Physics And Chemistry Of The Earth's Interior

The physics and chemistry of the Earth's interior are fundamental for our understanding of the Earth's composition,origin,evolution,and geodynamics.Seismic wave is one of the most important tools to explore the structure of the Earth's interior and constrain the physical and chemical processes.Elastic properties of mantle minerals at simultaneously high P-T conditions are key parameters for the interpretation of the composition and evolution of the Earth's interior from the seismic velocities.On the other hand,the physics and chemistry of the Earth's interior can be also derived from the natural samples.With the development of technology,more and more stable isotopes have been applied to investigate the composition and processes in the Earth's interior.The equilibrium isotope fractionation factors are key parameters for the understanding of fractionation mechanisms,which are crucial for the stable isotope geochemistry.In this thesis,we obtain the elastic properties of some mantle minerals under the mantle conditions using first-principles calculations.Combining seismological observations with elastic data,we explore the water distribution in the mantle transition zone and the origin of lower-mantle heterogeneities.Meanwhile,we also obtain the equilibrium fractionation factors for many isotope systems based on the density functional theory,providing a guideline for stable isotope geochemistry.Water can dissolve into the structure of anhydrous mantle minerals as an impurity.It is potentially present in the mantle transition zone(MTZ)at the several weight percent level on the basis of its known solubility in wadsleyite and ringwoodite.Whether the transition zone is wet or dry fundamentally impacts our understanding of the processes such as the deep-water cycle,melting,and geochemical cycling,mantle convection,and the formation of seismic low-velocity zones.Seismological observations can be used to decipher the water content in the MTZ,however,the elastic properties of hydrous wadsleyite and ringwoodite at the P-T conditions of MTZ are still absent in the literature.Here we calculated the elasticity of hydrous wadsleyite and ringwoodite at simultaneously high P-T conditions based on the density functional theory(DFT)and evaluated the water effect on the velocities of wadsleyite and ringwoodite.Together with previous determinations for the olivine elasticity,we simultaneously modeled the density and seismic velocity jumps across the olivine-wadsleyite transition.Our modeling can reconcile the 410-km discontinuity with a pyrolitic mantle if the water content in wadsleyite is?0.9 wt.%and the olivine is at its storage capacity of?500 ppm,indicating the presence of a hydrous melt phase exists at depths just above the phase transition.Meanwhile,combining the seismic observations with our data for hydrous ringwoodite,we also found that the lower part of the pyrolitic MTZ is locally water-rich(>0.5 wt.%)and moderately hydrous on average(-0.2 wt%).This demonstrates that the water content likely varies with depth in the MTZ.We also calculated the water partition coefficient between wadsleyite and ringwoodite,which is?1.7 at the phase boundary.The presence of water cannot increase the velocity contrasts between wadsleyite and ringwoodite,whereas it can significantly promote their impendence contrasts and reduce the pressure width of the coexisting loop of two phases.Thus,the local occurrence of 520-km discontinuity likely indicates the presence of water in the middle part of MTZ.Due to the large water partition coefficient between ringwoodite and bridgmanite(?30),the lower mantle has been suggested to be nearly dry.However,seismological studies have found numerous seismic heterogeneities with different length scales in the lower mantle.The subducted oceanic crust that has a distinctive chemical composition from the lower mantle was proposed as the main chemical origin for some of these heterogeneities.We also determined the velocity and density of oceanic crust along different mantle geotherms.Our results indicate that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica,highly consistent with the features of detected mid-mantle seismic scatterers.After this phase transition in silica,the oceanic crust has relatively higher wave velocities than those of the ambient mantle even when its temperature profile is+1000 Kelvin hotter than the ambient mantle,ruling out the possibility that the large low shear velocity provinces(LLSVPs)mainly originates from the subducted oceanic crust.Instead,the accumulation of the subducted oceanic crust along relatively cold geotherm can produce high velocity heterogeneities(?2%)in the lower mantle imaged by seismic tomography.On the other hand,the physics,chemistry,and the evolution of the Earth's interior can be also recorded by isotopic signatures,and the equilibrium isotope fractionation factors are key parameters for the isotope applications in geochemical processes.In general,most elements naturally occur as the formation of solid solutions and their contents can vary in a wide range.However,the concentration effect on the equilibrium isotope fractionation has been ignored.Based on the DFT calculations,we predicted the equilibrium fractionation(103ln?)of Mg-Ca isotopes among carbonate minerals and 103ln? of Ca isotope between orthopyroxene(opx)and clinopyroxene(cpx).We found that concentration of Ca and Mg in carbonates have significant effects on the 103ln? of 26Mg/24Mg and 44Ca/40Ca between other carbonates and dolomite.Meanwhile,I also found that the 103ln? of 44Ca/40Ca between opx and cpx also strongly depends on Ca content in opx,while it is not sensitive to Ca concentration in opx when Ca content is lower than a threshold value.Because most naturally occurring minerals are solid solutions with variable chemical compositions,the concentration effect on the equilibrium isotope fractionation should be widely considered in isotope geochemistry.Furthermore,with the consideration of concentration effect,we calculated the equilibrium Ti isotope fractionation factors among major Ti-bearing minerals.Our results show that limited Ti isotope fractionation is observed among clinopyroxene,orthopyroxene,olivine,and pyrope,whereas the one between Fe-Ti oxides and silicate minerals is up to-0.67‰ at 1200 K.This demonstrates that Fe-Ti oxides are important fractionating during magma differentiation.More importantly,we have established a new method to predict the equilibrium isotope fractionation between fluids/melts and minerals based on ab initio molecular dynamics simulations.This method was used to calculate the equilibrium Mg isotope fractionations between aqueous Mg2+ and carbonates and our predictions are highly consistent with the reliable experimental data.Compared to aqueous Mg2+,carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes.These data provide a guideline for applications of Mg isotopes in aqueous geochemical processes,such as the continental weathering and the global Mg cycle.