Early planetesimal differentiation and late accretion shaped Earth's nitrogen budget.

The relative roles of protoplanetary differentiation versus late accretion in establishing Earth's life-essential volatile element inventory are being hotly debated. To address this issue, we employ first-principles calculations to investigate nitrogen (N) isotope fractionation during Earth's accretion and differentiation. We find that segregation of an iron core would enrich heavy N isotopes in the residual silicate, while evaporation within a H2-dominated nebular gas produces an enrichment of light N isotope in the planetesimals. The combined effect of early planetesimal evaporation followed by core formation enriches the bulk silicate Earth in light N isotopes. If Earth is comprised primarily of enstatite-chondrite-like material, as indicated by other isotope systems, then late accretion of carbonaceous-chondrite-like material must contribute ~ 30-100% of the N budget in present-day bulk silicate Earth. However, mass balance using N isotope constraints shows that the late veneer contributes only a limited amount of other volatile elements (e.g., H, S, and C) to Earth.

Chalcogen isotopes reveal limited volatile contribution from late veneer to Earth.

The origin of Earth's volatile elements is highly debated. Comparing the chalcogen isotope ratios in the bulk silicate Earth (BSE) to those of its possible building blocks, chondritic meteorites, allows constraints on the origin of Earth's volatiles; however, these comparisons are complicated by potential isotopic fractionation during protoplanetary differentiation, which largely remains poorly understood. Using first-principles calculations, we find that core-mantle differentiation does not notably fractionate selenium and tellurium isotopes, while equilibrium evaporation from early planetesimals would enrich selenium and tellurium in heavy isotopes in the BSE. The sulfur, selenium, and tellurium isotopic signatures of the BSE reveal that protoplanetary differentiation plays a key role in establishing most of Earth's volatile elements, and a late veneer does not substantially contribute to the BSE's volatile inventory.

Primitive noble gases sampled from ocean island basalts cannot be from the Earth's core.

Noble gas isotopes in plumes require a source of primitive volatiles largely isolated in the Earth for 4.5 Gyrs. Among the proposed reservoirs, the core is gaining interest in the absence of robust geochemical and geophysical evidence for a mantle source. This is supported by partitioning data showing that sufficient He and Ne could have been incorporated into the core to source plumes today. Here we perform ab initio calculations on the partitioning of He, Ne, Ar, Kr and Xe between liquid iron and silicate melt under core forming conditions. For He our results are consistent with previous studies allowing for substantial amounts of He in the core. In contrast, the partition coefficient for Ne is three orders of magnitude lower than He. This very low partition coefficient would result in a 3He/22Ne ratio of ~103 in the core, far higher than observed in ocean island basalts (OIBs). We conclude that the core is not the source of noble gases in OIBs.

Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core-mantle boundary.

The lowermost portion of Earth's mantle (D″) above the core-mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3 postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da = 1,000 Db; Db ≈ Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da = 10,000 Dc; Dc = 10,000 Db) in the natural MgSiO3 system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3 postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″.

The phase diagrams of KCaF3 and NaMgF3 by ab initio simulations.

ABF3 compounds have been found to make valuable low-pressure analogues for high-pressure silicate phases that are present in the Earth's deep interior and that may also occur in the interiors of exoplanets. The phase diagrams of two of these materials, KCaF3 and NaMgF3, have been investigated in detail by static ab initio computer simulations based on density functional theory. Six ABF3 polymorphs were considered, as follows: the orthorhombic perovskite structure (GdFeO3-type; space group Pbnm); the orthorhombic CaIrO3 structure (Cmcm; commonly referred to as the "post-perovskite" structure); the orthorhombic Sb2S3 and La2S3 structures (both Pmcn); the hexagonal structure previously suggested in computer simulations of NaMgF3 (P63/mmc); the monoclinic structure found to be intermediate between the perovskite and CaIrO3 structures in CaRhO3 (P21/m). Volumetric and axial equations of state of all phases considered are presented. For KCaF3, as expected, the perovskite phase is shown to be the most thermodynamically stable at atmospheric pressure. With increasing pressure, the relative stability of the KCaF3 phases then follows the sequence: perovskite â†’ La2S3 structure â†’ Sb2S3 structure â†’ P63/mmc structure; the CaIrO3 structure is never the most stable form. Above about 2.6 GPa, however, none of the KCaF3 polymorphs are stable with respect to dissociation into KF and CaF2. The possibility that high-pressure KCaF3 polymorphs might exist metastably at 300 K, or might be stabilised by chemical substitution so as to occur within the standard operating range of a multi-anvil press, is briefly discussed. For NaMgF3, the transitions to the high-pressure phases occur at pressures outside the normal range of a multi-anvil press. Two different sequences of transitions had previously been suggested from computer simulations. With increasing pressure, we find that the relative stability of the NaMgF3 phases follows the sequence: perovskite â†’ CaIrO3 structure â†’ Sb2S3 structure â†’ P63/mmc structure. However, only the perovskite and CaIrO3 structures are stable with respect to dissociation into NaF and MgF2.

Modeling the melting of multicomponent systems: the case of MgSiO3 perovskite under lower mantle conditions.

Knowledge of the melting properties of materials, especially at extreme pressure conditions, represents a long-standing scientific challenge. For instance, there is currently considerable uncertainty over the melting temperatures of the high-pressure mantle mineral, bridgmanite (MgSiO3-perovskite), with current estimates of the melting T at the base of the mantle ranging from 4800 K to 8000 K. The difficulty with experimentally measuring high pressure melting temperatures has motivated the use of ab initio methods, however, melting is a complex multi-scale phenomenon and the timescale for melting can be prohibitively long. Here we show that a combination of empirical and ab-initio molecular dynamics calculations can be used to successfully predict the melting point of multicomponent systems, such as MgSiO3 perovskite. We predict the correct low-pressure melting T, and at high-pressure we show that the melting temperature is only 5000 K at 120 GPa, a value lower than nearly all previous estimates. In addition, we believe that this strategy is of general applicability and therefore suitable for any system under physical conditions where simpler models fail.

Core formation and core composition from coupled geochemical and geophysical constraints.

The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.

A seismologically consistent compositional model of Earth's core.

Earth's core is less dense than iron, and therefore it must contain "light elements," such as S, Si, O, or C. We use ab initio molecular dynamics to calculate the density and bulk sound velocity in liquid metal alloys at the pressure and temperature conditions of Earth's outer core. We compare the velocity and density for any composition in the (Fe-Ni, C, O, Si, S) system to radial seismological models and find a range of compositional models that fit the seismological data. We find no oxygen-free composition that fits the seismological data, and therefore our results indicate that oxygen is always required in the outer core. An oxygen-rich core is a strong indication of high-pressure and high-temperature conditions of core differentiation in a deep magma ocean with an FeO concentration (oxygen fugacity) higher than that of the present-day mantle.

Strong premelting effect in the elastic properties of hcp-Fe under inner-core conditions.

The observed shear-wave velocity VS in Earth's core is much lower than expected from mineralogical models derived from both calculations and experiments. A number of explanations have been proposed, but none sufficiently explain the seismological observations. Using ab initio molecular dynamics simulations, we obtained the elastic properties of hexagonal close-packed iron (hcp-Fe) at 360 gigapascals up to its melting temperature Tm. We found that Fe shows a strong nonlinear shear weakening just before melting (when T/Tm > 0.96), with a corresponding reduction in VS. Because temperatures range from T/Tm = 1 at the inner-outer core boundary to T/Tm ≈ 0.99 at the center, this strong nonlinear effect on VS should occur in the inner core, providing a compelling explanation for the low VS observed.

Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties.

Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D'') is important to understand the formation and dynamics of the Earth's mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core-mantle interaction, the presence of remnants of subducted material and that D'' is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core-mantle boundary temperature and pressure conditions. Here we explore the efficacy of this 'post-perovskite' phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D'' discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.

Subducted banded iron formations as a source of ultralow-velocity zones at the core-mantle boundary.

Ultralow-velocity zones (ULVZs) are regions of the Earth's core-mantle boundary about 1-10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10-20 per cent for compressional waves and 10-30 per cent for shear waves. It is also thought that such regions have an increased density of about 0-20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt, core-mantle reaction zones or underside sedimentation from the core. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core-mantle boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earth's interior by subduction, sink to the bottom of the mantle and may explain all of the observed features of ULVZs.

Electronic structure study of the high-pressure vibrational spectrum of FeS2 pyrite.

Plane-wave density functional calculations are used to investigate the pressure dependence of the geometry and Gamma-point phonons of FeS(2) pyrite up to 150 GPa. The linear response method is employed to calculate the vibrational properties. Raman-active modes are in excellent agreement with the experimental data available up to 50 GPa,(1) and we predict the evolution with pressure of the IR-active modes for which no high-pressure spectroscopic data have been reported so far. Over the wide pressure range investigated here, all vibrational frequencies depend nonlinearly on pressure; their pressure dependence is quantified by determining the full set of mode Grüneisen parameters and their pressure derivatives.

Computational mineral physics and the physical properties of perovskite.

The inherent uncertainties in modern first-principles calculations are reviewed using geophysically relevant examples. The elastic constants of perovskite at lower-mantle temperatures and pressures are calculated using ab initio molecular dynamics. These are used in conjunction with seismic tomographic models to estimate that the lateral temperature contrasts in the Earth's lower mantle are 800 K at a depth of 1000 km, and 1500 K at a depth of 2000 km. The effect of Al(3+) on the compressibility of MgSiO(3) perovskite is calculated using three different pseudopotentials. The results confirm earlier work and show that the compressibility of perovskites with Al(3+) substituted for both Si(4+) and Mg(2+) is very similar to the compressibility of Al(3+)-free perovskite. Even when 100% of the Si(4+) and Mg(2+) ions are replaced with Al(3+), the bulk modulus is only 7% less than that for Al(3+)-free perovskite. In contrast, perovskites where Al(3+) substitutes for Si(4+) only and that are charge balanced by oxygen vacancies do show higher compressibilities. When corrected to similar concentrations of Al(3+), the calculated compressibilities of the oxygen-vacancy-rich perovskites are in agreement with experimental results.