Elasticity Of SiC And TiC At High Temperature And Pressure:Implications For Ultra High Velocity Zones At The Core-mantle Boundary

In the past few decades,seismological studies have demonstrated many complex heterogeneous structures near the core-mantle boundary(CMB).A full understanding of these heterogeneous structures is very important for understanding the nature of mantle convection and the global heat and chemical transport and evolution.Among them,structures with low seismic velocity anomalies,such as the Large Low Shear Velocity Provinces(LLSVPs)and Ultra Low Velocity Zones(ULVZs),have attracted intense attention,and their compositions and causes have been widely discussed.However,some recent seismological studies have found that there are some High Velocity Layer(HVL)or even Ultra High Velocity Zones(UHVZs)near the CMB,and their shear wave velocities are as high as 3%～18%.By analyzing various possible causes of Ultra High Velocity Zones,it is found that they are more likely to be thermochemical anomalies arising from the remnants of the magma ocean.What mineral may be responsible for the heterogeneous structure of the core-mantle boundary is an urgent question.The equations of state and elastic properties of SiC and TiC at high pressure and temperature are obtained by first-principles calculations based on density functional theory(DFT).The calculated results are in good agreement with the published experimental results and fill the gap in the study of the elastic properties at high pressure and temperature.With the increase of pressure,SiC changes from the B3 phase with zinc-blende structure to the B1 phase with rock-salt structure,which is the same as rock-salt structure TiC.The solid solution is composed of SiC and TiC with the same structure,the wave velocities of which increase with the increase of SiC content.The SiC-TiC solid solution with SiC content of 20%～60%at the CMB can generate a high shear wave velocity anomaly of 3%～18%.We conclude that SiC and TiC with the same rock-salt structure formed dense solid solution and sank together to the bottom of the mantle during the cooling crystallization of the magma ocean,which is likely to be the source of Ultra High Velocity Zones(UHVZs)near the CMB.In addition,some studies have shown that the mantle of many carbon-rich planets is not silicate mantle,but mainly C and SiC,so the properties of SiC at high temperature and pressure contributes to deepen the understanding of the interior of carbon-rich planets.