Geochemistry of mantle–core differentiation at high pressure

Iron alloys of volatile elements in the deep Earth's interior

Investigations into the compositional model of the Earth, particularly the atypical concentrations of volatile elements within the silicate portion of the early Earth, have attracted significant interest due to their pivotal role in elucidating the planet's evolution and dynamics. To understand the behavior of such volatile elements, an established 'volatility trend' has been used to explain the observed depletion of certain volatile elements. However, elements such as Se and Br remain notably over-depleted in the silicate Earth. Here we show the results from first-principles simulations that explore the potential for these elements to integrate into hcp-Fe through the formation of substitutional alloys, long presumed to be predominant constituents of the Earth's core. Based on our findings, the thermodynamic stability of these alloys suggests that these volatile elements might indeed be partially sequestered within the Earth's core. We suggest potential reservoirs for volatile elements within the deep Earth, augmenting our understanding of the deep Earth's composition.

Towards molecular alloys: computational and experimental studies on (p-NCC6F4CNSeSeN)x(p-NCC6F4CNSSN)1-x

The β-phase of the radical p-NCC6F4CNSSN (1β) crystallizes in the orthorhombic space group Fdd2 and orders as a canted antiferromagnet with TN = 36 K. Computational studies (B3LYP or M06-2X functional with the cc-pVTZ-PP(-F)+basis set) of the microscopic nearest-neighbour magnetic exchange coupling in 1β and in the hypothetical isomorphous phase of the selenium radical p-NCC6F4CNSeSeN (2β) revealed that replacement of S by Se should lead to a significant enhancement in the magnetic ordering temperature by ca. 20% (B3LYP) - 30% (M06-2X). Recrystallization of 2 from solution or via vacuum sublimation afforded only the known diamagnetic, dimeric phase, 2α. Computational studies indicated that both the molecular geometry and charge distribution for 1 and 2 are extremely similar and experimental approaches to form alloys of the general form 11-x2x were explored: attempts to cosublime 1 and 2in vacuo were unsuccessful, forming only 1β due to the low volatility of 2. Crystallization of pure 1 by solution evaporation was found to afford polymorph 1α (triclinic, P1̄) selectively, irrespective of the solvent employed (CH2Cl2, MeCN, PhMe or THF) but 1α transformed to 1β upon subsequent vacuum sublimation. Crystallization of 1 in the presence of 2 (up to 20 mol%) from solution evaporation was examined. At 20 mol% there was clear evidence for formation of both 1α and 2α as distinct crystallographic phases by powder X-ray diffraction (PXRD) but some evidence for doping of 2 into 1α at low concentration (≤15 mol percent) was observed. Attempts to sublime a sample of 10.920.1 led to phase separation with the isolation of needle-shaped crystals of pure 1β characterized by X-ray diffraction.

A partially equilibrated initial mantle and core indicated by stress-induced percolative core formation through a bridgmanite matrix

The Earth's core formation mechanism determines the siderophile and light elements abundance in the Earth's mantle and core. Previous studies suggest that the sink of massive liquid metal through a solid silicate mantle resulted in an unequilibrated core and the lower mantle. Here, we show that percolation can be an effective core formation mechanism in a convective mantle and modify the compositions of the lower mantle and the core through partial equilibration between them. This grain-scale metal flow has a high velocity to meet the time constraint of core formation. The Earth's core could have been enriched with light elements, and the abundance of the moderately siderophile elements in the mantle could have been elevated to the current value during this process. The trapped core-forming melt in the mantle during the stress-induced percolation can also explain the highly siderophile element abundance in the Earth's mantle.

Behavior of light elements in iron-silicate-water-sulfur system during early Earth's evolution

Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D₂O) as Mg(OD)₂ in the ideal composition (Fe–MgSiO₃–D₂O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)₂, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeD_x) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x <  ~ 0.4). Both H and S can be incorporated into solid Fe and other light elements could have dissolved into molten iron hydride and/or FeS during the later process of Earth’s evolution.

A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres

Magma oceans were once ubiquitous in the early solar system, setting up the initial conditions for different evolutionary paths of planetary bodies. In particular, the redox conditions of magma oceans may have profound influence on the redox state of subsequently formed mantles and the overlying atmospheres. The relevant redox buffering reactions, however, remain poorly constrained. Using first-principles simulations combined with thermodynamic modeling, we show that magma oceans of Earth, Mars, and the Moon are likely characterized with a vertical gradient in oxygen fugacity with deeper magma oceans invoking more oxidizing surface conditions. This redox zonation may be the major cause for the Earth's upper mantle being more oxidized than Mars' and the Moon's. These contrasting redox profiles also suggest that Earth's early atmosphere was dominated by CO2 and H2O, in contrast to those enriched in H2O and H2 for Mars, and H2 and CO for the Moon.

Deep magma ocean formation set the oxidation state of Earth's mantle

The composition of Earth's atmosphere depends on the redox state of the mantle, which became more oxidizing at some stage after Earth's core started to form. Through high-pressure experiments, we found that Fe²⁺ in a deep magma ocean would disproportionate to Fe³⁺ plus metallic iron at high pressures. The separation of this metallic iron to the core raised the oxidation state of the upper mantle, changing the chemistry of degassing volatiles that formed the atmosphere to more oxidized species. Additionally, the resulting gradient in redox state of the magma ocean allowed dissolved CO₂ from the atmosphere to precipitate as diamond at depth. This explains Earth's carbon-rich interior and suggests that redox evolution during accretion was an important variable in determining the composition of the terrestrial atmosphere.

Giant impacts stochastically change the internal pressures of terrestrial planets

Pressure is a key parameter in the physics and chemistry of planet formation and evolution. Previous studies have erroneously assumed that internal pressures monotonically increase with the mass of a body. Using smoothed particle hydrodynamics and potential field method calculations, we demonstrate that the hot, rapidly rotating bodies produced by giant impacts can have much lower internal pressures than cool, slowly rotating planets of the same mass. Pressures subsequently increase because of thermal and rotational evolution of the body. Using the Moon-forming impact as an example, we show that the internal pressures after the collision could have been less than half that in present-day Earth. The current pressure profile was not established until Earth cooled and the Moon receded, a process that may take up to tens of millions of years. Our work defines a new paradigm for pressure evolution during accretion of terrestrial planets: stochastic changes driven by impacts.

Diamond Anvil Cell Partitioning Experiments for Accretion and Core Formation: Testing the Limitations of Electron Microprobe Analysis

Metal-silicate partitioning studies performed in high-pressure, laser-heated diamond anvil cells (DAC) are commonly used to explore element distribution during planetary-scale core-mantle differentiation. The small run-products contain suitable areas for analysis commonly less than tens of microns in diameter and a few microns thick. Because high spatial resolution is required, quantitative chemical analyses of the quenched phases is usually performed by electron probe microanalysis (EPMA). Here, EPMA is being used at its spatial limits, and sample thickness and secondary fluorescence effects must be accounted for. By using simulations and synthetic samples, we assess the validity of these measurements, and find that in most studies DAC sample wafers are sufficiently thick to be characterized at 15 kVacc. Fluorescence from metal-hosted elements will, however, contaminate silicate measurements, and this becomes problematic if the concentration contrast between the two phases is in excess of 100. Element partitioning experiments are potentially compromised; we recommend simulating fluorescence and applying a data correction, if required, to such DAC studies. Other spurious analyses may originate from sources external to the sample, as exemplified by 0.5 to &gt;1 wt% of Cu arising from continuum fluorescence of the Cu TEM grid the sample is typically mounted on.

Synthesis of Xenon and Iron-Nickel Intermetallic Compounds at Earth's Core Thermodynamic Conditions

Using in situ synchrotron x-ray diffraction and Raman spectroscopy in concert with first principles calculations we demonstrate the synthesis of stable Xe(Fe,Fe/Ni)_{3} and XeNi_{3} compounds at thermodynamic conditions representative of Earth's core. Surprisingly, in the case of both the Xe-Fe and Xe-Ni systems Fe and Ni become highly electronegative and can act as oxidants. The results indicate the changing chemical properties of elements under extreme conditions by documenting that electropositive at ambient pressure elements could gain electrons and form anions.

Earth's volatile contents established by melting and vaporization

The silicate Earth is strongly depleted in moderately volatile elements (e.g. Pb, Zn, In, alkalis) relative to CI chondrites, the meteorites which compositionally most closely resemble the Sun1. Qualitatively the depletion “trend” may be explained by accretion of 10-20% of a volatile-rich body to a reduced volatile-free protoEarth2,3 followed by partial extraction of some elements to the core1. Several issues remain, however, notably the overabundance of In in silicate Earth which leads to questions about the sources of Earth’s volatiles4,5. Here we have examined the melting processes which attended accretion on Earth and precursor bodies and performed vaporisation experiments under conditions of fixed temperature and oxygen partial pressure. We find that the pattern of volatile element depletion in silicate Earth is consistent with partial melting and vaporisation rather than with simple accretion of a volatile-rich chondrite-like body. We argue that melting and vaporisation on precursor bodies and possibly during the giant moon-forming impact6–8 was responsible for establishing the observed abundances of moderately volatile elements in the Earth.

Iron isotopic fractionation between silicate mantle and metallic core at high pressure

The +0.1‰ elevated ⁵⁶Fe/⁵⁴Fe ratio of terrestrial basalts relative to chondrites was proposed to be a fingerprint of core-mantle segregation. However, the extent of iron isotopic fractionation between molten metal and silicate under high pressure–temperature conditions is poorly known. Here we show that iron forms chemical bonds of similar strengths in basaltic glasses and iron-rich alloys, even at high pressure. From the measured mean force constants of iron bonds, we calculate an equilibrium iron isotope fractionation between silicate and iron under core formation conditions in Earth of ∼0–0.02‰, which is small relative to the +0.1‰ shift of terrestrial basalts. This result is unaffected by small amounts of nickel and candidate core-forming light elements, as the isotopic shifts associated with such alloying are small. This study suggests that the variability in iron isotopic composition in planetary objects cannot be due to core formation.

Terrestrial basalts have a unique iron isotopic signature taken as fingerprints of core formation. Here, high pressure studies show that force constants of iron bonds increase with pressure similarly for silicate and metals suggesting interplanetary isotopic variability is not due to core formation.

An early geodynamo driven by exsolution of mantle components from Earth's core

Terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, stripping iron-loving elements from the silicate mantle to the metallic core1–3, and leaving rock-loving components behind. Here we performed experiments showing that at high enough temperature, Earth’s major rock-loving component, magnesium oxide, can also dissolve in core-forming metallic melts. Our data clearly point to a dissolution reaction, and are in agreement with recent DFT calculations4. Using core formation models5, we further show that a high-temperature event during Earth’s accretion (such as the Moon-forming giant impact6) can contribute significant amounts of magnesium to the early core. As it subsequently cools, the ensuing exsolution7 of buoyant magnesium oxide generates a substantial amount of gravitational energy. This energy is comparable to if not significantly higher than that produced by inner core solidification8 — the primary driver of the Earth’s current magnetic field9–11. Since the inner core is too young12 to explain the existence of an ancient field prior to ~1 billion years, our results solve the conundrum posed by the recent paleomagnetic observation13 of an ancient field at least 3.45 Gyr old.

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.

Siderophile element constraints on the origin of the Moon

Discovery of small enrichments in (182)W/(184)W in some Archaean rocks, relative to modern mantle, suggests both exogeneous and endogenous modifications to highly siderophile element (HSE) and moderately siderophile element abundances in the terrestrial mantle. Collectively, these isotopic enrichments suggest the formation of chemically fractionated reservoirs in the terrestrial mantle that survived the putative Moon-forming giant impact, and also provide support for the late accretion hypothesis. The lunar mantle sources of volcanic glasses and basalts were depleted in HSEs relative to the terrestrial mantle by at least a factor of 20. The most likely explanations for the disparity between the Earth and Moon are either that the Moon received a disproportionately lower share of late accreted materials than the Earth, such as may have resulted from stochastic late accretion, or the major phase of late accretion occurred prior to the Moon-forming event, and the putative giant impact led to little drawdown of HSEs to the Earth's core. High precision determination of the (182)W isotopic composition of the Moon can help to resolve this issue.

Structural change in molten basalt at deep mantle conditions

Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth's interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.

An in situ approach to study trace element partitioning in the laser heated diamond anvil cell

Data on partitioning behavior of elements between different phases at in situ conditions are crucial for the understanding of element mobility especially for geochemical studies. Here, we present results of in situ partitioning of trace elements (Zr, Pd, and Ru) between silicate and iron melts, up to 50 GPa and 4200 K, using a modified laser heated diamond anvil cell (DAC). This new experimental set up allows simultaneous collection of x-ray fluorescence (XRF) and x-ray diffraction (XRD) data as a function of time using the high pressure beamline ID27 (ESRF, France). The technique enables the simultaneous detection of sample melting based to the appearance of diffuse scattering in the XRD pattern, characteristic of the structure factor of liquids, and measurements of elemental partitioning of the sample using XRF, before, during and after laser heating in the DAC. We were able to detect elements concentrations as low as a few ppm level (2-5 ppm) on standard solutions. In situ measurements are complimented by mapping of the chemical partitions of the trace elements after laser heating on the quenched samples to constrain the partitioning data. Our first results indicate a strong partitioning of Pd and Ru into the metallic phase, while Zr remains clearly incompatible with iron. This novel approach extends the pressure and temperature range of partitioning experiments derived from quenched samples from the large volume presses and could bring new insight to the early history of Earth.

Fast accretion of the earth with a late moon-forming giant impact

Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.

Accretion and core formation: constraints from metal-silicate partitioning

Experimental metal-silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core-mantle boundary in the accreting Earth, yielding a maximum pressure of 44GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures decrease to a maximum of 31GPa and it is extremely difficult to match the calculated mantle contents of the highly incompatible elements Nb and W to those observed. Progressive oxidation is required to fit the observed mantle contents of vanadium. All of the scenarios discussed above point to progressive oxidation having occurred as the Earth grew. The Earth appears to be depleted in Mn relative to the chondritic reference.

Partitioning experiments in the laser-heated diamond anvil cell: volatile content in the Earth's core

The present state of the Earth evolved from energetic events that were determined early in the history of the Solar System. A key process in reconciling this state and the observable mantle composition with models of the original formation relies on understanding the planetary processing that has taken place over the past 4.5Ga. Planetary size plays a key role and ultimately determines the pressure and temperature conditions at which the materials of the early solar nebular segregated. We summarize recent developments with the laser-heated diamond anvil cell that have made possible extension of the conventional pressure limit for partitioning experiments as well as the study of volatile trace elements. In particular, we discuss liquid-liquid, metal-silicate (M-Sil) partitioning results for several elements in a synthetic chondritic mixture, spanning a wide range of atomic number-helium to iodine. We examine the role of the core as a possible host of both siderophile and trace elements and the implications that early segregation processes at deep magma ocean conditions have for current mantle signatures, both compositional and isotopic. The results provide some of the first experimental evidence that the core is the obvious replacement for the long-sought, deep mantle reservoir. If so, they also indicate the need to understand the detailed nature and scale of core-mantle exchange processes, from atomic to macroscopic, throughout the age of the Earth to the present day.

The redox state of the mantle during and just after core formation

Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal-silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe2O3 content of the mantle that probably occurred in the first 1Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe2O3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal-silicate partition coefficients for O and Si, we have modelled core-mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe2O3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe2O3 during core formation, FeO would have disproportionated to produce Fe2O3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe2O3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

Accretion of the Earth and segregation of its core

The Earth took 30-40 million years to accrete from smaller 'planetesimals'. Many of these planetesimals had metallic iron cores and during growth of the Earth this metal re-equilibrated with the Earth's silicate mantle, extracting siderophile ('iron-loving') elements into the Earth's iron-rich core. The current composition of the mantle indicates that much of the re-equilibration took place in a deep (> 400 km) molten silicate layer, or 'magma ocean', and that conditions became more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to the Earth's core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of the smaller planetesimal building blocks.

The physical and chemical composition of the lower mantle

This article reviews some of the recent advances made within the field of mineral physics. In order to link the observed seismic and density structures of the lower mantle with a particular mineral composition, knowledge of the thermodynamic properties of the candidate materials is required. Determining which compositional model best matches the observed data is difficult because of the wide variety of possible mineral structures and compositions. State-of-the-art experimental and analytical techniques have pushed forward our knowledge of mineral physics, yet certain properties, such as the elastic properties of lower mantle minerals at high pressures and temperatures, are difficult to determine experimentally and remain elusive. Fortunately, computational techniques are now sufficiently advanced to enable the prediction of these properties in a self-consistent manner, but more results are required.A fundamental question is whether or not the upper and lower mantles are mixing. Traditional models that involve chemically separate upper and lower mantles cannot yet be ruled out despite recent conflicting seismological evidence showing that subducting slabs penetrate deep into the lower mantle and that chemically distinct layers are, therefore, unlikely.Recent seismic tomography studies giving three-dimensional models of the seismic wave velocities in the Earth also base their interpretations on the thermodynamic properties of minerals. These studies reveal heterogeneous velocity and density anomalies in the lower mantle, which are difficult to reconcile with mineral physics data.

Conditions for noncollinear instabilities of ferromagnetic materials

Two criteria have been identified here which determine whether a magnetic metal orders in a collinear (e.g., ferromagnet) or noncollinear (e.g., spin-spiral) arrangement. These criteria involve the ratio between the strength of the exchange interaction and the width of the electron bands, as well as Fermi-surface nesting between spin-up and spin-down sheets of the Fermi surface. Based on our analysis we predict that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g., high pressure experiments.

Partitioning of oxygen during core formation on the Earth and Mars

Core formation on the Earth and Mars involved the physical separation of metal and silicate, most probably in deep magma oceans. Although core-formation models explain many aspects of mantle geochemistry, they have not accounted for the large differences observed between the compositions of the mantles of the Earth (approximately 8 wt% FeO) and Mars (approximately 18 wt% FeO) or the smaller mass fraction of the martian core. Here we explain these differences as a consequence of the solubility of oxygen in liquid iron-alloy increasing with increasing temperature. We assume that the Earth and Mars both accreted from oxidized chondritic material. In a terrestrial magma ocean, 1,200-2,000 km deep, high temperatures resulted in the extraction of FeO from the silicate magma ocean owing to high solubility of oxygen in the metal. Lower temperatures of a martian magma ocean resulted in little or no extraction of FeO from the mantle, which thus remains FeO-rich. The FeO extracted from the Earth's magma ocean may have contributed to chemical heterogeneities in the lowermost mantle, a FeO-rich D" layer and the light element budget of the core.

Iron-silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle

The boundary between the Earth's metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition. To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle is first required. Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core-mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron-silicon alloy, with up to 5 wt% silicon. At pressures of 85-140 GPa, however, iron and SiO2 do not react and iron-silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core-mantle boundary (owing to reactions between iron and silicate), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region.

Chemical interaction of Fe and Al(2)O3 as a source of heterogeneity at the Earth's core-mantle boundary

Seismological studies have revealed that a complex texture or heterogeneity exists in the Earth's inner core and at the boundary between core and mantle. These studies highlight the importance of understanding the properties of iron when modelling the composition and dynamics of the core and the interaction of the core with the lowermost mantle. One of the main problems in inferring the composition of the lowermost mantle is our lack of knowledge of the high-pressure and high-temperature chemical reactions that occur between iron and the complex Mg-Fe-Si-Al-oxides which are thought to form the bulk of the Earth's lower mantle. A number of studies have demonstrated that iron can react with MgSiO3-perovskite at high pressures and high temperatures, and it was proposed that the chemical nature of this process involves the reduction of silicon by the more electropositive iron. Here we present a study of the interaction between iron and corundum (Al(2)O3) in electrically- and laser-heated diamond anvil cells at 2,000-2,200 K and pressures up to 70 GPa, simulating conditions in the Earth's deep interior. We found that at pressures above 60 GPa and temperatures of 2,200 K, iron and corundum react to form iron oxide and an iron-aluminium alloy. Our results demonstrate that iron is able to reduce aluminium out of oxides at core-mantle boundary conditions, which could provide an additional source of light elements in the Earth's core and produce significant heterogeneity at the core-mantle boundary.

The Earth's 'missing' niobium may be in the core

As the Earth's metallic core segregated from the silicate mantle, some of the moderately siderophile ('iron-loving') elements such as vanadium and chromium are thought to have entered the metal phase, thus causing the observed depletions of these elements in the silicate part of the Earth. In contrast, refractory 'lithophile' elements such as calcium, scandium and the rare-earth elements are known to be present in the same proportions in the silicate portion of the Earth as in the chondritic meteorites-thought to represent primitive planetary material. Hence these lithophile elements apparently did not enter the core. Niobium has always been considered to be lithophile and refractory yet it has been observed to be depleted relative to other elements of the same type in the crust and upper mantle. This observation has been used to infer the existence of hidden niobium-rich reservoirs in the Earth's deep mantle. Here we show, however, that niobium and vanadium partition in virtually identical fashion between liquid metal and liquid silicate at high pressure. Thus, if a significant fraction of the Earth's vanadium entered the core (as is thought), then so has a similar fraction of its niobium, and no hidden reservoir need be sought in the Earth's deep mantle.

92Nb-(92)Zr and the Early Differentiation History of Planetary Bodies

The niobium-92-zirconium-92 ((92)Nb-(92)Zr) extinct radioactive decay system (half-life of about 36 million years) can place new time constraints on early differentiation processes in the silicate portion of planets and meteorites. Zirconium isotope data show that Earth and the oldest lunar crust have the same relative abundances of (92)Zr as chondrites. (92)Zr deficits in calcium-aluminum-rich inclusions from the Allende meteorite constrain the minimum value for the initial (92)Nb/(93)Nb ratio of the solar system to 0.001. The absence of (92)Zr anomalies in terrestrial and lunar samples indicates that large silicate reservoirs on Earth and the moon (such as a magma ocean residue, a depleted mantle, or a crust) formed more than 50 million years after the oldest meteorites formed.

Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum

The high-pressure solubility in silicate liquids of moderately siderophile 'iron-loving' elements (such as nickel and cobalt) has been used to suggest that, in the early Earth, an equilibrium between core-forming metals and the silicate mantle was established at the bottom of a magma ocean. But observed concentrations of the highly siderophile elements--such as the platinum-group elements platinum, palladium, rhenium, iridium, ruthenium and osmium--in the Earth's upper mantle can be explained by such a model only if their metal-silicate partition coefficients at high pressure are orders of magnitude lower than those determined experimentally at one atmosphere (refs 3-8). Here we present an experimental determination of the solubility of palladium and platinum in silicate melts as a function of pressure to 16 GPa (corresponding to about 500 km depth in the Earth). We find that both the palladium and platinum metal-silicate partition coefficients, derived from solubility, do not decrease with pressure--that is, palladium and platinum retain a strong preference for the metal phase even at high pressures. Consequently the observed abundances of palladium and platinum in the upper mantle seem to be best explained by a 'late veneer' addition of chondritic material to the upper mantle following the cessation of core formation.

Greenhouse warming by CH4 in the atmosphere of early Earth

Earth appears to have been warm during its early history despite the faintness of the young Sun. Greenhouse warming by gaseous CO2 and H2O by itself is in conflict with constraints on atmospheric CO2 levels derived from paleosols for early Earth. Here we explore whether greenhouse warming by methane could have been important. We find that a CH4 mixing ratio of 10(-4) (100 ppmv) or more in Earth's early atmosphere would provide agreement with the paleosol data from 2.8 Ga. Such a CH4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an important component of the biota at that time. Elimination of the methane component of the greenhouse by oxidation of the atmosphere at about 2.3-2.4 Ga could have triggered the Earth's first widespread glaciation.

An interconnected network of core-forming melts produced by shear deformation

The formation mechanism of terrestrial planetary cores is still poorly understood, and has been the subject of numerous experimental studies. Several mechanisms have been proposed by which metal--mainly iron with some nickel--could have been extracted from a silicate mantle to form the core. Most recent models involve gravitational sinking of molten metal or metal sulphide through a partially or fully molten mantle that is often referred to as a 'magma ocean'. Alternative models invoke percolation of molten metal along an interconnected network (that is, porous flow) through a solid silicate matrix. But experimental studies performed at high pressures have shown that, under hydrostatic conditions, these melts do not form an interconnected network, leading to the widespread assumption that formation of metallic cores requires a magma ocean. In contrast, here we present experiments which demonstrate that shear deformation to large strains can interconnect a significant fraction of initially isolated pockets of metal and metal sulphide melts in a solid matrix of polycrystalline olivine. Therefore, in a dynamic (non-hydrostatic) environment, percolation remains a viable mechanism for the segregation and migration of core-forming melts in a solid silicate mantle.

Implications of Mars Pathfinder data for the accretion history of the terrestrial planets

Accretion models of the terrestrial planets often assume planetary bulk compositions with nonvolatile element abundance ratios equivalent to those of C1 carbonaceous chondrites. The moment of inertia factor of Mars reported by the Pathfinder team is inconsistent with a bulk planet C1 Fe/Si ratio or Fe content, which suggests that C1 chondrite accretion models are insufficient to explain the formation of Mars and the other terrestrial planets. Future planetary accretion models will have to account for variations in bulk Fe/Si ratios among the terrestrial planets.

Percolation of core melts at lower mantle conditions

Experiments at high pressure and temperature to determine the dihedral angle of core melts in lower mantle phases yielded a value of approximately 71 degrees for perovskite-dominated matrices. This angle, although greater than the 60 degrees required for completely efficient percolation, is considerably less than the angles observed in mineral matrices at upper mantle pressure-temperature conditions in experiments. In other words, molten iron alloy can flow much more easily in lower mantle mineralogies than in upper mantle mineralogies. Accordingly, although segregation of core material by melt percolation is probably not feasible in the upper mantle, core formation by percolation may be possible in the lower mantle.