Fe-rich Core Research Articles

An unusually low-density super-Earth transiting the bright early-type M-dwarf GJ 1018 (TOI-244)

Context. Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, recent discoveries are revealing a new group of low-density planets that are inconsistent with that description. Aims. We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright (K = 7.97 mag), nearby (d = 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days. Methods. We used Markov chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. Our model includes a planetary component and Gaussian processes aimed at modeling the correlated stellar and instrumental noise. Results. We find TOI-244 b to be a super-Earth with a radius of Rp = 1.52 ± 0.12 R⊕ and a mass of Mp = 2.68 ± 0.30 M⊕. These values correspond to a density of ρ = 4.2 ± 1.1 g cm−3, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a 479−96+128 km thick hydrosphere over a 1.17 ± 0.09 R⊕ solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition. Conclusions. With a 8% precision in radius and 12% precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations.

A comparative study of metastable phase separation for undercooled liquid Fe35Cu65 alloy under natural and forced cooling conditions

The influences of the cooling style on metastable phase separation (MPS) kinetics of undercooled liquid Fe35Cu65 alloy have been systematically investigated by glass fluxing technique and lattice-Boltzmann method. MPS was observed in the undercooling range of 13–285 K. With a rise in the undercooling, phase separation time increased linearly. Meanwhile, a microsegregation morphology, which appeared as Fe-rich globules near the sample surface and well-defined αFe dendrites far from the sample surface, transformed into the eccentric core-shell macrosegregation morphology composed of a floating Fe-rich core and a sinking Cu-rich shell. Under the forced cooling condition, the macrosegregation formation required a larger critical undercooling. In contrast with the naturally cooled sample, the phase separation time of forcedly cooled sample was much shorter, and both the eccentric level of the core-shell macrosegregation and the volume fraction of Fe-rich zone were smaller at the same undercooling. The MPS and microstructure evolution of liquid Fe35Cu65 alloy under the natural and forced cooling conditions were numerically simulated and experimentally confirmed. Theoretical analyses revealed that the fluid dynamics caused by the Stokes motion and Marangoni migration of Fe-rich globules were strongly dependent on the cooling style, the size and the position of Fe-rich globules during the MPS.

FeCo nanoparticles with different compositions as electrocatalysts for oxygen evolution reaction in alkaline solution

FeCo nanoparticles with different surface-layer compositions were synthesized using the polyol method. The compositional difference between the surface layer and the core arises from the dissimilarity in the reduction potentials of Fe and Co at the synthesis temperature. The FeCo nanoparticles consisted of an Fe-rich FeCo alloy core and a Co-rich FeCo oxide layer. The size of the FeCo nanoparticles was approximately 207 nm, and Fe59Co41 (S4) showed optimal intrinsic catalytic activity for an oxygen evolution reaction. The overpotential of S4 was 285 mV at 10 mA/cm2 in 1 M KOH solution, which is much lower than that of the other samples. In addition, after chronopotentiometry at 10 mA/cm2 for 24 h in 1 M KOH, the overpotential at 10 mA/cm2 increased from 285 to 294 mV. The enhancement of the catalyst properties was attributed not only to the synergetic effect of the metal core and oxide layer, but also to the optimal iron doping effect.

Hydrogen solubility in FeSi alloy phases at high pressures and temperatures

AbstractLight elements alloying with metallic Fe can change the properties and therefore play a key role in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser heating combined with high-energy synchrotron X-ray diffraction, we studied reactions between FeSi and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation. The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities to understand geochemical behaviors of H during core formation in future experiments. The low-H content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into the Fe-rich core.

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe2O4 nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm2 in 0.1 M KOH compared to RuO2 (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe2O4 (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO2 showed a gradual increase in overpotential to maintain 10 mA/cm2 after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.

Iron isotope fractionation at the core–mantle boundary by thermodiffusion

The D” layer at the base of the Earth’s mantle exhibits anomalous seismic properties, which are attributed to heat loss from and chemical interaction with the underlying molten Fe-rich outer core. Here we show that mass transfer due to temperature variations within the D” layer could lead to resolvable fractionation of iron isotopes. We constrain the degree of isotope fractionation by experiments on core-forming Fe alloy liquids at 2100–2300 K and 2 GPa, which demonstrate that heavy Fe isotopes preferentially migrate towards lower temperature and vice versa. We find that this isotope fractionation occurs rapidly due to the high mobility of iron, which reaches 0.013 ± 0.002‰ (2σ) per degree per amu at steady state. Numerical simulations of mantle convection capturing the evolution of a basal thermal boundary layer show that iron isotope fractionation immediately above the core–mantle boundary can reach measurable levels on geologic timescales and that plumes can entrain this fractionated material into the convecting mantle. We suggest that such a process may contribute to the heavy Fe isotope composition of the upper mantle inferred from mantle melts (basalts) and residues (peridotites) relative to chondrites. That being the case, non-traditional stable isotope systems such as Fe may constrain the interactions between the core and mantle. Iron isotopic fractionation at the core–mantle boundary due to thermal diffusion may partly explain the iron isotope composition of the upper mantle, according to high-temperature experiments and numerical simulations.

U, Th, and K partitioning between metal, silicate, and sulfide and implications for Mercury's structure, volatile content, and radioactive heat production

AbstractThe distribution of heat-producing elements (HPE) potassium (K), uranium (U), and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner solar system. To investigate the abundances of HPE in Mercury’s interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th, and K partitioning between metal, silicate, and sulfide (Dmet/sil and Dsulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts, respectively. Also, at low oxygen fugacity (log fO2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. Dmet/sil for U, Th, and K increases with decreasing oxygen fugacity, while DUmet/sil and DKmet/sil increase when the metal is enriched and depleted in O or Si, respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O, and C) in Fe metal and sulfide. We calculated chemical compositions of Mercury’s core after core segregation, for a range of fO2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S, and negligible C. Also if core-mantle separation happened at a fO2 lower than IW-4, the bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th, and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th, and K reservoirs into account. Without any sulfide layer, or if Mercury’s core segregated at a higher fO2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance if core segregation occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s and Earth’s values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at an fO2 close to, or lower than, IW-6 both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury’s surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar to or lower than the Earth’s and Venus’s, suggesting volatile depletion in the inner solar system. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.

Experimental Modeling of Silicate and Carbonate Sulfidation under Lithospheric Mantle P,T-Parameters

Interactions of mantle silicates with subducted carbonates, sulfides, and sulfur-rich fluids are experimentally simulated in the olivine-ankerite-sulfur and olivine-ankerite-pyrite systems using a multi-anvil high-pressure split-sphere apparatus at 6.3 GPa and range of 1050–1550 °C. Recrystallization of Fe,Ni-bearing olivine and ankerite in a sulfur melt was found to be accompanied by sulfidation of olivine and carbonate, involving partial extraction of metals, carbon, and oxygen into the melt, followed by the formation of pyrite (±pyrrhotite), diopside, and Fe-free carbonates. The main features of metasomatic alteration of Fe,Ni-olivine by a reduced sulfur fluid include: (i) a zonal structure of crystals (Fe-rich core, Mg-rich rim); (ii) inclusions of pyrite and pyrrhotite in olivine; (iii) certain Raman spectral characteristics of olivine. At T > 1350 °C, two immiscible melts, a predominantly sulfur melt with dissolved components (or a Fe–Ni–S–O melt) and a predominantly carbonate one, are generated. The redox interaction of these melts leads to the formation of metastable graphite (1350–1550 °C) and diamond growth (1550 °C). The studied olivine-ankerite-sulfur and olivine-ankerite-pyrite interactions may be considered as the basis for simulation of metasomatic processes accompanied by the formation of mantle sulfides during subduction of crustal material to the silicate mantle.

A novel synthetic strategy of Fe@Cu core-shell microsphere particles by integration of solid-state immiscible metal system and low wettability

Herein, a novel preparation route of bimetallic spherical core-shell particles of immiscible metal system was developed and demonstrated through the synthesis of Fe@Cu core–shell microsphere particles. Copper ferrite (CuFe2O4) was used as precursor for the preparation of the Core–shell structured Fe@Cu microsphere using the solid-state immiscibility of Fe and Cu and the low wettability strategy of the liquid-solid interface of graphite at 1403 K. The results showed that phase separation occurred between Cu and Fe, where Cu tended to move towards the surface of the particles and Fe was more concentrated in the center of the particles, due to the lower surface energy of liquid Cu with respect to solid Fe. This was followed by formation of metal droplets located on loose graphite flakes on the spherical Fe@Cu due to the solid-liquid interfacial tension. The as-prepared Fe@Cu microsphere exhibited good spherical shapes and smooth surfaces. The cross-sectional micrographics and element mapping of the spherical particles confirmed the evenly distributions of the Fe-rich core and Cu-rich shell. This strategy is promising for the fabrication of core–shell structured spherical particles for immiscible alloys.

Atom Probe Tomography Unveils Formation Mechanisms of Wear-Protective Tribofilms by ZDDP, Ionic Liquid, and Their Combination.

The development of advanced lubricant additives has been a critical component in paving the way for increasing energy efficiency and durability for numerous industry applications. However, the formation mechanisms of additive-induced protective tribofilms are not yet fully understood because of the complex chemomechanical interactions at the contact interface and the limited spatial resolution of many characterizing techniques currently used. Here, the tribofilms on a gray cast iron surface formed by three antiwear additives are systematically studied; a phosphonium-phosphate ionic liquid (IL), a zinc dialkyldithiophosphate (ZDDP), and an IL+ZDDP combination. All three additives provide excellent wear protection, with the IL+ZDDP combination exhibiting a synergetic effect, resulting in further reduced friction and wear. Atom probe tomography (APT) and scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) were used to interrogate the subnm chemistry and bonding states for each of the tribofilms of interest. The IL tribofilm appeared amorphous and was Fe, P, and O rich. Wear debris particles having an Fe-rich core and an oxide shell were present in this tribofilm and a transitional oxide (Fe2O3)-containing layer was identified at the interface between the tribofilm and the cast iron substrate. The ZDDP+IL tribofilm shared some of the characteristics found in the IL and ZDDP tribofilms. Tribofilm formation mechanisms are proposed on the basis of the observations made at the atomic level.

Genesis of the Weibao banded skarn Pb-Zn deposit, Qimantagh, Xinjiang: Insights from skarn mineralogy and muscovite 40Ar-39Ar dating

The Weibao Pb-Zn deposit is located in the Qimantagh area, Xinjiang (NW China). At Weibao, stratiform Pb-Zn ore bodies are hosted by the skarn-altered Mesoproterozoic Langyashan Formation. Both of the skarn rocks and Pb-Zn ores are banded. The skarn bands are normally 0.5 to 5cm wide and comprise alternating garnet and pyroxene zones. Systematic mineralogical studies indicate that the Weibao banded skarn was formed by infiltrative metasomatism of magmatic fluids in a disequilibriated geochemical system, rather than by the bimetasomatism between marble and Si-Al units or the metamorphism of calc-silicate-rich submarine exhalative beds. This infiltrative metasomatic process can be further divided into four stages, namely the prograde skarn, retrograde skarn, sulfide and carbonate stage. Three garnet types (Garnet I, II and III) and two pyroxene types (Pyroxene I and II) have been identified at Weibao. Garnet I has commonly Fe-rich core and Al-rich rim (Ad34Gr65 to Ad96Gr3), while Garnet II has a wider composition range (Ad23Gr73 to Ad97Gr2; average: Ad59Gr41). Garnet III is compositionally more homogeneous and Mn-rich (Ad17Sp3Gr80 to Ad55Sp6Gr39; average: Ad47Sp4Gr49). Pyroxene I is diopside-rich (Hd20Jo1Di79 to Hd36Jo2Di62; average Hd29Jo1Di70), while Pyroxene II has lower diopside component (Hd34Jo3Di63 to Hd55Jo13Di32; average: Hd48Jo5Di47). Garnet I and Pyroxene I may have formed by a small volume of high-temperature and volatile-rich fluids exsolved from the magmas at its initial crystallization stage. Further fractionation generated abundant magmatic fluids that reacted with the impure carbonates through infiltrative metasomatism, forming Garnet II and Pyroxene II in the banded skarn. Garnet III occurs as garnet+sphalerite+galena+calcite veinlets crosscutting the early-stage pyroxene and garnet zones, and may have been a product of late hydrothermal superimposition.Sulfide stage is the main ore-forming stage at Weibao. Sulfide minerals in this stage precipitated mainly through replacing anhydrous (e.g., pyroxene) and hydrous (e.g., amphibole) minerals, or as interstitials among early-formed minerals. A minor amount of the sulfides also occurs as sulfide veins. The sulfide precipitation may have been controlled chiefly by physicochemical changes of ore-forming fluids and the characteristics of early-formed skarn minerals. Specifically, hydrothermal replacement of skarn minerals has provided the space for sulfide to precipitate and thus controlled their distribution, and further shaped the mineralization into a banded structure. 40Ar-39Ar dating of the syn-mineralization muscovite yielded a Late Triassic age (ca. 226Ma), which was coeval to the massive post-collisional intermediate-felsic granitoids emplacement in the Qimantagh area. Magmatic fluids evolved from these intermediate-felsic granitoids have induced the regional-scale magmatic hydrothermal mineralization, including that at Weibao.

Northwest Africa 5790: Revisiting nakhlite petrogenesis

Northwest Africa 5790, the latest nakhlite find, is composed of 58vol.% augite, 6% olivine and 36% vitrophyric intercumulus material. Its petrology is comparable to previously discovered nakhlites but with key differences: (1) Augite cores display an unusual zoning between Mg# 54 and 60; (2) Olivine macrocrysts have a primary Fe-rich core composition (Mg#=35); (3) The modal proportion of mesostasis is the highest ever described in a nakhlite; (4) It is the most magnetite-rich nakhlite, together with MIL 03346, and exhibits the least anisotropic fabric. Complex primary zoning in cumulus augite indicates resorption due to complex processes such as remobilization of former cumulates in a new magma batch. Textural relationships indicate unambiguously that olivine was growing around resorbed augite, and that olivine growth was continuous while pyroxene growth resumed at a final stage. Olivine core compositions (Mg#=35) are out of equilibrium with the augite core compositions (Mg# 60–63) and with the previously inferred nakhlite parental magma (Mg#=29). The presence of oscillatory zoning in olivine and augite precludes subsolidus diffusion that could have modified olivine compositions. NWA 5790 evidences at least two magma batches before eruption, with the implication that melt in equilibrium with augite cores was never in contact with olivine. Iddingsite is absent.Accordingly, the previous scenarios for nakhlite petrogenesis must be revised. The first primary parent magmas of nakhlites generated varied augite cumulates at depth (Mg# 66–60) as they differentiated to different extents. A subsequent more evolved magma batch entrained accumulated augite crystals to the surface where they were partly resorbed while olivine crystallized. Trace element variations indicate unambiguously that they represent consanguineous but different magma batches. The compositional differences among the various nakhlites suggest a number of successive lava flows. To account for all observations we propose a petrogenetic model for nakhlites based on several (at least three) thick flows. Although NWA 5790 belongs to the very top of one flow, it should come from the lowest flow sampled, based on the lack of iddingsite.

New Fe-based soft magnetic amorphous alloys with high saturation magnetization and good corrosion resistance for dust core application

Fe-rich amorphous dust core alloys with minor-addition of Mo and/or Cr were synthesized because of the industrial request of mass production of high efficient low core loss Fe-based dust cores. It was found that the present Fe-(Cr, Mo)-P-B-Si-(C) amorphous alloy ribbons exhibit good soft magnetic properties of above 1.5 T for saturation magnetization, below 10 A/m for coercivity and rather good corrosion resistance in 3%NaCl aqueous solution. Besides, the amorphous alloy ribbons exhibit good bending ductility in as-spun and optimally annealed states. It is noticed that these characteristics are much superior to those for commercial Fe-Cr-Nb-P-B-Si soft magnetic dust core alloy (SENNTIX). In addition, the annealing treatment was noticed to cause the improvement of corrosion resistance without detriment to bending ductility. The new Fe-based soft magnetic alloys with good combined characteristics are expected to be used as the basic material as high-efficient soft magnetic dust cores in a high frequency range.

Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements

It is now widely accepted that the asteroid 4-Vesta has an Fe-rich metallic core, but the composition of the core and the conditions prevailing during core–mantle differentiation are poorly constrained. In light of new constraints on Vesta’s geophysical and geochemical properties obtained by the DAWN mission, we have re-examined the conditions at which core–mantle differentiation in Vesta may have occurred by linking the estimated mantle depletions of siderophile elements P, Co, Ni, Cu, Ga, Ge, Mo and W in the vestan mantle to newly derived predictive equations for core–mantle partitioning of these elements. We extend the number of elements previously considered in geochemical modeling of vestan core formation and use published metal–silicate partitioning data obtained at low pressures to characterize the dependence of metal/silicate partition coefficients (D) on pressure, temperature, oxygen fugacity and composition of the silicate and metallic melt. In our modeling we implement newly derived mantle depletions of P, Co, Ni and Ga through analysis of published HED meteorite analyses and assess two contrasting bulk compositional models for Vesta.Modeling results using Monte Carlo simulations constrain vestan core formation to have occurred at mildly reducing conditions of approximately 2log units below the iron–wüstite (IW) buffer (ΔIW=−2.05±0.20) if the two most likely bulk compositions (binary mixtures of H+CM or H+CV chondritic meteorites) are considered, assuming a temperature range between 1725 and 1850K and a sulfur-free pure Fe core. If the core is assumed to be sulfur-rich (15wt.% S) as predicted by the latter bulk compositional models, observed depletions for all eight siderophile elements can be simultaneously satisfied at ΔIW=−2.35±0.10 and 1725–1850K for the H+CV bulk composition and ΔIW=−2.30±0.15 and 1725–1850K for the H+CM bulk composition. More reducing conditions are not consistent with the observed siderophile element depletions in the vestan mantle, independent of the sulfur content of the vestan core and of the bulk compositional model chosen.Our analysis shows that a previously proposed shallow magma ocean on Vesta during core formation is not consistent with the observed mantle depletion of Ga for the two considered bulk compositions irrespective of core composition. Instead, our results are consistent with the existence of a deep magma ocean during core formation on Vesta, requiring >50–100% mantle melting.

Experimental investigation and numerical simulation on liquid phase separation of ternary Fe-Sn-Si/Ge monotectic alloy

The liquid phase separation of small Fe-Sn-Si/Ge alloy droplets under reduced-gravity condition is investigated experimentally by free fall technique and theoretically by lattice Boltzmann method. In the drop tube experiments, the Fe-Sn-Si/Ge monotectic alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquid temperatures for a few seconds. Finally, the molten alloy melt is ejected out from the small orifice of a quartz tube by high pressure jetting gas of He and dispersed into numerous tiny droplets, which are rapidly solidified during free fall in a protecting He gas environment. These droplets benefit from the combined advantages of high undercooling, containerless state and rapid cooling, which can provide an efficient way to study the liquid phase separation of high-temperature alloys in microgravity. In order to efficiently reproduce the dynamic process of phase separation inside drop tube equipment, the effects of surface segregation and Marangoni convection are introduced into the interaction potential of different liquids within lattice Boltzmann theory. Based on this modified model, the dynamic mechanism of phase separation can be sufficiently analyzed and the phase separation patterns can be realistically simulated. Experimental results demonstrate that conspicuous liquid phase separations have taken place for both Fe-Sn-Si and Fe-Sn-Ge alloy droplets and the corresponding morphologies are mainly characterized by core-shell and dispersed structures. The phase separation process can be modulated by the third-element addition. As the Si element of Fe-Sn-Si alloy is replaced by the Ge element with the same fraction, the distribution order of Fe-rich and Sn-rich zones is reversed within core-shell structure. A core-shell structure composed of a Fe-rich core and a Sn-rich shell is frequently observed in Fe-Sn-Si alloy droplets whereas the Fe-Sn-Ge alloy droplets tend to form a core-shell structure consisting of a Sn-rich core and a Fe-rich shell. Theoretical calculations show that the droplet cooling rate is closely related to droplet size: a smaller alloy droplet has a higher cooling rate. The liquid L2(Sn) phase always nucleates preferentially and forms tiny globules prior to solid Fe phase. Stokes motion can be greatly weakened in this experiment and the Marangoni migration dominates the globule movement in the process of liquid phase separation. Furthermore, the intensity of Marangoni convection within Fe-Sn-Ge alloy droplets is significantly stronger than that inside Fe-Sn-Si alloy droplets. Numerical simulations reveal that the cooling rate, Marangoni convection and surface segregation play the important roles in determining the selection of core-shell configurations and the formation of dispersed structures. Ultrahigh cooling rate contributes to forming the dispersed structures. When the Marangoni convection proceeds more drastically than the surface segregation, the minor liquid phase with a smaller surface free energy migrates to droplet center and occupies the interior of droplet, otherwise most of the minor phases appear around the periphery of droplet.

Liquid-liquid phase separation of freely falling undercooled ternary Fe-Cu-Sn alloy.

The active modulation and control of the liquid phase separation for high-temperature metallic systems are still challenging the development of advanced immiscible alloys. Here we present an attempt to manipulate the dynamic process of liquid-liquid phase separation for ternary Fe47.5Cu47.5Sn5 alloy. It was firstly dispersed into numerous droplets with 66 ~ 810 μm diameters and then highly undercooled and rapidly solidified under the containerless microgravity condition inside drop tube. 3-D phase field simulation was performed to explore the kinetic evolution of liquid phase separation. Through regulating the combined effects of undercooling level, phase separation time and Marangoni migration, three types of separation patterns were yielded: monotectic cell, core shell and dispersive structures. The two-layer core-shell morphology proved to be the most stable separation configuration owing to its lowest chemical potential. Whereas the monotectic cell and dispersive microstructures were both thermodynamically metastable transition states because of their highly active energy. The Sn solute partition profiles of Fe-rich core and Cu-rich shell in core-shell structures varied only slightly with cooling rate.

The melting curve of Ni to 1 Mbar

The melting curve of Ni has been determined to 125 GPa using laser-heated diamond anvil cell (LH-DAC) experiments in which two melting criteria were used: firstly, the appearance of liquid diffuse scattering (LDS) during in situ X-ray diffraction (XRD) and secondly, plateaux in temperature vs. laser power functions in both in situ and off-line experiments. Our new melting curve, defined by a Simon–Glatzel fit to the data where TM(K)=[(PM18.78±10.20+1)]1/2.42±0.66×1726, is in good agreement with the majority of the theoretical studies on Ni melting and matches closely the available shock wave melting data. It is however dramatically steeper than the previous off-line LH-DAC studies in which determination of melting was based on the visual observation of motion aided by the laser speckle method. We estimate the melting point (TM) of Ni at the inner-core boundary (ICB) pressure of 330 GPa to be TM=5800±700 K(2σ), within error of the value for Fe of TM=6230±500 K determined in a recent in situ LH-DAC study by similar methods to those employed here. This similarity suggests that the alloying of 5–10 wt.% Ni with the Fe-rich core alloy is unlikely to have any significant effect on the temperature of the ICB, though this is dependent on the details of the topology of the Fe–Ni binary phase diagram at core pressures. Our melting temperature for Ni at 330 GPa is ∼2500 K higher than that found in previous experimental studies employing the laser speckle method. We find that those earlier melting curves coincide with the onset of rapid sub-solidus recrystallization, suggesting that visual observations of motion may have misinterpreted dynamic recrystallization as convective motion of a melt. This finding has significant implications for our understanding of the high-pressure melting behaviour of a number of other transition metals.

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.

Enhancement of local superconductivity in ferromagnetic FeCrB metallic glass by Ar+ ion irradiation

We have reinforced local superconductivity in ferromagnetic Fe67Cr18B15 metallic glasses by ion irradiation. Superconductivity in this medium appears due to the presence of large-scale layered clusters of metallic Fe–Cr phase, 150–230 Å in size, with a ferromagnetic (or superparamagnetic) Fe-rich core and nonmagnetic Cr-rich superconducting shell. Here we show that due to the intensification of concentration phase separation in the Fe–Cr clusters under ion (Ar+) irradiation, the volume of the superconducting phase increases from the initial 0.4–0.5% up to 7–8%. After irradiation, the resistivity jump Δρ/ρ in the temperature range T = 3.1–3.6 K increases ∼14 times, reaching 19%, as compared to 1.36% for the initial sample. In the interval of T = 3.1–3.6 K, the rate of resistance change reaches 79% K−1 for the irradiated sample instead of 3.6% K−1 for the initial sample. In the same temperature interval, the rate of magnetoresistance change increases from 3% K−1 for the initial sample up to 70% K−1 after irradiation.

Ferric-iron-rich tourmaline from the Darasun gold deposit, Transbaikalia, Russia

We have studied tourmaline of the schorl – dravite – povondraite series in a sample from the Darasun porphyry-style gold deposit, Chita region, Transbaikalia, Russia. A combination of analytical procedures has been used to determine composition in terms of major elements (electron microprobe), Fe 2+ and Fe 3+ (Mossbauer spectroscopy), the probable position of Fe 3+ in the Fe-rich core of a separated grain (FTIR), the Li content (ICP–MS), and conditions of formation (fluid inclusions in coeval quartz). A significant amount of Fe 3+ has been directly determined by Mossbauer spectroscopy (83% of iron is Fe 3+ over the averaged sample). The evolution of chemical composition of the tourmalines studied here can be generalized in the following model. The first generation of low-Al and high-Fe “oxy-dravite” – povondraite (Al~3, Fe~4, Ca~0.2, □ apfu ) is followed by a second generation of higher-Al and lower-Fe “oxy-dravite” (Al~6, Fe~1.5, Ca~0.1, □~0.2 apfu ), and the third generation consists of dravite having the highest Al and the lowest Fe contents (Al > 6, Fe apfu ). Depletion of the second and third generations in Fe is caused by deposition of early sulfides. The evolution in chemical composition of the tourmalines studied here is different from that of redeposited tourmalinites in Western Carpathians, where tourmaline evolves from schorl–dravite to povondraite. The chemical composition of the Darasun tourmalines is different from Fe 3+ -rich tourmalines from evaporite formations in low K, and is similar in Mg content. At the same time, the major type of homovalent substitution (AlFe −1 ) in the tourmalines examined is similar to that in tourmalines from Sn- and Cu-porphyry deposits, and may be accepted as a prospecting guide for the porphyry-style deposits. This finding of povondraite is the third in the world and the first in hydrothermal gold deposit. The fluid inclusions in the associated quartz indicate that that first-stage tourmaline precipitated from boiling fluid at low pressure. Boiling favored the increasing oxygen activity, and resulted in the increase of Fe 3+ in that tourmaline.