Fe-Ni Melt Research Articles

Where did the largest diamonds grow? The experiments on percolation of Fe-Ni melt through olivine matrix in the presence of hydrocarbons

Recently it was found that large natural diamonds can grow from a metal liquid. One of the principal issues of the proposed hypothesis is the formation of so-called “pockets” filled with Fe-Ni melt and hydrocarbons in the Earth's mantle. The existing models of Fe migration imply percolation of liquid melt through interconnected interstices between silicate minerals, although these models face several fundamental problems in explaining the process of penetration of Fe melt between solid crystalline phases like silicate and oxide minerals. The aim of the present study is to contribute to the mechanism of Fe-Ni melt migration, and to elucidate the evolution of the “pockets” in the presence of hydrocarbons. The experiments were performed using a high-pressure apparatus “BARS” at pressures 3 and 5 GPa, and temperature 1600 °C. A silicate matrix consisting of natural olivine grains was used. The interstices in olivine were filled with anthracene that decomposes under high P-T into a complex hydrocarbon fluid. Percolation of Fe-Ni (64/36 wt%) melt through the interstices was demonstrated which occurred at relatively high rates. The basis of the proposed mechanism is “solubility-enhanced infiltration”: Fe-Ni occupies the space filled with light elements or substances that are soluble in the melt. It is suggested that the following simple, but efficient mechanism supports the growth of large diamonds as well as their resorption and storage within silicate mantle of the Earth for a long time.

Characterization of carbon phases in Yamato 74123 ureilite to constrain the meteorite shock history

Abstract The formation and shock history of ureilite meteorites, a relatively abundant type of primitive achondrites, has been debated for decades. For this purpose, the characterization of carbon phases can provide further information on diamond and graphite formation in ureilites, shedding light on the origin and history of this meteorite group. In this work, we present X-ray diffraction and micro-Raman spectroscopy analyses performed on diamond and graphite occurring in the ureilite Yamato 74123 (Y-74123). The results show that nano- and microdiamonds coexist with nanographite aggregates. This, together with the shock-deformation features observed in olivine, such as mosaicism and planar fractures, suggest that diamond grains formed by a shock event (≥15 GPa) on the ureilitic parent body (UPB). Our results on Y-74123 are consistent with those obtained on the NWA 7983 ureilite and further support the hypothesis that the simultaneous formation of nano- and microdiamonds with the assistance of a Fe-Ni melt catalysis may be related to the heterogeneous propagation and local scattering of the shock wave. Graphite geothermometry revealed an average recorded temperature (Tmax) of 1314 °C (±120 °C) in agreement with previously estimated crystallization temperatures reported for graphite in Almahata Sitta ureilite.

Thermodynamic Assessment of Liquid Fe-Ni-C Alloy Using Modified Quasichemical Model

The liquid solution properties in the Fe-Ni and Fe-Ni-C systems have been thermodynamically assessed using the modified quasichemical model in the pair approximation. The asymmetry of the Fe-Ni and Fe-Ni-C alloy melts was verified by measuring the melting points of Fe-Ni binary alloys on the Ni-rich side, and the C solubility limit in Fe-Ni melt over the entire concentration range at 1500°C and 1600°C. The solubility minimum of C and the maximum partial enthalpy of mixing of C in the ternary Fe-Ni-C system occurred at near the maximum short-range ordering composition of the binary Fe-Ni liquid solution. The extrema of the partial properties of C in the ternary Fe-Ni-C system were successfully reproduced with only one constant adjustable ternary parameter.

Evidence for Local Shock Melting in Seymchan Meteorite

Pallasite Seymchan contains three morphological types of olivine grains, which occur in a FeNi matrix and have rounded, angular, and fragmental shape. Sometimes the rounded grains form clusters. The texture and mineralogical features of the olivine cluster in pallasite Seymchan were studied by optical microscopy, EMPA, and SEM. The data obtained indicate an imposed impact event in the Seymchan history, which led to local melting and rounding of initially angular olivine grains, as well as to melting of adjacent regions of the host FeNi metal. Local impact melting reasonably explains the observed textures and the coexistence of three morphological types of olivine in the pallasite Seymchan. Configuration of the intergranular boundaries in the olivine cluster indicates the phenomenon of coalescence. The phosphate–metal–sulfide films decorating the intergranular boundaries in the cluster represent the binding medium necessary for the coalescence of the olivine melt drops in the FeNi melt. The texture of the films demonstrates a liquid immiscibility in phosphate–metal–troilite melt. The metal–troilite assemblage in the films differs in texture and chemical composition from that of ordinary chondrites.

Diamond Crystallization at High Pressure: The Relative Efficiency of Metal-Graphite and Metal-Carbonate Systems

Data on the interaction of the Fe–Ni melt with CaCO3 and graphite at 5 GPa and 1400°С under the thermogradient conditions used in experiments on the growth of diamond on the BARS high-pressure apparatus are presented. The phase composition and component composition of the fluid captured by diamonds in the form of inclusions were studied by gas chromatography–mass spectrometry (GC–MS). Diamonds were synthesized from graphite. During the interaction of the Fe–Ni melt with CaCO3, Ca–Fe oxides and (Fe, Ni)3C carbide were formed. The stability of heavy hydrocarbons under the experimental conditions was confirmed. It was established that the composition of the fluid in synthesized diamonds is close to the composition of the fluid from inclusions in some natural diamonds. Nevertheless, it was concluded that crystallization of large diamonds under natural conditions is hardly possible due to the filling of the main crystallization volume with refractory oxide phases.

Interaction of an Fe–Ni Melt with Anthracene (C14H10) in the Presence of Olivine at 3 GPa: Fluid Phase Composition

The first results on the interaction between an Fe–Ni melt and anthracene (С14Н10) in the presence of olivine at 3 GPa and 1500°С and on the study of the component composition of the fluid generated in this process are presented. The stability of aliphatic hydrocarbons in the implemented conditions is confirmed experimentally. It is established that, under these conditions, crystallization of high-magnesian olivines occurs (Fo = 97–98 mol %). The composition of the fluid is similar to the composition of the fluid from inclusions in synthetic diamonds. The conditions implemented in the experiment might have occurred at the early stages of the Earth’s evolution.

Ferrovolcanism on metal worlds and the origin of pallasites

As differentiated planetesimals cool, their cores can solidify from the outside-in, as evidenced by paleomagnetic measurements and cooling rate estimates of iron meteorites. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward core growth would likely be achieved by growth of solid FeNi dendrites. Growth of FeNi dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt percent sulfur as FeNi continues to solidify. Here we show that regions of residual sulfur-enriched FeNi melt in the core attain sufficient excess pressures to propagate via dikes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the plantesimals surface. We refer to these processes collectively as ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of FeNi metal. Ferrovolcanic eruptions may explain the observations that Psyche has a bulk density inconsistent with iron metorites yet shows evidence of a metallic surface composition.

Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite–taenite interfaces

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500°C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100–6600°C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite–taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite–taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite–taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52wt.% while Niα decreases from 7 to 4wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe–Ni–P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using Niγ/Niα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion of Wasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe–Ni melt and crustal rocks, cooled at uniquely slow rates.

Structure and dynamics of undercooled FeNi

Molecular dynamics simulation has been performed to explore the structural and dynamical properties of undercooled FeNi melt based on the embedded atom method (EAM), namely due to G. Bonny. The simulated partial pair correlation function (PPCF) and coordination number (CN) of liquid FeNi indicate that no stronger interaction of heterogenic atom pairs than those of the same atom pairs happen in melts. FeNi melt is close to an ideal mixing system without chemical short-range order (CSRO). The undercooled FeNi exhibits icosahedral short-range order (ISRO) that is reflected directly as a large number of 1551, 1541 bonded pairs as well as snapshot of icosahedron clusters. The mean squared displacement (MSD) and self diffusion coefficient of Fe and Ni is quite similar, which is consistent with the strong glass formation liquid of Ni36Zr64. Our work gives the detailed structure and dynamics information of undercooled FeNi melt.

Diffusion coefficient of carbon in Fe–Ni alloy during synthesis of diamond under high temperature and high pressure

Synthetic diamond particles were prepared under high temperature and high pressure using arrayed seeds. A dense Fe–Ni alloy shell covered each diamond seed during synthesis; the growth of diamond particles was controlled by the diffusion of carbon through the metallic shell. The diffusion coefficient of carbon through Fe–Ni melt at 1600 K and 5.5 GPa is about 5×10−6 cm2/s, with an activation energy for diffusion of 336 kJ/mol.

Diffusivities of an Al–Fe–Ni melt and their effects on the microstructure during solidification

A systematical investigation of the diffusivities in an Al–Fe–Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al–Ni, Al–Fe, Fe–Ni and Al–Fe–Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al–Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method.

Thermodynamics of titanium and nitrogen in an Fe-Ni melt

The equilibrium solubility of titanium and nitrogen in Fe-Ni melts was measured in the presence of pure solid TiN under various nitrogen pressures in the temperature range of 1843 to 1923 K. The activity coefficients of titanium and nitrogen relative to a 1 mass pct standard state in liquid iron were calculated from the experimental results for Fe-Ni alloys of nickel contents up to 30 mass pct. Nickel decreases the activity coefficient of titanium, but it increases the activity coefficient of nitrogen in an Fe-Ni-Ti-N melt. Therefore, the effect of nickel on the solubility product of TiN is not significant. The first- and second-order interaction parameters of nickel on titanium (e Ti Ni and r Ti Ni , respectively) were determined to be −0.0115 and 0 at 1873 K, respectively. Similarly, the interaction parameters of nickel on nitrogen (e N Ni and r N Ni , respectively) were determined to be 0.012 and 0, respectively, at 1873 K. The temperature dependence of these interaction parameters was also determined.

Insulating `nanocables': Invar Fe–Ni alloy nanorods inside BN nanotubes

Here we present the results on synthesis, structural and chemical analysis of insulating boron nitride (BN) nanotubes (NTs) which have been filled with conducting Invar Fe–Ni alloy (∼60 at.% Fe; ∼40 at.% Ni) nanorods. The result was accomplished by a two-step process: (i) carbon (C) NTs containing Invar alloy nanoparticles placed at the tube tips were synthesized by plasma-assisted chemical vapor deposition (CVD) on an Invar Fe–Ni alloy substrate; and (ii) the material was heated to the melting point of the alloy (1723 K) in a flow of B 2O 3 and N 2 gases and held for 30 min. During this second stage, simultaneous filling of NTs with the Fe–Ni melt through capillarity and chemical modification of C tubular shells to form BN tubules occurred.

Sinoite (Si2N2O); crystallization from EL chondrite impact melts

Sinoite (Si2N2O) was previously observed only in EL6 chondrites and recently modeled as having formed over geologic time scales at metamorphic temperatures of ;950 8C. I found several ;10‐210 mm-sized subhedral and euhedral grains of twinned, optically zoned sinoite associated with euhedral enstatite and euhedral graphite within impact-melted portions of QUE94368, the first EL4 chondrite. The presence of sinoite within a type 4 chondrite mitigates against the metamorphic model of sinoite formation; it seems more likely that sinoite crystallized from a liquid. During impact melting of EL material, N 2 may have been released from lattice defects in sulfides whereupon it reacted with reduced Si dissolved in the metallic Fe-Ni melt and with fine-grained or molten silica derived from the silicate fraction of the EL assemblage. The N that formed the sinoite was derived from the silicate melt or from temporary, melt-filled cavities constructed from unmelted EL material in which the nitrogen fugacity may have reached ;40 to 130 bars (0.004 to 0.013 GPa). Sinoite in EL6 chondrites may have formed either metamorphically, as previously proposed, or by means of crystallization from an impact melt, as in QUE94368. In the latter case, sinoite-bearing EL6 chondrites would be annealed impact-melt breccias.

Phase selection, growth, and interface kinetics in undercooled Fe-Ni melt droplets

An electromagnetic levitation facility is used to containerlessly process Fe-Ni droplets; undercoolings ΔT of up to 300 K were achieved. Thermal measurements during solidification showed two types of recalescence behaviour for alloys containing between 7.5 and 17 at% Ni: A single recalescence step for ΔT< ΔT * (primary growth of the stable ccp phase) and two recalescence steps for Δ> ΔT * (primary growth of the metastable bcc phase, and the subsequent transformation of the bcc phase and solidification of the stable ccp phase). The critical undercooling ΔT * strongly increases with the Ni-content. The growth velocities at which the primary dendrites propagate through the droplet have been measured for a number of Fe-Ni alloy compositions. The velocities reflect the phase selection, i.e. primary bcc phases grow markedly more slowly than primary ccp phases. Thermodynamic modelling (CALPHAD) and an analysis of the velocity data within current theories of dendrite growth is undertaken to describe nucleation and growth behaviour. The results suggest that the metastable phase is nucleated in preference at high undercoolings because of its lower solid-liquid interface energy and that the kinetics at the bcc-liquid interface is considerably more sluggish than the kinetics at the ccp-liquid interface.

The spatiotemporal pattern of FeNi metal melt solidification in space and the mechanism of its crystallization

The microstructure study of the shock-remelt-recrystallized metal in the Yanzhuang chondrite has revealed that the tetra-concentric-ring growth structure is the fundamental spatiotemporal pattern of the FeNi melt solidification in space in the state far from the thermodynamic equilibriun. The rapid growth and interaction of tetra-concentric-ring growth structures led to the formation of octahedral FeNi metal dentrites. The tetra-concentric-ring growth of FeNi metal may be a fundamental way by which octahedrite is formed from recrystaltization of FeNi metal melt under microgravity conditions.

A trace-element study of iron meteorites

The technique of neutron activation and γ counting on a lithium-drifted germanium semiconductor detector has been used in measuring concentrations of cobalt, copper, arsenic, gallium, iridium, and gold in 33 iron meteorites. Similar data on 25 other iron meteorites are available in the literature. These meteorites are grouped according to similarities in their trace-element content. Out of 23 samples classified as hexahedrites and coarse octahedrites, 22 can be assigned to three groups corresponding to the structural classifications as normal hexahedrites, coarse octahedrites, and a group intermediate between these two. Within these groups, as Ni increases, the elements Co, Cu, As, Ga, and Au increase while Ir decreases. This is thought to represent trace-element behavior during crystallization and crystal settling from a melt of a homogeneous chemical composition. A discontinuity involving the elements As, Au, Ga, and Ir exists between the coarse octahedrites and medium octahedrites. The medium octahedrites are also characterized by their heterogeneity. Only 8 out of 17 can be assigned to groups. Out of 10 fine octahedrites, 5 have uniform elemental compositions. This group differs from the medium octahedrites in Ni, Co, and Ga. These differences in trace-element abundance are ascribed to differences in composition of the Fe-Ni melt from which the meteorites crystallized. At least three melts, varying chiefly in Ni and Ga, are required to account for the compositions of all the iron meteorites. Six Ni-rich ataxites show no uniformity in their trace-element abundances. They probably represent the end stage of crystallization from several sources. Two structurally anomalous meteorites, Nedagolla and N'Goureyma, are unusually low in trace elements. This may have resulted from a remelting of a normal iron meteorite during break-up of the meteorite parent body.