Experimental determination of carbon solubility in Fe-Ni-S melts

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ScienceDirect Geochimica et Cosmochimica Acta 225 (2018) 66–79 www.elsevier.com/locate/gca

Experimental determination of carbon solubility in Fe-Ni-S melts Zhou Zhang a,b,⇑, Patrick Hastings a, Anette Von der Handt a, Marc M. Hirschmann a b

a Department of Earth Sciences, 150 Tate Hall, University of Minnesota, 55455, USA Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego 92037, USA

Received 10 May 2017; accepted in revised form 8 January 2018;

Abstract To investigate the effect of metal/sulfide and Ni/Fe ratio on the C storage capacity of sulfide melts, we determine carbon solubility in Fe-Ni-S melts with various (Fe + Ni)/S and Ni/Fe via graphite-saturated high-pressure experiments from 2–7 GPa and 1200–1600 °C. Consistent with previous results, C solubility is high (4–6 wt.%) in metal-rich sulfide melts and diminishes with increasing S content. Melts with near M/S = 1 (XS > 0.4) have <0.5 wt.% C in equilibrium with graphite. C solubility is diminished modestly with increased Ni/Fe ratio, but the effect is most pronounced for S-poor melts, and becomes negligible in near-monosulfide compositions. Immiscibility between S-rich and C-rich melts is observed in Ni-poor compositions, but above
18 wt.% Ni there is complete miscibility. Because mantle sulfide compositions are expected to have high Ni concentrations, sulfide-carbide immiscibility is unlikely in natural mantle melts. An empirical parameterization of C solubility in Ni-Fe-S melts as a function of S and Ni contents allows estimation of the C storage capacity of sulfide in the mantle. Importantly, as the metal/sulfide (M/S) ratio of the melt increases, C storage increases both because C solubility increases and because the mass fraction of melt is enhanced by addition of metal from surrounding silicates. Under comparatively oxidized conditions where melts are near M/S = 1, as prevails at <250 km depth, bulk C storage is <3 ppm. In the deeper, more reduced mantle where M/S increases, up to 200 ppm C in typical mantle with 200 ± 100 ppm S can be stored in Fe-Ni-S melts. Thus, metal-rich sulfide melts are the principal host of carbon in the deep upper mantle and below. Residual carbon is present either as diamond or, if conditions are highly reduced and total C concentrations are low, solid alloy. Ó 2018 Elsevier Ltd. All rights reserved. Keywords: Deep carbon; Metal-rich sulfide melt; Mantle redox

1. INTRODUCTION Determining the petrologic storage of carbon in the mantle is an important consideration in global carbon cycling. Because the different potential phases of carbon have notably different geochemical and geophysical characters, understanding the predominant carbon-bearing phase (s) in the mantle as a function of depth, temperature, and oxygen fugacity is a perquisite for determining how carbon

⇑ Corresponding author at: Department of Earth Sciences, 150 Tate Hall, University of Minnesota, 55455, USA. E-mail addresses: [email protected], zhangzhou333@gmail. com (Z. Zhang).

https://doi.org/10.1016/j.gca.2018.01.009 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved.

affects and is affected by the dynamics and chemistry of Earth’s interior. Because storage in mantle silicates is minimal (Shcheka et al., 2006), carbon resides chiefly in accessory phases, with possible hosts depending on oxygen fugacity (fO2). Under oxidizing conditions, carbon is present chiefly as carbonate, carbonated melt, and/or CO2 vapor. Under reduced conditions, it may reside in diamond, graphite, carbide, carbide melt, crystalline or molten Fe-rich alloy, or in sulfide melt (Dasgupta and Hirschmann, 2010). The oxidation state of the mantle becomes more reduced with depth compared to standard buffers such as fayalitemagnetite quartz (FMQ) or iron-wu¨stite (IW) (Frost and McCammon, 2008; Stagno et al., 2013). Consequently,

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the principal C-bearing phase transitions from carbonate to a reduced phase. The low oxygen fugacity expected for the deep upper mantle stabilizes reduced C phases, and it could be expected that C would reside chiefly as diamond (e.g., Stagno et al., 2013). However, low fO2 also translates to high activities of Fe and Ni, resulting in possible precipitation of Fe-Ni alloy at depth (Frost et al., 2004; Rohrbach et al., 2007, 2011; Frost and McCammon, 2008; Rohrbach and Schmidt, 2011) and diamond and crystalline Fe-rich alloy are not mutually stable. Rather, the two react to form carbide, which will be crystalline at low temperatures, and along the mantle geotherm, liquid (Wood, 1993; Lord et al., 2009; Rohrbach et al., 2014). Molten sulfide is also stable in the convecting mantle (Bockrath et al., 2004; Zhang et al., 2015; Zhang and Hirschmann, 2016) and iron-rich sulfide and carbide liquids are partially or entirely miscible, depending on temperature, pressure, and composition (Tsymbulov and Tsemekhman, 2001; Dasgupta et al., 2009; Deng et al., 2013). This indicates that sulfide liquid is a plausible host for a significant portion of the C in the mantle (Dasgupta and Hirschmann, 2010; Tsuno and Dasgupta, 2015; Zhang et al., 2015). An important influence on the storage of C in Fe-Ni-S melts is the metal sulfide ratio, [M/S = atomic (Fe + Ni)/ S]. Sulfide melts in the shallow upper mantle have stoichiometry close to that of monosulfide (M/S  1), depending chiefly on fS2 but also on the abundance of Cu, Ni. As conditions become more reduced with depth, activities of Fe and Ni in the melts grow, resulting in increased M/S (e.g. Hsieh et al., 1987; Hsieh and Chang, 1987). The solubility of C in Fe-Ni-S melts is greatly enhanced at greater M/S, rising from 0.1–0.3 wt.% for M/S near unity (Zhang et al., 2015), to 4–6 wt.% at M/S of 20, and so the magnitude of C storage in mantle sulfide increases in the deeper mantle. There are several previous experimental studies C solubility in Fe-Ni-S melt (Wang et al., 1991; Wood, 1993; Tsymbulov and Tsemekhman, 2001; Dasgupta et al., 2009; Deng et al., 2013; Tsuno and Dasgupta, 2015; Li et al., 2016; Zhang and Hirschmann, 2016), but the accuracy of C analyses in quenched metal-rich sulfide melts is not uniformly high, and the data needed for quantitative modeling remain insufficient, particularly for Nibearing compositions. In this study, we present new experiments further establishing the carbon solubility in Fe-Ni-S melts as a function of composition, temperature and pressure. We use these to parameterize the solubility of C and apply the results to evaluate the contribution of sulfide melts to storage of C in the mantle. 2. METHODS 2.1. Starting materials Starting materials were Fe-Ni-S mixtures with different Ni/(Fe + Ni) and (Fe + Ni)/S ratios. Owing to challenges in homogenizing powders with strongly contrasting physical properties (Zhang and Hirschmann, 2016), the mixtures were prepared from phases of similar hardness and density (phase purity provided in parentheses): Fe (99.9%), FeS2

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(99.9%), Ni (99.8%), and Ni3S2 (99.8%). Weighed powders were ground under ethyl alcohol in a mortar and pestle for 30 min to produce completely homogeneous batches and then dried at 110 °C in a vacuum oven for 5 min. To avoid oxidation and contamination, dried Fe-Ni-S mixtures were stored in a sealed glass container within a sealed glass desiccator. Previous analysis of quenched crystalline sulfide shows minimal oxidation or hydration (Zhang and Hirschmann, 2016). Carbon standards (Fe3C and Fe7C3) used for electron microprobe analyses were prepared in a similar procedure from stoichiometric mixtures of Fe and C by careful weighing (Dasgupta and Walker, 2008). 2.2. Experimental methods Experiments at 2–3.5 GPa and 1200–1600 °C were performed in an end loaded piston cylinder (PC) apparatus at the University of Minnesota using the experimental assemblies, procedures, and calibrations described by Xirouchakis et al. (2001) and Zhang and Hirschmann (2016). PC experiments used ½ inch BaCO3 assemblies with MgO spacers and straight-walled graphite heaters. High purity graphite rods were cut to 3 mm in length and a 4 mm outer diameter. Four holes (1 mm diameter, 2 mm length) were drilled into the graphite rods, allowing for the loading of four samples in each experimental run. The graphite capsules also provide carbon-saturated conditions. A 1 mm thick graphite lid covered and sealed the top. All experiments were performed at temperatures
100–400 °C above sulfide liquidus. Similar with previous studies, equilibrium between the sulfide melt and the enclosing graphite capsule was approached rapidly (Dasgupta and Walker, 2008; Kiseeva and Wood, 2013; Wood and Kiseeva, 2015). Experimental durations were 0.5–24 h. Experiments at 5–7 GPa and 1600 °C were performed in a 1000-ton Walker-style octahedral multianvil (MA) apparatus using an 18/12 assembly with cast MgO/Cr2O3 octahedra and integrated gasket fins, with calibrations described in Dasgupta et al. (2004) and Zhang and Hirschmann (2016). Straight-walled graphite furnaces and MgO spacers were used for all MA experiments. Graphite capsules were employed, 2 mm in length with an external diameter of
1.8 mm, with four drilled holes (
0.4 mm diameter, 1 mm length) for experimental charges A 0.4 mm thick graphite lid covered and sealed the top (Fig. 1). PC and MA run temperatures were measured using type B (Pt70Rh30/Pt94Rh6) and type D (W97Re3/W75Re25) thermocouples, respectively. Temperature was controlled by a Eurotherm controller to ±10 °C. Both PC and MA experiments were quenched by power termination. Carbide standards were synthesized in MgO capsules. Fe3C was synthesized in the piston cylinder at 1200 °C and 2 GPa; Fe7C3 was synthesized in the multianvil at 7.5 GPa and 1350 °C using the ‘‘large volume” 18 mm octahedral assembly described by Withers et al. (2011). 2.3. Analytical methods Experiments were analyzed using the JEOL JXA-8900R electron microprobe analyzer (EPMA) at the University of

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Fig. 1. Experimental run products in graphite capsules pressed into an indium mount for electron microprobe analysis.

Minnesota. Wavelength-dispersive X-ray analysis of carbon was performed using a multilayer crystal (LDE2 with 2d =
10 nm). Samples and standards were polished with Al2O3 pads to a finish of 0.3 mm and pressed in indium. Because the Fe-Ni-S phases and graphite are conductive, neither the standards nor samples required a conductive coating (Fig. 1). Both ends of the samples and standards were polished to ensure that electrical contact was established through the bottom of the capsules. Accurate carbon analysis by EPMA is challenging because of ubiquitous hydrocarbon contamination in the microprobe deriving from vacuum greases, pump oil, and sputtered epoxy (Robaut et al., 2006; Dasgupta and Walker, 2008). Additionally, the shape of the C ka peak is strongly affected by bonding effects while proximity of higher order interferences must be carefully monitored when choosing background positions. The background positions for carbon were 8/+13.5 JEOL spectrometer units relative to the carbon Ka peak, selected after careful repeat wavescans of standards and unknowns. Protocols for analytical standards and methods will be described in detail below. 2.3.1. Analytical standards Analysis of carbon by EPMA requires both primary and secondary standards, owing to the delicacy of carbon analyses and, in particular, the sensitivity of the C X-ray peak location and shape to the bonding environment (Bastin and Heijligers, 1986). We chose synthetic Fe3C (6.67 wt.% C) as our primary carbon standard. Secondary standards were synthetic Fe7C3 (8.43 wt.% C), NIST SRM-663 Chromium-Vanadium steel standard (0.57 wt.% C), Fe metal (Alfa-Aesar 99.995 wt.% Fe), and FeS2 (Alfa-Aesar 46.55 wt.% Fe) and Ni-metal (Alfa-Aesar, 99.995 wt.% wt.% Ni). As Fe-carbides are not always stoichiometric (Walker et al., 2013), we confirmed the homogeneity and stoichiometry of our carbide standards by (1) powder

XRD, (2) point analyses of carbon counts at different areas within a carbide standard and between different pieces of carbide standards, and (3) a comparison of the carbon ratios of the Fe3C and Fe7C3 standards. These three practices suggest that the Fe3C and Fe7C3 standards approach perfect stoichiometry within analytical error. Standards for Fe, Ni, S and O analyses were Fe metal (99.995 wt.%), Ni metal (99.995 wt.%), FeS2 (53.45 wt.% S) and magnetite (Fe3O4), respectively. Natural troilite (FeS) from Crescent City, California was used as a secondary standard for Fe and S. Oxygen was measured using a multilayer crystal (LDE1 with 2d =
6 nm). WDS scans were performed on standards to allow the interferences between C, Fe, Ni, S and O on the unknowns to be established. There is an interference from Fe Ll on the low side and Ni L-lines on the high side of carbon Ka, without other interferences being found. Matrix corrections were made using ZAF. Carbon was quantified using carbide standards and correcting for blanks (see Section 3.2 below for details). Single channel analyzer scans were executed to help eliminate low and high-energy noise and optimize our X-ray signal output. Carbon contamination in microprobe vacuum chambers is time dependent owing to cracking of hydrocarbons in the path of the electron beam (Robaut et al., 2006; Dasgupta and Walker, 2008). To minimize C contamination (Bastin and Heijligers, 1986; Robaut et al., 2006), we used a liquid N2 anti-contamination system close to the EPMA stage, and refilled it every 3 h during each analytical session. Additionally, we tracked the rate of C counts during analyses, following the procedure of Dasgupta and Walker (2008). We found optimum counting times of 10 s on peak and 5 s on background for our instrument. To determine the optimum analytical conditions, we monitored the intensity of C counts as a function of beam voltage and current while analyzing the Fe3C standard. Optimum peak/background ratios of 20–22 were found for voltages of 9–12 kV and a current of 80 nA (Fig. 2) and so, we analyzed standards and unknowns at 11 kV and 80 nA, conditions that are also appropriate to ensure

Fig. 2. Analytical conditions for carbon using the UMN JEOLJXA 8900, optimizing the signal to background ratio for C Ka counts using a multi-layer crystal (LDE2) with 2d =
10 nm. Peak/ background of the carbon peak while analyzing the synthesized Fe3C standard gives optimal values at approximately 9–12 kV and 80 nA. Quantitative analyses of C in standards and unknowns in this study were performed at 11 kV and 80 nA.

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sufficient excitation of Ni and Fe X-ray ka lines. The resulting peak/background ratio compares favorably with maximum values of 13 found by Dasgupta and Walker (2008). Beam sizes of 0, 10, 20 and 50 mm were used for standard and sample compositional analyses, with the same beam size used for calibration and analyses to avoid Braggdefocusing effects. 3. RESULTS 3.1. Texture and phase compositions All experiments were carried out above the liquidus temperatures in the system Fe-Ni-S, producing only sulfide melts (+graphite) quenched into crystals with wormlike intergrowth or dendritic texture (Zhang et al., 2015). Experiments with bulk sulfur compositions between 2 and 27 wt. % and Ni/(Ni + Fe) atomic ratio <0.20 produced two immiscible melts (+graphite; e.g. A992-2 and B519-1 experimental charges; Fig. 3B and C). More sulfur-rich (S > 30 wt.%) or Ni-rich (Ni > 18 wt.% or Ni/(Ni + Fe) >0.20) bulk compositions produced a single melt (+graphite; Fig. 3A and D).

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Quenched high-temperature phases have heterogeneous compositions. Relatively Ni-rich phases show dendritic textures and are more heterogeneous within a given small area (e.g. within a 10 mm diameter; Fig. 3C and D), requiring a larger beam size to approximate the bulk composition (Brenan and Caciagli, 2000; Brenan, 2003). Ni-free phases are less heterogeneous, and therefore could potentially be analyzed with a focused beam (Fig. 3A and B). Oxygen contents of quenched Fe-Ni-S melts range from 0 to 1.13 wt.%. In samples comprising two immiscible melts (+graphite), the metal- and carbon-rich melt is oxygenpoor (e.g. 0.04 wt.% in sample A1025-1) whereas the sulfur-rich and carbon-poor melt contains significantly more oxygen (e.g. 0.75 wt.% in sample A1025-1). Samples with a single melt (+graphite) contain less than 0.34 wt.% O. Oxygen analytical uncertainty is greater in more heterogeneous melts quenched from higher temperatures. 3.2. Carbon content calibration During our optimized EPMA analyses (Section 2.3), we used wavescans and analyses of carbide standards and carbon blanks to calibrate carbon concentration based on

Fig. 3. Backscattered electron images of quenched experimental Fe-Ni-S melts of varying compositions. (A) Sample A986-2, FeS melt; darker gray areas are quenched sulfides enriched in oxygen. Darkest gray regions are graphite crystals. (B) Sample A992-2, two Ni-free immiscible melts. (C) Sample B519-1, two immiscible melts with YNi
0.1. (D) Sample, B519-2, single homogeneous melt with YNi
0.3.

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observed carbon counts (Figs. 4 and 5). We apply this calibration to calculate the carbon content of unknown samples as: C unknown ¼

ðI unknown  I blank Þ ; tan h

ð1Þ

where I unknown and I blank are the carbon counting intensities of the unknown and the corresponding blank, respectively, and tan h is the slope of the carbon content–carbon count calibration (Fig. 5). Repeated analyses of the carbon blank standards at least every 2 h (after waiting 30 min following addition of liquid N2 to the anticontamination unit) revealed carbon abundances of 0.47 ± 0.08 wt.% (n = 352, 1 s.d.) on the pure Fe metal standard and 0.24 ± 0.07 wt.% (n = 248, 1 s.d.) on the FeS2 standard (Fig. 6). Carbon counts on the pure Fig. 6. Analyses of carbon on a pure Fe standard (Fe 99.995 wt.%, 352 analyses) and a FeS2 standard (248 analyses), using LDE2 crystal (2d =
10 nm), at 11 kv voltage and 80 nA current. Compilation includes data collected over many separate analytical sessions and indicate a persistent and reproducible carbon contamination with 1 standard deviation of 0.47 ± 0.08 wt.% for Fe and 0.24 ± 0.07 wt.% for FeS2. These apparent concentrations are calculated from the carbon counts in each analysis, multiplied by the slope from Fig. 5 by assuming that zero hydrocarbon contamination on a carbon-free standard corresponds to zero counts.

Fig. 4. Carbon wavelength scans of carbide standards, carbon blanks and samples using LDE2 crystal (2d =
10 nm), at 11 kv voltage and 80 nA current.

Ni standard are similar to those on the Fe standard, but not sufficiently numerous to produce meaningful statistics. The Fe, Ni, and FeS2 standards are nearly carbon-free based on their certificates of analysis. However, at the same beam conditions, carbon counts from the FeS2 carbon blank are approximately half of those from Fe-metal and Ni-metal carbon blanks We interpret the difference in C counts between standards to reflect greater thermal conductivities of Fe and Ni compared to FeS2, resulting in a systematic (and measurably different) contamination due to differential aggregation of hydrocarbons in the microprobe sample chamber (Bastin and Heijligers, 1986; Moretto and Hoffmann, 1994). We correct for hydrocarbon contamination during sample analyses according to the bulk sulfur content. To account for this apparent S effect, the carbon blank intensity ðI blank Þ used in Eq. (1) is approximated based on the S content of the unknown sample: I blank ¼

Fig. 5. Relationship of carbon counts per second and carbon concentration established from the carbon standards (Fe7C3 and Fe3C) and NIST steel 633, using LDE2 crystal (2d =
10 nm), at 11 kv voltage and 80 nA current. Pure Fe metal and FeS2 were analyzed as carbon blanks; the carbon counts on the blanks are a result of hydrocarbon aggregation due to differences in the thermal conductivity of Fe and FeS2. Note that the C standards are colinear with Fe-metal, rather than FeS2, consistent with the assumption that Fe is the appropriate standard for blank correction of S-free samples.

I Fe  I FeS2 ; ðS unknown =0:5Þ

ð2Þ

where I Fe and I FeS2 are the carbon intensities of pure Fe and pure FeS2, and S unknown is the sulfur mole fraction of the unknown. This calculation and the origin of the difference between carbon blank counts on pure Fe and FeS2 will be discussed in Section Section 4.1. As detailed below, sulfur-rich samples have low carbon solubility and so the hydrocarbon correction has a greater effect on the accuracy of the carbon content measurement. For sulfur-poor samples with higher carbon solubility, the hydrocarbon contamination has a smaller relative impact for the more carbon-rich high M/S melt. The small intersession variability of the carbon blank suggests that the precision of the reported C concentrations is <0.2 wt.%.

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3.3. Carbon solubility in Fe-Ni-S melts To avoid confusion, we define Xi as the mole fraction of i element i, or atomic ratio of FeþNiþS in the Fe-Ni-S melts, where i is the element of Fe, Ni or S and Ni and X S þ X Fe þ X Ni ¼ 1; YNi is the atomic ratio of NiþFe X Ni Y Ni ¼ X Ni þX Fe ; M/S is the atomic ratio of metal/sulfur or X Fe þX Ni . XS

In the discussion below, X and Y symbols all refer to compositions of sulfide melt. All experiments analyzed by EPMA are listed in Table 1. Carbon solubility in Fe-Ni-S melts ranges from 0.5 to 6 wt. %, and is greatest for high M/S melts: carbon solubility varies from 4.62 to 5.80 wt.% at XS = 0; 2.25–4.23 wt.% at XS = 0.1 ± 0.02; 1.92–3.58 wt.% at XS = 0.15 ± 0.02; and 0.09–0.28 wt.% at XS = 0.40 ± 0.02 (Fig. 7A). Substitution of Ni for Fe (increasing YNi) affects carbon solubility differently depending on M/S in the Fe-Ni-S melts. At high M/S (XS < 0.1), substitution of Ni for Fe depresses carbon solubility in the melts (Fig. 7B). Ni substitution in melts with moderate M/S (XS = 0.1–0.3) either moderately depresses or slightly increases carbon solubility. For Fe-Ni-S melts with M/S  1 (XS > 0.3), the effect of Ni substitution on carbon solubility could not be distinguished because carbon solubility is low (<1.0 wt.%) in all such melts (Fig. 7B). Distinct effects of temperature and pressure on carbon solubility are not evident from our experiments at 2–7 GPa and 1200–1600 °C (Fig. 7A).

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the unknowns (Fig. 7), the magnitude of sulfur mass absorption effect on carbon correction is less than <0.1 wt.% per sample. In this study, sulfur mass absorption effect on carbon was not applied. However, future experiments that verify the sulfur mass absorption effects on carbon concentration by EPMA with controlled sulfur content and carbon content standards would be worthwhile. 4.2. Effect of M/S on C solubility in sulfide melts We compare our experimental results with previous studies of C content in Fe-Ni-S melts at carbon-saturated conditions (Fig. 8; Wang et al., 1991; Wood, 1993; Tsymbulov and Tsemekhman, 2001; Corgne et al., 2008; Dasgupta et al., 2009, 2013; Deng et al., 2013; Tsuno and Dasgupta, 2015; Zhang et al., 2015). Carbon solubility trends (as a function of melt M/S) are broadly similar across most studies: C solubility is greatest in high M/S melts, and diminishes as melts approach M/S = 1. In detail, however, the 100 kPa (room pressure) studies of Wang et al. (1991) and Tsymbulov and Tsemekhman (2001) report lower C solubilities than the others, which were all conducted at high pressure, whereas Dasgupta et al. (2009) report higher values (Fig. 8). In the interval 0.1 < XS < 0.4, the latter are sufficiently different from all other determinations that they should be discounted, though the specific reasons for the discrepancies are not clear. 4.3. Effect of Ni on immiscibility and C in sulfide melts

4. DISCUSSION 4.1. Effective blank for C analyses The apparent C content measured on Fe metal is similar to that reported in previous studies (0.35 wt.%, Hirayama and Fujii, 1993; 0.5 wt.%, Dasgupta and Walker, 2008; 0.4 wt.%, Nakajima et al., 2009), but we find it significant that direct comparison between the Fe and FeS2 standards at the same beam conditions reveal much lower apparent concentrations in the FeS2, even though the intrinsic C in each is thought to be far below the apparent concentrations. As noted above, the different C blanks for metals and FeS2 are likely attributable to differences in thermal conductivity, and although we could not precisely quantify the thermal conductivity of unknown samples due to combined effects of Ni, C, and O, we make first order estimates of sample thermal conductivities based on sulfur concentrations and apply Eq. (2) to correct for hydrocarbon contamination. For sulfur-rich (sulfide-like) and carbon-poor samples, it is crucial to apply the hydrocarbon correction because hydrocarbon contamination has a greater effect on the total carbon counts of the sample and therefore accuracy of the carbon content measurement. Additionally, sulfur has a mass absorption effect on carbon X-ray intensities. According to Monte Carlo simulations using CALCZAF (Armstrong et al., 2013), the effects of mass absorption on the effective carbon concentration is <5% for S-poor (S < 10 wt.%) unknowns and more appreciable <20% for S-rich (S > 25 wt.%) unknowns. Because sulfur and carbon contents correlate negatively in

There is a well-documented miscibility gap between Srich and C-rich melts in the Fe-S-C ternary (Wood, 1993; Dasgupta et al., 2009; Deng et al., 2013). This gap narrows with increasing pressure until absent above 10 GPa, 1700 ° C (Deng et al., 2013). For melts with small amounts of Ni (<6.60 wt.% or YNi < 0.1), the miscibility gap closes at 6.2 GPa, 2000 °C (Corgne et al., 2008). However, sulfide melts in the Earth’s mantle tend to be more enriched in Ni, have YNi varying from 0.1 to 0.9 (Frost and McCammon, 2008; Aulbach et al., 2009), and the miscibility gap for such compositions is less well-characterized. Except for Tsymbulov and Tsemekhman (2001) and this study, previous investigations have investigated sulfide melts with narrow ranges of YNi, e.g. YNi = 0.05–0.10 in Corgne et al. (2008), YNi = 0.53–0.64 in Tsuno and Dasgupta (2015). This incomplete coverage has not revealed the systematic effect of Ni on the miscibility gap. Here, we explore the effects of YNi on immiscibility in sulfide melts based on the relatively limited datasets provided by Tsymbulov and Tsemekhman (2001) and this study. We observed immiscibility in some of the experiments (Fig. 3B and C), but at YNi > 0.20 or Ni > 18 wt.%, quenched crystals indicate that there was only a single homogeneous melt (+graphite) before quenching (Fig. 3D). At 100 kPa (room pressure), Tsymbulov and Tsemekhman (2001) similarly observed closure of the miscibility gap, but at a more Ni-rich composition (
25 wt.% Ni or YNi
0.30) (Fig. 9), the difference most likely owing to pressure. Therefore, whether mantle Fe-Ni-S melts stabilize as two immiscible melts or a single melt depends not only

No.

P (GPa)

T (°C)

D (hrs)

Ba (mm)

nb

Phase

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Table 1 Electron microprobe analysis of Fe-Ni-S melts quenched from high temperature. YNic

Xsd

Fe (wt.%)e

Ni (wt.%)

S (wt.%)

O (wt.%)

C (wt.%)

Total (wt.%)

1600 1600 1600 1600

0.5 0.5 0.5 0.5

50 50 50 50

18 22 20 22

0.21 0.39 0.57 0.80

– – – –

74.09(0.37) 56.07(1.03) 38.99(1.63) 17.82(0.48)

20.67(0.38) 38.02(1.65) 55.37(2.36) 77.07(1.52)

– – – –

0.07(0.08) 0.05(0.04) 0.09(0.07) 0.09(0.09)

5.87(0.35) 5.43(0.43) 5.25(0.43) 4.62(0.55)

100.70 99.57 99.70 99.60

Ni-free samples A986-1 2.0 A986-2 2.0 A992-1 2.0 A992-2 2.0 A992-2 2.0 A992-3 2.0 A992-3 2.0 A993 2.0

1600 1600 1600 1600 1600 1600 1600 1600

48.0 48.0 0.5 0.5 0.5 0.5 0.5 0.5

0 0 0 0 0 0 0 0

67 58 28 37 72 61 56 66

– – – – – – – –

0.53 0.49 0.41 0.04 0.41 0.03 0.41 0.04

60.53(0.30) 63.34(0.44) 70.34(0.90) 92.46(0.71) 70.65(0.82) 92.46(0.54) 70.50(0.54) 91.95(0.97)

– – – – – – – –

39.34(0.25) 35.78(0.57) 28.62(0.64) 2.63(0.18) 28.80(0.85) 2.15(0.37) 28.91(0.62) 2.52(0.63)

– – – – – – – –

0.22(0.07) 0.32(0.14) 0.28(0.06) 5.60(0.18) 0.32(0.08) 5.90(0.22) 0.25(0.10) 5.80(0.60)

100.09 99.44 99.24 100.69 99.77 100.51 99.66 100.27

Starting material 5 wt.% sulfur with varying YNi A1025-1 2.0 1600 0.5 A1025-1 2.0 1600 0.5 A1025-2 2.0 1600 0.5 B499-1 2.0 1600 1.0 B521-1 2.0 1600 0.5 A1025-3 2.0 1600 0.5 A1025-4 2.0 1600 0.5

10 10 10 50 10 10 10

27 10 22 60 64 49 54

C-rich S-rich

0.09 0.15 0.33 0.50 0.61 0.72 0.91

0.03 0.38 0.08 0.08 0.08 0.07 0.08

83.65(0.66) 61.30(1.72) 59.69(0.75) 44.78(0.53) 34.74(1.39) 24.61(1.11) 08.35(1.39)

8.95(0.20) 11.75(1.57) 30.39(0.75) 47.32(1.30) 57.69(1.17) 67.96(0.85) 83.72(1.17)

1.92(0.33) 26.37(1.56) 5.11(0.86) 4.81(0.45) 4.83(0.45) 4.69(0.40) 4.95(3.47)

0.04(0.10) 0.75(0.35) 0.08(0.04) 0.03(0.09) 0.05(0.10) 0.11(0.08) 0.03(0.07)

5.69(0.60) 0.35(0.10) 4.23(0.29) 2.86(0.20) 2.77(0.28) 3.52(0.36) 2.72(0.25)

100.24 100.52 99.50 99.80 100.08 100.88 99.76

Starting material 10 wt.% sulfur with varying YNi B518-1 2.0 1600 0.5 B518-1 2.0 1600 0.5 A1047-1 2.0 1600 2.0 A1047-1 2.0 1600 2.0 B521-2 2.0 1600 0.5 A1047-2 2.0 1600 2.0 A1047-2 2.0 1600 2.0 B518-2 2.0 1600 0.5 B499-2 2.0 1600 1.0 B521-3 2.0 1600 0.5 B518-3 2.0 1600 0.5 B518-4 2.0 1600 0.5

20 20 20 20 20 20 20 50 50 50 50 50

43 10 16 20 26 14 15 44 36 62 28 38

C-rich S-rich C-rich S-rich S-rich* C-rich S-rich

0.09 0.13 0.14 0.20 0.23 0.23 0.30 0.28 0.52 0.63 0.70 0.91

0.04 0.42 0.04 0.38 0.37 0.05 0.38 0.15 0.16 0.16 0.15 0.15

83.35(0.96) 60.65(2.19) 78.49(0.78) 56.84(1.51) 55.62(1.95) 69.38(1.65) 50.41(1.29) 61.35(0.75) 40.82(0.45) 31.83(0.65) 25.86(0.39) 07.59(0.29)

8.83(0.53) 9.58(2.57) 13.06(0.49) 15.12(0.99) 17.81(2.02) 22.31(1.95) 22.41(2.09) 25.22(0.91) 45.87(1.14) 55.96(0.94) 62.54(2.00) 80.51(1.29)

2.67(0.67) 28.96(1.54) 2.86(0.36) 26.26(1.30) 25.19(1.68) 3.48(1.35) 25.76(1.44) 10.31(0.71) 10.52(0.96) 10.02(0.86) 9.40(0.96) 9.46(0.67)

0.01(0.10) 0.93(0.33) 0.08(0.03) 0.91(0.40) 0.81(0.24) 0.07(0.04) 0.68(0.08) 0.04(0.02) 0.07(0.10) 0.08(0.18) 0.02(0.13) 0.08(0.16)

5.14(0.26) 0.22(0.15) 4.99(0.31) 0.41(0.10) 0.60(0.24) 4.78(0.63) 0.25(0.07) 3.58(0.19) 2.68(0.18) 2.08(0.43) 2.12(0.26) 1.92(0.13)

100.01 100.34 99.47 99.54 100.02 100.02 99.51 100.50 99.96 99.97 99.95 99.55

Starting material 15 wt.% sulfur with varying YNi B519-1 2.0 1600 0.5 B519-1 2.0 1600 0.5 A1047-1 2.0 1600 0.5 A1047-1 2.0 1600 0.5 A1047-2 2.0 1600 2.0

20 20 20 20 20

13 19 18 12 16

C-rich S-rich C-rich S-rich C-rich

0.07 0.11 0.12 0.16 0.15

0.05 0.39 0.04 0.39 0.04

83.98(0.94) 64.08(0.67) 80.30(0.58) 60.20(2.08) 76.93(1.00)

7.14(0.45) 8.03(0.63) 11.66(0.34) 12.49(2.62) 14.67(1.46)

3.65(0.76) 26.47(0.98) 2.53(0.34) 26.15(1.81) 2.94(1.75)

0.26(0.04) 0.85(0.15) 0.16(0.02) 0.86(0.53) 0.13(0.03)

5.51(0.16) 100.54 0.17(0.06) 99.60 5.06(0.84) 99.71 0.09(0.07) 99.79 4.82(0.40) 99.49 (continued on next page)

S-rich* C-rich S-rich C-rich S-rich C-rich*

Z. Zhang et al. / Geochimica et Cosmochimica Acta 225 (2018) 66–79

Sulfur-free samples B522-1 2.0 B522-2 2.0 B522-3 2.0 B522-4 2.0

Table 1 (continued) Phase

YNic

Xsd

Fe (wt.%)e

Ni (wt.%)

S (wt.%)

O (wt.%)

C (wt.%)

Total (wt.%)

20 50 50 50 50

15 57 66 35 28

S-rich

0.22 0.32 0.53 0.71 0.88

0.37 0.25 0.26 0.22 0.24

56.92(1.80) 54.30(1.15) 36.79(1.61) 22.96(0.43) 9.61(1.87)

16.71(1.96) 26.87(0.96) 43.02(0.86) 60.45(1.34) 72.55(1.24)

25.29(1.18) 16.44(1.24) 17.80(1.19) 14.89(0.92) 15.39(1.58)

0.83(0.40) 0.15(0.14) 0.60(0.20) 0.18(0.09) 0.07(0.12)

0.20(0.10) 1.78(0.14) 1.84(0.21) 1.98(0.20) 1.44(0.27)

99.95 99.54 100.05 100.46 99.06

Starting material 35 wt.% sulfur with varying YNi A995 2.0 1600 1.0 50 B496 2.0 1600 1.5 50

50 86

0.34 1.00

0.49 0.48

41.64(0.79) –

22.36(0.68) 64.86(0.64)

35.08(0.54) 34.30(0.86)

0.08(0.02) 0.02(0.02)

0.38(0.14) 0.40(0.12)

99.54 99.58

Samples at 2 GPa 1500 °C A1259-1 2.0 1500 A1259-2 2.0 1500 A1259-3 2.0 1500 A1259-4 2.0 1500 A1259-5 2.0 1500 A1259-6 2.0 1500 A1259-7 2.0 1500 A1259-7 2.0 1500 A1259-8 2.0 1500 A1259-8 2.0 1500

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

50 50 50 50 50 50 10 10 20 20

8 6 7 7 7 6 6 4 6 4

0.22 0.00 0.19 0.34 0.18 0.50 0.10 0.12 0.18 0.23

0.00 0.02 0.07 0.15 0.06 0.14 0.04 0.37 0.06 0.35

73.23(0.56) 92.62(0.25) 72.06(0.33) 56.25(0.22) 74.12(0.65) 43.39(0.78) 84.17(0.86) 63.99(0.89) 74.94(0.21) 57.46(1.20)

21.79(0.72) – 18.03(0.17) 30.63(0.47) 17.26(0.68) 44.76(0.42) 9.30(0.68) 9.11(1.06) 17.07(0.33) 17.86(1.57)

– 1.51(0.16) 4.59(0.16) 10.21(0.24) 3.88(0.88) 9.20(0.45) 2.69(1.03) 25.69(1.26) 4.29(0.21) 23.69(2.00)

0.00(0.05) 0.00(0.03) 0.01(0.03) 0.01(0.02) 0.01(0.05) 0.02(0.05) 0.01(0.03) 0.68(0.03) 0.01(0.02) 0.73(0.39)

5.27(0.14) 4.96(0.01) 4.84(0.03) 2.48(0.08) 5.04(0.15) 2.80(0.20) 3.70(0.20) 0.54(0.16) 3.73(0.10) 0.25(0.20)

100.29 99.09 99.53 99.58 100.31 100.17 99.88 100.01 100.05 99.99

Samples at 2 GPa 1300 °C B535-1 2.0 1300 A1260-1 2.0 1300 B535-2 2.0 1300 A1260-2 2.0 1300 A1260-2 2.0 1300 A1260-3 2.0 1300 A1260-3 2.0 1300 B535-3 2.0 1300 B535-3 2.0 1300 B535-4 2.0 1300 B535-4 2.0 1300

10.0 4.0 10.0 4.0 4.0 4.0 4.0 10.0 10.0 10.0 10.0

20 50 20 50 50 50 50 20 20 20 20

18 30 34 29 14 30 10 38 41 28 28

0.00 0.33 0.50 0.19 0.22 0.09 0.12 0.11 0.14 0.18 0.19

0.03 0.17 0.15 0.12 0.34 0.05 0.37 0.09 0.37 0.10 0.35

92.52(0.74) 56.67(0.94) 43.05(1.93) 70.77(0.75) 57.41(0.64) 82.68(0.52) 63.84(2.52) 79.35(1.16) 62.96(1.41) 73.65(0.87) 60.33(1.41)

– 29.57(0.68) 45.10(2.24) 17.30(0.32) 17.04(1.59) 8.65(0.20) 9.36(0.52) 10.52(0.31) 11.11(0.89) 17.46(1.41) 15.30(1.71)

1.89(0.48) 11.16(1.26) 10.14(2.31) 8.05(0.75) 22.82(4.55) 3.67(0.37) 25.42(3.03) 6.29(1.11) 25.34(2.70) 6.67(1.44) 23.26(1.41)

0.09(0.05) 0.09(0.08) 0.06(0.12) 0.06(0.04) 0.64(0.16) 0.02(0.07) 0.64(0.12) 0.01(0.04) 0.76(0.17) 0.07(0.07) 0.84(0.26)

4.81(0.15) 2.48(0.16) 2.31(0.67) 3.54(0.15) 1.03(0.25) 4.14(0.12) 0.41(0.59) 3.58(0.25) 0.44(0.25) 2.25(0.23) 0.42(0.13)

99.13 99.97 100.66 99.73 98.94 99.16 99.67 99.75 100.60 100.10 100.15

Samples at 2 GPa 1200 °C A1049-1 2.0 1200 B525-1 2.0 1200 B525-1 2.0 1200 B525-2 2.0 1200 A1019 2.0 1200 B525-3 2.0 1200 B525-3 2.0 1200 B524-4 2.0 1200 A1044-1 2.0 1200 A1044-2 2.0 1200 A1044-2 2.0 1200

18.0 24.0 24.0 24.0 41.0 24.0 24.0 24.0 24.0 24.0 24.0

20 20 20 20 20 20 20 50 20 20 20

30 30 12 58 15 40 20 48 42 40 28

0.00 0.09 0.12 0.20 0.50 0.09 0.12 0.27 0.24 0.13 0.17

0.03 0.08 0.39 0.12 0.19 0.08 0.37 0.18 0.17 0.06 0.38

92.38(0.30) 80.83(1.01) 62.56(1.57) 70.20(1.96) 41.69(1.45) 81.43(0.55) 64.93(2.18) 61.45(0.61) 65.89(0.84) 80.29(0.74) 59.73(1.16)

– 8.61(0.29) 9.25(1.21) 17.92(2.18) 43.81(1.64) 8.96(0.43) 9.01(1.73) 24.15(1.24) 21.38(1.29) 12.23(0.49) 12.86(1.57)

1.95(0.20) 5.56(1.18) 26.58(2.04) 8.02(1.50) 12.45(2.34) 5.02(0.70) 25.06(7.36) 11.56(1.39) 11.03(1.23) 3.94(0.76) 25.80(1.72)

0.07(0.03) 0.13(0.02) 1.13(0.33) 0.15(0.04) 0.12(0.22) 0.02(0.01) 0.31(0.16) 0.27(0.10) 0.22(0.07) 0.18(0.02) 0.64(0.16)

5.57(0.31) 4.22(0.34) 0.23(0.19) 3.60(0.61) 1.97(0.37) 3.78(0.27) 0.20(0.20) 1.98(0.14) 1.96(0.17) 4.22(0.30) 0.13(0.19)

99.97 99.35 99.75 99.89 100.03 99.21 99.51 99.41 100.48 100.86 99.16

T (°C)

A1047-2 B519-2 B498 B519-3 B519-4

2.0 2.0 2.0 2.0 2.0

1600 1600 1600 1600 1600

D (hrs) 2.0 0.5 1.0 0.5 0.5

C-rich*

C-rich S-rich C-rich S-rich C-rich*

C-rich S-rich C-rich S-rich C-rich S-rich C-rich S-rich

C-rich S-rich

C-rich S-rich

C-rich S-rich

73

nb

P (GPa)

Z. Zhang et al. / Geochimica et Cosmochimica Acta 225 (2018) 66–79

Ba (mm)

No.

0.07 0.10 0.07 0.09 0.19 0.16 0.21 0.11 0.15 0.18

0.10 0.38 0.09 0.41 0.26 0.10 0.35 0.05 0.38 0.29

82.09(1.99) 64.46(1.66) 82.61(3.41) 64.18(1.86) 65.12(1.43) 75.14(0.97) 59.43(1.95) 81.89(0.84) 61.22(2.97) 64.60(1.86)

6.85(0.66) 7.89(0.90) 6.55(0.99) 6.96(0.73) 15.98(1.11) 15.56(0.71) 16.17(1.76) 10.14(0.48) 11.45(1.55) 14.80(1.33)

6.80(1.94) 26.27(2.21) 5.97(2.10) 28.64(1.68) 17.63(1.54) 6.23(0.81) 23.41(1.57) 3.62(0.53) 25.71(2.01) 19.14(2.22)

0.04(0.16) 0.96(0.48) 0.03(0.20) 0.88(0.60) 0.49(0.19) 0.04(0.03) 0.66(0.11) 0.17(0.09) 0.67(0.23) 0.04(0.12)

4.07(0.30) 0.30(0.10) 4.13(0.20) 0.24(0.06) 1.70(0.09) 2.82(0.17) 0.24(0.08) 4.39(0.26) 0.26(0.17) 1.03(0.31)

99.85 99.88 99.29 100.90 100.92 99.79 99.91 100.21 99.31 99.61

0.21 0.00 0.00 0.00 0.00 0.19 0.09 0.18 0.34 0.51

0.00 0.04 0.39 0.05 0.37 0.09 0.07 0.15 0.16 0.16

74.86(0.54) 92.08(0.58) 71.42(1.41) 92.71(0.80) 71.71(1.02) 72.74(0.59) 81.88(0.91) 70.28(0.60) 56.52(0.64) 41.23(1.17)

20.32(0.55) – – – – 17.57(0.27) 8.70(0.10) 16.72(0.42) 30.26(1.25) 45.22(1.20)

– 2.67(0.21) 27.12(1.32) 3.53(0.37) 25.92(0.77) 6.05(0.36) 4.44(0.74) 10.55(0.48) 10.90(0.91) 10.85(1.57)

0.15(0.08) 0.19(0.03) 0.74(0.03) 0.16(0.04) 0.75(0.12) 0.19(0.07) 0.13(0.03) 0.14(0.01) 0.29(0.08) 0.17(0.03)

5.39(0.38) 4.81(0.40) 0.42(0.09) 5.18(0.06) 0.77(0.16) 3.68(0.16) 4.05(0.10) 3.20(0.28) 2.72(0.38) 2.62(0.19)

100.72 99.75 99.70 101.57 99.15 100.23 99.21 100.88 100.69 100.09

12 12 16 14

0.21 0.00 0.48 0.22

0.00 0.09 0.15 0.23

74.18(0.22) 89.08(0.27) 44.65(0.34) 63.54(0.58)

20.88(0.22) – 43.31(0.34) 18.52(0.58)

– 6.33(0.12) 10.31(0.90) 15.06(0.73)

0.20(0.12) 0.26(0.08) 0.17(0.01) 0.38(0.07)

5.37(0.23) 4.92(0.05) 2.66(0.09) 1.84(0.09)

100.62 100.59 101.09 99.33

15 14 10 12

0.19 0.00 0.49 0.22

0.00 0.13 0.16 0.22

76.01(0.81) 87.31(0.68) 43.09(0.80) 63.88(0.62)

18.71(0.81) – 43.56(0.80) 18.69(0.62)

– 8.89(0.15) 10.43(0.77) 14.89(0.29)

0.34(0.03) 0.20(0.03) 0.08(0.03) 0.23(0.03)

5.59(0.33) 4.31(0.46) 2.75(0.13) 2.04(0.34)

100.65 100.71 99.92 99.73

Z. Zhang et al. / Geochimica et Cosmochimica Acta 225 (2018) 66–79

1200 1200 1200 1200 1200 1200 1200 1200 1200 1200

24.0 24.0 24.0 24.0 18.0 24.0 24.0 24.0 24.0 24.0

10 50 50 50 20 20 20 20 20 50

25 58 21 34 34 40 28 44 38 51

Samples at 3.5 GPa 1400 °C A1257-1 3.5 1400 A1258-1 3.5 1400 A1258-1 3.5 1400 A1257-2 3.5 1400 A1257-2 3.5 1400 A1257-3 3.5 1400 A1258-2 3.5 1400 A1257-4 3.5 1400 A1258-3 3.5 1400 A1258-4 3.5 1400

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

50 50 10 50 50 50 50 50 50 50

15 12 15 12 16 14 16 15 15 14

Samples at 5 GPa 1600 °C M786-1 5.0 1600 M786-2 5.0 1600 M786-3 5.0 1600 M786-4 5.0 1600

1.0 1.0 1.0 1.0

50 50 50 50

Samples at 7 GPa 1600 °C M789-1 7.0 1600 M789-2 7.0 1600 M789-3 7.0 1600 M789-4 7.0 1600

11.0 1.0 1.0 1.0

50 50 50 50

a

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

74

B524-1 B524-1 B525-3 B525-4 A1049-2 A1044-3 A1044-3 A1044-4 A1044-4 B524-2

C-rich S-rich C-rich S-rich C-rich S-rich C-rich S-rich

C-rich S-rich C-rich S-rich

B is beam size. n is the number of analysis. c YNi is Ni/(Ni + Fe) molar ratio in Fe-Ni-S melts. d XS is S/(Ni + Fe+S) molar ratio in Fe-Ni-S melts. e One sigma uncertainties reported in parentheses. * Coexisting S-rich or C-rich melt was also present, but not analyzable owing to dimensions or polishing damage. Those experiments without indications of ‘‘S-rich” or ‘‘C-rich” produced only one homogeneous melt. b

Z. Zhang et al. / Geochimica et Cosmochimica Acta 225 (2018) 66–79

75

Fig. 7. Carbon solubility in Fe-Ni-S melts as a function of sulfur mole fraction, XS, and metal/sulfur atomic ratio, M/S. (A) All samples at various P-T conditions. (B) Variations of C solubility with symbols keyed to Ni content. Curves in (B) are empirical fits to the data according to Eq. (3). The thickness of the curves in (B) corresponds to error margin calculated from analytical uncertainties shown in (A).

gest an empirical parameterization of carbon solubility (CFe-Ni-S in wt.%) as a function of both YNi and XS: C FeNiS ¼ a  ð0:5  X S Þ2  ðb  Y Ni Þ þ 0:2 ð0 < X S 6 0:5Þ; ð3Þ

Fig. 8. Compilation of experimental studies of C concentrations in graphite-saturated Fe-Ni-S melts as a function of S mole fraction (XS). Experimental results from this and other studies are plotted with different symbols, with errors in most cases smaller than the width of the symbols. The abbreviations in the symbol key correspond to the following sources: W91-Wang et al. (1991); W93-Wood (1993); T01-Tsymbulov and Tsemekhman (2001); C06Chabot et al. (2008); C08-Corgne et al. (2008); D09-Dasgupta et al. (2009); DL13-Deng et al. (2013); DR13-Dasgupta et al. (2013); T15-Tsuno and Dasgupta (2015); Z15-Zhang et al. (2015), Li et al. (2016). The C solubility observations that are consistent with the broad trend defined by our results are from experiments with wide ranges in YNi, temperature (920–1700 °C), and pressure (100 kPa– 20 GPa).

on pressure and temperature, but also YNi. As most natural mantle sulfide melts have YNi >0.2, we do not expect sulfide-carbide immiscibility in most instances in the mantle. For S-poor melts, Ni diminishes the solubility of C (Hastings, 2012; Rohrbach et al., 2014). But as illustrated in Fig. 7B, Ni/Fe ratios influence solubility less as S concentrations increases. Because increasing Ni content narrows the sulfide-carbide melt miscibility gap, more Ni-rich sulfide melts have increased tolerance for C. This effect and the effect of Ni on solubility in metal-rich systems trade off, such that the net reduction of Ni on C solubility becomes less pronounced in sulfur-rich melts (Fig. 7B). Here, we sug-

where the values of a and b correspond to the following two conditions. (1) a1 = 8.2 and b1 = 2.8 with the incorporation of experimental data from previous studies (100 kPa–20 GPa and 920–1700 °C) plotted in Fig. 8. (2) a2 = 5.9 and b2 = 3.2, with the experimental data from this study (2–7 GPa and 1200–1600 °C) plotted in Fig. 7A. This relation reflects several key aspects of the experimental results: (1) CFe-Ni-S diminishes in approximate proportion to the square of XS; (2) Ni strongly reduces C solubility when XS is low, but has only a modest effect when XS is high; (3) carbon solubility decreases to a low value (0.2 wt.%) at near M/S = 1, but is measurably greater than the carbon detection limit. This equation was fit solely from experimental results from this study because inter-laboratory analytical variations in C contents are large (Fig. 8), obscuring systematic relationships. 4.4. Carbon storage in Fe-Ni-S melts and diamond precipitation from sulfide melts The empirical carbon solubility relation given by Eq. (3) alloys calculation of the C/S ratio of a sulfide melt with a given composition (Xs and YNi). Combined with a fixed bulk S content of a mantle rock, the sulfide melt C/S ratio allows estimation of the mass of carbon potentially stored in the Fe-Ni-S melts in various parts of the mantle. Estimates for the average mantle sulfur abundances derived chiefly from analyses of peridotite or PGE-S systematics of basalt fall in the range of 190–250 ppm (Morgan, 1986; McDonough and Sun, 1995; Be´zos et al., 2005; Wang and Becker, 2013). In contrast, estimates based on S/Dy ratios of mid-ocean ridge basalts are uniformly lower, in the range of 100–150 ppm (Saal et al., 2002; Salters and Stracke, 2004; Le Roux et al., 2006; Shimizu et al., 2016). However, the low platinum group element (PGE) concentrations of MORB require that that most

76

Z. Zhang et al. / Geochimica et Cosmochimica Acta 225 (2018) 66–79

Fig. 9. (A) Carbon versus Ni and (B) Sulfur versus Ni concentrations (Fe ± Ni)-S-C melts illustrating the region of immiscibility (shaded regions) between C-rich and S-rich melts at low Ni concentrations. At mantle conditions, carbide and sulfide melts exhibit complete miscibility above
18 wt.% Ni. Experimental data are from this study at 1200–1600 °C and 2–7 GPa and Tsymbulov and Tsemekhman (2001) (Tsy1400 ° C) at 1400 °C and 100 kPa.

sources maintain sulfide saturation during melting (Be´zos et al., 2005) and comparison of S concentrations in MORB to sulfur solubilities indicate that sulfide is mostly saturated in the MORB source as well as during MORB differentiation (Wallace and Carmichael, 1992). Therefore, S should not behave according to Henry’s Law during MORBsource partial melting and fractionation and so there is reason to doubt whether S/Dy ratios of MORB reflect that of their source. In the following calculations, we consider mantle with S contents between 100 and 300 ppm. Inclusions trapped in olivine and diamond indicate that the sulfide in the mantle from depths <250 km has XS
0.4–0.6 and YNi
0.2–0.6 (Eggler et al., 1993; Bulanova et al., 1996; Westerlund et al., 2006; Aulbach et al., 2009; Lorand et al., 2013). In the presence of reduced carbon, such compositions are molten or partially molten in the shallow asthenosphere and the deeper parts of the continental lithosphere (Zhang et al., 2015). For these compositions, experimental constraints suggest that the C/S of C-saturated Fe-Ni-S melts is <0.01 and so for a bulk mantle S abundance of 100–300 ppm, this corresponds to <3 ppm C storage in Fe-Ni-S melts. This is a small fraction of the C in the lithosphere (
100 ppm, as judged from a global survey of mantle xenoliths, Deines, 2002) or in the depleted asthenosphere (10–30 ppm; e.g., Dasgupta and Hirschmann, 2010; Michael and Graham, 2015; Rosenthal et al., 2015). Consequently, reduced carbon occurs chiefly as graphite/diamond at these depths. In the deeper portions of the upper mantle, where conditions become more reduced relative to the IW and NiNiO buffers, C storage in sulfide increases. The diminishing fO2 causes the activities of Fe and Ni increase according to the reactions, Fe2 SiO4 ¼ ðolivineÞ

Fe

ðFeNiS meltÞ

1 þ FeSiO3 þ O2 ; 2 ðopxÞ

ð4Þ

Ni2 SiO4 ¼ ðolivineÞ

Ni

ðFeNiS meltÞ

1 þ NiSiO3 þ O2 2 ðopxÞ

ð5Þ

The oxygen produced by reactions (4) and (5) is expressed as increased Fe2O3 in garnet or other silicates (Frost et al., 2004; Rohrbach et al., 2007). The resulting increase in Fe and Ni at more reduced conditions produces increased M/S in the metal-rich sulfide melt and so enhances higher carbon storage. For example, metal-rich sulfide melts with M/S ratios of 4–15 have been documented as in sub-lithospheric diamonds originating from 360 to 750 km depth (Smith et al., 2016). Additionally, the YNi in sulfide melts decreases with diminishing fO2, as more Fe is converted to metal in the sulfide than Ni. In the following exercise, carbon solubility is calculated at YNi = 0.2 using Eq. (3). As the available S is fixed by mantle bulk composition, greater Fe and Ni in the melt increases the mass of sulfide present. For example, for mantle with 200 ppm S, a FeNi sulfide melt with M/S = 1 or S  35 wt.% will constitute a mode of 0.05 wt.%, but a metal-enriched melt (M/S = 10, S  5 wt.%) will have a mode close to 0.4%. Combined with the greatly enhanced C solubility in liquids with high M/S (Fig. 8), the increased mode of sulfide results in substantial C storage in sulfide melts. For mantle with 100–300 ppm S, sulfide can become a significant or predominant host of C in reduced mantle. As shown in Fig. 10, sulfide could be the sole carbon host in depleted mantle with 10–30 ppm C, if M/S were >3 (Xs < 0.25). Sulfide melt could also be significant hosts of C in more C-rich regions of the mantle, such as sources of oceanic island basalts with 30–500 ppm C (Dasgupta and Hirschmann, 2010; Rosenthal et al., 2015). For example, under conditions sufficiently reducing such that M/S > 9 or Xs > 0.1, as may occur in the lower mantle where Fe-rich alloy is stable (Frost et al., 2004), a metal-rich sulfide melt in a mantle source with 200 ppm S

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5. CONCLUDING REMARKS

Fig. 10. Carbon storage in Fe-Ni-S melts in the reduced mantle as a function of XS in Fe-Ni-S melts. Carbon storage curves are calculated in three steps: (i) the carbon solubility in Fe-Ni-S is calculated using Eq. (3) and YNi = 0.2; (ii) at a given XS, the C/S weight ratio is calculated from the carbon solubility and sulfur content of the Fe-Ni-S melt (iii) carbon storage in the bulk rock is calculated from C/S given mantle concentrations of S = 100, 200 or 300 ppm. For MORB-source mantle with 10–30 ppm C, all of the C will be stored in metal-rich sulfide melt for M/S ratios >3. For OIB-source mantle with 30–500 ppm C, substantial fractions of C will be stored in metal-rich sulfide melt with high M/S. For example, for mantle with 100 ppm C and 200 ppm S, all C is in metal-rich sulfide melt if M/S > 7.6. The arrows show schematically the effect of oxidation, resulting in decreased M/S on C solubility. Such decreases could lead to precipitation of macrodiamonds from metal-rich sulfide melt.

may store up to 126 ppm C. However, in cases where mantle is very reduced, the high Fe activity in metal-rich sulfide melt may cause Fe-Ni solid precipitation. Metal-rich sulfide melts are possible sources for precipitation of macrodiamonds, particularly in the sublithospheric mantle (Tsuno and Dasgupta, 2015; Smith et al., 2016). Different than lithospheric sulfide melt, sub-lithospheric sulfide melt could have high M/S and therefore high carbon solubility (Figs. 7 and 9). These reduced metal-rich sulfide melts with high M/S and high dissolved C could be subjected to more oxidizing conditions either because of the effects of changing pressure on silicate-sulfide equilibrium (e.g., Eqs. (4) and (5)), or because it migrates from a comparatively reduced region to more oxidizing lithology, as might occur if the melt came into contact with a eclogitic, pyroxenitic, or peridotitic domain that has been influenced by subduction. Such a change in fO2 would lower the metal activity (Eqs. (4) and (5)), and drive the melt to a lower M/S ratio. This would both diminish the total mass of sulfide liquid and decrease the C solubility, as illustrated by the arrows in Fig. 10. The combined effects could lead to precipitation of diamond. For example, for a rock with 200 ppm S, a metal-rich sulfide melt beginning with a composition XS of 0.08 oxidizing to a composition equal to XS =0.18 would precipitate 131 ppm C (relative to the bulk rock) as diamond. Such a process could account for the large macrodiamonds of sublithospheric origin associated with eclogite (Smith et al., 2016).

High-pressure experiments varying XNi and M/S in sulfide melt saturated in graphite suggest that: (1) the miscibility gap between S-rich and Fe-C-rich liquids at high pressure closes above 18 wt.% Ni, (2) M/S exerts a strong control on carbon solubility in Fe-Ni-S melts, and (3) increased Ni diminishes carbon solubility, but chiefly when XS is low. Application of an empirical model for carbon solubility as a function of XS and wt.% Ni suggests that in the deep upper mantle metal-rich sulfide melt is the sole host of carbon in the depleted mantle (10–30 ppm C) if M/S > 3 (XS < 0.25) and could be a principal host of C (up to 126 ppm C with XS = 0.1 at S = 200 ppm) in more C-rich regions, such as the sources of oceanic island basalts. For deep upper mantle or transition zone domains more carbon-rich than the carbon solubility of Fe-Ni-S melts, the residual carbon is present as diamond. For highly reduced mantle domains possible in the lower mantle, FeNi-S melt is the principal host of C, and could coexist with metallic alloy. ACKNOWLEDGEMENTS We thank Tony Withers and Jed Mosenfelder for assistance with high pressure experiments and Fred Davis for discussions. We are grateful for the constructive reviews from Arno Rohrbach and two anonymous reviewers. The first author benefited from a University of Minnesota Doctoral Dissertation Fellowship and a Scripps Institution of Oceanography Postdoctoral Fellowship during completion of this work. We gratefully acknowledge the support of grants NSF EAR1119295 and EAR1426772.

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