| Water is a key component in making Earth habitable,and has been involved in the formation and evolution of the Earth because very small amounts of water can change the physical properties of Earth’s materials.Water lowers the melting temperature of materials,resulting in magmatism/volcanism,weakens the Earth’s materials by hydrolysis,facilitates recrystallization,and enhances diffusion.The subducting slabs serve as a bridge for material exchange between the surface and the Earth’s interior and could carry a large amount of water into the deep Earth,and hydrous minerals at different regions and depths in subducting slabs are the major water carriers.The lower mantle is the sphere with the highest volume fraction of the Earth’s interior,the physical properties of hydrous minerals in slabs of the lower mantle are of great significance to our understanding of the Earth’s deep water.However,it still lacks the elastic properties of hydrous minerals under lower-mantle conditions.In this study,we conducted firstprinciples calculations based on density functional theory to investigate the elastic properties of phase H,δ-AlOOH,and ε-FeOOH under the lower-mantle conditions and evaluate the effects of water on wave velocities and density in harzburgite layer and oceanic crust of subducting slabs.This work provides new evidence for water transport in the lower mantle.Phase H(MgSiO4H2),one of the lower mantle’s dense hydrous magnesium silicates(DHMSs),may form and exist in cold slabs and is crucial in carrying water into the deep mantle.Its sound velocities and density are crucial for inferring the midmantle water cycling via seismic approaches.Here we obtain the elastic and thermodynamic properties of phase H and discuss the effect of the Mg-Si disorder on elasticity.The density of phase H is～15%and～6%lower than that of bridgmanite and periclase,respectively.The dehydration reaction from phase H to bridgmanite,which may occur at the depth of～1300-1700 km in cold slabs,will cause an increase of 1.0%,2.7%,and 15%at 1500 km on VP,VS,and density,respectively.The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of～17%in the mid-mantle,which can provide an explanation for some seismic discontinuities detected by previous studies.We have estimated the velocities and density of the hydrous harzburgite by the hydration of bridgmanite to form phase H,further constraining the potential water content in local regions of the subducted slabs.Meanwhile,phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones.δ-AlOOH(AlOOH)has high thermal stability,which usually forms in the mafic oceanic crust and remains stable to the core-mantle boundary.We have calculated δAlOOH with symmetric hydrogen bonds,i.e.Pnnm structure,to obtain the thermoelastic properties under the lower-mantle conditions.The calculated equation of state is in good agreement with the experiments at various temperatures and pressures,and the deviations are less than 1.0%.Besides silica,δ-AlOOH has the fastest wave velocities in the lower mantel,while the density of δ-AlOOH is at least 0.5 g/cm3 lower than other major lower-mantle minerals,simply higher than that of phase H.Based on the experimental compositions,we combined high-quality elastic data of the main minerals to obtain the velocities and density of hydrous MORB and anhydrous MORB at the pressure range of 30-120 GPa.The results show that the hydrous MORB has lower density but higher velocities in most depths,and the differences have a positive correlation with the water content,that is,the content of δ-AlOOH.Compared with other lithologies,the hydration of MORB increases the velocities,and further increases the seismic contrast with the surrounding mantle.Therefore,the hydrous MORB can explain the fast seismic scatterers observed in the lower mantle,but cannot explain the large-scale low-velocity anomalous regions at the core-mantle boundary.ε-FeOOH(FeOOH)can form solid solutions with δ-AlOOH and phase H,and this system affects the water content and physical properties of the Earth’s interior.We first evaluated the effects of different exchange-correlation functionals on this iron-bearing mineral and proved the necessity of Hubbard U correction.With LDA+U,ε-FeOOH undergoes a high-spin to low-spin transition at～44 GPa under static conditions accompanied by a volume collapse of～12%.We obtained the fractions of low spin state at wide temperature and pressure ranges.The width of spin transition,that is,the mixed states,increases with temperature.The beginning point of the spin transition moves toward higher pressure with temperature.We can further calculate the equation of state and elastic modulus of the mixed state under the lower-mantle conditions and rigorously constrain the valid range of QHA.ε-FeOOH has significantly low velocities and high density at the conditions where it can exist stably.At the top of the lower mantle,its VP and VS are about 30%and 50%lower than most minerals,respectively,and its density is about 10%higher than most minerals.At the range of spin transition,VP descends sharply,and the difference with other minerals reaches more than 60%.Therefore,the local enrichment of ε-FeOOH in the mid-mantle(1000-1500 km)may explain the seismic scatters with low VP and VS or with high density.The unique properties of velocities and density of ε-FeOOH can be detected as evidence for water transport in the lower mantle. |
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