The effect of metal composition on Fe–Ni partition behavior between olivine and FeNi-metal, FeNi-carbide, FeNi-sulfide at elevated pressure

Chemical Geology 221 (2005) 207 – 224 www.elsevier.com/locate/chemgeo

The effect of metal composition on Fe–Ni partition behavior between olivine and FeNi-metal, FeNi-carbide, FeNi-sulfide at elevated pressure Astrid Holzheid *, Timothy L. Grove Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA Received 21 August 2004; received in revised form 5 May 2005; accepted 30 May 2005

Abstract Metal–olivine Fe–Ni exchange distribution coefficients were determined at 1500 8C over the pressure range of 1 to 9 GPa for solid and liquid alloy compositions. The metal alloy composition was varied with respect to the Fe/Ni ratio and the amount of dissolved carbon and sulfur. The Fe/Ni ratio of the metal phase exercises an important control on the abundance of Ni in the olivine. The Ni abundance in the olivine decreases as the Fe/Ni ratio of the coexisting metal increases. The presence of carbon (up to ~ 3.5 wt.%) and sulfur (up to ~ 7.5 wt.%) in solution in the liquid Fe–Ni-metal phase has a minor effect on the partitioning of Fe and Ni between metal and olivine phases. No pressure dependence of the Fe–Ni-metal–olivine exchange behavior in carbon- and sulfur-free and carbon- and sulfur-containing systems was found within the investigated pressure range. To match the Ni abundance in terrestrial mantle olivine, assuming an equilibrium metal–olivine distribution, a sub-chondritic Fe/Ni-metal ratio that is a factor of 17 to 27 lower than the Fe/Ni ratios in estimated Earth core compositions would be required, implying higher Fe concentrations in the core forming metal phase. A simple metal–olivine equilibrium distribution does not seem to be feasible to explain the Ni abundances in the Earth’s mantle. An equilibrium between metal and olivine does not exercise a control on the problem of Ni overabundance in the Earth’s mantle. The experimental results do not contradict the presence of a magma ocean at the time of terrestrial core formation, if olivine was present in only minor amounts at the time of metal segregation. D 2005 Elsevier B.V. All rights reserved. Keywords: Partitioning; Metal; Sulfide; Carbide; Olivine; Core formation

1. Introduction * Corresponding author. Current address: Westfa¨lische WilhelmsUniversita¨t Mu¨nster, Institut fu¨r Mineralogie, Corrensstraße 24, 48149 Mu¨nster, Germany. Tel.: +49 251 833 3493; fax: +49 251 833 8397. E-mail address: [email protected] (A. Holzheid). 0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2005.05.005

It is generally accepted that the Earth’s core and mantle are formed by separation of metal from silicate phases in the very early history of the Earth (e.g., Yin et al., 2002; Kleine et al., 2002; Scho¨nberg et al.,

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2002). To develop hypotheses of the core–mantle separation process and the chemical evolution of the Earth, knowledge of the partition behavior between the coexisting phases metal, metal sulfide, solid silicate and silicate melt is required. Core forming metal or metal sulfide will remove siderophile (metal-loving, e.g., Fe, Ni, Co, noble metals) and/or chalcophile (sulfide-loving, e.g., Cu, Pb, Zn) elements from the silicate proto-mantle into the separating core forming metallic phase. The concentration of siderophile and chalcophile elements preserved in the silicate mantle, e.g., in mantle olivine, may provide a record of core forming processes. Experimentally determined partitioning behavior of siderophile and chalcophile elements between metal, metal sulfide, solid silicate and silicate melt may shed light on core forming processes. In the past years, several experimental studies have focused on the determination of metal–silicate melt partition coefficients of siderophile and chalcophile elements at ambient and elevated pressure and temperatures ranging from around 1200 8C to more than 2000 8C (see Walter et al., 2000 for a review up to 1999; Jaeger and Drake, 2000; Chabot and Agee, 2002; Bouhifd and Jephcoat, 2003; Kegler et al., 2004, 2005). A number of partition coefficients of siderophile and chalcophile elements between solid metal and metal sulfide (see Chabot and Jones, 2003; Chabot et al., 2003 for compilation of experimental studies) or between silicate melt and metal sulfide (e.g., MacLean and Shimazaki, 1976; Rajamani and Naldrett, 1978; Li and Agee, 1996, 2001), at ambient and elevated pressure, are available in the literature. A few experimental studies mostly at ambient pressure focus on the determination of partitioning of siderophile and chalcophile elements between solid silicate and metal sulfide or coexisting solid silicate, silicate liquid and metal sulfide with mainly olivine as solid silicate phase (e.g., Fleet et al., 1977, 1981; Fleet and MacRae, 1987, 1988; Ehlers et al., 1992; Seifert et al., 1988; Gaetani and Grove, 1997). However, the effects of the light elements on partitioning of siderophile elements between solid silicate with core forming metal (e.g., H, C, O, S, Si) are not well known, and experimental studies are required to characterize their influence on core forming processes. We therefore systematically investigated the partitioning of Fe and Ni between Fe–Ni dominated metallic phases and olivine as function of pressure and metal

composition at isothermal conditions. The metal composition was varied in respect of the Fe/Ni ratio and the amount of dissolved carbon and sulfur in the Fe– Ni-metal phase. From these data, the influence of carbon and sulfur on the metal–olivine partition behavior of Fe and Ni and in addition thermodynamic properties of binary Fe–Ni alloys close to the solid to liquid transition of the alloys are derived.

2. Experiments 2.1. Experimental techniques Experiments were performed in 0.5-in. solid-medium piston cylinder apparati (Boyd and England, 1960) at pressures from 1 to 2.6 GPa and 1500 8C and in the MIT Walker-type multi-anvil (MA) at pressures of 4 and 8.8 GPa and 1500 8C. Starting material consisted of mixtures of metal alloys and finely ground San Carlos olivine single crystals. The mixing proportions were ~ 40 vol.% metal and ~ 60 vol.% olivine. Up to six types of metal alloys were used with Fe/Ni ratios ranging from 9:91 to 91:9 by weight. In two sets of experiments, the metals were initially enriched by adding up to 30 wt.% carbon (dFNC-OlT experiments) and 20 wt.% sulfur (dFNS-OlT experiments), respectively. Commercially available powder of Ni (puriss., Fluka, Switzerland), ground Fe-sponge (specpure, Johnson Matthey, USA), ground sulfur (99.9995%, Johnson Matthey, USA) and ground graphite (Ultra Carbon, USA) was used. 2.1.1. Piston cylinder experiments In piston cylinder (PC) experiments, the starting material was packed into MgO capsules (ID 2.5 mm, bsampleQ length 1.8 mm) that were drilled into 0.25in. diameter MgO rods (length 15.9 mm). In most of the charges, additional pure olivine powder was packed on top of the starting mixture. The MgO rods were positioned slightly longitudinally off-center in the graphite furnace. The offset between the thermocouple and the sample resulted in a temperature correction of 50 8C. The temperature gradient across the experimental charge is estimated to be ~ 10 8C. The graphite furnace was contained in a sleeve of pressed BaCO3 pressure medium. The temperature of the charge was measured and controlled using a

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W3Re97–W25Re75 thermocouple. Slight differences in sample geometry between runs lead to an uncertainty of at most F 7 8C in absolute run temperature at any fixed reference point from charge to charge (Bartels et al., 1991). All PC assemblies were pressurized cold to ~ 0.7 GPa, heated to 865 8C at 100 8C/min and pressurized to the final pressure, while the temperature was kept constant at 865 8C for 6 min. The temperature was then raised to the run temperature at a rate of 50 8C/min, while the final pressure was kept on the sample. The pressure was calibrated against the reaction: anorthite + gehlenite + corundum = Ca-tschermakite (Hays, 1966). Pressure reproducibility is judged to be F0.1 GPa from reproducibility of this phase transition in multiple calibration runs. Run durations were 21 to 40 h. Solid or liquid metal coexisted with crystalline olivine in C- and S-free experiments (FN-Ol). Liquid carbonrich metal coexisted with crystalline olivine in Ccontaining experiments (FNC-Ol). Liquid metal sulfide coexisted with crystalline olivine in S-containing experiments (FNS-Ol). The charges were quenched by turning off the power. The sample temperature dropped below 200 8C within the first seconds after turning off the power to the graphite heater. The sample pressure was released instantaneously. All experimental charges were mounted in epoxy, cut longitudinally through the center of the MgO capsule and polished as microprobe sections. Run parameters of the PC experiments are given in Table 1. 2.1.2. Multi-anvil experiments In multi-anvil (MA) experiments, the starting material was packed into MgO capsules (OD 4.9 mm, ID 2.2 mm, capsule length 4 mm, bsampleQ length 2.5 mm). The MgO capsules were placed on pedestals and covered by wafers manufactured from MgO. The sample assemblies were positioned in the center of an 18/11 Ceramacast 584-OSR (Aremco Products, USA) assembly. Rhenium foil (thickness 0.06 mm) was used as heater. The temperature of the charge was measured and controlled using a W3Re97–W25Re75 thermocouple. All MA assemblies were pressurized cold to the desired final pressure. The temperature was then raised to the run temperature with a temperature ramp of 100 8C/min, while the final pressure was kept on the sample. The

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pressure was calibrated over the interval of 2 to 9 GPa using the reactions: quartz-coesite and coesitestishovite (Parman et al., 1997). Reproducibility of identical sample pressures of different experiments and pressure uncertainties are assumed to be F0.5 GPa near 10 GPa (~ 5% error) (Parman et al., 1997). A temperature gradient of about 40 8C/mm across the experimental charge is assumed (Agee et al., 1995). Run durations were 24 h. Solid metal coexisted with crystalline olivine in C- and S-free experiments (FN-Ol). Liquid carbon-rich metal coexisted with crystalline olivine in C-containing experiments (FNC-Ol). The charges were quenched by turning off the power. The sample temperature dropped below 200 8C within the first few seconds after turning off the power to the rhenium foil heater. The sample pressure was released gradually over a period of 12 h. All experimental charges were mounted in epoxy, cut longitudinally through the center of the MgO capsule and polished as microprobe sections. Run parameters of the MA experiments are given in Table 1. 2.2. Analytical techniques Back-scattered electron (BSE) imaging and energy-dispersive spectrometry (EDS) analyses were used to distinguish between the coexisting phases in the experimental charges. Quantitative analyses were obtained by wavelength-dispersive spectrometry (WDS) using a JEOL 733 Superprobe electron microprobe with 15 kV accelerating voltage and a beam current of 10 nA. Spot size ranged from 1 to 10 Am depending on the analyzed phases. The data were reduced with the CITZAF program (Armstrong, 1995). Counting times were 40 s for major elements (Mg, Si, Fe) in the olivine phases and for all components in the metallic phases. Longer counting times (up to 80 counting on peak) were used for the trace element analyses of Ni in the olivine phases. Secondary fluorescence effects of Ni in olivine from adjacent metal grains as described by Ehlers et al. (1992) were avoided by analyzing olivine grains that were at least 50 Am away from any visible metal phases. All sulfur-containing metal phases and most of the metal carbide phases quenched into two immiscible metal phases, a S- or C-poor metal and S- or C-rich

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Table 1 Experimental parameters and chemical compositions of run products

a

P [GPa]

Time [h]

SiO2

1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500

0.9 1.0 1.0 1.2 1.5 1.6 1.6 1.8 1.9 2.0 4.0 8.8 0.9 1.0 1.1 1.6 1.6 1.6 1.6 1.8 2.1 1.0 1.1 1.1 1.6 1.9 2.1 2.2 2.2 2.2 4.0 8.8

22.1 25.6 24.2 21.7 23.6 21.5 22.3 22.6 24.4 23.6 24.0 24.0 24.8 24.2 24.2 24.0 25.9 24.8 39.0 20.0 23.7 24.1 24.0 23.4 22.0 23.7 24.0 23.5 24.8 29.0 24.0 24.0

40.2 40.9 40.5 39.4 40.0 41.0 40.6 40.4 40.2 41.3 40.6 40.4 41.9 40.4 41.0 40.5 40.8 41.2 41.8 41.4 40.9 41.5 41.3 41.1 40.6 40.6 40.3 41.3 40.4 42.1 40.2 40.6

FeO (4)b (1) (2) (6) (3) (1) (2) (4) (1) (1) (1) (1) (2) (2) (1) (3) (2) (2) (2) (1) (1) (1) (1) (1) (2) (1) (4) (3) (2) (8) (2) (1)

9.80 9.66 10.8 9.54 9.64 8.84 9.17 9.23 10.0 7.69 8.80 8.88 4.66 7.96 8.62 9.52 10.2 7.50 2.99 7.97 7.92 7.40 6.41 8.06 9.44 8.89 9.22 7.80 8.74 7.77 8.55 8.94

MnO (25) (10) (5) (13) (20) (24) (7) (35) (2) (31) (12) (8) (34) (42) (31) (17) (4) (47) (10) (27) (24) (21) (33) (23) (16) (9) (24) (53) (33) (47) (16) (10)

0.110 0.137 0.144 0.100 0.108 0.150 0.144 0.116 0.097 0.108 0.091 0.111 0.098 0.112 0.096 0.147 0.143 0.076 0.084 0.099 0.131 0.112 0.111 0.134 0.135 0.106 0.087 0.100 0.097 0.097 0.114 0.092

(18) (4) (6) (6) (8) (8) (6) (4) (4) (9) (4) (6) (10) (14) (6) (8) (5) (10) (8) (10) (9) (10) (8) (4) (7) (3) (6) (17) (13) (11) (13) (5)

MgO

NiO

48.6 50.3 48.7 50.4 49.3 50.4 49.8 50.8 48.6 50.2 50.0 49.9 53.0 49.1 50.9 49.8 50.2 51.9 54.0 50.2 50.0 50.4 52.9 50.8 50.2 49.8 50.1 51.2 50.5 49.5 48.6 49.8

0.079 0.440 0.134 0.043 0.155 0.759 0.696 0.045 0.140 1.66 1.10 1.16 0.049 1.10 0.543 0.121 0.112 0.115 0.039 0.905 1.04 1.32 0.673 1.06 0.160 0.259 0.201 0.121 0.219 0.178 1.15 1.07

(6) (1) (5) (11) (4) (2) (3) (3) (2) (2) (1) (2) (3) (5) (3) (6) (4) (4) (3) (3) (2) (2) (3) (2) (3) (1) (3) (5) (5) (7) (3) (1)

(19) (21) (25) (6)c (20)c (63) (37) (5) (23) (13) (7) (7) (7)c (12) (22)c (11) (8)c (28) (4)c (60) (6) (10) (16)c (7) (23) (17) (26)c (5)c (27)c (32)c (12) (4)

Total olivine

Fe

98.7 101.5 100.3 99.5 99.2 101.1 100.4 100.5 99.2 101.0 100.5 100.5 99.7 98.7 101.2 100.1 101.4 100.8 99.0 100.5 99.9 100.7 101.5 101.1 100.6 99.6 99.9 100.5 100.0 99.6 98.6 100.5

95.4 51.9 88.1 94.9 95.5 22.7 25.1 93.9 91.8 13.0 14.4 15.6 88.1 10.0 48.1 85.9 87.4 91.4 90.5 24.0 18.0 14.2 44.1 15.4 94.4 84.9 87.8 88.3 87.2 86.9 14.4 18.6

Ni (5) (5) (6) (4) (4) (6) (9) (7) (7) (7) (3) (5) (2) (3) (1) (10) (8) (3) (10) (1) (1) (1) (13) (1) (3) (1) (1) (2) (2) (1) (14) (9)

4.39 48.1 11.0 4.94 4.53 77.0 73.9 6.62 7.45 87.4 87.0 86.2 8.93 89.5 50.6 12.8 11.7 2.77 3.43 75.1 80.6 85.5 56.3 85.1 4.22 12.1 9.20 8.21 10.0 9.58 85.2 81.1

S or C (53) (5) (6) (28) (41) (6) (8) (58) (68) (8) (3) (5) (8) (2) (1) (11) (8) (49) (29) (1) (2) (1) (14) (1) (25) (2) (7) (15) (1) (11) (15) (11)

2.93 (27)d bd.l.e 1.23 (15) 0.035 (6) 0.029 (4) 5.83 (12) 6.08 (41) 0.014 (3) 0.032 (13) 0.547 (69) 0.052 (24) bd.l. 1.35 (15) 3.09 (8) 3.01 (14) 3.51 (18) 2.84 (12) 3.54 (11) 1.07 (19) 0.78 (27)

Total metal 99.8 100.0 99.1 99.8 100.0 99.7 99.0 100.5 99.3 100.4 101.4 101.8 99.4 99.5 100.1 98.7 99.1 98.9 99.0 99.1 98.6 100.2 100.5 100.5 100.0 100.0 100.0 100.0 100.0 100.0 100.7 100.5

Additional olivine was placed on the metal–olivine starting mixture. Numbers in parentheses are 1-j deviations of the mean. The entry should be read 40.2 wt.% F 0.4 wt.%. c Analyzed olivine was b50 Am away from metal phase. d Sulfur and carbon concentrations in the metal phases are average concentrations as all metal sulfides and most metal carbides quenched into S- or C-poor and S- or C-rich phases (see Section 2.2 for more detail). e Below detection limit. b

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FN-Ol 2a FN-Ol 12a FN-Ol 11a FN-Ol 3a FN-Ol 1a FN-Ol 7a FN-Ol 8a FN-Ol 5a FN-Ol 13a FN-Ol 10a FN-Ol 8.50 FN-Ol 8.53 FNS-Ol 10a FNS-Ol 7a FNS-Ol 12a FNS-Ol 1a FNS-Ol 3a FNS-Ol 4 FNS-Ol 2a FNS-Ol 5a FNS-Ol 9a FNC-Ol 19 FNC-Ol 23 FNC-Ol 21 FNC-Ol 16a FNC-Ol 26a FNC-Ol 12a FNC-Ol 2 FNC-Ol 3 FNC-Ol 1 FNC-Ol 8.48 FNC-Ol 8.52

T [8C]

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metal component. In some of the experimental charges, both metal phases, i.e., S-poor and S-rich metal phases or C-poor and C-rich metal phases, were analyzed simultaneously using a defocused beam (10 Am). In all other experimental charges, average sulfur concentrations in the metal sulfides and carbon concentrations in the metal carbides were determined by modal abundances of S-poor and Srich phases or C-poor and C-rich phases using backscattered electron image analyses software and mole fractions of the different phases, similar to the method described in Chabot and Drake (1997). In general, the C content in carbon-rich metal phases was calculated by difference using the measured concentrations of Fe and Ni in the metal. Quantitative carbon analyses of some of the FNC-Ol experimental charges were performed using x-ray wavelength dispersive analysis and Ni/C pseudocrystals. Carbon concentrations determined by difference and by quantitative carbon analyses agree within less than 5-j uncertainties. Chemical compositions of all coexisting phases in the experimental charges are given in Table 1.

3. Experimental results 3.1. FeNi-metal–olivine experiments Crystalline olivine coexisted with solid or liquid metal in the experiments depending on the pressure of the experiment. Run temperatures in all experiments were at 1500 8C. Fig. 1a shows a digital overview backscattered image of a representative experimental charge of FN-Ol. The top of the charge corresponds to the top of the image. The brightest phase is iron sulfide, the phase of intermediate brightness is olivine and the darkest phase is the MgO-crucible. Additional olivine is placed on top of the metal–olivine starting mixture. Despite run temperatures below the melting point of the metal phases in some of the experiments, the FeNi alloys were homogeneous in composition throughout the experimental charge. The FeNi alloys after the experiment cover a compositional range from 13 to 96 wt.% Fe and 4 to 87 wt.% Ni, respectively. Both NiO and FeO contents in the coexisting olivine crystals are influenced by the Fe and Ni proportions

Fig. 1. Backscattered electron photomicrographs of the experiments FN-Ol 10 (a), FNS-Ol 9 (b) and FNC-Ol 12 (c). The top of the images corresponds to the top of the charges. The phases visible in the charges are in decreasing order of brightness metal phases (metallic alloy in FN-Ol 10, metal sulfide in FNS-Ol 9 and metal carbide in FNC-Ol 12), crystalline olivine and MgO that surrounds the charge as capsule material. Crystalline olivine coexisted with Fe–Ni alloy (FN-Ol experiments), liquid Fe–Ni sulfide (FNS-Ol experiments), or liquid Fe–Ni carbide (FNC-Ol experiments) during the experiment. Additional olivine is placed on top of the metal– olivine starting mixture in FN-Ol 10 (a). The black phase at the bottom of the experimental charge FNC-Ol 12 is excess carbon (c).

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of the metal phase. The olivines were homogeneous and similar in their chemical composition within the individual experimental charges. The NiO content varied from 430 ppm to ~ 1.7 wt.%, while the FeO content varied only between ~ 6.9 and 10.8 wt.% when comparing olivines of all experimental charges. Fig. 2a illustrates the correlation of the Fe/Ni ratio of the metal phases with the Fe/Ni ratio of the coexisting olivine phases. The Fe/Ni ratio of the olivine phases increases with increasing Fe/Ni ratio of the metal phases. The solid line represents the best fit line. The parameters of the least square regression are given in the figure. No pressure dependence of the Fe–Ni-metal–olivine exchange behavior was found within the investigated pressure range from ~ 1 GPa to ~ 8.8 GPa as indicated by the different symbols in Fig. 2a. 3.2. Sulfur-containing FeNi-metal–olivine experiments Crystalline olivine coexisted with liquid metal in S-containing experiments. Fig. 1b shows a digital overview photomicrograph of FNS-Ol 9 as a representative experimental charge. The top of the charge corresponds to the top of the image. The brightest phase is iron sulfide, the phase of intermediate brightness is olivine and the darkest phase is the MgO-crucible. The bright spots in the MgO capsule are tiny molten elemental sulfur blebs. Sulfur melts at temperatures above 120 8C at ambient pressure. As the PC experiments were first pressurized cold to ~ 0.7 GPa and than heated to 865 8C with 100 8C/ min, molten sulfur was present in the charges shortly after increasing the temperature. It is most likely that the MgO of the capsule material was not compressed to its final density at a hydrostatic pressure of only 0.7 GPa. Therefore, some sulfur could have migrated out of the actual experimental charge into the surrounding MgO capsule in this very early stage of the PC-experiment. This sulfur is then visible as blebs inside the MgO in the photomicrographs. In some of the experiments, this early depletion of sulfur by migration into the MgO was significant and the resulting metal phase was nearly sulfur-free, even if the initial starting mixture contained up to 20 wt.% S. In general, all sulfur-containing metal phases quenched into two immiscible metal phases, a S-poor

metal (b 0.05 wt.% S) and S-rich metal component close to stoichiometric FeS. Fig. 3a shows a digital photomicrograph of the quench texture of experiment FNS-Ol 2. The metal sulfide phases after the experiments cover a compositional range from 10 to 89 wt.% Fe, ~ 3 to 90 wt.% Ni and up to ~ 7.5 wt.% S. The coexisting olivines were homogeneous in their chemical composition and were identical in composition within the individual charges. The NiO content in the olivine phases depends on the Ni amount of the metal phase and varied from ~ 390 ppm to ~ 2 wt.%. The FeO concentration in the olivine varied from 3 to 10 wt.% when comparing olivines of all experimental charges. Fig. 2b illustrates the correlation of the Fe/Ni ratio of the metal phases with the Fe/Ni ratio of the coexisting olivine phases for each individual experiment. The Fe/Ni ratio of the olivine phases increases with increasing Fe/Ni ratio of the FeNi-sulfide phases. The solid line represents the best fit line. The parameters of the least square fit are given in the figure. In analogy to the FN-Ol experiments, pressure independence of the Fe–Nimetal–olivine exchange reaction was found within the investigated pressure range. 3.3. Carbon-containing FeNi-metal–olivine experiments Crystalline olivine coexisted with liquid metal in C-containing experiments. Excess carbon was present as a third phase in the experiments with initially more than 11 wt.% C in the metal mixture. Fig. 1c shows a digital overview photomicrograph of FNCOl 12 as a representative experimental charge. The top of the charge corresponds to the top of the image. The brightest phase is iron carbide, the phase of intermediate brightness is olivine, the darker grey phase is the MgO-crucible and the black phase at the bottom of the MgO-crucible is excess carbon. The upper part of the sample consists of an olivine layer that was placed on top of the metal– olivine starting mixture. Some metal migrated into this upper layer along olivine grain boundaries and metal also accumulated in the upper right corner of the experimental charge. Excess carbon with large olivine grains and finely dispersed metal is concentrated at the bottom of the charge. The liquid metallic phase quenched into a single crystalline metal

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Fig. 2. Dependences of the Fe/Ni ratio in the olivine on the Fe/Ni ratio of the binary FeNi phase (a), the ternary FeNiS phase (b) and the ternary FeNiC phase (c) at isothermal conditions (1500 8C). Experiments at similar pressures have identical symbols. The solid line represents the least square fit. The equations of the best fit and the correlation coefficients (r 2) are given in the individual figure.

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Fig. 3. Backscattered electron photomicrographs of the quench texture of experiments FNS-Ol 2 (a) and FNC-Ol 1 (b). The quench texture of the metal carbide in FNC-Ol 1 is similar to the iron sulfide quench texture in FNS-Ol 2, inferring an immiscibility gap in the Fe–C systems in analogy to the Fe–S system.

carbide phase at carbon concentrations of less than 2 wt.% C. At higher carbon concentrations, the liquid metal carbide does not crystallize into a single metal phase. A C-poor metal (b 0.5 wt.% C in the FeNi alloy) and a C-rich metal with up to 5 wt.% carbon are present in these charges. The Ni content in the Crich metal is up to a factor of 1.8 higher compared to the C-poor metal. Fig. 3b shows a digital photomicrograph of the quench texture of experiment FNCOl 1. The quench texture is similar to iron sulfide quench textures, inferring a miscibility gap in the Fe–C systems similar to that in the Fe–S system (e.g., Hansen and Anderko, 1958; Wood, 1993). The metal carbide phases after the experiment cover a compositional range from 10 to ~ 95 wt.% Fe, 4 to 90 wt.% Ni and up to 3.5 wt.% C. The solubility limit of carbon in the liquid metal carbide phase increases with increasing Fe/Ni ratio of the

metal phase. No systematic correlation of the carbon solubility limit with pressure was found within the investigated pressure range. The dominant effect appears to be compositional, i.e., the Fe/Ni ratio of the metal phase on the carbon solubility limit is more influential. The solubility limits of carbon in pure iron and pure nickel at 1500 8C and ambient pressure are ~ 5 wt.% C in pure Fe and ~ 0.7 wt.% C in pure Ni (Hansen and Anderko, 1958). The carbon saturation limit that we measure is lower than that predicted at 1 atm. The measured carbon solubilities in the FeNi-C alloys are 0.91 to 0.77 lower than the limits predicted by Hansen and Anderko (1958). There was no significant change of carbon solubility with increasing pressure. The coexisting olivines were homogeneous in their chemical composition and were identical in composition within the individual charges. The NiO content in the olivine strongly depends on the Ni content of the metal phase and ranges from ~ 900 ppm to ~ 1.4 wt.%. The FeO content in the olivine was 7.5 F 2 wt.% in the majority of all experimental charges. No distinct correlation between FeO content of the olivine and Fe content of the metal was therefore found. Fig. 2c illustrates the correlation of the Fe/Ni ratio of the metal phases with the Fe/Ni ratio of the coexisting olivine phases for each individual experiment. The Fe/Ni ratio of the olivine phases increases with increasing Fe/Ni ratio of the FeNi-carbide phases. The solid line represents the best fit line. The parameters of the least square fit are given in the figure. In analogy to the FN-Ol and FNS-Ol experiments, pressure independence of the Fe–Ni-metal–olivine exchange reaction was found within the investigated pressure range. 3.4. Attainment of equilibrium Although attainment of equilibrium has not been demonstrated by reversal experiments, the following constraints suggest attainment of equilibrium. (1) All olivine phases present in the experimental charges do not have any zoning of Fe or Ni from core to rim, regardless of the starting composition of the solid or liquid metal phases. The coexisting olivines were identical in composition within the individual charges. This implies significant chemical reaction in the crystalline silicate phases. (2) Binary solid or liquid Fe–

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Fig. 4. Correlation between the molar exchange ratios Fe / (Fe + Ni) of olivine and coexisting metal phases at constant temperature (1500 8C) and pressure (~1.6 GPa) for FN-Ol (a), FNS-Ol (b) and FNC-Ol (c) experiments. Crossing tie lines would indicate a lack of complete equilibration between the coexisting phases.

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Ni-metal phases and all liquid metal carbide phases that quenched into a single crystalline metal carbide phase were homogeneous in their chemical composition and were identical in composition within the individual charges. Some of the carbon-containing metal phases and all of the sulfur-containing metal phases quenched into two immiscible metal phases. The individual immiscible metal phases were again homogeneous in their chemical compositions. (3) The molar Fe / (Fe + Ni) exchange ratios of coexisting olivine and binary Fe–Ni-metal phases, coexisting olivine and Fe–Ni-metal sulfide phases, and coexisting olivine and Fe–Ni-metal carbide phases yield consistent slopes of non-crossing tie lines. This is graphically illustrated in Fig. 4a–c for FN-Ol, FNS-Ol and FNCOl experiments. The achievement of regular and systematic partitioning with no crossing of tie lines is an evidence for equilibrium. In addition, reproducibility of experimental results was tested by repeating experiments at identical run conditions (e.g., experiments FN-Ol 7 and FN-Ol 8; experiments FNS-Ol 1 and FNS-Ol 2; experiments FNC-Ol 3 and FNC-Ol 1).

4. Thermodynamic modelling 4.1. Thermodynamic properties of binary FeNi alloys The Ni and Fe interaction between the coexisting metal and olivine can be described independently of oxygen fugacity that prevailed during the experiment by the Ni–Fe exchange reaction met 2Nimet þ Fe2 SiOol þ Ni2 SiOol 4 ¼ 2Fe 4

ð1Þ

The condition of equilibrium, expressed in terms of the chemical potentials l, is ol met ol 2lmet Ni þ lfayalite ¼ 2lFe þ lNiolivine

ð2Þ

ol ol ol with l ol fayalite = l fayalite 8 + RT ln a fayalite; l Ni-olivine = ol ol met met l Ni-olivine 8 + RT ln a Ni-olivine; l Fe = l Fe 8 + RT ln a met Fe ; met p met l met = l 8 + RT ln a ; where l is the chemical Ni c Ni Ni potential of component c (Fe, Ni, fayalite, Ni-olivine) in phase p (met: metal and ol: olivine), l cp 8 is the chemical potential in standard state (1 bar, 298 K) and a cp is the activity of component c in phase p. Temperature T is in Kelvin.

Replacing activities and chemical potentials and rearranging Eq. (2) yields met ol  DGo =ð 2RT Þ ¼ lncmet Fe  lncNi þ lncNiolivine
 met ol   lncol fayalite þ ln XFe XNiolivine  . ol met Xfayalite XNi Þ ð3Þ

where X cp is the mole fraction of component c (Fe, Ni, fayalite, Ni-olivine) in phase p (met: metal and ol: olivine), c cp is the activity coefficient of component c in phase p and DG o is the Gibbs energy of reaction in standard state. Calculated activity coefficients of Ni-olivine and fayalite components in the olivine based on the studies by, e.g., Wood and Kleppa (1981), Davidson and Mukhopadhyay (1984), Seifert et al. (1988) and Hirschmann and Ghiorso (1994), show only slight negative deviations from ideality for c ol Ni-olivine and positive deviations for c ol fayalite. Expression (3) can be therefore simplified to h  met met ol  DGo =ð 2RT Þ ¼ lncmet Fe  lncNi þ ln XFe XNiolivine  ol i met = Xfayalite XNi ð4Þ We have used two approximations for the nonideal behavior of the metal alloys. Replacing the activity coefficients of Fe and Ni in the metal alloy, met c met Fe and c Ni , by interaction Margules parameters (W G’s) for both symmetric and asymmetric regular solution models. Margules parameter—mole fraction expressions of binary symmetric regular solution  met 2 RT lncmet ð5Þ Fe ¼ WG XNi  met 2 RT lncmet Ni ¼ WG XFe

ð6Þ

Margules parameter- mole fraction expressions of binary asymmetric regular solution  FeNi  NiFe   met 2 met þ 2XFe WG  WGFeNi XNi RT lncmet Fe ¼ WG ð7Þ  NiFe  FeNi   met 2 met þ 2XNi WG  WGNiFe XFe RT lncmet Ni ¼ WG ð8Þ

A. Holzheid, T.L. Grove / Chemical Geology 221 (2005) 207–224

yields to the following expressions for the two solution models of FeNi alloys. Binary symmetric regular solution, i.e., W FeNi = G NiFe WG = WG h   ol i met ol met  RT ln XFe XNiolivine = Xfayalite XNi h   met 2 i 1 met 2 ¼ DGo þ WG XNi  XFe 2

ð9Þ

with O DG o as intercept and W G as Margules fitting parameter slope. Eq. (9) corresponds to a linear equation y ¼ ao þ a1 x 1

ð10Þ ln[(X met Fe d

ol met y =  RT X ol Ni-olivine) / (X fayalite d X Ni )], o met 2 met 2 DG , a 1 = W G, and x 1 = [(X Ni ) -(X Fe ) ].

where ao = O A linear dependence of the Fe/Ni-metal–olivine mole fraction ratio is expected as a function of DG o and W G. This is graphically illustrated in Fig. 5. The intercept–symbolized as grey dashed line–corresponds to O DG o, while the slope of the least square regression (solid line) represents the interaction of the Fe and Ni components in the FeNi alloy (i.e., the symmetric Margules parameter W G).

217

The Gibbs energy term is constant as long as the exchange reaction between coexisting metal and olivine can be described by reaction (1). p Binary asymmetric regular solution, i.e., W FeNi G NiFe WG h . ol i met ol met Xfayalite XNi  RT ln XFe XNiolivine h 1 met met met met ¼ DGo þ WGFeNi XNi  2XFe XNi XNi 2 i h  met 2 met met met met  2 XFe þ 2XFe X Ni þ WGNiFe XFe  XFe i  met 2 þ 2 XNi ð11Þ with O DG o as intercept and W GFeNi and W GNiFe as independent Margules parameters. Eq. (11) corresponds to a polynomial equation y ¼ ao þ a1 x 1 þ a2 x 2

ð12Þ

ln[(X met Fe d

ol met where y =  RT X ol Ni-olivine) / (X fayalited X Ni )], o FeNi NiFe a o = O DG = preset, a 1 = W G , a 2 = W G , x 1 = met met 2 X Nimet[X Nimet  2X met and x 2 = X met Fe X Ni  2(X Fe ) ], Fe met met met met 2 [ X Fe + 2X Fe X Ni + 2(X Ni ) ]. The Fe and Ni Margules parameters W GFeNi and NiFe W G of an asymmetric regular binary FeNi solution

Fig. 5. Graphic illustration of Eq. (5). The intercept of the least square fit of the linear dependence corresponds to half of the Gibbs energy of reaction in standard state DG8. The slope of the regression equals the value of the Margules parameter W G (W G = W GFeNi = W GNiFe) of the binary symmetric regular FeNi solution.

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Table 2 Gibbs free energy terms and Margules parameters of binary Fe–Ni regular solutions [J/mol]

DG8 W GFeNi W GNiFe r2

Fe–Ni regular solutions Symmetric

Asymmetric

86614 F 3198 16123 F 2259

86614 F 3198 15293 F 6149 16464 F 3239 0.81

0.81

DG8: Gibbs free energy; W GFeNi , W GNiFe : Margules parameters of binary Fe–Ni regular solutions; r 2 : correlation coefficients of least square fits.

can be obtained by a multiple regression with a preset intercept (O DG o) and the two independent variables W GFeNi and W GNiFe. The Gibbs energy of reaction in standard state DG o and the Margules parameter of the binary symmetric and asymmetric regular FeNi solution models are listed in Table 2.

Fe and Ni activities and corresponding activity coefficients of the alloy composition for binary symmetric and asymmetric regular solution models are calculated after Eqs. (5)–(8) at our experimental conditions of 1500 8C and 1 to 8.8 GPa. In Fig. 6a and b Fe and Ni activities, a Femet and a Nimet, and activity coefficients, c Femet and c Nimet, are plotted as function of the alloy composition for binary symmetric and asymmetric regular solution models at 1500 8C and pressures of 1 to 8.8 GPa. The solid lines are best fit lines through the activities and activity coefficients, respectively, based on our experimental results. Dashed lines in Fig. 6a represent the expected relationship between activities and composition of an ideal solid solution, i.e., Raoultian ideal solution with c Fe = c Ni = 1. Both the activities of Fe and Ni that are derived from our experimental data exhibit positive deviations from Raoultian behavior. The difference between activities of Fe and Ni for asymmet-

Fig. 6. Variation of the activities (a) and activity coefficients (b) of Fe and Ni components in the binary FeNi alloys with Fe composition of the metal phase at constant temperature (1500 8C) and pressures ranging from 0.9 to 8.8 GPa. The solid lines are best fit lines through the experimentally derived activities and activity coefficients. Gray solid lines are activities and activity coefficients calculated after Swartzendruber et al. (1991). The dashed line in (a) represents the activity variation with composition based on Raoultian ideal solid solution behavior (c Fe = c Ni = 1).

A. Holzheid, T.L. Grove / Chemical Geology 221 (2005) 207–224

ric solution models (open symbols) and for symmetric solution models (closed symbols) is insignificant. Therefore, we conclude that a symmetric regular solution is sufficient to describe the non-ideal behavior of the FeNi alloys produced in our experiments. In addition literature activities and activity coefficients of Fe and Ni at 1500 8C are plotted as gray solid lines in Fig. 6a and b. Equation G ex(fcc) in Table 6 in Swartzendruber et al. (1991) was used to calculate the literature activities and activity coefficients. While our experimental data exhibit significant positive deviations from Raoultian behavior, activities and activity coefficients derived from Swartzendruber et al. (1991) exhibit negative deviations from Raoultian behavior. The thermodynamic model in Table 6 in Swartzendruber et al. (1991) is mainly derived from measurements of Fe and Ni activities in solid alloys at 1200 8C and in liquid alloys at 1600 8C (see Swartzendruber et al., 1991 for references used to derive the thermodynamic model). The activity measurements in the alloys are performed well within the solid or liquid stability fields of the FeNi alloy. However, all run temperatures in our study were only 50 to 150 8C below the melting point of the FeNi alloys, except for FN-Ol 2 that was slightly above the metal melting point. Our experimentally derived activities and activity coefficients of binary FeNi alloys reflect therefore the relationship between metal composition and activities or activity coefficients close to the solid to liquid transition of the alloys. The deviation of our experimentally derived activities and activity coefficients of binary FeNi alloys to activities and activity coefficients of binary FeNi alloys derived from Swartzendruber et al. (1991) might reflect in general the complexity of thermodynamic properties of components in solid solutions close to solid to liquid transition (e.g., Woodruff, 1973).

5. Fe and Ni partitioning between metal and olivine The partitioning of Fe and Ni between coexisting FeNi-rich metal phases and olivine can be described by the oxygen fugacity independent Fe–Ni exchange reaction met 2Femet þ Ni2 SiOol þ Fe2 SiOol 4 ¼ 2Ni 4

ð13Þ

219

or expressed only as mole fractions met ol met ol þ XNiolivine ¼ XNi þ Xfayalite XFe

ð14Þ

The corresponding two-component metal–olivine exchange distribution coefficient K DNi–Fe of the oxygen fugacity independent reaction (13) is NiFemet=ol

KD

    ¼ XNi =XNiolivine = XFe =Xfayalite

ð15Þ

Expressing the experimentally derived partition behavior of Fe and Ni between metal and olivine by K DNi–Fe-values allows therefore direct comparison of the experimental data without considering oxygen fugacities, which had prevailed during the experiments. However, non-ideality of the Fe and Ni components in the alloys that coexist with the olivine in the experiments is not taken into account in expression (15). The Fe–Ni exchange reaction between metal and olivine (Eq. (13)) should be therefore expressed in activities rather than in mole fractions (Eq. (14)), i.e. ol met ol amet Fe þ aNiolivine ¼ aNi þ afayalite

ð16Þ

We than obtain as a Ni–Fe exchange distribution coefficient K DNi–Fe*    met ol ol KDNiFe * ¼ amet Ni =aNiolivine = aFe =afayalite Þ

ð17Þ

Replacing the activities of the elements in the metal and olivine phases by a = X d c (X is mole fraction and c is activity coefficient) yields  met met ol  cNi =XNiolivine col KDNiFe * ¼ XNi Niolivine   met met ol ð18Þ = XFe cFe =Xfayalite col fayalite Rearranging the equation yields

 ol ol met KDNiFe * ¼ KDNiFe cmet c =c c Ni fayalite Niolivine Fe

ð19Þ

where K DNi–Fe is the corresponding two-component metal–olivine exchange distribution coefficient of Eq. (15). Assuming only small deviations from ideality for ol c ol Ni-olivine and c fayalite, as pointed out by Wood and Kleppa (1981), Davidson and Mukhopadhyay (1984),

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Table 3 Activity coefficients of binary symmetric and asymmetric Ni–Fe alloys and C-factor of Eq. (20) Symmetric solution c Ni–Fe c Ni–Fe C sym. Ni Fe FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol FN-Ol

2 12 11 3 1 7 8 5 13 10 8.50 8.53

2.73 1.36 2.40 2.70 2.72 1.06 1.08 2.61 2.57 1.02 1.02 1.03

1.00 1.27 1.01 1.00 1.00 1.89 1.81 1.00 1.01 2.27 2.21 2.17

2.72 1.07 2.37 2.69 2.72 0.56 0.60 2.60 2.55 0.45 0.46 0.47

Asymmetric solution sol.

c Ni–Fe c Ni–Fe C asym. Ni Fe 2.77 1.34 2.41 2.74 2.76 1.06 1.07 2.64 2.59 1.02 1.02 1.03

1.00 1.28 1.01 1.00 1.00 1.87 1.80 1.00 1.01 2.21 2.16 2.12

sol.

2.77 1.05 2.38 2.73 2.76 0.56 0.60 2.63 2.58 0.46 0.47 0.48

sym. sol. : c Ni–Fe , c Ni–Fe calc. after Eqs. (5) and (6), C sym. sol. = Ni Fe Ni–Fe Ni–Fe c Ni / c Fe ; asym. sol.: c Ni–Fe , c Ni–Fe calc. after Eqs. (7) and (8), Ni Fe Ni–Fe C asym. sol. = c Ni–Fe / c . Ni Fe

Seifert et al. (1988) and Hirschmann and Ghiorso (1994), simplifies Eq. (19) to   met KDNiFe* ¼ KDNiFe cmet ¼ KDNiFe C ð20Þ Ni =cFe where C is the ratio of the activity coefficients of Ni and Fe in the metal phase. The factor C is a measure

of the change in the distribution coefficients when considering the non-ideality of Ni and Fe in the metal phase. Experimentally derived activity coefficients of Ni and Fe in the binary Ni–Fe metals and values of factor C are listed in Table 3. Activity coefficients of Ni and Fe are based on FN-Ol experiments, i.e., binary FeNi alloys coexisted with olivine, and are calculated after Eqs. (5) and (6) for symmetric solutions and Eqs. (7) and (8) for asymmetric solutions. Values of factor C are calculated after Eq. (20). According to the value of C, exchange distribution coefficients K DNi–Fe* will change by a factor of ~ 2.5 for Fe-rich binary FeNi alloys (90 F 5 at.% Fe). K DNi– Fe * values will remain similar to K DNi–Fe values at FeNi alloys with equal proportions of Fe and Ni (51 F 4 at.% Fe) and will be lower by a factor of ~ 2 for Ni-rich FeNi alloys (23 F 1 at.% Fe and b 18 at.% Fe). K DNi–Fe*-values are plotted as function of pressure at constant temperature (1500 8C) as closed symbols in Fig. 7. The symbol’s shape is assigned to different Fe content in the binary FeNi alloys (see values above). Any pseudo-correlation of the Fe/Ni ratios of the metal phase diminished as result of consideration of non-ideal behavior of Ni and Fe in FeNi

Fig. 7. K DNi–Fe*-values of FN-Ol experiments (solid symbols) and of studies by Ehlers et al. (1992) (diamonds), Seifert et al. (1988) (1420 8C: triangles down, 1600 8C: triangles up) and Brey (unpublished data, personal communication) (circles) are plotted as function of pressure. The horizontal solid line represents the range of present Earth’s core–olivine Ni–Fe ratio.

A. Holzheid, T.L. Grove / Chemical Geology 221 (2005) 207–224

alloys. In addition to these data, K DNi–Fe*-values calculated from partition data available in the literature are plotted as open symbols in Fig. 7. At lower pressure, the pressure influence on the partition behavior appears to be more pronounced than at high pressures. This might be attributed to the solid to liquid transition in the FeNi solid solutions. Liquid metal coexisted with solid olivine in all experiments of Ehlers et al. (1992) and in experiments of Seifert et al. (1988) at 1420 8C. Solid metal coexisted with solid olivine in the 1600 8C experiments of Seifert et al. (1988), in experiments of Brey (unpublished data) and in the binary FeNi alloy experiments of our study. Variations in solid metal–solid olivine K DNi–Fe*-values are less than an order of magnitude within the investigated pressure range from ambient pressure to about 9 GPa. Significant pressure dependences of the Fe and Ni exchange behavior between solid metal and olivine phases within the investigated pressure range might be therefore excluded based on our data and literature data.

6. Implications of Ni and Fe partition behavior to planetary core formation scenarios A question still under debate is whether the Ni abundance in the Earth’s mantle reflects an equilibrium distribution between coexisting metal and silicate phases or a stepwise accumulation/accretion of chondritic material (see Walter et al., 2000 for compilation of concepts of core formation). In the late sixties of the last century, Ringwood pointed out that Ni is overabundant in the Earth’s mantle and that the observed Ni content in the Earth’s mantle might not be consistent with equilibrium distribution between metal and silicate phases (e.g., Ringwood, 1966). Liquid or solid metal–liquid silicate partition coefficients at ambient pressure and temperature conditions of the upper mantle are orders of magnitude too high compared to the Earth’s core–mantle ratio. Liquid metal–liquid silicate partition coefficients of Ni, Co and Fe were experimentally determined at pressures of 2 to 20 GPa using piston cylinder devices and multi-anvil devices (Li and Agee, 1996, 2001; Righter et al., 1997; Righter and Drake, 1999; Chabot and Agee, 2002). Bouhifd and Jephcoat (2003) report Ni–Fe and Co–Fe metal– liquid silicate partition data based on laser-heated diamond-anvil cell experiments up to pressures of

221

42 GPa. Calculated Ni–Fe and Co–Fe metal–liquid silicate exchange partition coefficients, based on the liquid metal–liquid silicate partition coefficients of Ni, Co and Fe, decrease with increasing pressure and temperature, i.e., Ni and Co become less siderophile with increasing pressure and temperature. Ni–Fe and Co–Fe metal–liquid silicate exchange partition coefficients appear even to converge at pressures of about 28 GPa (Li and Agee, 1996) and higher (45 GPa, e.g., Bouhifd and Jephcoat, 2003), corresponding to today’s Earth upper mantle–lower mantle region. The authors therefore concluded that the absolute abundance of Ni, but also of Co, in today’s Earth mantle is consistent with a metal–silicate equilibrium at the base of a 750 to 1200 km deep magma ocean. Kegler et al. (2004, 2005) reinvestigated the effect of pressure and temperature on the solid metal–liquid silicate and liquid metal–liquid silicate partitioning behavior of Ni, Co and Fe. The influence of pressure on Ni–Fe and Co–Fe liquid or solid metal–liquid silicate exchange partition coefficients is more pronounced at lower than at higher pressures with a distinct change in pressure dependence at about 4 to 5 GPa. A change in the coordination number of Co2+ and Ni2+ in silicate melts with increasing pressure might be the cause of the change in pressure influence as already suggested by Keppler and Rubie (1993). At pressures higher than 5 GPa, the pressure influence diminishes, leaving the ratio of Ni–Fe to Co–Fe metal–silicate exchange partition coefficients more or less independent of pressure. The data of Kegler et al. (2004, 2005) do not support models of metal– silicate equilibrium distribution to explain the Ni abundances in the Earth’s mantle. However, as partially molten silicate matrices during core formation processes are also likely scenarios, metal–solid silicate interactions have to be considered as well. Metal– solid silicate interactions, expressed as K DNi–Fe* (see Eq. (20)) are plotted as function of pressure in Fig. 7. In addition to K DNi–Fe*-values of this study’s FN-Ol experiments (solid symbols), K DNi–Fe*-values of studies by Ehlers et al. (1992), Seifert et al. (1988) and Brey (unpublished data, personal communication) are plotted in the figure. The range of olivine Fe/Ni compositions that would be in equilibrium with a metal phase similar to that present in the Earth’s core (i.e., chondritic Fe/Ni) is symbolized as a horizontal bar. The majority of all K DNi–Fe*-values is more

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than one order of magnitude larger than present day Ni–Fe ratios within the pressure range covered by experiments. To match the Ni abundance in terrestrial mantle olivine, assuming an equilibrium metal–olivine distribution, a Fe/Ni-metal ratio of 0.7 to 0.9 would be required. This sub-chondritic ratio is a factor of 17 to 27 lower than Fe/Ni ratios in estimated Earth core compositions (e.g., Ringwood, 1977; Wa¨nke, 1981; Alle`gre et al., 1995; Wa¨nke and Dreibus, 1997). A simple metal–solid silicate equilibrium distribution is therefore unable to explain the Ni abundances in the Earth’s mantle. If olivine was present in only minor amounts at the time of metal segregation, then equilibrium between metal and olivine is not relevant to the problem of Ni overabundance in the Earth’s mantle and the experimental results do not contradict metal–silicate equilibrium at the base of a 750 to 1200 km deep magma ocean at the time of terrestrial core formation as suggested by, e.g., Li and Agee (1996), Righter et al. (1997), Righter and Drake (1999) and Li and Agee (2001). However, metal– liquid silicate partitioning data of Ni, Co and Fe at elevated temperature and pressure by Kegler et al. (2004, 2005) and kinetical constraints on metal–silicate separation scenarios, proposed by Rubie et al. (2003), question models of metal–silicate equilibrium at the base of a magma ocean. Kegler et al. (2004, 2005) report a change in pressure influence on Ni and Co metal–silicate partition behavior (see above for more detail). Convergence of partition coefficients cannot be assumed at any elevated pressure. Rubie et al. (2003) investigated the kinetics of metal–silicate equilibration. They conclude that chemical equilibration cannot occur between ponded liquid metal and overlying liquid silicate at the base of a magma ocean. The required equilibration times would be two to three orders of magnitude greater than the lifetime of the magma ocean. In contrast, small metal droplets (~ 1 cm) would equilibrate with the surrounding silicate liquid as they sink through the magma ocean. However, this implies metal–silicate equilibration at lower pressures. But metal–silicate partition coefficients at lower pressures are orders of magnitude too high compared to the Earth’s core–mantle ratio. The Ni content in the Earth’s mantle is therefore not consistent with a chemical equilibrium of metal droplets with silicate as the droplets sink through the magma ocean. Other mechanisms such as addition of oxidized

material to the growing Earth after the metal core had formed (e.g., Ringwood, 1984; Wa¨nke et al., 1984; O’Neill, 1991) or even inefficient core formation with later oxidation of metal that remained in the mantle during separation of core forming metal from the mantle silicate (Jones and Drake, 1986) might be considered as possible models to explain the high mantle abundances of Ni and also of the other moderately siderophile elements.

Acknowledgments We thank Gerhard Brey, Frankfurt/Main, Germany, for providing the solid FeNi-metal–solid olivine experimental charges. Thorough reviews by Nancy Chabot and an anonymous referee led to significant improvement of this paper. The research was performed while the first author held a research scholarship supported by the German Science Foundation (DFG). Support for this research was also provided by NASA Grants NAG5-9525 and NAG5-13051 to TLG. [SG]

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