Core Segregation Research Articles

Non-chondritic sulphur isotope composition of the terrestrial mantle

Core-mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth's sulphur should at present reside in the core, which implies that the residual silicate mantle should exhibit fractionated (34)S/(32)S ratios according to the relevant metal-silicate partition coefficients, together with fractionated siderophile element abundances. However, Earth's mantle has long been thought to be both homogeneous and chondritic for (34)S/(32)S, similar to Canyon Diablo troilite, as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous (34)S/(32)S ratios, directly correlated to the strontium and neodymium isotope ratios (87)Sr/(86)Sr and (143)Nd/(144)Nd. These isotope trends are compatible with binary mixing between a low-(34)S/(32)S ambient mantle and a high-(34)S/(32)S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ(34)S of -1.28 ± 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic (34)S/(32)S ratio for the silicate Earth could be accounted for by a core-mantle differentiation record in which the core has a (34)S/(32)S ratio slightly higher than that of chondrites (δ(34)S = +0.07‰). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core-mantle differentiation.

Dynamics of non-spherical particles in a rotating drum

Considerable amount of experimental work has been performed to elucidate the behavior of granular flow inside a rotating drum and it has yet to be clearly understood. However, a majority of these investigations have involved spherical or nearly spherical particles. The extent of the experiments involving non-spherical particles previously carried out was limited by the experimental technique used for the investigation or the inability to single out the effect of the particle shape. In this work, the radioactive particle tracking technique (RPT) is adapted to follow large non-spherical particles inside a rotating drum. The particles consist of pharmaceutical tablets containing a suitable compound, thus enabling their use as a tracer particle. Three crucial aspects of particle dynamics inside a rotating drum are studied: residence time in the active and passive layers, mixing and segregation, as well as axial dispersion. The results obtained for non-spherical particles are compared to those which would be predicted using models developed for spherical of nearly spherical particles. For the different shapes studied in this work, it is found that particles having an aspect ratio greater than two can lead to significant deviations in velocity profile and residence time. In addition, the mixing of different shaped particles is observed to lead to unexpected core segregation patterns. Lastly, it is found that the non-spherical particle higher degree of spatial orientation in the active layer leads to a lower axial dispersion coefficient than the ones obtained with spherical particles.

The oxygen isotope composition of earth's oldest rocks and evidence of a terrestrial magma ocean

Analysis of Hadean and Archean rocks for 16O‐17O‐18O isotopes demonstrates that the Terrestrial Mass Fractionation Line of oxygen isotopes has had the same slope and intercept for at least the past 4.0 and probably for as long as 4.2 Ga. The homogenization of oxygen isotopes required to produce such long‐lived consistency was most easily established by mixing in a terrestrial magma ocean. The measured identical oxygen isotope mass fractionation lines for Earth and Moon suggest that oxygen isotope reservoirs of both bodies were homogenized at the same time during a giant moon‐forming impact. But other sources of heat for global melting cannot be excluded such as bolide impacts during early accretion of proto‐Earth, the decay of short‐lived radioactive isotopes, or the energy released during segregation of core from mantle.

Assessing uncertainty in geochemical models for core formation in Earth

Unraveling the conditions at which Earth's metallic iron core formed yields important information about Earth's early accretion and differentiation history. Multi-variable statistical modeling of siderophile element partitioning between core-forming metallic liquids and silicate melts form the basis for physical models of core formation. While it seems clear that core segregation in a deep peridotitic magma ocean is generally consistent with many mantle siderophile element abundances, there is considerable disparity among extant physical models in terms of the key parameters of pressure, temperature and oxygen fugacity at which the core formed. Moreover, there is ongoing debate over whether simple single-stage equilibrium or more complex multi-stage accretion models are required by the partitioning data. Here we consider how variations in the statistical regression of partitioning data affect the outcomes of physical models for core formation. Taking extant experimental data sets for four well-studied siderophile elements (Ni, Co, W and V) as examples, we find that the regression model exerts a fundamental control on physical model outcomes. Further, the experimental data are currently too imprecise to discriminate among various single-stage and continuous core formation scenarios. Progress in the development of physical models requires better isolation of the independent variables that affect partition coefficients and verification of activity models at high pressure and temperature in order to reduce the global uncertainty in multi-variable statistical models.

Experimental evidence for the absence of iron isotope fractionation between metal and silicate liquids at 1 GPa and 1250–1300 °C and its cosmochemical consequences

Iron isotope fractionation during metal–silicate differentiation has been proposed as a cause for differences in iron isotope compositions of chondrites, iron meteorites and the bulk silicate Earth. Stable isotope fractionation, however, rapidly decreases with increasing temperature. We have thus performed liquid metal–liquid silicate equilibration experiments at 1250–1300°C and 1GPa to address whether Fe isotope fractionation is resolvable at the lowest possible temperatures for magmatic metal–silicate differentiation. A centrifuging piston cylinder apparatus enabled quantitative metal–silicate segregation. Elemental tin or sulphur was used in the synthetic metal-oxide mixtures to lower the melting temperature of the metal. The analyses demonstrate that eight of the 10 experimental systems equilibrated in a closed isotopic system, as was assessed by varying run durations and starting Fe isotope compositions. Statistically significant iron isotope fractionation between quenched metals and silicates was absent in nine of the 10 experiments and all 10 experiments yield an average metal–silicate fractionation factor of 0.01±0.04‰, independent of whether graphite or silica glass capsules were used. This implies that Fe isotopes do not fractionate during low pressure metal–silicate segregation under magmatic conditions. In large bodies such as the Earth, fractionation between metal and high pressure (>20GPa) silicate phases may still be a possible process for equilibrium fractionation during metal–silicate differentiation. However, the 0.07±0.02‰ heavier composition of bulk magmatic iron meteorites relative to the average of bulk ordinary/carbonaceous chondrites cannot result from equilibrium Fe isotope fractionation during core segregation. The up to 0.5‰ lighter sulphide than metal fraction in iron meteorites and in one ordinary chondrite can only be explained by fractionation during subsolidus processes.

Core Tester Iron Losses Segregation by Finite Element Modeling

A new designed core tester and its feeding and control devices are presented in this paper. With this workbench, mounted single phase induction motor stators can be magnetically fed with controlled induction waveforms in order to evaluate their quality in the industrial environment. As the measured data involves the machine stator as well as the core of the testing apparatus, Finite Element and iron losses modeling are employed in this paper in order to distinguish the losses of the stator under test and the losses of measurement device itself. Therefore, the main goals of this paper are to propose a new tester design as well as a numerical way allowing the separation of the losses in the stator under test and the losses inherent to the tester itself.

Morphology Evolution of PS-b-P2VP Diblock Copolymers via Supramolecular Assembly of Hydroxylated Gold Nanoparticles

We report on the strong segregation of core–shell Au nanoparticles, with a shell layer consisting of a random copolymer brush of styrene and vinylphenol (PS-r-PVPh-SH), in poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer. Because of the formation of multiple hydrogen bonds between the hydroxyl groups within the shell of the nanoparticles and the pyridine group in PS-b-P2VP, the Au nanoparticles were strongly localized into P2VP domains with a very high volume fraction of nanoparticles (ϕp ∼ 0.53). The spatial distribution of Au nanoparticles, observed by transmission electron microscopy (TEM), is compared with results of previous experiments where homopolymers were blended with block copolymers. If the diameter d of the nanoparticles is much less than the width D of the P2VP lamellar domains, these nanoparticles are more uniformly distributed across the P2VP domain than if d is comparable to D, in which case the nanoparticles are pushed toward the center of the P2VP domains. This behavior is ...

Properties of particleboard made from kenaf ( Hibiscus cannabinus L.) as function of particle geometry

An examination of particle geometry was carried out after the kenaf stems were chipped and flaked and the identified geometry was compared with that of rubberwood. The particle geometries were then classified into several classes; 10 for kenaf core, 7 for kenaf bast, and 7 for rubberwood. The results indicate that most of kenaf core particles were of rectangular or nearly rectangular shapes whilst kenaf bast was in the form of curl and kneel form. Overall, board made from a combination of 70% rubberwood (RW) and 30% kenaf (K) was characterized by greater strength and stability than boards comprising 100% kenaf. These results provide enough evidence that combining rubberwood with kenaf particles imparts better mechanical and physical properties of the admixture RW–K board. Nonetheless, segregation of kenaf core and kenaf bast is necessary since both materials produced different particle geometries thus require different processing parameters. Between the two, kenaf core has better compatibility with rubberwood and shows superior performance than kenaf bast.

Crystallization of a basal magma ocean recorded by Helium and Neon

Interpretation of the noble gas isotopic signature in hotspots is still controversial. It suggests that relatively primitive material remains untapped in the deepest mantle, even while mantle convection and sub-surface melting efficiently erase primordial heterogeneities. A recent model suggests that significant differentiation and fractionation affects the deepest mantle following the formation of a dense basal magma ocean (BMO) right after core segregation (Labrosse et al., 2007). Here we explore the consequences of the crystallization of a BMO for the noble gas evolution of the mantle. The crystals extracted from a BMO upon cooling generate dense chemical piles at the base of the mantle. We show that if the solid–melt partition coefficients of He and Ne are > 0.01 at high pressure and temperature, He and Ne isotopic ratios in pile cumulates can be pristine like. Hence, the entrainment of modest amounts of BMO cumulate in mantle plumes (< 10%) potentially explains the primitive-like He and Ne signatures in hotspots. Because pile material can be depleted in refractory elements while simultaneously enriched in noble gasses, our model forms a viable hypothesis to explain the complex relationship between He and refractory isotopic systems in Earth's interior.

Stress-modulated composition in the vicinity of dislocations in nearly lattice matched AlxIn1−xN/GaN heterostructures: A possible explanation of defect insensitivity

Evidence of composition fluctuations around threading dislocations at scales ranging from atomic distances to tens of nanometers is provided by $z$-contrast imaging, strain measurement, and energy dispersive x-ray spectroscopy in Al${}_{x}$In${}_{1\ensuremath{-}x}$N/GaN heterostructures. The atomic core rings of edge-type dislocations are shown to lie across highly antisymmetric elemental environments, and the indium-rich pit centers of mixed dislocation are found to lie on the tensile side of their atomic core ring. The observed composition fluctuations around pure-edge dislocations are compared with an elastostatic free energy model calculation and a good qualitative and quantitative agreement is obtained. Hydrostatic stress is shown to be their principal cause: Tensile stress regions are indium rich and compressive stress regions are aluminum rich. We show that the stress field of a mixed dislocation can impact the composition of the alloy more than a hundred nanometers away from its core. Indium core segregation on pure-screw threading dislocation is also evidenced and explained by the model, as shear stress is also expected to affect composition. Furthermore, threading dislocations are shown to bend less in the Al${}_{x}$In${}_{1\ensuremath{-}x}$N alloy than in GaN, suggesting that they are ``pinned'' by stress-induced fluctuations. Such concentration modulations can have an important impact on optical and electrical properties of Group-III nitride devices that generally contain a high dislocation density (in the $10{}^{8}$ to $10{}^{10}$ cm${}^{\ensuremath{-}2}$ range). We propose that stress-induced composition modulation could be the origin of defect insensitivity in indium-containing nitride ternary alloys.

Global Variability in the Platinum-group Element Contents of Komatiites

Platinum-group element (PGE) concentrations in komatiites of similar age but different petrological affinity, lithofacies type and emplacement style are remarkably similar when the effects of olivine fractionation and accumulation are accounted for. All komatiites display a slight enrichment of PPGE (Rh, Pt and Pd) over IPGE (Ir, Ru and Os) attributable to limited retention of IPGE in their mantle sources, coupled with saturation in Ir–(Os) alloys. Significant differences and trends exist, however, including subtle provinciality in Pt/Pd ratios, and in particular a systematic linear increase in komatiite PGE contents with age from the oldest at >3·5 Ga to the late Archaean group at c. 2·7 Ga, regardless of petrogenetic type. There is a linear secular trend of increasing Pt at a given MgO content and in Pt/Ti from 3500 Ma to 2700 Ma across all komatiite types. Barberton-type komatiites younger than 3000 Ma are generally slightly depleted in Pt and Pd relative to Munro-type. Munro-type komatiites from 3200–3500 Ga greenstone belts of the east Pilbara Craton of Western Australia are strongly depleted in PGE relative to younger belts, but similar to Barberton-type komatiites of the same age. Munro-type and Barberton-type komatiites from the 2900 Ma greenstone belts of the Youanmi Terrane in the Yilgarn Craton have similar PGE contents. Age, rather than komatiite type, appears to be the dominant control. These observations lead us to the hypothesis that the age control is related to a secular variation in the PGE content of komatiite mantle source regions. We envisage that Early Archaean komatiites could have been derived from plumes sourced in the deep mantle, which had been largely stripped of its highly siderophile element budget during core segregation and as yet incompletely replenished in PGE by the Late Veneer. Late Archaean komatiites have higher PGE contents as a result of progressive mixing-in of Late Veneer material over the period between the Late Heavy Bombardment (4·3–3·9 Ga) and 2·7 Ga, at which time the mantle had become effectively homogeneous with respect to PGE. The relative constancy of gamma-osmium values in komatiites throughout the Archaean is consistent with this hypothesis.

The effect of Si on metal–silicate partitioning of siderophile elements and implications for the conditions of core formation

We have determined the liquid metal–liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5 GPa/1923 K and 6 GPa/2123 K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal–silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal. The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth ( Georg et al., 2007; Fitoussi et al., 2009), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed. We use our new data in conjunction with published metal–silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal–silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5 GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., Wade and Wood, 2005). The core–mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3 wt.% and 0.15%, respectively.

Silicon isotopes in the inner Solar System: Implications for core formation, solar nebular processes and partial melting

We report Si isotopic data on a suite of terrestrial mantle-derived samples, meteorites and a lunar sample. Our data on co-existing mantle minerals, peridotites and basalts demonstrate lack of any resolvable high temperature fractionation during igneous processes. We show that the δ 30Si of the bulk silicate Earth (BSE) is identical, within analytical uncertainties, to carbonaceous and ordinary chondrites (CHUR). Based on our data the difference between δ 30Si BSE and δ 30Si CHUR is 0.035 ± 0.035. Whole-rock differentiated meteorites from different parent bodies (Mars, Vesta) and a lunar breccia sample also show similar δ 30Si suggesting broad-scale Si isotope homogeneity in the inner Solar System with an average δ 29Si = −0.20 ± 0.01 and δ 30Si = −0.39 ± 0.02 relative to the NBS28 Si isotope standard. A difference between δ 30Si BSE and δ 30Si CHUR of 0.035, as observed in our study, translates to less than 1.67 wt.% Si in the core considering a continuous accretion model whereas estimates using a batch model are even lower. Within uncertainties (±0.035‰) in the δ 30Si difference between the BSE and CHUR, a maximum of 3.84 wt.% Si could be present in the Earth’s core whereas at δ 30Si BSE–δ 30Si CHUR = 0, there is no requirement of Si in the Earth’s core. Such low Si in the core necessitates the presence of other light elements in the core to explain its density deficit. Our data also places constraints on the oxidation state of the Earth’s mantle during core segregation. The uncertainties in estimating the concentration of oxidized Fe in the mantle during the first 90% of accretion arise from uncertainties in the estimates of the equilibrium partition coefficient of silicon between metal and silicate at conditions relevant to core formation. For δ 30Si BSE–δ 30Si CHUR = 0.035 ± 0.035, the concentration of oxidized Fe in the mantle during the first 90% of accretion could be as low as ∼1%. However, at δ 30Si BSE–δ 30Si CHUR = 0, the Si isotope data do not require any change in the mantle concentration of oxidized Fe during accretion from the present day value of 6.26%.

Coupled 182 W- 142 Nd constraint for early Earth differentiation

Recent high precision (142)Nd isotope measurements showed that global silicate differentiation may have occurred as early as 30-75 Myr after the Solar System formation [Bennett V, et al. (2007) Science 318:1907-1910]. This time scale is almost contemporaneous with Earth's core formation at approximately 30 Myr [Yin Q, et al. (2002) Nature 418:949-952]. The (182)Hf-(182)W system provides a powerful complement to the (142)Nd results for early silicate differentiation, because both core formation and silicate differentiation fractionate Hf from W. Here we show that eleven terrestrial samples from diverse tectonic settings, including five early Archean samples from Isua, Greenland, of which three have been previously shown with (142)Nd anomalies, all have a homogeneous W isotopic composition, which is approximately 2epsilon-unit more radiogenic than the chondritic value. By using a 3-stage model calculation that describes the isotopic evolution in chondritic reservoir and core segregation, as well as silicate differentiation, we show that the W isotopic composition of terrestrial samples provides the most stringent time constraint for early core formation (27.5-38 Myr) followed by early terrestrial silicate differentiation (38-75 Myr) that is consistent with the terrestrial (142)Nd anomalies.

Metal–silicate silicon isotope fractionation in enstatite meteorites and constraints on Earth's core formation

Silicon has long been considered a possible light element in Earth 's core. If differences in 30Si/ 28Si ratios between metal (core) and silicate (mantle and crust) can be quantified, silicon isotopes may be used to constrain the amount of this element in the core, and in so doing elucidate the conditions that attended Earth 's differentiation. We investigate Si-isotope fractionation between metal and silicate in metal-rich enstatite meteorites as an analogue for Earth 's differentiation. We report here a 5 to 6‰ difference in the 30Si/ 28Si ratio between Si in metal and Si in silicate in the aubrites (enstatite achondrites) Mount Egerton and Norton County. The meteorites are believed to have derived from enstatite chondrites by melting and thermal metamorphism with final equilibration at 1200 and 1130 ± 80 K, respectively. Using the measured silicate–metal Si-isotope fractionation in these rocks we obtain a temperature dependence for fractionation of Δ 30 S i silicate-metal = 7.64 × 10 6 ± 0.47 1 σ T 2 , in agreement with independent experimental and theoretical determinations. The measured silicate–metal fractionation suggests a ∼ 0.8‰ difference in the 30Si/ 28Si ratio between Earth 's core and mantle at P/ T conditions relevant to core formation. Our results, based on thermodynamic calculations for Si solubility in iron-rich metal and the measured Si-isotopic silicate–metal fractionation, imply at least ∼ 6 wt.% Si in the core (depending on the exact Δ 30Si BSE-chondrite value). The Si-isotope data also require that oxygen fugacity in the lower mantle increased during or after the process of core segregation by 1 to 2 log units.

Surface segregation and stability of core–shell alloy catalysts for oxygen reduction in acid medium

Density functional theory is used for the evaluation of surface segregation, trends for dissolution of Pt surface atoms in acid medium, and oxygen reduction reaction activity of core-shell materials, containing a monolayer of platinum over a monometallic or bimetallic core. Two groups of cores are investigated: Pt/X with X = Ir, Au; Pd, Rh, Ag; Co, Ni, Cu; and Pt/Pd(3)X, with X = Co, Fe, Cr, V, Ti, Ir, Re. It is found that all the 4d and 5d pure cores may serve as stable cores, and their beneficial effect on the Pt monolayer may be further tuned by alloying the core to another element, here chosen from 3d or 5d groups. The Pd(3)X cores enhance the stability of the surface Pt atoms both in vacuum and under adsorbed oxygen; however the high oxygen philicity of some of the X elements induces their surface segregation that may cause surface poisoning with oxygenated species and their dissolution in acid medium.

The behaviour of tungsten during mantle melting revisited with implications for planetary differentiation time scales

Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os–Ir alloy phase, whose stability is dependent on fO 2 of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the 53 Mn– 53 Cr and 26 Al– 26 Mg systems.

Centrifuge assisted percolation of Fe–S melts in partially molten peridotite: Time constraints for planetary core formation

The mechanism which segregates molten Fe–S into metallic cores of planetary bodies is still not fully understood. Due to the high interfacial energy and wetting angle between Fe–S melts and silicate mantle minerals, the continuous percolative flow of such melts cannot be efficient for the core segregation in planetary bodies. A series of percolation experiments has been realized on a partially molten fertile garnet peridotite, employing a centrifuging piston cylinder. A high temperature garnet peridotite with Mg# ~0.90 composed of 60 vol.% olivine, 15 vol.% orthopyroxene, 6 vol.% clinopyroxene and 19 vol.% garnet has been used as the silicate matrix. Peridotite powders with the 100–200 or 20–30 µm grain size were mixed with 5–30 vol.% Fe–S of eutectic composition Fe 70S 30. The aggregates were centrifuged at 500–700 g at temperatures below and above the melting point of the peridotite. The centrifuge experiments revealed a negligible percolation of Fe–S melts through the unmolten peridotite matrix. Only at T > 1260 °C, i.e. above the solidus of the peridotite, and starting with 5 vol.% of Fe 70S 30 the vertical melt gradient achieved 1–2 vol.%/mm. In samples with 15 vol.% Fe 70S 30 the vertical separation achieved 2–2.5 vol.%/mm after 10 h of centrifuging at 500 g. An increase in the degree of partial silicate melting in the peridotite leads to an increase of the Fe–S separation rate from the peridotite matrix. Fe–S contents >10 vol.% cause an increase of the Fe–S melt droplet size and of the effective percolation velocity of Fe–S melt. A threshold dividing fast (>10 cm per year) and slow percolations (<1 mm per year) of Fe –S melt is found around 14–15 vol.% of Fe 70S 30. The experimentally determined permeabilities of Fe–S melt in the unmolten peridotite with 7–10 vol.% of Fe 70S 30 melt are 10 − 18 –10 − 19 m 2, which is 2–3 orders of magnitude lower than the values calculated previously from static experiments. The presence of the silicate melt increases the segregation velocity of Fe–S melt in a partially molten peridotite by more than one order of magnitude with respect to the unmolten peridotite matrix. This could provide an effective segregation of Fe–S melt in a planetary mantle down to 2.5 vol.% of residual Fe–S melt. The extremely slow percolation of Fe–S melt in the absence of the partial silicate melting precludes a scenario of metallic core formation via percolation before temperatures allow a substantial partial melting of mantle silicates in planetary bodies.

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

Abstract The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3–6 log units below the iron–wustite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO 2 being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core–mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe–S–Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe–S liquid, poor in Si; (ii) a middle layer of Fe–Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.

Melting of the Indarch meteorite (EH4 chondrite) at 1 GPa and variable oxygen fugacity: Implications for early planetary differentiation processes

In order to derive constraints on planetary differentiation processes, and ultimately the formation of the Earth, it is required to study a variety of meteoritic materials and to investigate their melting relations and elemental partitioning at variable pressures, temperatures, and oxygen fugacities ( f O2). This study reports the first high pressure (HP) and high temperature (HT) investigation of an enstatite chondrite (Indarch). Four series of experiments exploring various f O2 conditions have been carried out at 1 GPa in a piston–cylinder apparatus using the EH4 chondrite Indarch. We show that temperature and redox conditions have important effects on the phase equilibria of the meteorite: the solidus and liquidus temperatures of the silicate portion increase with decreasing f O2, and the stability fields of various phases are modified. Olivine and pyroxene are stable around 1.5 log f O2 unit below the iron–wüstite buffer (IW−1.5), whereas quartz and pyroxene is the stable assemblage under the most reducing conditions, between IW−5.0 and IW−4.0, due to reduction of the silicate. While these changes are occurring in the silicate, the metal gains Si from the silicate, (Fe, Mg, Mn, Ca, Cr)-bearing sulfides are observed at f O2 less than IW−4, and the partitioning of Ni and Mo are both affected by the presence of Si in Fe–S–C liquids. The f O2 has also a significant effect on the liquid metal–liquid silicate partitioning behavior of Si and S, two possible light elements in planetary cores, and of the slightly siderophile elements Cr and Mn. With decreasing f O2, S becomes increasingly lithophile, Si becomes increasingly siderophile, and Cr and Mn both become strongly siderophile and chalcophile. The partitioning behavior of these elements places new constraints on models of core segregation for the Earth and other differentiated bodies.