Metal-silicate Partitioning Research Articles

Solidus and liquidus profiles of chondritic mantle: Implication for melting of the Earth across its history

We investigated the melting properties of a synthetic chondritic primitive mantle up to core–mantle boundary (CMB) pressures, using laser-heated diamond anvil cell. Melting criteria are essentially based on the use of X-rays provided by synchrotron radiation. We report a solidus melting curve lower than previously determined using optical methods. The liquidus curve is found between 300 and 600 K higher than the solidus over the entire lower mantle. At CMB pressures (135 GPa), the chondritic mantle solidus and liquidus reach 4150 (± 150) K and 4725 (± 150) K, respectively. We discuss that the lower mantle is unlikely to melt in the D″-layer, except if the highest estimate of the temperature profile at the base of the mantle, which is associated with a very hot core, is confirmed. Therefore, recent suggestions of partial melting in the lowermost mantle based on seismic observations of ultra-low velocity zones indicate either (1) a outer core exceeding 4150 K at the CMB or (2) the presence of chemical heterogeneities with high concentration of fusible elements. Our observations of a high liquidus temperature as well as a large gap between solidus and liquidus temperatures have important implications for the properties of the magma ocean during accretion. Not only complete melting of the lower mantle would require excessively high temperatures, but also, below liquidus temperatures partial melting should take place over a much larger depth interval than previously thought. In addition, magma adiabats suggest very high surface temperatures in case of a magma ocean that would extend to more than 40 GPa, as suggested by siderophile metal–silicate partitioning data. Such high surface temperature regime, where thermal blanketing is inefficient, points out to a transient character of the magma ocean, with a very fast cooling rate.

Prediction of metal–silicate partition coefficients for siderophile elements: An update and assessment of PT conditions for metal–silicate equilibrium during accretion of the Earth

Experimental studies of the partitioning of siderophile elements between metallic and silicate liquids have provided fundamental constraints on the early history and differentiation conditions of the Earth. With many new studies even in the last 20 yr, several models have emerged from the results, including low pressure equilibrium, high pressure equilibrium, and combined high and low pressure multi-stage models. The reasons – silicate melt composition, pressure effects on silicate melt structure, different methods for calculating metal activity coefficients, and the role of deep mantle phases – for the multitude of resulting models have not been specifically addressed before, yet are critical in evaluating the more likely and realistic models. The four reasons leading to the divergence of results will be discussed and evaluated. The behavior of the moderately siderophile elements Ni and Co will be compared using several approaches, each of which results in the same conclusion for Ni and Co. This consistency will eliminate the supposition that one or the other approaches gives a more accurate answer for element partitioning. Newly updated expressions for 11 elements are then derived and presented and applied to the early Earth to evaluate the idea of a late stage equilibration between a core forming metal and silicate melt (or magma ocean). It is possible to explain all 11 elements at conditions of 27–33 GPa, 3300–3600 K, ΔIW = − 1, for peridotite and a metallic liquid containing 10% of a light element. The main difference between the current result and several other recent modeling efforts is that Mn, V, and Cr are hosted in deep mantle phases as well as the core. The other elements – Ni, Co, Mo, W, P, Cu, Ga, and Pd – are hosted in core, and detailed modeling here shows the importance of accounting for oxygen fugacity, silicate and metallic liquid compositions, as well as temperature and pressure. The idea of late stage metal–silicate equilibrium at a restricted pressure and temperature range leaving a chemical finger print on the upper mantle remains viable.

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.

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.

Partitioning of Mo, P and other siderophile elements (Cu, Ga, Sn, Ni, Co, Cr, Mn, V, and W) between metal and silicate melt as a function of temperature and silicate melt composition

Metal–silicate partition coefficients can provide information about the earliest differentiation histories of terrestrial planets and asteroids. Systematic studies of the effects of key parameters such as temperature and melt composition are lacking for many elements. In particular, data for Mo is scarce, but given its refractory nature, is of great value in interpreting metal–silicate equilibrium. Two series of experiments have been carried out to study Mo and P partitioning between Fe metallic liquid and basaltic to peridotitic silicate melt, at 1 GPa and temperatures between 1500 and 1900 °C. Because the silicate melt utilized was natural basalt, there are also measurable quantities of 9 other siderophile elements (Ni, Co, W, Sn, Cu, Mn, V, Cr, Ga and Zn). The Ni and Co data can be used to assess consistency with previous studies. In addition, the new data also allow a first systematic look at the temperature dependence of Cu, Ga, Sn, Cr, Mn V and W for basaltic to peridotitic melts. Many elements exhibit an increase in siderophile behavior at higher temperature, contrary to popular belief, but consistent with predictions from thermodynamics. Using these new data we examine D Mo met/sil and D P met/sil in detail and show that increasing temperature causes a decrease in the former and an increase in the latter, whereas both increase with MgO content of the silicate melt. The depletions of Mo and P in the mantle of the Earth can be explained by metal–silicate equilibrium at magma ocean conditions — both elements are satisfied at PT conditions of an intermediate depth magma ocean for the Earth 22.5 GPa and 2400 °C.

Core formation and metal–silicate fractionation of osmium and iridium from gold

Both core formation and the late addition of extraterrestrial material have been invoked to explain the abundances and relative proportions of iron-loving elements in the Earth’s mantle. High-temperature experiments suggest that the concentration of gold is consistent with core formation, but the amounts of osmium and iridium require later inputs of extraterrestrial material. The abundances of the highly siderophile elements as well as their relative proportions1 in the mantle deviate from those predicted by equilibrium partitioning between metal and silicate during the formation of the Earth’s core. This discrepancy is generally explained by invoking the addition of a late veneer of extraterrestrial material to the mantle after core formation was complete2. Recently reported partition coefficients for gold, platinum and palladium3,4,5 could result in mantle abundances consistent with equilibrium partitioning. However, whether these results can be extrapolated to all highly siderophile elements, and thereby preclude the need for a late veneer, remains to be verified. Here we use high-temperature experiments to determine the metal–silicate partition coefficients for osmium, iridium and gold. On the basis of our estimates, equilibrium partitioning during core formation can explain the observed concentration of gold in the mantle, but not that of osmium and iridium. We conclude that not all highly siderophile elements were affected by core formation in the same way, and that the abundances of elements such as osmium and iridium require the addition of a late veneer.

Oxygen as a light element: A solution to single-stage core formation

The abundances of siderophile elements in the silicate Earth indicate that Earth's iron-rich core probably formed at high pressure and high temperature. A popular model of core formation considers that the concentrations of several moderately siderophile elements are consistent with a scenario of simple single-stage equilibration at the base of a magma ocean. However, recent work using temperature sensitive partitioning data for V and Nb have casted doubt on this interpretation since the required basal temperature would greatly exceed that of the mantle solidus. Here we show that single-stage core formation event could explain the mantle contents of siderophile elements best constrained by experiment (Ni, Co, V, Mn, Cr, and Nb) provided that the core contains a few weight percents of oxygen. Our calculations, based on partitioning and metallurgy data, reveal that V and Nb become significantly less siderophile with increasing the O content of core-forming materials, while the behaviour of Ni, Co, Cr and Mn is little affected. Since the other likely light element candidates C, Si and S do not drastically influence the siderophile behaviour, we conclude that a simple-equilibration scenario is a viable hypothesis only if O contributes partially to the core density deficit. This interpretation is consistent with the W budget of the silicate Earth and recently published W metal–silicate partitioning data. The presence of a few weight percents of oxygen in the core is also in agreement with recent high-pressure high-temperature solubility measurements in molten iron equilibrated with perovskite and ferropericlase.

Evidence for high-pressure core-mantle differentiation from the metal–silicate partitioning of lithophile and weakly-siderophile elements

Liquid Fe metal–liquid silicate partition coefficients for the lithophile and weakly-siderophile elements Ta, Nb, V, Cr, Si, Mn, Ga, In and Zn have been measured in multianvil experiments performed from 2 to 24 GPa, 2023–2873 K and at oxygen fugacities of −1.3 to −4.2 log units relative to the iron-wüstite buffer. Compositional effects of light elements dissolved in the metal liquid (S, C) have been examined and experiments were performed in both graphite and MgO capsules, specifically to address the effect of C solubility in Fe-metal on siderophile element partitioning. The results were used to examine whether there is categorical evidence that a significant portion of metal–silicate equilibration occurred under very high pressures during core-mantle fractionation on Earth. Although the depletion of V from the mantle due to core formation is significantly greater than that of Nb, our results indicate that both elements have similar siderophile tendencies under reducing conditions at low pressures. With increasing pressure, however, Nb becomes less siderophile than V, implying that average metal–silicate equilibration pressures of at least 10–40 GPa are required to explain the Nb/V ratio of the mantle. Similarly the moderately-siderophile, volatile element ratios Ga/Mn and In/Zn are chondritic in the mantle but both volatility and core-mantle equilibration at low pressure would render these ratios strongly sub-chondritic. Our results indicate that pressures of metal–silicate partitioning exceeding 30–60 GPa would be required to render these element ratios chondritic in the mantle. These observations strongly indicate that metal–silicate equilibration must have occurred at high pressures, and therefore support core-formation models that involve deep magma oceans. Moreover, our results allow us to exclude models that envisage primarily low-pressure (<1 GPa) equilibration in relatively small planetary bodies. We also argue that the core cannot contain significant U as this would require metal–silicate equilibration at oxygen fugacities low enough for significant amounts of Ta to have also been extracted from the mantle. Likewise, as In is more siderophile than Pb but similarly volatile and also quite chalcophile it would have been difficult for Pb to enter the core without reversing the relative depletions of these elements in the mantle unless metal–silicate equilibration occurred at high pressures >20 GPa.

Hf–W chronology of the accretion and early evolution of asteroids and terrestrial planets

The 182Hf– 182W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their 26Al– 26Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of 26Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf–W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf–W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained. The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf–W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf–W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO 2 in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical 182W/ 184W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of 182Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical 182W/ 184W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle. Hafnium–tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the 182W/ 184W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of 182Hf. In contrast, no 182W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with 147Sm– 143Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a 146Sm– 142Nd age of 50–200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf–W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of 146Sm– 142Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined 182W– 142Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf–W age for the formation of the Moon.

Effect of pressure, temperature, and oxygen fugacity on the metal-silicate partitioning of Te, Se, and S: Implications for earth differentiation

We have measured liquid Fe metal–liquid silicate partitioning ( D i) of tellurium, selenium, and sulfur over a range of pressure, temperature, and oxygen fugacity (1–19 GPa, 2023–2693 K, fO 2 −0.4 to −5.5 log units relative to the iron-wüstite buffer) to better assess the role of metallic melts in fractionating these elements during mantle melting and early Earth evolution. We find that metal-silicate partitioning of all three elements decreases with falling FeO activity in the silicate melt, and that the addition of 5–10 wt% S in the metal phase results in a 3-fold enhancement of both D Te and D Se. In general, Te, Se, and S all become more siderophile with increasing pressure, and less siderophile with increasing temperature, in agreement with previous work. In all sulfur-bearing experiments, D Te is greater than D Se or D S, with the latter two being similar over a range of P and T. Parameterized results are used to estimate metal-silicate partitioning at the base of a magma ocean which deepens as accretion progresses, with the equilibration temperature fixed at the peridotite liquidus. We show that during accretion, Te behaves like a highly siderophile element, with expected core/mantle partitioning of >10 5, in contrast to the observed core/mantle ratio of ∼100. Less extreme differences are observed for Se and S, which yielded core/mantle partitioning 100- to 10 times higher, respectively, than the observed value. Addition of ∼0.5 wt% of a meteorite component (H, EH or EL ordinary chondrite) is sufficient to raise mantle abundances to their current level and erase the original interelement fractionation of metal-silicate equilibrium.

Metal–silicate partitioning of tungsten at high pressure and temperature: Implications for equilibrium core formation in Earth

Tungsten partitioning between liquid metal and liquid silicate, D(W), from 1723–2673 K, 0.5–18 GPa, and over a wide range of metal and silicate compositions provides constraints on planetary core formation models. We find that W partitioning is extremely sensitive to the carbon content of the metal alloy, becoming about an order of magnitude more siderophile at carbon saturation. Activity-composition corrections based on interactions between W–C and Fe–C in metal solution shift calculated D(W) to more lithophile values, and calculated oxygen fugacities ( fO 2) to more oxidizing values, than uncorrected data. W exists as a highly charged cation in silicate solution, displaying a mixture of oxidation states from + 4 to + 6 in experiments at fO 2 of ∼ 0.5–2 log units below the iron–wüstite buffer. At constant fO 2, the average calculated valence decreases with pressure from ∼ + 5.5 at 0.5 GPa to ∼ + 4 at 11–18 GPa. As a result of its high oxidation state, W partitioning is strongly dependent on silicate melt polymerization and fO 2. In contrast to some previous results, we find that D(W) may decrease slightly in response to increasing temperature in the pressure range of our experiments. Pressure exerts a non-monotonic effect: D(W) increases with pressure up to ∼ 3 to 4 GPa, but decreases at higher pressures. Previous models for the effects of pressure and temperature on W partitioning that conflict with our results appear to result from a conflation of the intensive parameters of pressure, temperature, and carbon content. The mantle abundance of W could have been set by single-stage metal–silicate equilibration along the liquidus in a deep peridotite magma ocean at pressures from 20–50 GPa, and at oxygen fugacities consistent with the mantle's present iron budget (IW− 2 to − 2.5). Equilibration at higher pressure is viable if the core-forming metal contained a significant, but not unreasonable, abundance of carbon (∼ 2 wt.%). Recent continuous accretion models involving multi-stage metal-silicate equilibration in a deepening magma ocean with progressive oxidation of the silicate remain permissible given our new treatment of W partitioning data.

Low osmium solubility in silicate at high pressures and temperatures

A new methodology to experimentally investigate metal/silicate partitioning for Os ( D Os metal/silicate) under high P– T conditions was devised that utilizes metal and silicate components with natural Os abundances and disparate 187Os/ 188Os. The isotopic contrast in starting components allows examination of isotopic exchange between metal and silicate. As is common for metal–silicate partitioning experiments involving highly siderophile elements (HSE), tiny nuggets of metal appeared in the silicate melts. Linear trends in the 187Os/ 188Os versus 1/ 188Os diagram for different silicate chunks from individual experimental charges permit projections of data to nugget-free silicate concentrations. The Os concentrations and isotope compositions in pure silicates reveal that the nuggets remove Os from the silicate melt while percolating through the melt, but contribute limited Os back to the silicate melt. Our data suggest that the rate of chemical equilibration regarding Os concentration between metal and silicate is much faster than that for isotopic exchange. Results for multiple experiments indicate extremely low Os solubility in silicate melt ( D Os metal/silicate > 2 × 10 5) at pressures as high as 2 GPa, and temperatures as high as 2400 °C. These results indicate that metal–silicate segregation during planetary differentiation at high temperatures and pressures may leave silicate that is extremely depleted in Os compared to the mantle abundances of Os observed, and that isotopic exchange of HSE between planetary cores and silicate mantles subsequent to primordial differentiation may be limited.

Experimental studies of metal–silicate partitioning of Sb: Implications for the terrestrial and lunar mantles

The terrestrial mantle has a well defined Sb depletion of ∼7 ± 1 ( Jochum and Hofmann, 1997), and the lunar mantle is depleted relative to the Earth by a factor of ∼50 ± 5 ( Wolf and Anders, 1980). Despite these well defined depletions, there are few data upon which to evaluate their origin—whether due to volatility or core formation. We have carried out a series of experiments to isolate several variables such as oxygen fugacity, temperature, pressure, and silicate and metallic melt compositions, on the magnitude of D Sb met / sil . The activity of Sb in FeNi metal is strongly composition dependent such that solubility of Sb as a function of fO 2 must be corrected for the metal composition. When the correction is applied, Sb solubility is consistent with 3+ valence. Temperature series (at 1.5 GPa) shows that D Sb met / sil decreases by a factor of 100 over 400 °C, and a pressure series exhibits an additional decrease between ambient pressure (100 MPa) and 13 GPa. A strong dependence upon silicate melt composition is evident from a factor of 100 decrease in D Sb met / sil between nbo/t values of 0.3 and 1.7. Consideration of all these variables indicates that the small Sb depletion for the Earth’s mantle can be explained by high PT equilibrium partitioning between metal and silicate melt D Sb met / sil . The relatively large lunar Sb depletion can also be explained by segregation of a small metallic core, at lower pressure conditions where D Sb met / sil is much higher (2500).

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.

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.

Platinum-Group Elements in Cosmochemistry

Research Article| August 01, 2008 Platinum-Group Elements in Cosmochemistry Herbert Palme Herbert Palme *Institut für Geologie und Mineralogie, Universität zu Köln Zülpicherstrasse 49b, 50674 Köln, Germany E-mail: herbert.palme@uni-koeln.de Search for other works by this author on: GSW Google Scholar Elements (2008) 4 (4): 233–238. https://doi.org/10.2113/GSELEMENTS.4.4.233 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Herbert Palme; Platinum-Group Elements in Cosmochemistry. Elements 2008;; 4 (4): 233–238. doi: https://doi.org/10.2113/GSELEMENTS.4.4.233 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract In a cooling solar nebula, five of the six platinum-group elements (PGE) condense as refractory-metal alloys at temperatures above the condensation of Fe-Ni metal. Non-refractory Pd condenses in solid solution with Fe-Ni. Such refractory alloys are preserved in some meteorites, although they are often highly altered. The high resistance of PGE to oxidation leads to efficient extraction with metallic Fe-Ni during metal segregation and core formation. Experimentally determined PGE metal-silicate partition coefficients predict lower contents of PGE in planetary silicates than are found, supporting a late addition of PGE components. PGE are particularly useful as tracers of impacting extraplanetary materials in the strongly PGE-depleted crusts of the Earth and other planets. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The effects of core formation on the Pb- and Tl- isotopic composition of the silicate Earth

We have performed metal-silicate partitioning experiments at 2 GPa and 1650–2180 °C to investigate the behaviour of Pb and Tl during terrestrial core formation. The aim was to test the hypothesis that metal core formation followed by late sulphide addition to the core resulted in the concentrations and isotopic compositions of Pb and Tl in the silicate Earth. We investigated D Pb met/sil and D TL met/sil as functions of the sulphur content of the metal and measured the equilibrium Tl isotope fractionation between the coexisting phases. Lead is moderately siderophile under the likely conditions (initially reducing [Wade, J., Wood, B.J., Core formation and the oxidation state of the Earth, Earth Planet. Sci. Lett. 236(2005) 78–95.]) of core segregation on Earth so that the μ( 238U/ 204Pb) of the bulk silicate Earth should have increased by a factor of 6.5 ( D Pb ∼ 13) as the core separated. In the case of Tl, core segregation should have reduced the Tl concentration of the BSE by about 50%. Neither the Pb nor Tl isotopic compositions of the bulk silicate Earth can, however, be completely explained by S-free iron core formation. Thallium isotopes were found not to be significantly fractionated by metal or sulphide separation from silicate. Addition of sulphur to the metal greatly increases metal-silicate partition coefficients for both Pb and Tl. D Pb met/sil increases by a factor of 15 and D TL met/sil by a factor of 45 as S increases from 0 to 35% in the metal phase. This means that extraction of sulphide from a molten mantle would result in D Pb sulph/sil of ∼ 40 and D TL sulph/sil of ∼ 60. We used the latter results to calculate the effects of late sulphide extraction on the Pb and Tl isotopic compositions of the silicate Earth. For a bulk Earth with μ of 0.7 addition of 1.6% sulphide to the core 100–140 Myr after the beginning of the solar system is sufficient to displace the Pb-isotopic composition of the silicate Earth into the region indicated by estimates in the literature. The Tl concentration and ɛ 205Tl of bulk silicate Earth are also consistent with this extent of sulphide addition. Raising μ of the Earth reduces the amount of sulphide addition to the core needed to satisfy the Pb and Tl isotopic constraints but for values of μ above 0.9 (< 0.65% sulphide) the sulphide has insufficient effect. Separation of sulphide from a mixture of silicate crystals and melt would, however, because of the incompatible nature of Pb and Tl amplify the sulphide effect. We show that, for initial μ of 0.7, as little as 0.1% sulphide extraction (at 150–200 Myr) from a mixture of 91% crystals and 9% melt would be sufficient to displace the isotopic compositions of Pb and Tl of the BSE to the required values. Late volatile loss of Pb and Tl is, therefore, not required by the partitioning and currently available isotopic data.

Core formation and the oxidation state of the Earth: Additional constraints from Nb, V and Cr partitioning

We have combined metal–silicate partitioning data from the literature with new experimental results at 1.5–8 GPa and 1480–2000 °C to parameterize the effects of pressure, temperature and composition on the partitioning of V, Cr and Nb between liquid Fe metal (with low S and C content) and silicate melt. Using information from the steelmaking literature to correct for interactions in the metal phase, we find that, for peridotitic silicate melts, metal–silicate partition coefficients are given by: log D Nb met/sil = 2.5 log D Fe met/sil + 1.57 - 11 , 930 T - 114 ( ± 43 ) P T ± 0.41 log D V met/sil = 1.5 log D Fe met/sil + 0.582 - 6493 T - 62 ( ± 19 ) P T ± 0.08 log D Cr met/sil = log D Fe met/sil + 0.643 - 4232 ( ± 538 ) T - 22 ( ± 13 ) P T where D i met/sil is the partition coefficient for element i. Partitioning of V and Cr is insensitive to silicate melt composition, but Nb shows a considerable compositional effect. The new data enable us to examine different models of terrestrial accretion and core formation. If we fix the Fe content of the mantle at the current value and use the Ni content of the mantle to estimate pressure of equilibration then temperatures about 1200 °C above the silicate liquidus are required to match vanadium partitioning to the current concentration of V in the mantle. Under these conditions the core concentrations of Si (∼15%) and Nb (>60% of Earth’s budget) are implausibly high. A more realistic approach is to assume that the metal of accreting planetesimals equilibrated at the base of a deep magma ocean whose temperature was close to the silicate liquidus. As the magma ocean deepened in proportion to the size of the Earth the metal was continuously extracted to the core without further re-equilibration in the lowermost mantle. In this case the V, Cr and Nb contents of core and mantle can easily reproduce the expected values provided the Earth became more oxidized as it accreted (O’Neill H. S. (1991) The origin of the Moon and the early history of the Earth—a chemical model. 2. The Earth Geochim. Cosmochim. Acta 55(4), 1159–1172; Wade J. and Wood B. J. (2005) Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, 78–95; Wänke H. and Dreibus G. (1988) Chemical-composition and accretion history of terrestrial planets. Philos. Trans. Roy. Soc. Lond. A-Math. Phys. Eng. Sci. 325(1587), 545–557). Increasing oxidation requires the oxidized Fe content of the mantle to increase from 0.5% to 1.0% to the current value of 6.26% as the Earth grew. This model , with magma ocean thickness corresponding to 35% of mantle depth, reproduces the calculated core–mantle partitioning of Ni, and Co and yields a Si content of the core of approximately 6%, in good agreement with cosmochemical estimates.

New Ni and Co metal-silicate partitioning data and their relevance for an early terrestrial magma ocean

It is generally assumed that the abundances of the siderophile (metal-loving) elements in the Earth's mantle are the result of core formation in the early Earth. In the most popular model the low contents of siderophile elements in the Earth's mantle are the result of metal-silicate equilibration followed by metal accumulation at the base of a magma ocean. This model is primarily based on the pressure dependence of Ni and Co metal-silicate partition coefficients first determined by Li and Agee [Li, J., Agee, C.B., 1996. Geochemistry of mantle–core differentiation at high pressures. Nature 381, 686–689]. We have undertaken a further test of this hypothesis by performing a large number of additional partition experiments with the two key elements Ni and Co at a variety of temperatures and a pressure range extending from ambient pressure to 25 GPa (~750 km depth in the Earth). The unexpected result of the new experiments is the recognition of two pressure regimes in the temperature and pressure dependences of Ni–Fe and to a lesser extent of Co–Fe exchange metal-silicate partition coefficients, KDNi–Fe and KDCo–Fe. The strong pressure and temperature dependences of KD⁎Ni–Fe and KD⁎Co–Fe at atmospheric pressure change below about 5 GPa to much weaker dependences at higher pressures. As a consequence the pressure dependences of KDNi–Fe and KDCo–Fe cannot be regressed with a single fit over the entire pressure range. Fits based on partition data obtained in this work and from the literature between 5 GPa and 25 GPa were extrapolated to pressures and temperatures to lower mantle liquidus conditions. The Co–Fe partition coefficients required to satisfy the present mantle composition is reached at about 35 GPa, whereas the proper Ni–Fe ratio requires 45 to 50 GPa. At 55 GPa Ni and Co partition coefficients would be identical but the Ni- and Co-contents imposed on the Earth's mantle by metal-silicate equilibration would be too low. This would exclude single stage magma ocean models for producing approximately chondritic Ni/Co ratio in the Earth's mantle. However, the accuracy of the experimental data obtained here and from the literature is not sufficient to unambiguously exclude single stage equilibration. The results of this and other recent studies exclude that Fe-, Ni-, and Co-contents of the present upper mantle were established by metal-silicate equilibration at upper mantle conditions (<25 GPa).

Metal–silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion

We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7GPa and between 2123 and 2473K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle–core equilibrium value.