite indicate that grain size of bridgmanite and stress conditions at top of the lower mantle would be several millimeters and ~10⁵ Pa to realize viscosity of 10^21-22 Pa·s, respectively. This grain size of bridgmanite suggests that the main part of the lower mantle is isolated from the convecting mantle as primordial reservoirs.

Weak cubic CaSiO3 perovskite in the Earth's mantle

Cubic CaSiO₃ perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet^1,2,28. However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO₃ perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO₃, previously investigated at room temperature^3,4, we find that cubic CaSiO₃ perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO₃ perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO₃ perovskite governs the dynamics of subducting slabs. Weak CaSiO₃ perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO₃ perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle^5,6, or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific^7-9.

Extrinsic Anisotropy of Two-Phase Newtonian Aggregates: Fabric Characterization and Parameterization

Rocks of the Earth's crust and mantle commonly consist of different minerals with contrasting mechanical properties. During progressive, high-temperature (ductile) deformation, these rocks develop extrinsic mechanical anisotropy linked to strain partitioning between different minerals, amount of accumulated strain, and bulk strain geometry. Extrinsic anisotropy plays an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain- and rock-scale fabrics causing this anisotropy cannot be directly simulated in large-scale numerical simulations. For two-phase aggregates-a good rheological approximation of most Earth's rocks-we propose a method to indirectly approximate the extrinsic viscous anisotropy by combining (a) 3D mechanical models of rock fabrics, and (b) analytical effective medium theories. Our results confirm that weak inclusions induce substantial weakening by forming a network of weak thin layers with limited lateral connectivity. Consequently, even when the inclusion phase is extremely weak, structural weakening is not larger than 30-60%, less than in previous estimates. On the other hand, the presence of strong inclusions does not have a profound impact on the effective strength of the aggregate, and lineated fabrics only develop at relatively low viscosity contrasts. When rigid inclusions become clogged, however, the aggregate viscosity can increase over the theoretical upper bound. We show that the modeled grain-scale fabrics can be parameterized as a function of the bulk deformation and material phase properties and combined with analytical solutions to approximate the anisotropic viscous tensor.

Exploring microstructures in lower mantle mineral assemblages with synchrotron x-rays

Understanding dynamics across phase transformations and the spatial distribution of minerals in the lower mantle is crucial for a comprehensive model of the evolution of the Earth's interior. Using the multigrain crystallography technique (MGC) with synchrotron x-rays at pressures of 30 GPa in a laser-heated diamond anvil cell to study the formation of bridgmanite [(Mg,Fe)SiO₃] and ferropericlase [(Mg,Fe)O], we report an interconnected network of a smaller grained ferropericlase, a configuration that has been implicated in slab stagnation and plume deflection in the upper part of the lower mantle. Furthermore, we isolated individual crystal orientations with grain-scale resolution, provide estimates on stress evolutions on the grain scale, and report {110} twinning in an iron-depleted bridgmanite, a mechanism that appears to aid stress relaxation during grain growth and likely contributes to the lack of any appreciable seismic anisotropy in the upper portion of the lower mantle.

Six 'Must-Have' Minerals for Life's Emergence: Olivine, Pyrrhotite, Bridgmanite, Serpentine, Fougerite and Mackinawite

Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]2SiO4), bridgmanite ([Mg,Fe]SiO3), serpentine ([Mg,Fe,]2-3Si2O5[OH)]4), and pyrrhotite (Fe(1-x)S), are an essential requirement in planetary bodies to produce such disequilibria and, thereby, life. Yet only two minerals, fougerite ([Fe2+6xFe3+6(x-1)O12H2(7-3x)]2+·[(CO2-)·3H2O]2-) and mackinawite (Fe[Ni]S), are vital-comprising precipitate membranes-as initial "free energy" conductors and converters of such disequilibria, i.e., as the initiators of a CO2-reducing metabolism. The fact that wet and rocky bodies in the solar system much smaller than Earth or Venus do not reach the internal pressure (≥23 GPa) requirements in their mantles sufficient for producing bridgmanite and, therefore, are too reduced to stabilize and emit CO2-the staple of life-may explain the apparent absence or negligible concentrations of that gas on these bodies, and thereby serves as a constraint in the search for extraterrestrial life. The astrobiological challenge then is to search for worlds that (i) are large enough to generate internal pressures such as to produce bridgmanite or (ii) boast electron acceptors, including imported CO2, from extraterrestrial sources in their hydrospheres.

An improved setup for radial diffraction experiments at high pressures and high temperatures in a resistive graphite-heated diamond anvil cell

We present an improved setup for the experimental study of deformation of solids at simultaneous high pressures and temperatures by radial x-ray diffraction. This technique employs a graphite resistive heated Mao-Bell type diamond anvil cell for radial x-ray diffraction in combination with a water-cooled vacuum chamber. The new chamber has been developed by the sample environment group at PETRA III and implemented at the Extreme Conditions Beamline P02.2 at PETRA III, DESY (Hamburg, Germany). We discuss applications of the new setup to study deformation of a variety of materials, including ferropericlase, calcium perovskite, bridgmanite, and tantalum carbide, at high-pressure/temperature.

Texture Development and Stress–Strain Partitioning in Periclase + Halite Aggregates

Keywords: multiphase deformation; high pressure; texture; plasticity modeling

Competing Deformation Mechanisms in Periclase: Implications for Lower Mantle Anisotropy

Seismic anisotropy is observed above the core-mantle boundary in regions of slab subduction and near the margins of Large Low Shear Velocity Provinces (LLSVPs). Ferropericlase is believed to be the second most abundant phase in the lower mantle. As it is rheologically weak, it may be a dominant source for anisotropy in the lowermost mantle. Understanding deformation mechanisms in ferropericlase over a range of pressure and temperature conditions is crucial to interpret seismic anisotropy. The effect of temperature on deformation mechanisms of ferropericlase has been established, but the effects of pressure are still controversial. With the aim to clarify and quantify the effect of pressure on deformation mechanisms, we perform room temperature compression experiments on polycrystalline periclase to 50 GPa. Lattice strains and texture development are modeled using the Elasto-ViscoPlastic Self Consistent method (EVPSC). Based on modeling results, we find that { 110 } ⟨ 1 1 ¯ 0 ⟩ slip is increasingly activated with higher pressure and is fully activated at ~50 GPa. Pressure and temperature have a competing effect on activities of dominant slip systems. An increasing { 100 } ⟨ 011 ⟩ : { 110 } ⟨ 1 1 ¯ 0 ⟩ ratio of slip activity is expected as material moves from cold subduction regions towards hot upwelling region adjacent to LLSVPs. This could explain observed seismic anisotropy in the circum-Pacific region that appears to weaken near margins of LLVSPs.

Oxygen fugacities of extrasolar rocks: Evidence for an Earth-like geochemistry of exoplanets

Oxygen fugacity is a measure of rock oxidation that influences planetary structure and evolution. Most rocky bodies in the Solar System formed at oxygen fugacities approximately five orders of magnitude higher than a hydrogen-rich gas of solar composition. It is unclear whether this oxidation of rocks in the Solar System is typical among other planetary systems. We exploit the elemental abundances observed in six white dwarfs polluted by the accretion of rocky bodies to determine the fraction of oxidized iron in those extrasolar rocky bodies and therefore their oxygen fugacities. The results are consistent with the oxygen fugacities of Earth, Mars, and typical asteroids in the Solar System, suggesting that at least some rocky exoplanets are geophysically and geochemically similar to Earth.

The role of diffusion-driven pure climb creep on the rheology of bridgmanite under lower mantle conditions

The viscosity of Earth’s lower mantle is poorly constrained due to the lack of knowledge on some fundamental variables that affect the deformation behaviour of its main mineral phases. This study focuses on bridgmanite, the main lower mantle constituent, and assesses its rheology by developing an approach based on mineral physics. Following and revising the recent advances in this field, pure climb creep controlled by diffusion is identified as the key mechanism driving deformation in bridgmanite. The strain rates of this phase under lower mantle pressures, temperatures and stresses are thus calculated by constraining diffusion and implementing a creep theoretical model. The viscosity of MgSiO₃ bridgmanite resulting from pure climb creep is consequently evaluated and compared with the viscosity profiles available from the literature. We show that the inferred variability of viscosity in these profiles can be fully accounted for with the chosen variables of our calculation, e.g., diffusion coefficients, vacancy concentrations and applied stresses. A refinement of these variables is advocated in order to further constrain viscosity and match the observables.

Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle

Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain-induced intrinsic anisotropy. Here we quantify the potential contribution of grain-scale and rock-scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain-induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock-scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi-laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1-0.2% of Vp and Vs radial anisotropy). More generally, rock-scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain-scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid-ocean ridge basalt-rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2-3% Vp and 3-6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660-km discontinuity could be possibly related to grain-scale shape-preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.

Valence and spin states of iron are invisible in Earth's lower mantle

Heterogeneity in Earth's mantle is a record of chemical and dynamic processes over Earth's history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg_0.46Fe³⁺_0.53)(Si_0.49Fe³⁺_0.51)O₃ Bdg that Fe³⁺ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

Characterization by Scanning Precession Electron Diffraction of an Aggregate of Bridgmanite and Ferropericlase Deformed at HP-HT

Scanning precession electron diffraction is an emerging promising technique for mapping phases and crystal orientations with short acquisition times (10-20 ms/pixel) in a transmission electron microscope similarly to the Electron Backscattered Diffraction (EBSD) or Transmission Kikuchi Diffraction (TKD) techniques in a scanning electron microscope. In this study, we apply this technique to the characterization of deformation microstructures in an aggregate of bridgmanite and ferropericlase deformed at 27 GPa and 2,130 K. Such a sample is challenging for microstructural characterization for two reasons: (i) the bridgmanite is very unstable under electron irradiation, (ii) under high stress conditions, the dislocation density is so large that standard characterization by diffraction contrast are limited, or impossible. Here we show that detailed analysis of intracrystalline misorientations sheds some light on the deformation mechanisms of both phases. In bridgmanite, deformation is accommodated by localized, amorphous, shear deformation lamellae whereas ferropericlase undergoes large strains leading to grain elongation in response to intense dislocation activity with no evidence for recrystallization. Plastic strain in ferropericlase can be semiquantitatively assessed by following kernel average misorientation distributions.

Note: Modified anvil design for improved reliability in DT-Cup experiments

The Deformation T-Cup (DT-Cup) is a modified 6-8 multi-anvil apparatus capable of controlled strain-rate deformation experiments at pressures greater than 18 GPa. Controlled strain-rate deformation was enabled by replacing two of the eight cubic "second-stage" anvils with hexagonal cross section deformation anvils and modifying the "first-stage" wedges. However, with these modifications approximately two-thirds of experiments end with rupture of the hexagonal anvils. By replacing the hexagonal anvils with cubic anvils and, split, deformation wedge extensions, we restore the massive support to the deformation anvils that were inherent in the original multi-anvil design and prevent deformation anvil failure. With the modified parts, the DT-Cup has an experimental success rate that is similar to that of a standard hydrostatic 6-8 multi-anvil apparatus.

Methane: Fuel or Exhaust at the Emergence of Life?

As many of the methanogens first encountered at hydrothermal vents were thermophilic to hyperthermophilic and comprised one of the lower roots of the evolutionary tree, it has been assumed that methanogenesis was one of the earliest, if not the earliest, pathway to life. It being well known that hydrothermal springs associated with serpentinization also bore abiotic methane, it had been further assumed that emergent biochemistry merely adopted and quickened this supposed serpentinization reaction. Yet, recent hydrothermal experiments simulating serpentinization have failed to generate methane so far, thus casting doubt on this assumption. The idea that the inverse view is worthy of debate, that is, that methanotrophy was the earlier, is stymied by the "fact" that methanotrophy itself has been termed "reverse methanogenesis," so allotting the methanogens the founding pedigree. Thus, attempting to suggest instead that methanogenesis might be termed reverse methanotrophy would require "unlearning"-a challenge to the subconscious! Here we re-examine the "impossibility" of methanotrophy predating methanogenesis as in what we have termed the "denitrifying methanotrophic acetogenic pathway." Advantages offered by such thinking are that methane would not only be a fuel but also a ready source of reduced carbon to combine with formate or carbon monoxide-available in hydrothermal fluids-to generate acetate, a target molecule of the first autotrophs. And the nitrate/nitrite required for the putative oxidation of methane with activated NO would also be a ready source of fixed nitrogen for amination reactions. Theoretical conditions for such a putative pathway would be met in a hydrothermal green rust-bearing exhalative pile and associated chimneys subject to proton and electron counter gradients. This hypothesis could be put to test in a high-pressure hydrothermal reaction chamber in which a cool carbonate/nitrate/nitrite-bearing early acidulous ocean simulant is juxtaposed across a precipitate membrane to an alkaline solution of hydrogen and methane. Key Words: Green rust-Methanotrophy-Nitrate reduction-Emergence of life. Astrobiology 17, 1053-1066.

Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe²⁺ in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe³⁺/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe³⁺ in bridgmanite combined with the strong partitioning of Fe²⁺ into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.

A nearly water-saturated mantle transition zone inferred from mineral viscosity

An open question for solid-earth scientists is the amount of water in Earth's interior. The uppermost mantle and lower mantle contain little water because their dominant minerals, olivine and bridgmanite, have limited water storage capacity. In contrast, the mantle transition zone (MTZ) at a depth of 410 to 660 km is considered to be a potential water reservoir because its dominant minerals, wadsleyite and ringwoodite, can contain large amounts of water [up to 3 weight % (wt %)]. However, the actual amount of water in the MTZ is unknown. Given that water incorporated into mantle minerals can lower their viscosity, we evaluate the water content of the MTZ by measuring dislocation mobility, a property that is inversely proportional to viscosity, as a function of temperature and water content in ringwoodite and bridgmanite. We find that dislocation mobility in bridgmanite is faster by two orders of magnitude than in anhydrous ringwoodite but 1.5 orders of magnitude slower than in water-saturated ringwoodite. To fit the observed mantle viscosity profiles, ringwoodite in the MTZ should contain 1 to 2 wt % water. The MTZ should thus be nearly water-saturated globally.

High-pressure rotational deformation apparatus to 135 GPa

A large-strain, torsional deformation apparatus has been developed based on diamond anvil cells at high pressures, up to 135 GPa with a help of hard nano-polycrystalline diamond anvils. These pressure conditions correspond to the base of the Earth's mantle. An X-ray laminography technique is introduced for high-pressure in situ 3D observations of the strain markers. The technique developed in this study introduces the possibility of the in situ rheological measurements of the deep Earth materials under ultrahigh-pressure conditions.

Pure climb creep mechanism drives flow in Earth's lower mantle

At high pressure prevailing in the lower mantle, lattice friction opposed to dislocation glide becomes very high, as reported in recent experimental and theoretical studies. We examine the consequences of this high resistance to plastic shear exhibited by ringwoodite and bridgmanite on creep mechanisms under mantle conditions. To evaluate the consequences of this effect, we model dislocation creep by dislocation dynamics. The calculation yields to an original dominant creep behavior for lower mantle silicates where strain is produced by dislocation climb, which is very different from what can be activated under high stresses under laboratory conditions. This mechanism, named pure climb creep, is grain-size-insensitive and produces no crystal preferred orientation. In comparison to the previous considered diffusion creep mechanism, it is also a more efficient strain-producing mechanism for grain sizes larger than ca. 0.1 mm. The specificities of pure climb creep well match the seismic anisotropy observed of Earth's lower mantle.

Low viscosity and high attenuation in MgSiO3 post-perovskite inferred from atomic-scale calculations

This work represents a numerical study of the thermal activation for dislocation glide of the [100](010) slip system in MgSiO₃ post-perovskite (Mg-ppv) at 120 GPa. We propose an approach based on a one-dimensional line tension model in conjunction with atomic-scale calculations. In this model, the key parameters, namely, the line tension and the Peierls barrier, are obtained from density functional theory calculations. We find a Peierls stress σ_p = 2.1 GPa and a line tension Γ = 9.2 eV/Å, which lead to a kink-pair enthalpy (under zero stress) of 2.69 eV. These values confirm that this slip system bears a very low lattice friction because it vanishes for temperatures above approximately 500 K under mantle conditions. In the Earth’s mantle, high-pressure Mg-ppv silicate is thus expected to become as ductile as ferropericlase. These results confirm the hypothesis of a weak layer in the D″ layer where Mg-ppv is present. Easy glide along [100](010) suggests strong preferred orientations with (010) planes aligned. Highly mobile [100] dislocations are also likely to respond to stresses related to seismic waves, leading to energy dissipation and strong attenuation.

Spin and valence dependence of iron partitioning in Earth's deep mantle

We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth's lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth's mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.