Core-mantle Equilibration Research Articles

Experimental constraints on metal-silicate partitioning of chlorine and implications for planetary core formation

Chlorine (Cl) is critical for Earth’s habitability and an important tracer for volatile accretion processes. Yet its chemical behavior during core formation, one of the major events throughout planetary formation, is still poorly understood. This is primarily hindered by experimental challenges of reproducing such extreme pressure and temperature conditions and characterizing chemical compositions of recovered samples. Here we perform experiments on Cl partitioning between iron-rich metallic melt (core analog) and silicate melt (mantle analog) at temperatures of 1900–2400 K and pressures of 1–18 gigapascals to simulate core-mantle differentiation of terrestrial planets. More importantly, to avoid likely complications of Cl loss due to wet or oil polishing, we find it is critical to apply dry polishing to recovered samples as shown in previous work focusing on halogens. Our determined partition coefficients of Cl between metallic melt and silicate melt range from <0.003 to 1.38, which shows its siderophile (iron-loving) behavior for the first time. Moreover, we find Cl gradually prefers metallic melt as the increase of pressure, while confirming a positive effect of oxygen contents in metallic liquid. Considering plausible core formation scenarios for Earth and Mars, our results indicate that Cl abundance in Mars’ mantle could be explained by a single-stage core formation scenario. While for the Cl budget in Earth’s mantle, multi-stage core formation with partial core-mantle equilibrium may be required, and this would provide further constraints for dynamics of core formation and volatile accretion.

RETRACTED: Redox Evolution of the Crystallizing Terrestrial Magma Ocean and Its Influence on the Outgassed Atmosphere

Magma oceans (MOs) are episodes of large-scale melting of the mantle of terrestrial planets. The energy delivered by the Moon-forming impact induced a deep MO on the young Earth, corresponding to the last episode of core-mantle equilibration. The crystallization of this MO led to the outgassing of volatiles initially present in the Earth’s mantle, resulting in the formation of a secondary atmosphere. During outgassing, the MO acts as a chemical buffer for the atmosphere via the oxygen fugacity, set by the equilibrium between ferrous- and ferric-iron oxides in the silicate melts. By tracking the evolution of the oxygen fugacity during MO solidification, we model the evolving composition of a C-O-H atmosphere. We use the atmospheric composition to calculate its thermal structure and radiative flux. This allows us to calculate the lifetime of the terrestrial MO. We find that, upon crystallizing, the MO evolves from a mildly reducing to a highly oxidized redox state, thereby transiting from a CO- and H2-dominated atmosphere to a CO2- and H2O-dominated one. We find the overall duration of the MO crystallization to depend mostly on the bulk H content of the mantle, and to remain below 1.5 millions yr for up to nine Earth’s water oceans’ worth of H. Our model also suggests that reduced atmospheres emit lower infrared radiation than oxidized ones, despite of the lower greenhouse effect of reduced species, resulting in a longer MO lifetime in the former case. Although developed for a deep MO on Earth, the framework applies to all terrestrial planet and exoplanet MOs, depending on their volatile budgets.

Partitioning of noble gases (He, Ne, Ar, Kr, Xe) during Earth’s core segregation: A possible core reservoir for primordial noble gases

Noble gas isotopes in plume-influenced oceanic-island basalts (OIBs) clearly indicate the existence of distinct reservoirs somewhere in the deep Earth that are characterized by their more primitive noble gas isotopic signatures. As one of the largest differentiated layers on Earth, whether the Earth’s liquid outer core can preserve ‘primordial’ noble gases is fundamental for better understanding the Earth’s formation and the mantle evolution. Since Earth’s core is believed to have formed via liquid iron segregated from magma ocean during the planetary accretion stage, how much noble gases could have been incorporated into Earth’s core depends on the partitioning of noble gases between liquid metal and molten silicates. In this study, we perform thermodynamic integration (TI) calculations based on first-principles molecular dynamics (FPMD) simulation to investigate the partition coefficients of noble gases (NG: He, Ne, Ar, Kr, Xe) between liquid Fe and MgSiO3 melt. The simulations have been performed by adopting conditions that the pressures are from 10 ∼ 135 GPa and the temperatures from 2300 ∼ 5000 K to fully account for possible core-mantle equilibrium conditions. By analyzing the temperature and pressure dependence of the results, we suggest that the metal-silicate partitioning of noble gases (DNG) merely depends on the temperature, with extremely low DNG < 0.001 at temperatures < 2500 K (20 GPa), but increases to different degrees at high temperatures. The DNG tends to be more affected by temperature as the noble gas atomic mass increases, with the least temperature effect on the partition coefficient of He (DHe), which increases from ∼ 10−3 at 2300 K (10 GPa) to ∼ 10−1 (5000 K, 135 GPa) near the core-mantle boundary (CMB), and DNe = 10−6 ∼ 10−3, DAr = 10−6 ∼ 10−2, DKr = 10−7 ∼ 10−1 and DXe = 10−5 ∼ 100 from 2300 to 5000 K, respectively. Analysis of electronic interactions indicates that noble gases have positive chemical bonding with Fe, Mg, and Si atoms, and the bonding strengths increase with pressure as well as increasing noble gas atomic mass. Based on our calculated partition coefficients, we suggest that Earth’s core may have preserved significant amounts of noble gases considering the volatile-rich environment from which the core segregated.

Tracing Earth's Volatile Delivery With Tin

AbstractEarth's accretion history for volatile elements, and the origin of their depletions with respect to the Sun and primitive meteorites, continue to be debated. Two end‐member scenarios propose either that volatile elements were delivered during the main phases of accretion and differentiation, or that the Earth accreted from materials largely devoid of volatiles with late addition of volatile‐rich materials. Experiments evaluating the effect of metal–silicate equilibrium on elemental and isotopic distribution of volatile and siderophile elements such as Sn can help to distinguish between these scenarios. In this study, we have systematically investigated the relative influence of temperature, pressure, oxygen fugacity, and metal and silicate composition on the metal–silicate partioning behavior of Sn, from 2 to 20 GPa and 1,700 to 2,573 K, indicating that Sn siderophility noticeably decreases with temperature and S content of the metal but increases dramatically with pressure. A resolvable isotopic fractionation factor between metal and silicate suggests that core–mantle equilibrium temperatures (∼3,000 K) could potentially generate a Sn isotopic composition of the mantle lighter than the core by 150–200 ppm/amu. Core formation modeling shows that the volatiles were added during the last 10% of the accretion history. A final core containing 2.5 to 3.5 wt.% S is required. Furthermore, modeling of the BSE isotopic composition argues for a late Sn delivery on Earth with carbonaceous chondrite‐like material as the most likely source of volatiles. Therefore, both elemental and isotopic approaches converge toward an identical volatile accretion scenario, involving a late volatile delivery.

Earth’s volatile accretion as told by Cd, Bi, Sb and Tl core–mantle distribution

The timing and origin of volatile elements accretion on Earth has been and continues to be key questions, despite intense research scrutiny. Two end-member scenarios are usually proposed in which (1) volatile elements were delivered during the main phases of Earth’s accretion and underwent subsequent core–mantle differentiation, or (2) Earth accreted from largely dry and volatile-depleted material, with late addition of volatile-rich material after differentiation. Studying the behaviour of elements that are both volatile and siderophile in a metal–silicate equilibrium can help discriminate between those two scenarios by deconvolving the effect of siderophile processes such as the Earth’s differentiation from the effect of volatile processes. We report high-pressure and high-temperature metal–silicate equilibrium experiments that are used to trace the behaviour of four moderately siderophile and volatile elements: Cd, Bi, Sb and Tl. Experiments were performed in piston cylinder and multianvil presses between 2 and 20 GPa, from 1700 to 2600 K, in order to study the partitioning behaviour of these elements, including the relative influence of pressure, temperature, oxygen fugacity (fO2), and composition. Our results indicate that Cd, Bi, Sb and Tl partitioning coefficients are largely controlled by changes in temperature, pressure, fO2 and the S content of the metal phase. The pressure effect on Tl and Bi partitioning is measured for the first time and improves significantly the knowledge of Bi and Tl behaviour during core formation. Core formation modelling was used to reconcile the experimental data with observed abundances for different accretion scenarios. Homogeneous accretion with full core–mantle equilibration induces a massive segregation of Bi, Sb and Tl in the core, preventing reproduction of observed present-day mantle abundances. We find that a scenario in which the volatile elements are accreted in the last 10–20 % of the Earth’s accretion is the most suitable accretion process that is able to explain the abundances of Cd, Bi, Sb and Tl in Earth’s mantle. Partial core–mantle equilibration is necessary to reproduce Bi and Tl abundances. Our partitioning data also suggests that a 0.5 % chondritic late veneer may account for the Bi abundance measured in the bulk silicate Earth. These observations corroborate a growing wealth of evidence in support of this schematic heterogeneous accretion pathway.

Highly reduced accretion of the Earth by large impactors? Evidence from elemental partitioning between sulfide liquids and silicate melts at highly reduced conditions

The Earth may have formed at very reducing conditions through the accretion of (a) large reduced and differentiated impactor(s). Segregation of Fe-S liquids within these bodies would have left a geochemical mark on the mantles of reduced impactors and on the proto-Earth’s mantle. Here, we study the geochemical consequences of highly reduced accretion of the Earth by large impactors. New insights into the partitioning of trace elements between Fe-S liquid and silicate melt at (highly) reduced conditions (ΔIW = −5 to +1) were obtained by performing 21 high pressure experiments at 1 GPa and 1683–2283 K. The observed Fe-S liquid-silicate melt partitioning behavior is in agreement with thermodynamic models that predict a significant role for O in Fe-S liquid and S in the silicate melt.The experimental results were combined with literature data to obtain new and/or revised thermodynamic parameterizations that quantify the effects of composition and redox state on the elemental distribution between Fe-S liquids and highly reduced silicate melts. The results were used to assess which elements would most likely retain the geochemical signature of accretion of reduced impactors. Under the assumption of instantaneous core merging, impact delivery to the proto-Earth’s mantle was found to be significant (>10% of present-day BSE concentrations) only for S, Zn, Se, Te and Tl, whereas the abundances of the other elements remain largely unaffected.The results also show that present-day BSE S/Se, Se/Te, Tl/S and potentially In/Zn as well as their absolute abundances are inconsistent with their delivery by (a) large, highly reduced chondritic differentiated impactor(s) during terrestrial accretion. Continued core-mantle equilibration in the proto-Earth, volatility-related loss and/or post-accretion sulfide liquid segregation in the terrestrial magma ocean would further increase or not affect these discrepancies. We conclude that a significant contribution of (a) large (>10% of Earth’s mass) reduced and differentiated chondritic impactor(s) during accretion of the Earth is not reflected in the present-day S, Zn, Se, Te and Tl systematics of the terrestrial mantle. This suggests that significant overprinting of the primordial BSE S/Se, Se/Te and S/Tl signature could have occurred and/or (2) that the S/Se and Se/Te ratios were set by accretion of more oxidised CI-like materials.

Redox Processes in Early Earth Accretion and in Terrestrial Bodies

The Earth is a unique rocky planet with liquid water at the surface and an oxygen-rich atmosphere, consequences of its particular accretion history. The earliest accreting bodies were small and could be either differentiated and undifferentiated; later larger bodies had formed cores and mantles with distinct properties. In addition, there may have been an overall trend of early reduced and later oxidized material accreting to form the Earth. This paper provides an overview—based on natural materials in our Earthbound sample collections, experimental studies on those samples, and calculations and numerical simulations of differentiation processes—of planetary accretion, core–mantle equilibration, mantle redox processes, and redox variations in Earth, Mars, and other terrestrial bodies.

Anomalous 182W in high 3He/4He ocean island basalts: Fingerprints of Earth’s core?

The short-lived 182Hf-182W isotope system (t1/2 = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth’s accretionary and differentiation history. We report µ182W values (the deviation of 182W/184W of a sample from that of laboratory standards, in parts per million) and corresponding 3He/4He ratios for rocks from 15 different hotspots. These rocks are characterized by µ182W values that range from ∼0 to as low as −23 ± 4.5. For each volcanic system that includes rocks with negative µ182W values, the values tend to be negatively correlated with 3He/4He. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct µ182W-3He/4He characteristics. One reservoir has 3He/4He ≈ 8 R/RA and μ182W ≈ 0, which is indistinguishable from the convecting upper mantle. Based on high 3He/4He, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by µ182W ≈ 0, while the other is characterized by µ182W ≤ −23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while 182Hf was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative µ182W value of ∼−220. Thus, it is a candidate for the origin of the negative µ182W in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative µ182W most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the µ182W of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the µ182W signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high 3He/4He and µ182W = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to consist of (partially) molten lower mantle material. The negative µ182W signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative µ182W signature through core-mantle equilibration.

Core formation and geophysical properties of Mars

The chemical and physical properties of the interiors of terrestrial planets are largely determined during their formation and differentiation. Modeling a planet's formation provides important insights into the properties of its core and mantle, and conversely, knowledge of those properties may constrain formational narratives. Here, we present a multi-stage model of Martian core formation in which we calculate core–mantle equilibration using parameterizations from high pressure–temperature metal–silicate partitioning experiments. We account for changing core–mantle boundary (CMB) conditions, composition-dependent partitioning, and partial equilibration of metal and silicate, and we evolve oxygen fugacity (fO2) self-consistently. The model successfully reproduces published meteorite-based estimates of most elemental abundances in the bulk silicate Mars, which can be used to estimate core formation conditions and core composition. This composition implies that the primordial material that formed Mars was significantly more oxidized (0.9–1.4 log units below the iron–wüstite buffer) than that of the Earth, and that core–mantle equilibration in Mars occurred at 42–60% of the evolving CMB pressure. On average, at least 84% of accreted metal and at least 40% of the mantle were equilibrated in each impact, a significantly higher degree of metal equilibration than previously reported for the Earth. In agreement with previous studies, the modeled Martian core is rich in sulfur (18–19 wt%), with less than one weight percent O and negligible Si.We have used these core and mantle compositions to produce physical models of the present-day Martian interior and evaluate the sensitivity of core radius to crustal thickness, mantle temperature, core composition, core temperature, and density of the core alloy. Trade-offs in how these properties affect observable physical parameters like planetary mass, radius, moment of inertia, and tidal Love number k2 define a range of likely core radii: 1620–1870 km. Seismic velocity profiles for several combinations of model parameters have been used to predict seismic body-wave travel times and planetary normal mode frequencies. These results may be compared to forthcoming Martian seismic data to further constrain core formation conditions and geophysical properties.

Significant depletion of volatile elements in the mantle of asteroid Vesta due to core formation

Vesta, the second-largest body in the asteroid belt, is believed to be a prime example of a rocky protoplanet. Studies of the HED (Howardite–Eucrite–Diogenite) meteorite suite, considered to be sourced from the mantle of Vesta, provide insight into protoplanet differentiation processes. Eucrites and diogenites are depleted in many volatile elements, but the depletions of many volatile siderophile elements are not well constrained. Although previous work indicated that the Vestan core equilibrated with its mantle in a global melting event, the possible contribution of core formation to the volatile budget of HED meteorites has not been assessed to date. This prohibits an assessment of the overall volatile depletion systematics of Vesta.Here, we use a compilation of published volatile siderophile element (VSE; C, S, Zn, As, Se, Cd, In, Sb, Te, Tl, Pb, Bi) abundances in diogenites and eucrites to constrain their depletions in the Vestan mantle. We assess to what extent these depletions may be volatility-related, caused by partitioning into sulfides in the mantle, and/or a result of their preferential partitioning into the Vestan core during core-mantle equilibration. Our new estimates for VSE depletions in the Vestan mantle show no correlation with condensation temperature, suggesting the depletions are difficult to reconcile with partial condensation or degassing during accretionary processes only. Sulfide saturation of the diogenite and eucrite source region is also unlikely because of low indigenous S abundances in these samples compared to calculated S contents at sulfide saturation. Consideration of the metal-silicate partitioning behaviour of the VSE shows that most of these elements are sufficiently siderophile to explain a significant part of their observed depletions in the Vestan mantle by their preferential partitioning into a S-rich core.Moderately volatile elements As and Sb both appear more abundant in the Vestan mantle than expected from their highly siderophile behaviour in systems with basaltic melts. These apparent overabundances may be related to strong silicate melt compositional effects on their metal-silicate partitioning behaviour. On the other hand, Zn, and to a lesser extent Te and Tl are significantly more depleted than expected from metal-silicate partitioning alone. This could be evidence for volatile loss of these elements from Vesta through kinetic loss in a hard vacuum. Alternatively, the perceived loss of Zn may be the result of a Zn-poor Vestan bulk composition and/or partly by preferential partitioning of Zn into spinel. The additional depletion of Tl and by extension other highly chalcophile elements such as Se and Te may be explained by segregation of sulfides in the eucritic melt source region(s), if significant amounts of S degassed from eucritic melts after sulfide segregation.Our results underline the feasibility of planetary cores being a major reservoir for many volatile elements that are depleted in planetary mantles, and that are currently assumed to have suffered incomplete condensation during accretion or (partial) degassing after accretion.

The lunar core can be a major reservoir for volatile elements S, Se, Te and Sb

The Moon bears a striking compositional and isotopic resemblance to the bulk silicate Earth (BSE) for many elements, but is considered highly depleted in many volatile elements compared to BSE due to high-temperature volatile loss from Moon-forming materials in the Moon-forming giant impact and/or due to evaporative loss during subsequent magmatism on the Moon. Here, we use high-pressure metal-silicate partitioning experiments to show that the observed low concentrations of volatile elements sulfur (S), selenium (Se), tellurium (Te), and antimony (Sb) in the silicate Moon can instead reflect core-mantle equilibration in a largely to fully molten Moon. When incorporating the core as a reservoir for these elements, their bulk Moon concentrations are similar to those in the present-day bulk silicate Earth. This suggests that Moon formation was not accompanied by major loss of S, Se, Te, Sb from Moon-forming materials, consistent with recent indications from lunar carbon and S isotopic compositions of primitive lunar materials. This is in marked contrast with the losses of other volatile elements (e.g., K, Zn) during the Moon-forming event. This discrepancy may be related to distinctly different cosmochemical behavior of S, Se, Te and Sb within the proto-lunar disk, which is as of yet virtually unconstrained.

High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O

The distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100GPa and 5700K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54±5GPa and 3300–3400K. These conditions would result in 8.5±1.4wt% Si and 1.6±0.3wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature-oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density.

The solubility of platinum in silicate melt under reducing conditions: Results from experiments without metal inclusions

The solubility of Pt in silicate melt was investigated at conditions of 2073–2573K, 2GPa and ∼IW −1.5 to +3.5. These are the first measurements of Pt solubility under conditions more reducing than the iron-wüstite buffer (IW) which are demonstrably free from contamination by metal-inclusions. Pt solubility increases with increasing temperature and decreasing oxygen fugacity. The ability of carbon to enhance Pt solubility under reducing conditions (<IW +2) was tested by performing a set of isobaric, isothermal experiments which utilised graphite and PtIr alloy capsules. Analyses of the run-product glasses from these experiments show no correlation between dissolved C and Pt contents. In light of this result, the dissolution of Pt as a neutral species is suggested as being responsible for the observed relationship between solubility and fO2. A compilation of data from this study, Ertel et al. (2006) and Mann et al. (2012), suggests that pressure has no significant effect on solubility between 2 and 18GPa. Metal–silicate partition coefficients calculated from our Pt solubility data show that, for core–mantle equilibrium at IW −2, Pt concentrations in the primitive upper mantle (PUM) can be satisfied if the temperature of equilibration is >3500K. Under these conditions however, the estimated Pt/Os ratio is ∼40,000 times higher than that estimated for the PUM (Brandon et al., 2006). Instead, the PUM composition is generated most readily by metal–silicate equilibrium at more modest temperatures (∼3100K), followed by a late accretion of chondritic material subsequent to core formation.

Metal–silicate partitioning of sulphur, new experimental and thermodynamic constraints on planetary accretion

Partitioning of sulphur between liquid Fe-rich metals and silicates (DSmet/sil) was investigated at temperatures from 1800 °C to 2400 °C, pressures from 2 to 23 GPa and oxygen fugacities from 3.5 to 1.5 log units below the iron–wüstite buffer, using multi-anvil apparatus. The results are combined with previous experimental works to refine a multi-variable thermodynamic model of DSmet/sil. Sulphur appears to become more siderophile with increasing pressure and FeO content of the silicate melt, and less siderophile with increasing temperature and with Si, C, O, Fe and Ni contents of the metal. We then modelled the behaviour of sulphur in the course of planetary accretion, using different possible scenarios of mantle dynamics and evolution with time of oxygen fugacity. We investigated three end-member models for metal–silicate segregation of the incoming impactors: (i) the planetary mantle does not mix and is kept chemically stratified, (ii) the magma ocean is continuously mixed chemically, and (iii) both the magma ocean and the solid lower mantle are well mixed.We show that if S is accreted along the accretion, whatever the oxidation path, its distribution between core and mantle can lead to the observed S concentration of the mantle (200±80 ppm) and to the estimations of S content of the core (from its depletion in the mantle relative to the other elements with the same volatility). In the case of an Earth built with reduced material, to explain the present-day 200(±80) ppm S found in the mantle, it is necessary that both the magma ocean and the solid lower mantle mix at each major step of the planetary accretion. S could also be accreted in the last 10 to 20% of Earth's growth and reach its observed present terrestrial abundances if the magma ocean is chemically mixed along the accretion. Consequently, our models show that the S terrestrial abundances do not formally require an S accretion in a late veneer but can be explained by a core–mantle equilibration alone.

Lunar core formation: New constraints from metal–silicate partitioning of siderophile elements

Analyses of Apollo era seismograms, lunar laser ranging data and the lunar moment of inertia suggest the presence of a small, at least partially molten Fe-rich metallic core in the Moon, but the chemical composition and formation conditions of this core are not well constrained. Here, we assess whether pressure–temperature conditions can be found at which the lunar silicate mantle equilibrated with a Fe-rich metallic liquid during core formation. To this end, we combine measurements of the metal–silicate partitioning behavior of siderophile elements with the estimated depletion due to core formation in those elements in the silicate mantle of the Moon. We also explore how the presence of the light element sulfur (suggested by seismic models to be present in the core at concentrations of up to 6 wt%) in the lunar core affects core formation models.We use published metal–silicate partitioning data for Ni, Co, W, Mo, P, V and Cr in the lunar pressure range (1 atm–5 GPa) and characterize the dependence of the metal/silicate partition coefficients (D) on temperature, pressure, oxygen fugacity and composition of the silicate melt and the metal. If the core is assumed to consist of pure iron, core–mantle equilibration conditions that best satisfy lunar mantle depletions of five siderophile elements—Ni, Co, W, Mo and P—are a pressure of 4.5(±0.5) GPa and a temperature of 2200 K. The lunar mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if 6 wt% S is incorporated into the lunar core. Our results therefore suggest that metal–silicate equilibrium during lunar core formation occurred at depths close to the present-day lunar core–mantle boundary. This provides independent support for both the existence of a deep magma ocean in the Moon in its early history and the presence of significant amounts of sulfur in the lunar core.

Diffusion of transition metals in periclase by experiment and first-principles, with implications for core–mantle equilibration during metal percolation

Diffusion of the divalent transition metals in the deep mantle is important for understanding length scales of mantle heterogeneity, transport across the core–mantle boundary, and the degree of equilibration during core formation. Here we report the results of experiments and theoretical calculations on Ni, Co, Fe and Mn diffusion in periclase, the primary repository of these elements in Earth's lower mantle. The periclase used in the experiments was doped with 550ppm Al3+ to fix the cation vacancy concentration, and the experiments were performed at 2GPa and temperatures between 1673 and 2073K. The variation in diffusion coefficients among the transition metals was found to be less than a factor of three, and to increase in the order Ni<Co∼Mn<Fe. Theoretical results on the diffusion energetics, based on density functional theory with electron correlation effects accounted for using the GGA+U method, correctly predict the relative diffusivities, and yield absolute diffusion coefficients in reasonable agreement with the experimental results. A simple crystal field splitting model does not provide even qualitative guidance for the trends in relative diffusivity, but does show strong correlation with changes in migration energy due to spin transitions. The diffusion coefficients are rapid enough under mantle conditions to allow equilibration during core separation through the solid mantle, if the metal percolates through a grain-scale permeable network with channel spacing of centimeters or less.

Isotopic constraints on the age of the Earth’s core: Mutual consistency of the Hf-W and U-Pb systems

The estimation of the time of Earth’s core formation on the basis of isotopic systems with short-lived and long-lived parent nuclides gives significantly different results. Isotopic data for the 182Hf-182W system with a 182Hf half-life of approximately 9 Myr can be interpreted in such a way that the core was formed 34 Myr after the origin of the solar system assuming complete core-mantle equilibrium. Similar estimates on the basis of the U-Pb isotopic system suggest a significantly longer mean time of core formation of approximately 120 Myr. If the Earth’s core were formed instantaneously, both isotopic systems would have shown identical values corresponding to the true age. The discrepancy between the U-Pb and Hf-W systems can be resolved assuming prolonged differentiation of prototerrestrial material into silicate and metallic phases, which occurred not simultaneously and uniformly in different parts of the mantle. This resulted in the isotopic heterogeneity of the mantle, and its subsequent isotopic homogenization occurred slowly. Under such conditions, the mean isotopic compositions of W and Pb in the mantle do not correspond to the mean time of the separation of silicate and metallic phases. This is related to the fact that the exponential function of radioactive decay is strongly nonlinear at high values of the argument, and its mean value does not correspond to the mean value of the function. There are compelling reasons to believe that the early mantle was heterogeneous with respect to W isotopic composition and was subsequently homogenized by convective mixing. This follows from the fact that the lifetime of isotopic heterogeneities in the mantle is close to 1.8 Gyr for various long-lived isotopic systems. There is also no equilibrium between the mantle and the core with respect to the contents of siderophile elements. Because of this, the mean isotopic ratios of W and Pb cannot be used for the direct computation of the time of metal-silicate differentiation in the Earth. Such estimation requires more sophisticated models accounting for the duration of the differentiation process using several isotope pairs. Given the prolonged core formation, which has probably continued up to now, the question about its age becomes ambiguous, and only the most probable growth rate of the core can be estimated. The combined use of the U-Pb and Hf-W systems constrains the time of formation of 90% of the core mass between 0.12 and 2.7 billion years. These model estimates could have been realistic under the condition of complete disequilibrium between the silicate and metallic phases, which is as improbable as the suggestion of complete equilibrium between them on the whole Earth scale.

Rubidium isotopes in primitive chondrites: Constraints on Earth's volatile element depletion and lead isotope evolution

The bulk silicate Earth (BSE) shows substantial deficits in volatile elements compared to CI-chondrites and solar abundances. These deficits could be caused by pre-accretionary depletion in the solar nebula during condensation of solids, or by later heat-driven evaporation during collision of small bodies that later accreted to form the Earth. The latter is considered to result in isotope fractionation for elements with low condensation temperatures that correlates with the degree of depletion. Here, we report first high-precision isotope ratio measurements of the moderately volatile and lithophile trace element Rb. Data from seventeen chondrite meteorites show that their Rb isotope abundances are nearly indistinguishable from Earth, not deviating more than 1 per mil in their 87Rb/85Rb. The almost uniform solar system Rb isotope pool suggests incomplete condensation or evaporation in a single stage is unlikely to be the cause of the volatile element deficit of the Earth. As Rb and Pb have similar condensation temperatures, we use their different degrees of depletion in the BSE to address the mechanisms and timing of terrestrial volatile depletion. The Rb isotope data are consistent with a scenario in which the volatile budget of the Earth was generated by a mixture of a highly volatile-element depleted early Proto-Earth with undepleted material in the course of terrestrial accretion. Observed Pb and Rb abundances and U–Pb and Rb–Sr isotope systematics suggest that volatile addition occurred at approximately the same time at which last core–mantle equilibration was achieved. In line with previous suggestions, this last equilibration involved a second stage of Pb (but not Rb) depletion from the BSE. The timing of this second Pb loss event can be constrained to ~110Ma after the start of the solar system. This model supports a scenario with core storage of Pb in the aftermath of a putative Moon forming giant impact that also delivered the bulk of the volatile elements to the Earth.

Systematics of metal–silicate partitioning for many siderophile elements applied to Earth’s core formation

Superliquidus metal–silicate partitioning was investigated for a number of moderately siderophile (Mo, As, Ge, W, P, Ni, Co), slightly siderophile (Zn, Ga, Mn, V, Cr) and refractory lithophile (Nb, Ta) elements. To provide independent constrains on the effects of temperature, oxygen fugacity and silicate melt composition, isobaric (3 GPa) experiments were conducted in piston cylinder apparatus at temperature between 1600 and 2600 °C, relative oxygen fugacities of IW−1.5 to IW−3.5, and for silicate melt compositions ranging from basalt to peridotite. The effect of pressure was investigated through a combination of piston cylinder and multi-anvil isothermal experiments between 0.5 and 18 GPa at 1900 °C. Oxidation states of siderophile elements in the silicate melt as well as effect of carbon saturation on partitioning are also derived from these results. For some elements (e.g. Ga, Ge, W, V, Zn) the observed temperature dependence does not define trends parallel to those modeled using metal–metal oxide free energy data. We correct partitioning data for solute interactions in the metallic liquid and provide a parameterization utilized in extrapolating these results to the P– T– X conditions proposed by various core formation models. A single-stage core formation model reproduces the mantle abundances of several siderophile elements (Ni, Co, Cr, Mn, Mo, W, Zn) for core–mantle equilibration at pressures from 32 to 42 GPa along the solidus of a deep peridotitic magma ocean (∼3000 K for this pressure range) and oxygen fugacities relevant to the FeO content of the present-day mantle. However, these P– T– fO 2 conditions cannot produce the observed concentrations of Ga, Ge, V, Nb, As and P. For more reducing conditions, the P– T solution domain for single stage core formation occurs at subsolidus conditions and still cannot account for the abundances of Ge, Nb and P. Continuous core formation at the base of a magma ocean at P– T conditions constrained by the peridotite liquidus and fixed fO 2 yields concentrations matching observed values for Ni, Co, Cr, Zn, Mn and W but underestimates the core/mantle partitioning observed for other elements, notably V, which can be reconciled if accretion began under reducing conditions with progressive oxidation to fO 2 conditions consistent with the current concentration of FeO in the mantle as proposed by Wade and Wood (2005). However, neither oxygen fugacity path is capable of accounting for the depletions of Ga and Ge in the Earth’s mantle. To better understand core formation, we need further tests integrating the currently poorly-known effects of light elements and more complex conditions of accretion and differentiation such as giant impacts and incomplete equilibration.

A high field strength element perspective on early lunar differentiation

Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured 92Zr and 91Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in 182W vs. Hf/W and 142Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the 182W and 142Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the 182W vs. Hf/W and 142Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar 182Hf– 182W and 146Sm– 142Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.