Zizheng Gong

Evidence for an oxygen-depleted liquid outer core of the Earth

On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth’s liquid outer core consists of mainly liquid iron alloyed with about ten per cent (by weight) of light elements. Although the concentrations of the light elements are small, they nevertheless affect the Earth’s core: its rate of cooling, the growth of the inner core, the dynamics of core convection, and the evolution of the geodynamo. Several light elements—including sulphur, oxygen, silicon, carbon and hydrogen—have been suggested, but the precise identity of the light elements in the Earth’s core is still unclear. Oxygen has been proposed as a major light element in the core on the basis of cosmochemical arguments and chemical reactions during accretion. Its presence in the core has direct implications for Earth accretion conditions of oxidation state, pressure and temperature. Here we report new shockwave data in the Fe–S–O system that are directly applicable to the outer core. The data include both density and sound velocity measurements, which we compare with the observed density and velocity profiles of the liquid outer core. The results show that we can rule out oxygen as a major light element in the liquid outer core because adding oxygen into liquid iron would not reproduce simultaneously the observed density and sound velocity profiles of the outer core. An oxygen-depleted core would imply a more reduced environment during early Earth accretion.

Variations of Thermal Pressure for Solids along the Principal Hugoniot

The behavior of thermal pressure PTH for all kinds of solid materials was investigated using the lattice dynamics theory up to 500GPa. The results show that for most metals, ionic crystal and minerals, the thermal pressure is approximately independent on volume, whereas the thermal pressure of a few solids has strong dependence on volume. The volume dependence of thermal pressure has no relation with the chemical bonding type and crystal structure of materials, but is correlated with the Debye temperature ΘD and the second Gruneisen parameter q. The ratio of the thermal pressure to the total pressure (PTH /PTotal) along the Hugoniot keeps constant over a wide compression range, not only for non‐porous materials but also for porous materials within certain porosity, which could explain the existence of material constant parameter β along solid Hugoniot.

Sound Velocity of (Mg0.92, Fe0.08)SiO3 Perovskite up to 140 Shock Pressure and its Geophysical Implications

New experimental data of compressional sound velocity VP for natural polycrystalline Enstatite (Mg0.92, Fe0.08) SiO3 initial specimens were measured using the optical analyzer techniques, and our former data were also revised. The VP showed two discontinuities in the pressure range up to 140 GPa. Combination with previous Hugoniot research, three distinct regions appear for (Mg0.92, Fe0.08) SiO3 Enstatite: a low‐pressure phase (LPP, Enstatite) exists up to shock pressures about 64 GPa, while a mixed phase (MP, Enstatite+ Perovskite ) is possible near the high pressure part of this range; perovskite phase (PvP) was found between about 68 to 83 GPa; and then a new high‐pressure perovskite phase (NPvP) occurs at shock pressures higher than 85 GPa. Between 83–85GPa, VP showed a negative jump. The solid to solid microstructure transition of (Mg0.92, Fe0.08)SiO3 Perovskite, caused by the high‐spin to low spin HS‐LS transition of iron around 83 GPa pressure region, probably is responsible for the radial anomaly ...

A New Evidence of the Stability of (Mg, Fe)SiO3 Perovskite at Lower Mantle Conditions: Shock Recovery Experiments

By using a two‐stage light gas gun, 9 experiments of shock recovery experiments with initial sample of (Mg0.92, Fe0.08) SiO3 Enstatite (En) (7 experiments) and MgO+SiO2 (2 experiments) were conducted between 65 and 110 GPa shock pressure (the corresponding temperature is estimated as 2500∼5000K). The analysis of X‐Ray Diffraction (XRD) and Infrared (IR) for the shock recovered samples indicate that there is no possibility for the chemical decomposition of (Mg0.92, Fe0.08) SiO3 perovskite into SiO2 plus (Mg0.92, Fe0.08) O during shock compression. Our experiments support that (Mg, Fe) SiO3‐perovskite is stable at lower mantle P&T conditions.

An Empirical Material Constant and Equation of State on the Solids Hugoniot

A new material parameter β: β = (ρ0 − ρ00 pH/PH′)/ρ0ρ00 (1 − PH/PH′), where ρ is density and subscript 0 and 00 represent different initial density, and PH and PH′ represent Hugoniot pressure of ρ0 and ρ00 which compressed to the same density ρ, was find out to keep in constant along Hugoniot. For metal βmetal=1.217ρ0−0.884, where ρ0 is the ideal crystal density. By using of β, Hugoniot data of different initial density samples can be simply converted by: PH = PH′ ρ0 (βρ00 − 1)/ρ00 (βρ0 − 1), and a new empirical Equation to express Hugoniot State was obtained: pHVn = p0V0n, where n is constant. The properties and limitation of this empirical material parameter to be constant with variations of pressure and porosity were discussed.

Equation of State, Phase Stability of (Mg0.92, Fe0.08)SiO3 Perovskite from Shock Wave Study and Its Geophysical Implications

13 shots of shock compression data were measured for enstatite (Mg0.92, Fe0.08)SiO3 with initial density of 3.06g/cm3 up to 140 GPa, using impedance‐match method and electrical probe technique. The relationship between shock wave velocity D and particle velocity u can been described linearly by: D = 3.76 +1.48u(km/s), and no evidence of phase transition was shown in the experimental shock pressure range. Our experimental Hugoniot is about 7% denser than the model Hugoniot of (Mg0.92, Fe0.08)O (Mw.) plus SiO2(St.) calculated by additive principle. This excluded the possibility that chemical decomposition of enstatite with perovskite structure to oxides would happen during shock compression up to 140GPa. The Gruneisen parameter γobtained by fitted our experimental data to: γ=γ0 (ρ0/ρ)q, yields γ0=1.84, q=1.69, with ρ0=4.19g/cm3. By using the third‐order Birch‐Murnaghan finite strain equation of state (EOS), Our shock experimental data yield a zero‐pressure bulk modulus K0s=260.09GPa and pressure derivative ...