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Subduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high‐temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low‐temperature polymorph of serpentine, is stable under the P‐T conditions of cold subduction slabs (<260°C at 2 GPa), and its high‐temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P‐T conditions of subduction zones. Our static results agree with previous studies. Its high‐temperature velocities are much higher than previous experimental‐based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear‐wave splitting in some cold slabs such as Tonga. Plain Language Summary: Serpentine minerals are crucial water carriers in subduction zones. Their layered structure is responsible for their strong elastic anisotropy. However, their elastic properties, especially the anisotropy at high‐temperature conditions remain unclear. In this study, we determined the elasticity of lizardite, the low‐temperature polymorph of serpentine, at P‐T conditions of subduction zones using ab initio calculations. The resulting velocities agree with previous calculation studies but are much higher than previous experimentally‐based estimates. Lizardite has significant shear wave anisotropy. The large elastic anisotropy could account for the observed shear‐wave splitting in the subducting slabs. Key Points: The elastic properties of lizardite are determined at P‐T conditions corresponding to subduction zones using ab initio calculationsThe elastic anisotropy of lizardite for S wave is almost double that of antigoriteThe large elastic anisotropy of lizardite can account for the observed shear‐wave splitting in the subducting slabs near trenches [ABSTRACT FROM AUTHOR]

Single‐crystal elasticity of both α‐ and β‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli (Cij), we investigated the contribution of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α‐orthopyroxene and olivine displays compressional (VP) and shear‐wave (VS) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, VP and VS of this metastable wedge with β‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone. Plain Language Summary: Subducting slab plays a significant role in transportation the surface material to the Earth's deep interior. It is normally imaged as a high‐velocity body compared to the surrounding mantle. However, seismic studies detected the existence of 3%–5% low compressional‐wave velocity anomalies accompanied with strong seismic shear‐wave anisotropies within the subducting slab at the top transition zone in various locations of the Earth which cannot be explained by the presence of metastable olivine alone. Besides olivine, orthopyroxene could also remain metastable in the coldest harzburgite layer of the slab. Here we report experimental results on the single‐crystal elasticity of both α‐ and β‐orthopyroxene up to 20 GPa and 300 K. These experimental results allow us to provide a comprehensive evaluation on the velocity profiles of the coldest harzburgite layer of the slab. We found that the coldest harzburgite layer with 22–37 vol.% orthopyroxene and 62–78 vol.% olivine has the VP and VS 3.0%–4.4(6)% and 2.0%–4.5(6)% lower than those of the pyrolitic mantle in the mantle transition zone, respectively. The observed 3%–5% low‐velocity anomalies within the slab in the top transition zone should be explained by the metastable orthopyroxene and olivine instead of metastable olivine alone. Key Points: Single‐crystal elasticity of α‐ and β‐orthopyroxene was determined to 20 GPa and shows anomalous change across the phase transitionThe obtained results were used to model the velocity profiles of the coldest harzburgite layer of the sinking subduction slabWe find that the low‐velocity anomalies within the slab at the top transition zone should be caused by metastable orthopyroxene and olivine [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]

The thermal stability of hydrous minerals in Earth's deep interior is key to understanding the evolution and physicochemical states of the planet. The recently discovered pyrite‐type (Py) FeO2Hx (x ≤ 1) phase, which can be transformed from α/ε‐FeOOH at ∼80 GPa, is believed to be a crucial candidate in transporting water and hydrogen to the lowermost mantle through subducting slabs. Here, we examined the stability and decomposition behavior of FeOOH through a set of shock‐recovery experiments up to ∼70 GPa and ∼2,750 K. Our results show that FeOOH partially decomposes to iron oxides Fe2O3 and Fe3O4 at 35–70 GPa and 1,150–2,750 K, which indicates that H2O and O2 are released during the decomposition of FeOOH in subducting slabs. The released H2O and O2 may have altered the physical and chemical properties of the surrounding mantle and contributed to the oxidation of surface Earth. Plain Language Summary: The mineral goethite (α‐FeOOH), a primary component of rusts and bog iron ores, is widespread on our planet. Early studies show that FeOOH polymorphs could transport water and hydrogen to the lowermost mantle when the minerals are transformed into hydrogen‐bearing iron peroxides at ∼1,800 km depths. However, the thermal stability of FeOOH at the mid‐lower mantle equivalent pressure‐temperature conditions is poorly known. Here, we used a shock‐recovery approach to examine the stability of FeOOH at pressures up to ∼70 GPa and temperatures to ∼2,750 K. Our results show that FeOOH partially decomposes to form hematite and magnetite, releasing water and oxygen at the equivalent pressure‐temperature conditions of a subducting slab in the mid‐lower mantle, thus indicating a narrower FeOOH‐stable region in the lower mantle than previously proposed. The released oxygen via FeOOH decomposition may have provided extra oxidation power for the long‐term oxidation of Earth's surface. Key Points: The thermal stability of FeOOH has been investigated by shock recovery experiments up to ∼70 GPa and ∼2,750 KFeOOH partially decomposes and releases water and oxygen in the mid‐lower mantleThe deoxygenation of FeOOH is a potential sporadic oxygen source for the Great Oxidation Event of Earth [ABSTRACT FROM AUTHOR]

The Andean subduction zone is an excellent place to study deformation within a subducting slab as a function of depth, owing to the varying and well‐resolved geometry of the subducting Nazca slab beneath South America. Here we combine the results of source‐side shear wave splitting with the latest regional tomography model to isolate intraslab raypaths and determine the spatial distribution of anisotropy within the Nazca slab. We observe that in the upper mantle, the intraslab anisotropy appears strongest where the slab is most contorted, suggesting a strong link between anisotropy and subduction‐related slab deformation. We identify a second source of anisotropy (δt∼ 1 s) within the subducting slab at lower mantle depths (660–800 km). The surrounding mantle and transition zone appear largely isotropic, with deep anisotropy concentrated within the slab as it deforms while entering the higher‐viscosity lower mantle. Plain Language Summary: Few observations exist of how a tectonic plate deforms as it descends deep into the Earth's interior at a subduction zone. Carefully selected seismic waves that mostly travel through this subducting plate, or slab, provide some of the most direct measurements of how the slab behaves as it sinks through the upper mantle (0–410 km) and the mantle transition zone (410–660 km). Studying the polarization of seismic waves allows us to detect and infer the pattern of deformation within the Earth's interior. Using this technique, we find that the Nazca slab in the Andean subduction zone in South America has undergone internal deformation during the process of subduction, in particular where the slab's 3‐D shape changes. Furthermore, we find that the deeper Nazca slab (≥660 km) appears to undergo further deformation as it interacts with the stiffer uppermost lower mantle. Key Points: We find a notable presence of anisotropy within the Nazca slab in both the upper and lower mantleUpper mantle slab anisotropy is strongest where the slab geometry is most contorted and deformedWidespread anisotropy is observed at the base of Nazca slab where it penetrates the higher‐viscosity lower mantle [ABSTRACT FROM AUTHOR]