Thermoelastic Properties Of Aluminous Phases In MORB From First-principles Calculation:Implications For The Earth's Lower Mantle

Even though the bulk lower mantle is considered to be well-mixed above the D"layer,the heterogeneities in the Earth's lower mantle have also been disclosed by studies of seismology and geochemistry.However,the causes of these heterogeneities remain an open question.Among many interpretations,the subduction of slabs is generally invoked to explain the compositional heterogeneity and the regionally seismological anomalies,because seismic tomography and inclusions in super-deep diamonds indicate that subducted slabs can penetrate into the transition zone and even reach the lower mantle.In addition,the mid-ocean ridge basalt(MORB)in subducted slabs and the surrounding pyrolitic mantle are characterized by large variation in mineralogical composition and temperature.In the subducted MORB,an independent aluminous phase composes about 20-25%volumetrically at the topmost lower mantle.Therefore,understanding of the physical and chemical properties of the aluminous phase component is critical for evaluating the influences of subducted MORB on the mantle's heterogeneity.In the high-pressure experiments of natural MORB,the aluminous phase is presented in the form of complex solid solution,in which MgAl2O4 and NaAlSiO4 are two main end-members in composition.MgAl2O4 has two polymorphs,which are the calcium ferrite(CF)structure and a denser structure,calcium titanate(CT)structure.CF and CT only refer to calcium ferrite-and calcium titanate-type MgAl2O4 respectively I mention below for simplicity.Comprehensive understanding of the elastic properties of the mantle minerals,especially the thermoelastic properties of aluminous phases,is crucial to understand MORB-mantle interaction and lower mantle's heterogeneity.A couple of pioneering studies explored the athermal elastic and thermodynamic properties of the aluminous phases by using first-principles calculations.However,the influence of temperature should not be neglected.Due to experimental difficulties,there are still no direct laboratory measurements of thermoelastic properties(e.g.,shear modulus)of CF and CT at lower mantle pressure and temperature conditions.Theoretical methods are thus worthy of employment to solve the problem.First of all,lattice parameters of CF and CT at 30-70 GPa and 0-3000 K were calculated by using first-principles molecular dynamics simulations,and then used to fit the equations of state at ambient conditions and high temperatures.The density profiles of MORB at 2000 K and pressure from 35 to 60 GPa are calculated and compared with Preliminary Reference Earth Model(PREM)to infer the behavior of subducted MORB in the lower mantle.Consequently,the density of MORB at 2000 K is higher than those of PREM by about 2.5%,suggesting that subducted slabs have enough driving force for the downwelling into the lower mantle.Secondly,the temperature dependence of elastic constants and modulus of CF and CT is investigated.The corresponding wave velocities highlight the importance of knowing the elastic and seismic properties of mantle minerals at relevant temperatures and pressures.The seismic wave velocities of CF and CT confirm that the phase transition from CF to CT cannot explain the lower mantle seismic discontinuities.Finally,the seismic anisotropies of CF and CT are derived and compared with another end member of aluminous phases,i.e.,calcium ferrite-type NaAlSiO4,and bridgmanite.It turned out that at lower mantle,temperature enhances shear wave splitting of CT to the maximum of 31%.In addition,the two aluminous phases exhibit lower shear to compressional wave velocity heterogeneity ratio RS/P and higher body wave to shear wave velocity heterogeneity ratio R?/S,density to shear wave velocity heterogeneity ratio R?/S than seismological observations.Anisotropy compatible with the primary seismic observations of shear wave splitting in the lower mantle could be the result of lattice preferred orientation or shape preferred orientation for CF or CT in a sheared boundary layer.Lastly,considering the aluminous phases always exist as solid solutions in the lower mantle,the equations of state of CF-type NaMg2Al5SiO12(NaCF)were calculated,which mixed by MgAl2O4 and NaAlSiO4 at a ratio of 2:1,at ambient condition and high temperatures.The thermoelasticity of NaCF is also investigated at 30-60 GPa and 0-3000 K.The results of NaCF are similar to CF,except for some details.After Mg2+ and Al3+ are partially replaced by Na+ and Si4+,the elastic constant components and the anisotropy change differently.On the other hand,the shear wave splitting of aluminous phases affacts more by structure other than composition.The results indicate that one can use the thermoelastic properties of the end-members of the aluminous phase to estimate the thermoelastic properties of the complex solid solution in the lower mantle,but cannot attain the accurate results.