Ab initio simulations of iron-nickel alloys at Earth's core conditions

We report ab initio density functional theory calculations on iron–nickel (FeNi) alloys at conditions representative of the Earth's inner core. We test different concentrations of Ni, up to ∼39 wt% using ab initio lattice dynamics, and investigate the thermodynamic and vibrational stability of the three candidate crystal structures (bcc, hcp and fcc). First of all, at inner core pressures, we find that pure Fe transforms from the hcp to the fcc phase at around 6000 K. Secondly, in agreement with low pressure experiments on Fe–Ni alloys, we find the fcc structure is stabilised by the incorporation of Ni under core pressures and temperatures. Our results show that the fcc structure may, therefore, be stable under core conditions depending on the temperature in the inner core and the Ni content. Lastly, we find that within the quasi-harmonic approximation, there is no stability field for FeNi alloys in the bcc structure under core conditions

High temperature elastic anisotropy of the perovskite and post-perovskite polymorphs of Al2O3

Finite temperature ab initio molecular dynamics calculations were performed to determine the high temperature elastic and seismic properties of the perovskite and post-perovskite phases of pure end-member Al2O3. The post-perovskite phase exhibits very large degrees of shear-wave splitting. The incorporation of a few mole percent of Al2O3 into MgSiO3 is predicted to have little effect on the perovskite to post-perovskite phase transition pressure and seismic properties of the post-perovskite phase; although a small difference in shear-wave splitting may be observable

Water distribution in the lower mantle: Implications for hydrolytic weakening

The presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the Earth's lower mantle in what is generally known as “hydrolytic weakening”. This weakening will have profound effects on global convection, but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative. In order to constrain the likelihood of hydrolytic weakening being important in the lower mantle, we use first principles methods to calculate the partitioning of water (strictly protons) between mineral phases of the lower mantle under lower mantle conditions. We show that throughout the lower mantle water is primarily found either in the minor Ca-perovskite phase or in bridgmanite as an Al3+–H+ pair. Ferropericlase remains dry. However, neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small. We find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents (>1000 ppm). We conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle

The pressure medium as a solid-state oxygen buffer

We present a simple method to buffer oxygen fugacity at high pressures and high temperatures where the traditional 'double capsule' method is inappropriate. The pressure medium is doped with a metal which partially reacts with the free oxygen in the pore spaces of the, cell. The resultant finely intergrown metal-metal oxide assemblage buffers the oxygen fugacity in the sample as long as the capsule and furnace materials are oxygen permeable

Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

The elasticity of FexMg1 − xO was examined under lowermost mantle temperature and pressure conditions using density functional theory (DFT). The addition of iron decreases the shear modulus of MgO but has varying effects on the bulk modulus depending on the spin state of the iron. The spin state of iron in FexMg1 − xO is dependent on pressure and temperature but also on the concentration of iron. At 136 GPa, Fe in low concentrations (75%) it is nearly entirely in the high spin state. There is, as expected, a large decrease in seismic velocities with iron substitution. However, the effect of Fe is greater at high-temperatures than at low-temperatures, meaning it is difficult to extrapolate low-temperature experimental results. We cannot simultaneously match the density and seismic velocities of ULVZs with Fe-enriched ferropericlase. This is reflected in (dln Vs/dln Vp)T,P, which in ULVZs is generally observed to be about 3, but does not exceed about 1.5 for Fe-enriched periclase. A mixture of ferropericlase and ferrous perovskite can cause Vs decreases of up to 45%, which allows the range of ULVZ Vp, Vs and densities to be matched. We also find that (dln Vs/dln Vp)T,P increases up to as much as 3 but this value is strongly dependent on the bounds of the mixing geometry. We conclude, therefore, that the properties of ULVZs can be readily explained by a lower mantle with a single phase that is heavily enriched in Fe

The electrical conductivity and thermal profile of the Earth's mid-mantle

Electrical conductivity in the Earth's mantle is sensitive to temperature and chemical environment. Recent laboratory measurements of electrical conductivity are combined with candidate mantle geotherms to produce synthetic electrical conductivity profiles. These profiles are used to forward model the Earth's geomagnetic response function C, results of which are compared with the observed globally averaged response function at periods of 3.5 days to 4 months. Candidate lower mantle geotherms, representing whole-mantle and layered convection end-members, are compared using published electrical conductivity measurements on alumina-bearing and alumina-free perovskite in the conductivity models. Comparison of the predicted response functions with the observed geomagnetic response of the Earth shows that a) if lower mantle alumina is incorporated into perovskite, then the lower mantle must be cool, and b) if the alumina is not incorporated in perovskite then the results are only consistent with a hot lower mantle. In addition, the maximum alumina content of lower mantle MgSiO3 perovskite is constrained at 4%

Ferrous iron partitioning in the lower mantle

We used density functional theory (DFT) to examine the partitioning of ferrous iron between periclase and bridgmanite under lower mantle conditions. To study the effects of the three major variables — pressure, temperature and concentration — these have been varied from 0 to 150 GPa, from 1000 to 4000 K and from 0 to 100% total iron content. We find that increasing temperature increases KD, increasing iron concentration decreases KD, while pressure can both increase and decrease KD. We find that KD decreases slowly from about 0.32 to 0.06 with depth under lower mantle conditions. We also find that KD increases sharply to 0.15 in the very lowermost mantle due to the strong temperature increases near the CMB. Spin transitions have a large effect on the activity of ferropericlase which causes KD to vary with pressure in a peak-like fashion. Despite the apparently large changes in KD through the mantle, this actually results in relatively small changes in total iron content in the two phases, with View the MathML sourceXFefp ranging from about 0.20 to 0.35, before decreasing again to about 0.28 at the CMB, and View the MathML sourceXFebd has a pretty constant value of about 0.04–0.07 throughout the lower mantle. For the very high Fe concentrations suggested for ULVZs, Fe partitions very strongly into ferropericlase

Diffusion of noble gases in subduction zone hydrous minerals

Subduction of atmospheric noble gases has been considered to play an important role in altering the primordial isotopes of Earth’s mantle over geological time. Analysis of natural samples and experiments indicate that large quantities of noble gases can be dissolved in volatile-bearing hydrous minerals in the subduction slabs. To quantitatively investigate the recycling efficiency of noble gases and relevant consequences on the mantle noble gas isotopic evolution, the diffusivities of noble gases in these minerals are needed. In this study, diffusion of He, Ne, Ar, Kr and Xe in lizardite, antigorite and tremolite have been calculated by first-principles methods based on density functional theory. Our results disclose that diffusion is slower with increasing radius of the noble gas atom (DHe > DNe > DAr > DKr > DXe) as expected. The common ring-structures in hydrous silicate minerals provide incorporation sites for the noble gas atoms and control their mobility. The diffusion activation energies are 84.9, 157.3, 287.5, 347.4, 414.9 kJ/mol from He to Xe in lizardite, and despite the very similar lattice structure between lizardite and antigorite, the activation energies are found to be significantly higher in antigorite, which are 120.6, 267.3, 449.6, 497.9 and 550.0 kJ/mol, respectively. In tremolite, the energy barriers are 93.6, 158.2, 266.3, 322.2 and 385.0 kJ/mol, which are also found to be in very good agreement with available experimental values and similar to those in lizardite. We also calculated diffusion activation energies at higher pressures (1 GPa for liazardite, 3 GPa for antigorite and tremolite) to better understand how much noble gases can be preserved against diffusive loss during subduction. Our result show that the oceanic crust and the lithospheric mantle of the subduction slab play different roles in delivering noble gases into the mantle. We find that all Ar, Kr, Xe and possibly part of the Ne can be entrained by the serpentine-dominated lithospheric mantle into the deep mantle due to the high diffusive energy barriers in antigorite. In contrast, noble gases in the amphibole-enriched oceanic crust would be characterized by fractionated noble gas signature, with the concentrations of retained noble gases in the crust following their respective ionic radius (Ne < Ar < Kr < Xe)