Transport and Thermodynamic Properties of Iron Silicate Melts from Computational Simulation

Author:Sun Yi Cheng

Supervisor:Xu Shijin, zhou Huiqun


Degree Year:2018





Geochemical evidence suggests that the early Earth may have experienced several global melting events,leading to the formation of early continental crust and promoting chemical differentiation between the iron core and the silicate mantle.As an important carrier of migration for the materials and energy inside the earth,silicate melt plays an important role in the evolution of the earth.Its transport and thermodynamic properties determine the chemical and thermal evolution of magma ocean.Seismological studies indicate that there may be some partial melting regions within the Earth,such as plate subduction zone,asthenosphere low-velocity zone(LVZ),large low-shear-velocity provinces(LLSVPs),and core-mantle boundary(CMB).To obtain a deeper understanding of the geological processes in the molten region of the Earth,it is necessary to correctly understand the transport and thermodynamic properties of the silicate melt.Iron,as the most important metal element,has a non-negligible effect on the properties of silicate melts.The concentration of iron in the early stage of magma ocean may be higher than that in current mantle,and the partial melting regions in current mantle may be related to the presence of iron.At present,numerous studies of the dynamical properties of iron-free silicate melts have been carried out,but the research on iron silicate melts is still very rare.In this thesis,the classical molecular dynamics method and the first-principles method were used to study the iron end member of olivine and the iron end member of bridgmanite respectively.The main conclusions are listed as follows.The self-diffusion coefficient increases and viscosity decreases with increasing temperature,thus implying that temperature has a significant effect on the transport coefficients of Fe2SiO4 melt.The temperature dependence of the transport coefficients displays Arrhenius behavior below 5 GPa but exhibits a crossover from Arrhenius to non-Arrhenius behavior at higher pressure,probably due to the dynamical cooperativity caused by high coordination numbers.The pressure has inhibitory effect on the mobility of Fe2SiO4 melt,in which the viscosity increases with increasing pressure and the self-diffusion coefficient is reversed.The dependence of transport properties on the temperature and pressure of Fe2SiO4 melt similar to that of Mg2SiO4 melt,however,Fe2SiO4 melt is notably more viscous than Mg2SiO4 melt at high pressure as a result of the higher percentage content of bridging oxygen in the Fe2SiO4melt.Along magma ocean isentropes,we constrain the viscosity profiles of Fe2SiO4 melt in an early magma ocean.The viscosity of Fe2SiO4 melt increases by 1-2 orders of magnitude as pressure increases up to 50 GPa.Combined with the viscosity data of Mg2SiO4 melt,we suggest that the iron-rich silicate melts would have decelerated the cooling rate in the lower mantle and had a great effect on the physical properties of the lower mantle portion of the magma ocean.The high viscosity of iron-rich melt in the early magma ocean may be related to the formation of ULVZs.We obtain the P-T-V equation of state and thermodynamic parameters of FeSiO3 melt at high temperature and high pressure.The FeSiO3 liquid adiabats for given potential temperatures is calculated and compared to that of MgSiO3 liquid and found that a lower potential temperature can produce superliquidus.The crystallization pathway of a magma ocean is discussed according to different mantle liquidus.FeSiO3 melt has a slight change to the crystallization path,but the key still depends on the mantle liquidus.In addition,the 2500 K adiabat of FeSiO3 liquid has a temperature of 5360 K at the base of a completely melted mantle,which is 560 K~1100 K higher than the temperature of 2500 K adiabat of MgSiO3 liquid in the same location.Higher temperature would reduce the viscosity at the bottom of the mantle and has significant effect on the evolution of the bottom of the magma ocean.At the same time,we also obtained the detailed structural information and spin transition of iron in FeSiO3 melt under high temperature and high pressure.Through the entire mantle,the mean coordination numbers of Si-O for the FeSiO3 melt increases from 4 to 6 with increasing pressure,and the mean coordination numbers of Fe-0 can reach more than 7,which mean that the structures of FeSiO3 melt change significantly at the bottom of the mantle.Compared with the olivine melts,the degree of polymerization of FeSiO3 melt in the mantle is relatively higher,which will affect the transport properties of the FeSiO3 melt.We find that the transition from high-spin to low-spin of iron in FeSiO3 melt is linear,and the pressure interval of the spin transition exceeds 296 GPa.The calculated spin state of iron in FeSiO3 liquid is mainly high-spin state near the conditions of the core-mantle boundary.In combination with previous studies,the spin crossover of iron in silicate melts may promote the continuous enrichment of iron into the melts with increasing pressure throughout the mantle.