Elasticity Of Superhydrous Phase B And Phase D At The Mantle Temperatures And Pressures:Implications For Discontinuities In The Uppermost Lower Mantle

The presence of water can greatly affect the physical and chemical properties of minerals,melts,and fluids because a small amount of water can significantly change the melting point,electrical conductivity,viscosity,density and wave velocity of materials in the Earth's interior.Therefore,the water cycle in the interior of the Earth is one of the key processes controlling geodynamics and geochemical evolution.Studying the water cycle in the interior of the Earth is crucial for us to understand the interior of the Earth.Superhydrous phase B(ShyB)and phase D are considered as important candidates for transporting water into the transition zone and lower mantle since they are stable in the cold subduction zones of the uppermost lower mantle.Seismological observations have revealed that there are many local discontinuities or scatterers in the lower mantle,and the dehydration of hydrous phases may be one of the causes for these discontinuities or scatterers.Therefore,the elasticity of ShyB and phase D at high temperature and pressure are crucial for us in understanding the water cycle in the deep earth and the causes of the discontinuities or scatterers in the lower mantle.The thermodynamic properties and elasticity of superhydrous phase B(ShyB)and phase D at high pressure and temperature are calculated using first-principles calculations based on density functional theory with local density approximation(LDA)and generalized gradient approximation(GGA),respectively.In the uppermost lower mantle,the velocities and densities of ShyB are significantly lower than that of bridgmanite,the major mineral in the lower mantle.The Vp,Vs,and density of ShyB are 9.2%-10.1%,14.1%-14.4%,and 15.3%-16.0%lower than those of bridgmanite,respectively.The anisotropy of ShyB is unremarkable at relevant mantle conditions.The dehydration of ShyB into bridgmanite,periclase and water,which can occur at a depth around 800 km at a cold slab,will cause an increase of 7.5%,15.0%,and 12%on compressional velocity,shear velocity,and density,respectively.Thus the impedance contrasts for shear velocity by the dehydration of ShyB is?27%and such a strong signal may be detected by seismology even if the cold slab only contains a small amount of ShyB.The dehydration of ShyB could produce a local discontinuity at the depth of?800 km.The water released from the dehydration of ShyB may rise upward and promote the partial melt,which can further explain both the 800-km discontinuity and the low-velocity zones just above 800-km discontinuity in Western-Pacific Subduction Zones.Unlike ShyB,phase D does not exhibit low-velocity feature in the uppermost lower mantle.The velocities of phase D are slightly lower than those of bridgmanite by 0.5%-3.5%for VP and by 0%-2.0%for Vs in the uppermost lower mantle(660-1000 km).Thus,only the accumulation of large amounts of phase D could probably produce the observed low velocity anomalies in subduction zone regions of the uppermost lower mantle.The same as ShyB,phase D exhibits a low-density feature in the uppermost lower mantle,which is?14%less dense than bridgmanite.Previous studies have shown that the bulk modulus of phase D will suddenly increase at 30-40 GPa due to the hydrogen bond symmetrization.However,our research indicates that the calculated bulk modulus increases smoothly with increasing pressure and no sudden increase occurs at the range of 30-40 GPa.Although the anisotropy of ShyB is unremarkable at relevant mantle conditions,the strong anisotropy of phase D(?18%)at relevant mantle conditions suggests that the accumulation of phase D could provide an alternative interpretation for the observed seismic anisotropies in the Tonga slab below 660 km.Because phase D is?15%less dense than bridgmanite,the dehydration of phase D could produce large impedance contrasts at the depth of 660 km and 1000 km.This probably provides an explanation for some scatterers or discontinuities beneath the Mariana and Tonga subduction zones,especially those at the depth around 1000 km.Our study provides data on the wave velocity and density contrast caused by the dehydration of ShyB and phase D,indicating that the dehydration of ShyB and phase D may be related to the discontinuities and scatterers observed in the subduction zones of the uppermost lower mantle.It would be a strong evidence that water is transported into the lower mantle if future geophysics observations confirm that the discontinuous or scatterers observed in the uppermost lower mantle are indeed associated with the dehydration of ShyB and phase D.