Search

The seismic anisotropy observed in Earth's lowermost mantle (D″ layer) is thought to originate from the lattice‐preferred orientation (LPO) of mantle materials induced by plastic deformation. Ferropericlase is the second most abundant mineral in the lower mantle, and its LPO induced by deformation may significantly contribute to the seismic anisotropy in the D″ layer. The slip system(s) of ferropericlase controlling the LPO in the D″ layer is still controversial, owing to technical difficulties in conducting high‐pressure deformation experiments. We conducted shear deformation experiments on MgO under pressures of up to 120 GPa and reported the LPO after the deformation experiments. Our results indicate that {100}<011> is the dominant slip system in the D″ layer. The LPO developed during shear deformation would result in shear wave splitting with VSH > VSV, consistent with seismic observations in the regions expected to accommodate large shear strain by subducted slabs in the D″ layer. Plain Language Summary: Heat from Earth's core induces active thermal convection involving plastic flow in the Earth's solid mantle. Owing to plastic flow, minerals with elastic anisotropy in the mantle are thought to be aligned in certain directions (lattice‐preferred orientation, LPO) and generate the seismic anisotropy observed in the lowermost mantle. In particular, the LPO of ferropericlase, the second most abundant mineral in the lower mantle, has garnered attention as a likely candidate for attributing the seismic anisotropy of the lowermost mantle. To quantitatively interpret the dynamics of mantle convection in the lowermost mantle from seismic observations, it is essential to understand the LPO patterns of ferropericlase during deformation under the lowermost mantle conditions (for instance, extremely high‐pressure conditions at 1.2 million atmospheric pressure). However, owing to technical difficulties in conducting high‐pressure deformation experiments, there have been uncertainties in the deformation‐induced LPO of ferropericlase in the lowermost mantle. We conducted shear deformation experiments of MgO under the lowermost mantle pressure for the first time and demonstrated that the LPO of MgO produced by shear deformation is indeed consistent with the seismic anisotropy observed in parts of the lowermost mantle expected to accommodate large shear deformation by subducted slabs. Key Points: Strain rate controlled shear deformation experiments on MgO were conducted under lowermost mantle pressure (120 GPa) for the first timeLarge strain deformation experiments on MgO show that the {100}<011> slip is dominant in the D″ layerA {100}<011> slip of MgO by shear deformation results in VSH > VSV shear splitting, consistent with seismic observations in the D″ layer [ABSTRACT FROM AUTHOR]

Two large low velocity provinces (LLVPs) are observed in Earth's lower mantle, beneath Africa and the Pacific Ocean, respectively. The maximum height of the African LLVP is ∼1,000 km larger than that of the Pacific LLVP, but what causes this height difference remains unclear. LLVPs are often interpreted as thermochemical piles whose morphology is greatly controlled by the surrounding mantle flow. Seismic observations have revealed that while some subducted slabs are laterally deflected at ∼660–1,200 km, other slabs penetrate into the lowermost mantle. Here, through geodynamic modeling experiments, we show that rapid sinking of stagnant slabs to the lowermost mantle can cause significant height increases of nearby thermochemical piles. Our results suggest that the African LLVP may have been pushed more strongly and longer by surrounding mantle flows to reach a much shallower depth than the Pacific LLVP, perhaps since the Tethys slabs sank to the lowermost mantle. Plain Language Summary: Seismic observations have revealed the two large low velocity provinces (LLVPs) beneath Africa and the Pacific Ocean in Earth's lowest mantle. Growing evidence indicates that the maximum height of the African LLVP is up to ∼1,000 km taller than that of the Pacific LLVP, but the underlying cause of such a large difference in height remains uncertain. Geodynamic modeling studies have shown that the morphology of LLVPs is greatly controlled by their interactions with surrounding mantle flow which is mainly driven by subduction of cold slabs. Seismic observations have shown that while some subducted slabs flatten or stagnate at ∼660–1,200 km, other slabs arrive at the deepest mantle. With geodynamic modeling experiments, we show that catastrophic sinking of subducted slabs into the lower mantle can cause strong upwelling flows that in turn greatly increase the height of nearby LLVP‐like piles, while other piles can be less affected. This work suggests that the African LLVP may be enduring stronger nearby mantle flow to rise by the slab‐driven surrounding mantle flow than the Pacific LLVP. The African LLVP may start rising since the sinking of the Tethys slabs to the lowermost mantle. Key Points: Continuous sinking of voluminous subducted slabs to the lower mantle can push nearby large low velocity province (LLVP)‐like piles to rise significantlyEpisodic large increase of height is only possible for piles that are nearly neutrally buoyant and are relatively unstableThe African LLVP may have been strongly pushed by the Tethys slabs and reach a much larger height than the Pacific LLVP [ABSTRACT FROM AUTHOR]

The subduction of serpentinized slabs is the dominant process to transport "water" into Earth's mantle, and plays a pivotal role for subduction dynamics. Antigorite, the most abundant serpentine mineral in subduction settings, may imprint a seismic signature on serpentinized slabs, making them seismically distinguishable from the dry, non‐serpentinized ones. However, the complete single‐crystal elasticity of antigorite has not been experimentally constrained at high pressures, hindering the use of seismological approaches to detect serpentinization in subducting slabs. Here, we report the full elastic stiffness tensor of antigorite by single‐crystal Brillouin spectroscopy and X‐ray diffraction up to 7.71(5) GPa. We use our results to model seismic properties of antigorite‐bearing rocks and show that their seismological detectability depends on the geometrical relation between seismic wave paths and foliation of serpentinized rocks. In particular, we demonstrate that seismic shear anisotropy shows low sensitivity to serpentinization for a range of relevant geometries. Plain Language Summary: The subduction of serpentinized slabs plays a key role in the deep recycling of water into the Earth's interior. Antigorite is the main serpentine mineral in subducting slabs, and the most important carrier of water. Antigorite‐bearing rocks are predicted to have a distinct seismic signature, potentially allowing them to be detected with seismological approaches. However, our current knowledge on seismic properties of antigorite‐bearing rocks is limited, mostly hampered by a lack of experimental constraints on single‐crystal elasticity of antigorite at relevant pressures. In this study, state‐of‐the‐art techniques were employed to produce the first experimental description of the complete high‐pressure elasticity of antigorite single crystals. Our experimental data set was implemented in the modeling of seismic properties of antigorite‐bearing rocks at pressures relevant for subduction. Our results were used to discuss the relation between seismic wave path and shear wave anisotropy in serpentinized slabs, and challenge the use of shear wave splitting as a proxy for serpentinization in slabs. Key Points: Single‐crystal elasticity of antigorite at high pressures is determined by Brillouin spectroscopy and X‐ray diffraction experimentsSeismic signature of serpentinized slabs is constrained in a relevant composition‐pressure spaceSerpentinization in slabs may be undetectable through shear wave anisotropy [ABSTRACT FROM AUTHOR]

Oxyhydroxides like δ‐(Al,Fe)OOH may stabilize "water" in Mid‐Ocean Ridge Basalt (MORB) subducted into the Earth's lower mantle. The single‐crystal elasticity of δ‐(Al,Fe)OOH has not been experimentally constrained, hampering an accurate evaluation of the seismic detectability of this high‐pressure solid solution, and the presence of "water," in the deep Earth. Here, we report the first experimental single‐crystal elasticity results of δ‐(Al0.97Fe0.03)OOH measured by X‐ray diffraction and Brillouin spectroscopy. We use our results to compute seismic properties of hydrous and anhydrous MORB at pressures and temperatures expected in slabs at shallow lower mantle conditions. We show that hydrous MORB is less dense than anhydrous MORB, but has faster aggregate seismic velocities. This suggests that hydration in MORB has an effect on velocities opposite to that observed in other lithologies, and further indicates that hydration of MORB increases the seismic contrast to the background mantle. Plain Language Summary: Water can be delivered into the Earth's mantle via subduction of hydrous phases, playing an active role in global‐scale geological processes. Hence, tracking hydrous phases during subduction is pivotal to understand the geological evolution of our planet. At lower mantle depths, oxyhydroxides such as δ‐(Al,Fe)OOH are the primary hosts of water in subducted oceanic crust. Here, we investigated the single‐crystal elasticity of δ‐(Al0.97Fe0.03)OOH at pressures consistent with subduction in the upper mantle and transition zone. Our experimental results were used to evaluate the impact that hydration has on the physical properties of oceanic crust subducted into the lower mantle. For this purpose, we modeled density and aggregate properties of hydrous and anhydrous oceanic crust subducted into the lower mantle. Our modeling shows that hydration induces a reduction in the density of the modeled lithologies, but increases their aggregate velocities. Our results suggest that hydration may influence the subducting behavior of oceanic crust as well as their detectability through seismology. Key Points: Brillouin spectroscopy and X‐ray diffraction provide first constraints on the single‐crystal elasticity of δ‐(Al,Fe)OOH at mantle pressuresHydration decreases density but increases aggregate velocities of MORB in the shallow lower mantleHydration of MORB increases the seismic velocity contrast with the background mantle [ABSTRACT FROM AUTHOR]

Distributions of slabs within Earth's mantle are increasingly used to reconstruct past subduction zones, based on first‐order assumptions that slabs sink vertically after slab break‐off, and thus delineate paleo‐trench locations. Non‐vertical slab motions, which occur prior to break‐off, represent a potentially significant source of error for slab‐based plate reconstructions, but are poorly understood. We constrain lateral migration of the Indian slab and overlying India‐Asia collision zone by comparing tomographically imaged mantle structure with plate‐kinematic constraints. Following coupling of the Indian and Australian plates at the onset of collision, ∼1,000 km lateral migration of the Indian slab was driven by vertical subduction of the Australian slab. The sinking behaviors of individual slabs do not evolve in isolation, but instead influence, or are influenced by, other slabs in the same plate network. Hence, lateral slab migrations may be determined by interpreting the sinking behavior of slabs collectively, and with respect to plate kinematics. Plain Language Summary: To understand the links between plate tectonics and mantle processes, researchers must determine how tectonic plates have moved with respect the evolving mantle through geological time. To overcome this problem, recent studies use the locations of subducted slabs in the deep mantle to reconstruct plate motions, based on the hypothesis that slabs sink vertically through the mantle, and therefore mark the surface locations of past subduction zones. Here, we test slab sinking hypotheses, and their use in plate reconstruction modeling, by investigating the sinking kinematics of the subducting Indian and Australian slabs during the India‐Asia collision. Our analysis indicates that since onset of collision at ∼45–40 Ma, the Indian slab migrated laterally, ∼1,000 km northwards through the mantle, driven by subduction of the neighboring Australian slab. We arrive at this new interpretation because we interpret Indian and Australian slab kinematics collectively, and with respect to India‐Australia plate motions. Our study shows that the sinking behavior of one slab can influence that of another slab in the same network. Slab‐based plate reconstructions should therefore interpret slabs of the same network collectively, and with respect to plate motions, in order to constrain non‐vertical slab motions and avoid potentially significant plate reconstruction errors. Key Points: Subduction of Australian oceanic lithosphere drove northward motion of coupled India‐Australia plate since onset of collision at 45–40 MaBuoyant Indian continent stalled subduction of Indian slab whilst Australian slab subduction drove motion of coupled India‐Australia plateApproximately 1,000 km north lateral migration of Indian slab occurred to maintain compatibility with plate kinematics of coupled India‐Australia plate [ABSTRACT FROM AUTHOR]

Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite. Plain Language Summary: Earth's mantle convects like a fluid over geological time and it organizes mineral fabrics resulting in directional dependence of seismic velocities, that is seismic anisotropy. There is abundant evidence for flow‐induced seismic anisotropy at depths above about 400 km, but it is less clear if anisotropy is developed in the mantle transition zone at about 400–700 km deep. Here, we use seismic waves generated from the bottom and top of the transition zone to constrain anisotropy within the layer. Localized evidence of strong anisotropy is found beneath the Pacific Northwest near locations where prior imaging studies show gaps between subducted oceanic plate fragments. We propose that focused flow through constrictions like slab gaps may cause seismic anisotropy in the mantle transition zone. Key Points: PSH/PSV amplitude ratios between P660s and P410s are useful for constraining transition zone anisotropyEvidence for up to 2% mantle transition zone anisotropy is found beneath the Pacific Northwest of the USMantle flow between slab fragments may enhance development of transition zone anisotropy [ABSTRACT FROM AUTHOR]

The presence of water in minerals generally alters their physical properties. Ringwoodite is the most abundant phase in the lowermost mantle transition zone and can host up to 1.5–2 wt% water. We studied high‐pressure lattice thermal conductivity of dry and hydrous ringwoodite by combining diamond‐anvil cell experiments with ultrafast optics. The incorporation of 1.73 wt% water substantially reduces the ringwoodite thermal conductivity by more than 40% at mantle transition zone pressures. We further parameterized the ringwoodite thermal conductivity as a function of pressure and water content to explore the large‐scale consequences of a reduced thermal conductivity on a slab's thermal evolution. Using a simple 1‐D heat diffusion model, we showed that the presence of hydrous ringwoodite in the slab significantly delays decomposition of dense hydrous magnesium silicates, enabling them to reach the lower mantle. Our results impact the potential route and balance of water cycle in the lower mantle. Plain Language Summary: The physical properties of minerals are determined by the interaction of atoms in the crystal lattice. Water can be incorporated into the crystal structure and alter its behavior. Ringwoodite is a high‐pressure mineral that can host large quantities of water and is expected to be abundant in the lower part of Earth's mantle transition zone, a region ranging from 520 to 660‐km depth. Here we studied ringwoodite thermal conductivity, describing how effectively heat is transported through solids. Based on our measurements we determined that water in ringwoodite significantly slows down heat propagation. We performed computer simulations to investigate the large‐scale implications of our findings. For this purpose, we modeled a cold oceanic plate, entirely made of ringwoodite, which is surrounded by warm mantle. The delayed heat transport is sufficient to maintain low temperatures in the inner part of the oceanic plate and potentially preserve the hydrous minerals for an extended period of time. Key Points: Ringwoodite thermal conductivity is reduced by 40% due to the presence of 1.73 wt% water in the crystal structureLower thermal conductivity of hydrous ringwoodite might delay the breakdown of hydrous phases hosted in a subducting slabHydrous ringwoodite acts as a heat propagation barrier, supporting preservation of hydrous minerals down to the lower mantle [ABSTRACT FROM AUTHOR]