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.

Structure and Density of FeS at High Pressure and High Temperature and the Internal Structure of Mars

In situ x-ray diffraction measurements revealed that FeS, a possible core material for the terrestrial planets, transforms to a hexagonal NiAs superstructure with axial ratio (c/a) close to the ideal close-packing value of 1.63 at high pressure and high temperature. The high-pressure-temperature phase has shorter Fe-Fe distances than the low-pressure phase. Significant shortening of the Fe-Fe distance would lead to metallization of FeS, resulting in fundamental changes in physical properties of FeS at high pressure and temperature. Calculations using the density of the high-pressure-temperature FeS phase indicate that the martian core-mantle boundary occurs within the silicate perovskite stability field.

Dynamic compression of Earth materials

Shock wave techniques have been used to investigate the pressuredensity relations of metals, silicates, and oxides over the entire range of pressures present in the earth (3.7 x 10(6) bars at the center). In many materials of geophysical interest, such as iron, wüstite, calcium oxide, and forsterite, major shock-induced phase changes dominate the compression behavior below pressures of 10(6) bars. The shock wave data for the high-pressure phases of these minerals lead to important inferences about the composition of the lower mantle and outer, liquid core of the earth. The lower mantle of the earth appears to have a slightly higher density than is inferred to correspond to the behavior of an olivine-rich assembiage of the same composition as the upper mantle. The core has a density some 10 percent less than that of pure iron and may have 9 to 12 percent sulfur or about 8 percent oxygen by weight.

Thermal expansion of iron-rich alloys and implications for the Earth's core

Understanding the thermal-chemical state of the Earth's core requires knowledge of the thermal expansion of iron-rich alloys at megabar pressures and high temperatures. Our survey of literature revealed a significant lack of such data. We have determined the unit-cell parameters of the iron-sulfur compound Fe(3)S by using synchrotron x-ray diffraction techniques and externally heated diamond-anvil cells at pressures up to 42.5 GPa and temperatures up to 900 K. The zero-pressure thermal expansivity of Fe(3)S is determined in the form alpha = a(1) + a(2)T, where a(1) = 3.0 +/- 1.3 x 10(-5) K(-1) and a(2) = 2.8 +/- 1.5 x 10(-8) K(-2). The temperature dependence of isothermal bulk modulus ((partial differential)K(T,0)/(partial differential)T)(P) is estimated at -3.75 +/- 1.80 x 10(-2) GPa K(-1). Our data at 42.5 GPa and 900 K suggest that approximately 2.1 at. % (1.2 wt. %) sulfur produces 1% density deficit in iron. We have also carried out energy-dispersive x-ray diffraction measurements on pure iron and Fe(0.864)Si(0.136) alloy samples that were placed symmetrically in the same multi-anvil cell assemblies, using the SPring-8 synchrotron facility in Japan. Based on direct comparison of unit cell volumes under presumably identical pressures and temperatures, our data suggest that at most 3.2 at. % (1.6 wt. %) silicon is needed to produce 1% density deficit with respect to pure iron.

Constraints on the composition of the Earth's core from ab initio calculations

Knowledge of the composition of the Earth's core is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron. The outer core, however, is also thought to contain a significant fraction of light elements, because its density--as deduced from seismological data and other measurements--is 6-10 per cent less than that estimated for pure liquid iron. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron-sulphur mixture.

Models of the Earth's Core

Combined inferences from seismology, high-pressure experiment and theory, geomagnetism, fluid dynamics, and current views of terrestrial planetary evolution lead to models of the earth's core with the following properties. Core formation was contemporaneous with earth accretion; the core is not in chemical equilibrium with the mantle; the outer core is a fluid iron alloy containing significant quantities of lighter elements and is probably almost adiabatic and compositionally uniform; the more iron-rich inner solid core is a consequence of partial freezing of the outer core, and the energy release from this process sustains the earth's magnetic field; and the thermodynamic properties of the core are well constrained by the application of liquid-state theory to seismic and laboratory data.