Mineralogical Investigation On Lower Mantle Structure

Seismological studies have found numerous seismic heterogeneities with differ-ent length scales in the lower mantle including small-scale seismic scatters with size of several kilometers and Large Low Shear Velocity Provinces(LLSVPs)of the size of?1000 kilometers.For small-scale seismic scatters,many of them are found near subduction zone and has a size of several kilometers,the subducted oceanic crust that has a distinctive chemical composition from the pyrolitic lower mantle was proposed as the main chemical origin for these heterogeneities.However,the velocity and den-sity characteristics of subducted oceanic crust at the lower-mantle pressures and tem-peratures still remain unknown.For LLSVPs,we haven't known its composition and temperature structure and it's still bebatable whether they are pure thermal origin or thermochemical origin.In the first part of this study,the first-principles calculations are performed to obtain the elastic properties of calcium ferrite-type(CF-type)phases NaAlSi04,MgAl2O4 and Mg0.75Fe0.25Al2O4.Then combining this study and previous studies,the velocity and density of oceanic crust along different mantle geotherms are determined.The sub-ducted oceanic crust shows a large negative shear velocity anomaly at the phase bound-ary between stishovite and CaCl2-type silica,highly consistent with the features of de-tected mid-mantle seismic scatterers.After the phase transition of silica,the oceanic crust has relatively higher wave velocities than these of the ambient mantle even when its temperature profile is +1000 kelvin hotter than the ambient mantle,ruling out the pos-sibility that LLSVPs mainly originates from the subducted oceanic crust.The accumu-lation of the subducted oceanic crust can cause extremely high velocity heterogeneities(?2%)in the lower mantle imaged by seismic tomography.The velocity heterogeneities caused by the presence of MORB are noticeably depth-dependent,suggesting that the popular depth-independent random model for seismic scatterers throughout the lower mantle needs to be reconsidered.Our findings strongly support the presence of sub-ducted oceanic crust in the lower mantle.The current scenario of subducted oceanic crust has important implications for our interpretations of the interaction between the subducted slab and the lower mantle,the strength of mantle convection,and the geo-chemical signatures from deep mantle.In the second part of this study,based on the elastic data of bridgmanite,ferriperi-clase,calcium perovskite and post perovskite and three-dimensional tomography mod-els,Markov chain Monte Carlo(MCMC)method is used to sample composition and temperature of lower mantle with the depth range of 800 km-2800 km.The lower man-tle has a Mg/Si ratio close to 1 which is different from the Mg/Si raitio of upper mantle.This suggests lower mantle has a different compistion from upper mantle.One dimensi-nal temperature profile of this study is hotter than Brown's geotherm.LLSVPs are ther-mochemical anomalies which are hotter than ambient mantle and entrichd in bridge-manite and iron.For example,when using GyPSUM tomography model,at the depth of 2800 km,LLSVPs are 300 K hotter than surrounding mantle and LLSVPs are more enriched in iron and has much higher concentration of bridgmanite and consequently much less post perovskite than surrounding mantle;the S-wave anomaly are largely constributed by compositional heterogeneities;Thermal effect and compositional het-erogeneities almost contribute equaly to P-wave anomaly of LLSVPs.If adopting the same temperature as surrounding mantle,LLSVPs has a larger density than ambient mantle by a bit less than 1%.According to geodynamic studies,LLSVPs should be a stable dome-like structure and has a height meeting sesimic observations.