The Interpretation For Multiple "660" And The Inversion Of Temperature And Composition In The Lower Mantle Based On The Elasticity Of Akimotoite And Other Candidate Minerals

Exploring the temperature and composition of the deep Earth is an important research area in Solid Geophysics,which is of great importance to understanding the origin and evolution of the Earth.At present,the elastic properties of minerals under high pressures and high temperatures calculated by first-principles calculations have comparable precision to that of high-pressure experiments.Combining these high-precision elastic data with detailed seismic observations,we can obtain the fine structure of the deep interior of the Earth.The equation of state and elastic properties of akimotoite at simultaneously high pressures and high temperatures were obtained using first-principles calculations based on the density functional theory(DFT).The calculated results agree with the available experimental data.Combining our results with the elastic data of other minerals,we estimated the VP,Vs,and density contrasts caused by the akimotoite-related transitions.The velocity contrasts between akimotoite and bridgmanite are 4.6%and 8.3%for VP and Vs,respectively,which are only about half of those between majorite and akimotoite.Moreover,because both the akimotoite-bridgmanite and majorite-akimotoite transitions have broad phase boundaries,these phase transitions may not contribute to multiple discontinuities around?660 km depth in subduction zones as detected by seismic studies.Instead,the decomposition of pyrope into bridgmanite and corundum,which would occur in cold subduction zones with a sharp phase boundary and a large impedance contrast due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures,could be a more reasonable explanation for the discontinuity at?700-750 km in subduction zones.Furthermore,the transformation from high-pressure clinopyroxene to akimotoite at the depth of?600 km can increase the Vp,Vs,and density by 10.1%,14.8%,and 9.9%,respectively,indicating that the phase transition may account for the local discontinuity at?600 km in some subduction zones.In addition,the anisotropies of akimotoite are significantly higher than those of other major minerals at the base of the mantle transition zone and could be the origin of the seismic anisotropies detected in some subduction zones.Combining the elastic data of candidate minerals in the lower mantle calculated by first-principles calculations with seismic tomographic models,we applied Markov chain Monte Carlo method to obtain the spatial distribution of temperature and compositions in the lower mantle.The parameters include the temperature,the mass fraction of candidate minerals(ferropericlase,bridgmanite,post-perovskite,and calcium perovskite),and iron contents.After testing the parameter range,the step length,the total number of steps,and the frequency distribution of the parameters,we got the final results.The 1-D results show that the temperature of the lower mantle is significantly higher than that of the adiabatic geotherms,especially at the core-mantle boundary.The mass fraction of ferropericlase and Mg/Si in the lower mantle gradually decrease from the depth of?800 to?2000 km and then increase to the core-antle boundary.Furthermore,we analyze the difference in the inversion parameters between Large Low Shear Velocity Provinces(LLSVPs)and the surrounding area.For the depth profile of 2800 km,LLSVPs have a strong high-temperature anomaly compared with the surrounding area,which is compatible with the low contents of post-perovskite and the high contents of bridgmanite.Besides,its density is slightly higher,and its iron contents are higher than those of the surrounding area.Most parameters of the vertical profile along 25° S is similar to those of the 2800-km profile,but the upper part of LLSVPs shows a lower density than that of the surrounding area,which is opposite to the former result.