# Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion

Heng Chen, Bach Mai Nguyen, Frédéric Moynier

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Heng Chen, Bach Mai Nguyen, Frédéric Moynier. Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion. Meteoritics and Planetary Science, 2013, 48 (12), pp.2441-2450. 10.1111/maps.12229. insu-02918073

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# Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion

Heng CHEN<fnref n="1" />, Bach Mai NGUYEN<fnref n="1" />, and Frédéric MOYNIER<fnref n="1" />,<fnref n="2" />*

*(Received 29 November 2012; revision accepted 12 October 2013)*

**Abstract**–High-precision Zn isotopic compositions measured by MC-ICP-MS are documented for 32 iron meteorites from various fractionally crystallized and silicate-bearing groups. The δ⁶⁶Zn values range from –0.59‰ up to +5.61‰ with most samples being slightly enriched in the heavier isotopes compared with carbonaceous chondrites (0 < δ⁶⁶Zn < 0.5). The δ⁶⁶Zn versus δ⁶⁸Zn plot of all samples defines a common linear fractionation line, which supports the hypothesis that Zn was derived from a single reservoir or from multiple reservoirs linked by mass-dependent fractionation processes. Our data for Redfields fall on a mass fractionation line and therefore refute a previous claim of it having an anomalous isotopic composition due to nonmixing of nucleosynthetic products. The negative correlation between δ⁶⁶Zn and the Zn concentration of IAB and IIE is consistent with mass-dependent isotopic fractionation due to evaporation with preferential loss of lighter isotopes in the vapor phase. Data for the Zn concentrations and isotopic compositions of two IVA samples demonstrate that volatile depletion in the IVA parent body is not likely the result of evaporation. This is important evidence that favors the incomplete condensation origin for the volatile depletion of the IVA parent body.

## INTRODUCTION

Iron meteorites are mostly composed of metallic Fe and Ni, and are enriched in siderophile elements compared with chondrites (Mittlefehldt et al. 1998). They are classified into different groups according to their chemical compositions, especially the concentrations of Ga, Ge, and Ir, such that each group of meteorites is presumed to belong to one distinct parent body (Scott and Wasson 1976; Kracher et al. 1980; Wasson et al. 1980, 2007; Malvin et al. 1984; Wasson and Huber 2006). However, on the basis of their formation processes, the groups are often divided into two categories: magmatic groups and nonmagmatic groups. The magmatic groups (e.g., IIAB, IICD, IIIAB, IVA), which are largely free of silicates, display smooth elemental trends consistent with a fractional crystallization of a slowly cooling magma, and are thought to represent cores of asteroids, which were formed by fractional-crystallization processes (Scott

and Wasson 1975, 1976; Haack and Scott 1993). The silicate-bearing groups, which include IAB, IIE, and IIICD, contain abundant silicates and their elemental variations could not be explained by simple fractional crystallization. The origin for silicate-bearing irons is still debated, and models include unique core crystallization involving complex fractional crystallization, nucleation effects, and liquid immiscibility (McCoy et al. 1993), forming as pools of impact-produced melt near the base of a mega-regolith on a chondritic body (Wasson and Wang 1986), and breakup and reassembly of a partially differentiated object at its peak temperature (Benedix et al. 2000). It is clear that metal in the IAB, IIE, and IIICD groups was once molten; therefore, to avoid ambiguity, the term “silicate-bearing groups” has been adopted to substitute for “non-magmatic groups” in recent years; likewise “fractionally crystallized groups” has been adopted for “magmatic groups” (Haack and McCoy 2003; Goldstein et al. 2009).

<footnote n="1">¹Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA</footnote>

<footnote n="2">²Institut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, Paris 75005, France</footnote>

<footnote>*Corresponding author. E-mail: moynier@ipgp.fr</footnote>
The study of chemical compositions in iron meteorites has been used in the past to constrain the physical processes which led to the formation of these samples (Scott et al. 1973; Scott and Wasson 1976; Wasson et al. 1980; Malvin et al. 1984; Albarède et al. 2013). The recent development of multiple-collector inductively coupled-plasma mass-spectrometry (MC-ICP-MS) has allowed for high-precision measurements of nontraditional stable isotopes. These isotope ratios have been used as new tracers to study planetary formation and differentiation: Si (Georg et al. 2007; Fitoussi and Bourdon 2012; Pringle et al. 2013; Savage and Moynier 2013), Fe (Poitrasson et al. 2004; Wang et al. 2012), Ni (Moynier et al. 2007), Cu (Luck et al. 2003; Williams and Archer 2011; Bishop et al. 2012), Zn (Luck et al. 2005; Moynier et al. 2007; Paniello et al. 2012a), Ge (Luais 2007), Cd (Wombacher et al. 2008), and Sr (Moynier et al. 2010).

Zn is a lithophile element, which is preferentially retained in the silicate phases during planetary differentiation; therefore, Zn concentrations in fractionally crystallized irons are generally very low (<5 ppm) compared with the carbonaceous chondrites (>50 ppm). Models of Earth's chemical composition assert that Zn is present in only negligible quantities in the core (McDonough and Sun 1995). Zn is also a moderately volatile element with a condensation temperature of approximately 726 K (Lodders 2003). It has five stable isotopes: ⁶⁴Zn (49.17%), ⁶⁶Zn (27.73%), ⁶⁷Zn (4.04%), ⁶⁸Zn (18.45%), and ⁷⁰Zn (0.61%). In terrestrial rocks, Zn shows a narrow range of isotopic variations (Chen et al. 2013), indicating that isotopic fractionation during magmatic differentiation is rather limited (no more than 0.05 permil/amu for basaltic magma; Chen et al. 2013). Volatilization is the most important planetary formation process that can significantly fractionate isotopes with preferential loss of the light isotopes in the gas phase, leading to mass-dependent isotopic variability (Albarède 2004; Paniello et al. 2012a; Chen et al. 2013). Previous studies have used Zn isotope ratios to determine the impact-induced volatilization (Moynier et al. 2006; Paniello et al. 2012a, 2012b). Comparing the Zn isotope compositions in iron meteorites may help further our understanding of the physical and chemical conditions involved in the variation in the volatile element abundance among different groups. So far, all Zn isotopic fractionations measured in bulk solar system materials have followed a mass-dependent law (Moynier et al. 2009). One remarkable exception is the Redfields iron meteorite, for which Ghidan and Loss (2011) reported surprisingly large isotopic anomalies. These isotopic anomalies were interpreted to be the consequences of an incomplete mixing of nucleosynthetic products, similar to what

have been observed in the unusual FUN refractory inclusions from the Allende chondrite (Völkening and Papanastassiou 1990). However, such large isotopic anomalies are unexpected in differentiated materials such as iron meteorites; therefore, this observation, if correct, would have important consequences on our understanding of the homogenization of nucleosynthetic products and the preservation of isotopic anomalies during planetary differentiation.

Presently, there are very few works that focus on the isotopic composition of Zn in iron meteorites (Luck et al. 2005; Ghidan and Loss 2011) and none has measured the composition of the Zn-poor IVA group. This is mostly because of the very low Zn concentrations (few ppm) in most iron meteorites and even down to 0.1 ppm in the IVA group. Our goal in this work is to study high-precision Zn isotopic compositions of a variety of iron meteorite groups including IVAs to learn more about the planetary processes that had operated on their parent bodies and about the origin of the volatile depletion observed in some iron groups.

## SAMPLES AND ANALYTICAL METHODS

### Sample Description

A total of 32 samples from 10 meteorite groups were analyzed by MC-ICP-MS: seven IAB, four IIC, three IIB, six IID, three IIE, one IIF, three IIIAB, one IIIE, one IIIF, two IVA, and one ungrouped (see Table 1). These meteorite samples were obtained in the form of metal chips. For all the irons, large chips with no visible sulfides and silicate inclusions were selected. Some of these samples were cut in half for Cu and Zn isotope analysis, respectively, and the Cu isotope compositions were reported in Bishop et al. (2012). Zn data of Campo del Cielo, Casas Grandes, and Charcas obtained from Luck et al. (2005) were incorporated to further characterize the group IAB and IIIAB.

### METHODS

To extract an adequate amount of Zn (>300 ng) for isotopic analysis, 0.2 up to 2.0 g chips were obtained from each sample based on the Zn concentration estimated from previous literature. As low-temperature alteration and biological processes could generate detectable Zn isotopic fractionation (Albarède 2004), our samples were specifically selected from polished interior slices, free from fusion crust or from oxidation products. Metallic chips were first cleaned in diluted HCl, and then in deionized water in an ultrasonic bath to get rid of surface contamination. Cleaned samples
<table><caption>Table 1. Isotopic compositions and concentrations of Zn in iron meteorites. The isotopic composition is reported using the δ notation (parts per 1000 deviation from a terrestrial Zn standard JMC 3-0749 L), while the Zn concentrations are in ppm. The external reproducibility (2δ) of δ<sup>66</sup>Zn, δ<sup>67</sup>Zn, and δ<sup>68</sup>Zn are 0.03, 0.05, and 0.07‰, respectively. Cu isotopic compositions (‰) and Zn concentrations from literatures are also listed for comparison.</caption><thead><tr><th>Name</th><th>Museum Code</th><th>Group</th><th>δ<sup>66</sup>Zn</th><th>δ<sup>67</sup>Zn</th><th>δ<sup>68</sup>Zn</th><th>[Zn] ppm</th><th>Lit. [Zn] ppm</th><th>Lit. δ<sup>65</sup>Cu<sup>a</sup></th></tr></thead><tbody><tr><td>Anoka</td><td>ME 980</td><td>IAB</td><td>0.09</td><td>0.19</td><td>0.23</td><td>44.6</td><td></td><td>-0.26</td></tr><tr><td>Canyon Diablo</td><td>ME 3045</td><td>IAB</td><td>1.88</td><td>2.83</td><td>3.75</td><td>24.8</td><td>22.7<sup>b</sup></td><td>23.88<sup>c</sup></td></tr><tr><td>Wolsey</td><td>ME 3200</td><td>IAB</td><td>2.62</td><td>3.94</td><td>5.24</td><td>23.2</td><td></td><td></td></tr><tr><td>Odessa</td><td>ME 2388</td><td>IAB</td><td>1.96</td><td>2.98</td><td>3.91</td><td>22.2</td><td>13-26<sup>c</sup></td><td>29<sup>d</sup></td></tr><tr><td>Cosby's Creek</td><td>BM 19938</td><td>IAB</td><td>0.46</td><td>0.72</td><td>0.90</td><td></td><td></td><td>0.17</td></tr><tr><td>Deport</td><td>M 1.25</td><td>IAB</td><td>1.55</td><td>2.34</td><td>3.10</td><td>26.3</td><td></td><td>0.02</td></tr><tr><td>Carlton</td><td>ME 880</td><td>IAB</td><td>0.51</td><td>0.79</td><td>1.03</td><td>40.0</td><td></td><td>-0.3</td></tr><tr><td>Campo del Cielo<sup>b</sup></td><td></td><td>IAB</td><td>3.40</td><td>5.08</td><td>6.74</td><td></td><td></td><td>0.28</td></tr><tr><td>Kumerina</td><td>OHWAM 8506.6</td><td>IIC</td><td>0.17</td><td>0.34</td><td>0.37</td><td>0.3</td><td>0.38<sup>c</sup></td><td>0.22<sup>d</sup></td></tr><tr><td>Perryville</td><td>USNM 428</td><td>IIC</td><td>0.18</td><td>0.52</td><td>0.40</td><td>0.1</td><td></td><td>-1.06</td></tr><tr><td>Wiley</td><td>BM 1859,914</td><td>IIC</td><td>0.35</td><td>0.57</td><td>0.68</td><td>0.2</td><td></td><td>-1.23</td></tr><tr><td>Ballinoo</td><td>ME 980</td><td>IIC</td><td>-0.07</td><td>-0.15</td><td>-0.14</td><td>0.5</td><td>0.49<sup>d</sup></td><td>-0.44</td></tr><tr><td>DRPA 78008</td><td></td><td>IIB</td><td>0.20</td><td>0.31</td><td>0.41</td><td>0.4</td><td></td><td></td></tr><tr><td>DRPA 78009</td><td></td><td>IIB</td><td>0.26</td><td>0.47</td><td>0.55</td><td>0.5</td><td></td><td></td></tr><tr><td>DRPA 78005</td><td></td><td>IIB</td><td>0.66</td><td>1.04</td><td>1.37</td><td>0.3</td><td></td><td></td></tr><tr><td>Needles</td><td>USNM 43533</td><td>IID</td><td>1.17</td><td>1.76</td><td>2.27</td><td>2.2</td><td></td><td></td></tr><tr><td>Carbo</td><td>USNM 838</td><td>IID</td><td>1.56</td><td>2.28</td><td>3.01</td><td>2.5</td><td></td><td>-1.49</td></tr><tr><td>Wallapai</td><td>USNM 788</td><td>IID</td><td>1.08</td><td>1.57</td><td>2.11</td><td>0.4</td><td></td><td>-1.47</td></tr><tr><td>Elbogen</td><td>NHMV</td><td>IID</td><td>0.97</td><td>1.41</td><td>1.86</td><td>1.5</td><td></td><td></td></tr><tr><td>Bridgewater</td><td></td><td>IID</td><td>1.48</td><td>2.20</td><td>2.90</td><td>2.2</td><td></td><td></td></tr><tr><td>Arltunga</td><td>MET 43 AD</td><td>IID</td><td>0.85</td><td>1.18</td><td>1.57</td><td>2.67</td><td></td><td>-0.4</td></tr><tr><td>Watson</td><td>ME 3195</td><td>IIE</td><td>5.61</td><td>8.45</td><td>11.16</td><td>0.8</td><td></td><td>-0.57</td></tr><tr><td>Weekeroo Station</td><td>ASU 133a</td><td>IIE</td><td>2.08</td><td>3.14</td><td>4.20</td><td>1.1</td><td></td><td>-0.81</td></tr><tr><td>Arlington</td><td>USNM 3281</td><td>IIE</td><td>2.19</td><td>3.23</td><td>4.32</td><td>1.05</td><td></td><td></td></tr><tr><td>Monahans</td><td>ASU 256.13</td><td>IIF</td><td>0.17</td><td>0.15</td><td>0.32</td><td>11.48</td><td></td><td>-1.34</td></tr><tr><td>Henbury</td><td>ASU 193.156</td><td>IIIAB</td><td>1.38</td><td>2.14</td><td>2.77</td><td>1.44</td><td>1.2<sup>d</sup></td><td>-0.11</td></tr><tr><td>Gnowangerup</td><td>OHWAM 13580</td><td>IIIAB</td><td>0.46</td><td>0.32</td><td>0.77</td><td>34.83</td><td></td><td></td></tr><tr><td>Casas Grandes<sup>b</sup></td><td></td><td>IIIAB</td><td>-0.59</td><td>-0.8</td><td>-1.08</td><td>10.9</td><td></td><td>-0.46</td></tr><tr><td>Charcas<sup>b</sup></td><td></td><td>IIIAB</td><td>-0.5</td><td>-0.82</td><td>-0.99</td><td>2.2</td><td></td><td>-0.33</td></tr><tr><td>Brainard</td><td>NHMV</td><td>IIIAB</td><td>0.62</td><td>0.96</td><td>1.26</td><td>1.78</td><td></td><td></td></tr><tr><td>Aliskerovo</td><td>M649.1</td><td>IIIE</td><td>0.56</td><td>0.81</td><td>1.1</td><td>0.88</td><td></td><td>-0.23</td></tr><tr><td>Clark County</td><td>USNM 1304</td><td>IIIF</td><td>-0.10</td><td>-0.15</td><td>-0.19</td><td>0.18</td><td></td><td>-1.4</td></tr><tr><td>Gibeon</td><td>NHMV</td><td>IVA</td><td>0.07</td><td>0.15</td><td>0.17</td><td>0.17</td><td>0.07<sup>d</sup></td><td>-0.78</td></tr><tr><td>Yanhuitlan</td><td>M861.1</td><td>IVA</td><td>0.06</td><td>0.08</td><td>0.1</td><td>0.66</td><td></td><td>-1.26</td></tr><tr><td>Redfield</td><td>OHWAM 13142</td><td>UNGR</td><td>0.55</td><td>0.83</td><td>1.10</td><td>6.8</td><td>7.2<sup>c</sup></td><td>-0.86</td></tr></tbody></table>

<sup>a</sup>δ<sup>65</sup>Cu is from Bishop et al. (2012).

<sup>b</sup>Zn isotopic compositions and concentrations are from Luck et al. (2005).

<sup>c</sup>Ghidan and Loss (2011).

<sup>d</sup>Rosman and Delaeter (1974).

Samples are provided by: ME = The Field Museum, Chicago; M = Oscar Monnig Meteorite Collection, Texas Christian University; USNM = United States National Museum, Washington DC; ASU = Arizona State University, Tempe; BM = Natural History Museum, London; NHMV = Natural History Museum, Vienna; OHWAM = Western Australia Museum, Perth.

were placed in double-distilled 6 mol/L HCl in closed Teflon beakers at approximately 120 °C until complete dissolution.

Zinc was extracted from the solution following the anion-exchange chromatography procedure described in Moynier et al. (2006). The dissolved solute was dried under a heating lamp and then redissolved in 1.5 N

HBr. The sample's solution was loaded on to the teflon columns, which were preloaded with AG-1X8 (200-400 mesh) resin and cleaned with alternating cycles of 0.5 HNO<sub>3</sub> and H<sub>2</sub>O. The sample matrices were cleaned up by an additional 1.5 mol L<sup>-1</sup> HBr step, while the remaining Zn was finally collected from the column in 0.5 mol L<sup>-1</sup> HNO<sub>3</sub>. In a similar manner, two more
passes through smaller columns were performed on the collected Zn solution for better purification result, especially to remove any traces of Ni. As in 1.5 mol L⁻¹ HBr, neither Fe nor Ni forms anionic complexes; these elements are not retained by the resin and thus there is no danger of losing Zn due to the saturation of the column (Marechal and Albarede, 2002), even when approximately 2 g of IVA meteorites were loaded. This analytical method allows us to measure the Zn isotopic composition of even the most Zn-poor samples. In addition, the small volumes of acid used in this method could keep a low blank (<10 ng).

All Zn isotope analyses were performed on a Thermo-Finnigan Neptune MC-ICP-MS at Washington University in St. Louis following the procedure described in Paniello et al. (2012a).

Samples were introduced into the mass spectrometer in 0.1 mol L⁻¹ HNO₃ via a CPI PFA nebulizer attached to a glass spray chamber. The Zn concentration in each sample was determined by comparing the voltage peak heights of ⁶⁴Zn with that of a standard with known concentration (200 ppm). Then all samples and standards were adjusted to the same concentration (200 ± 20 ppm) for mass spectrometer analysis. Isotope intensities were collected simultaneously in static mode using a multiple faraday collector array. Thirty integrations of 5 s duration were acquired during sample measurements with an associated internal error on the ⁶⁶Zn/⁶⁴Zn of about 3–6 × 10⁻⁶. Typically, 150 ng Zn was used for each measurement. Ni interferences on mass 64 are corrected using ⁶²Ni, with a mass bias correction applied on the ⁶⁴Ni/⁶²Ni ratio (⁶⁴Ni/⁶²Ni = 0.25467). The Zn isotopic ratios are expressed as parts per 1000 deviations relative to the “Lyon” standard JMC-Lyon, which is the most broadly used standard for normalizing Zn isotopes:

$$ \delta^{x}Zn = \left[ \left( \frac{^{x}Zn}{^{64}Zn_{\text{sample}}} \right) / \left( \frac{^{x}Zn}{^{64}Zn_{\text{JMC-Lyon}}} \right) - 1 \right] \times 1000 $$

with x = 66, 67, or 68.

The internal precision on δ⁶⁶Zn (based on more than three repeat runs of the same sample solution during a single analytical session) is <±0.03% (2 SE). Replicate analyses of the same sample, carried out during different sessions, were performed for most of the samples. To evaluate a more robust external precision, we processed eight or more analyses (full procedural replicates) for some samples with high concentrations such as Anaka and Canyon Diablo. The 2 × standard error (2 SE) of the sample estimated from these replicates gives ±0.03‰ for δ⁶⁶Zn, ±0.05‰ for δ⁶⁸Zn, and ±0.07‰ for δ⁶⁸Zn for Anaka, which is the sample with largest number of replicates (see

<table><caption>Table 2. Zn isotopic composition of Anoka and Canyon Diablo to test precision, total 9 and 8 procedural replicates were prepared and analyzed.</caption><thead><tr><th>Samples</th><th>δ<sup>66</sup>Zn</th><th>δ<sup>67</sup>Zn</th><th>δ<sup>68</sup>Zn</th></tr></thead><tbody><tr><td>Anoka 1</td><td>0.04</td><td>0.10</td><td>0.11</td></tr><tr><td>Anoka 2</td><td>0.10</td><td>0.16</td><td>0.25</td></tr><tr><td>Anoka 3</td><td>0.09</td><td>0.19</td><td>0.22</td></tr><tr><td>Anoka 4</td><td>0.15</td><td>0.30</td><td>0.37</td></tr><tr><td>Anoka 5</td><td>0.01</td><td>0.04</td><td>0.06</td></tr><tr><td>Anoka 6</td><td>0.10</td><td>0.19</td><td>0.23</td></tr><tr><td>Anoka 7</td><td>0.14</td><td>0.24</td><td>0.31</td></tr><tr><td>Anoka 8</td><td>0.10</td><td>0.18</td><td>0.20</td></tr><tr><td>Anoka 9</td><td>0.11</td><td>0.27</td><td>0.28</td></tr><tr><td><strong>Average Anoka</strong></td><td><strong>0.09</strong></td><td><strong>0.19</strong></td><td><strong>0.23</strong></td></tr><tr><td><strong>2se Anoka</strong></td><td><strong>0.03</strong></td><td><strong>0.05</strong></td><td><strong>0.07</strong></td></tr><tr><td>Canyon Diablo 1</td><td>1.85</td><td>2.80</td><td>3.70</td></tr><tr><td>Canyon Diablo 2</td><td>1.85</td><td>2.76</td><td>3.69</td></tr><tr><td>Canyon Diablo 3</td><td>1.86</td><td>2.83</td><td>3.75</td></tr><tr><td>Canyon Diablo 4</td><td>1.93</td><td>2.93</td><td>3.88</td></tr><tr><td>Canyon Diablo 5</td><td>1.92</td><td>2.93</td><td>3.86</td></tr><tr><td>Canyon Diablo 6</td><td>1.88</td><td>2.83</td><td>3.74</td></tr><tr><td>Canyon Diablo 7</td><td>1.81</td><td>2.68</td><td>3.61</td></tr><tr><td>Canyon Diablo 8</td><td>1.91</td><td>2.90</td><td>3.81</td></tr><tr><td><strong>Average Canyon Diablo</strong></td><td><strong>1.88</strong></td><td><strong>2.83</strong></td><td><strong>3.75</strong></td></tr><tr><td><strong>2se Canyon Diablo</strong></td><td><strong>0.03</strong></td><td><strong>0.06</strong></td><td><strong>0.06</strong></td></tr></tbody></table>

se = standard error.

<caption>Table 2). The total blank (<10 ng) is negligible compared with the amount of Zn present in the samples (>300 ng). The long-term uncertainty in concentration is about ±10%.</caption>

## RESULTS

Zinc concentrations are reported in Table 1. The samples are generally in good agreement with those found in previous studies. Moreover, Zn concentrations of the same group members consistently fall within the same range: 22–45 ppm for IAB, 0.1–0.5 ppm for IIB and IIC, 0.4–2.7 ppm for IID, 0.8–1.1 ppm for IIE, 1.4–11 ppm for IIIAB, and 0.17–0.67 ppm for IVA. As expected, members of the silicate-bearing groups IAB and IIE are generally richer in Zn than the fractionally crystallized groups. Gnowangerup (IIIAB) is the only sample that has an abnormally high concentration of Zn (34 ppm) compared with the rest of the fractionally crystallized irons.

The δ⁶⁶Zn and δ⁶⁸Zn measurements of all samples are reported in Table 1 and plotted on Fig. 1. As expected, all samples fall on a mass-dependent fractionation line with a slope of 1.981 (R² = 0.999) on the three-isotope plot. In contradiction to Ghidan and Loss (2011), this study shows that Redfields’ Zn isotopic composition is mass-dependent just like any
<figure><img src="image_2.png" /><figcaption>Fig. 1. δ⁶⁶Zn versus δ⁶⁸Zn for different groups of iron meteorites. All samples fall onto the line of mass-dependent fractionation (with slope of 1.98). UNGR = Ungrouped meteorites.</figcaption></figure>

other bulk meteorite analyzed so far. The ranges of δ⁶⁶Zn values significantly overlap for the different meteorite groups, with the isotopically heaviest sample being the IIE Watson (δ⁶⁶Zn = 5.61‰) and the lightest being the III AB Casas Grandes (δ⁶⁶Zn = -0.59‰ as measured by Luck et al. 2005). Correspondingly, members of the groups IIE are most enriched in heavy isotopes and members of IIIAB are most enriched in light isotopes. The two members of the IVA group analyzed here (Gibeon and Yanhuitlan) show very low Zn concentrations (0.17 and 0.66 ppm), and almost identical isotope values (δ⁶⁶Zn = 0.07 and 0.06‰), which fall in the range of carbonaceous chondrites.

The δ⁶⁶Zn values as a function of Zn concentrations of all samples are shown in Fig. 2. Members of the IAB and IIE groups display a definite negative correlation of higher δ⁶⁶Zn with lower Zn concentration. Other samples seem to show relatively low Zn concentrations with no clear relationship with δ⁶⁶Zn. This finding is consistent with the relationship between concentration and δ⁶⁶Zn found in HED meteorites according to Paniello et al. (2012b).

Cu isotope data obtained from the same parent chip are also included in Table 1. When δ⁶⁶Zn is compared to δ⁶⁵Cu in Fig. 3, a strong positive correlation is observed in group IAB ($R^2$ = 0.909). In contrast, the fractionally crystallized groups IIC, IID, and IIIAB show some negative correlations ($R^2$ = 0.953, 0.572, and 0.898, respectively). Only these four groups for which we obtained both δ⁶⁶Zn and δ⁶⁵Cu data for three or more samples are able to define a trend. Overall, the samples seem to scatter with no correlation.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Zn concentration versus δ⁶⁶Zn plot. Only group IAB+IIE shows a definite negative trend; other groups show no correlation. UNGR = Ungrouped meteorites.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3. δ⁶⁶Zn versus δ⁶⁵Cu. The silicate-bearing group IAB shows resolvable positive trends, while the fractionally crystallized groups IIC and IID show loose negative trends. Other groups' trends are not definable due to lack of members. UNGR = Ungrouped meteorites.</figcaption></figure>

# DISCUSSION

## Mass-Dependent Fractionation and Redfields

Ghidan and Loss (2011) reported the Redfields iron meteorite to have an anomalous Zn isotopic composition, stating its deviation from the Zn standard IRMM 3702 to be 5.6 ± 0.4 for δ⁶⁶Zn,
4.4 ± 3.6 for δ⁶⁷Zn, and 21.0 ± 0.9 for δ⁶⁸Zn in unspiked samples. When normalized to ⁶⁸Zn/⁶⁴Zn, the isotope ⁶⁶Zn anomaly would be approximately -52ε units, which is significantly larger than the largest anomaly found in CAIs (+16.7ε in Allende FUN inclusion; Völkening and Papanastassiou 1990). In this work, we find the Zn isotopic composition of Redfields to be consistent with mass-dependent fractionation, with its isotopic deviations from the standard JMC 3-0749 L to be 0.55 ± 0.03, 0.83 ± 0.05, and 1.10 ± 0.07 for δ⁶⁶Zn, δ⁶⁷Zn, and δ⁶⁸Zn, respectively. Mass-dependent fractionations are expected when the heavier isotopes show larger fractionation effects with δ⁶⁸Zn = 2 × δ⁶⁶Zn and δ⁶⁷Zn = 1.5 × δ⁶⁶Zn, which is exactly what we observed. Therefore, Zn isotopic fractionation in Redfields is totally mass-dependent. Moreover, the Zn concentration of Redfields measured in this study (6.8 ppm) is very close to the isotope dilution measurement of Ghidan and Loss (2011) (7.2 ppm) ensuring that our Zn purification technique is working correctly. The difference in isotopic compositions might be due to the heterogeneity of the samples or problem with measurement and data analysis. However, extreme isotopic heterogeneity within a single iron meteorite that could lead to a large magnitude of anomalies as in this case is unlikely.

With the improvement of analytical precision, isotope anomalies of several refractory siderophile elements, e.g., Ni (Regelous et al. 2008; Steele et al. 2011), Mo (Burkhardt et al. 2011), Ru (Chen et al. 2010) have been found in iron meteorites. However, these isotope anomalies in iron meteorites are barely at the limit of the analytical resolutions, and smaller than what have been observed in CAIs and bulk chondrites. Until now, Zn isotopic anomalies have only been found in FUN calcium-aluminum-rich inclusions (CAIs) (Völkening and Papanastassiou 1990; Loss et al. 1994) and these mineral scale anomalies are ascribed to incomplete mixing of the products of stellar nucleosynthesis in the early solar system. Given the fact that Zn isotopic anomalies have never been observed in bulk chondrites and other differentiated meteorites (Moynier et al. 2009) and that iron meteorites are not a major Zn reservoir (due to the lithophile behavior of Zn), it would be very unlikely that iron meteorites would have preserved such large magnitude of Zn isotopic anomalies after early solar system mixing and asteroid differentiation. In addition, previous works on Zn isotopes also pointed out that the sources for terrestrial, lunar, Martian, and meteoritic zinc all come from a single reservoir or from multiple reservoirs linked by mass-dependent fractionation processes (Luck et al. 2005; Herzog et al. 2009). All of our samples

forming a straight line on the three-isotope plot (Fig. 1) support these conclusions.

Recent studies have observed that neutron irradiation from cosmic rays can alter the isotopic compositions of some elements in iron meteorites (Walker 2012); for example, neutron capture reactions create detectable Pt and Os anomalies in some irons with long exposure ages (Wittig et al. 2013; Kruijer et al. 2013). However, it is unlikely that the isotopic anomalies observed by Ghidan and Loss (2011) are the result of neutron irradiation, because the Zn isotope composition in most cosmic-ray damaged samples (e.g., Carbo [IID] with cosmic-ray exposure age of 850 Ma) is mass-dependent.

## δ⁶⁵Cu versus δ⁶⁶Zn Correlation

Although Zn and Cu are both moderately volatile elements, Zn is more volatile ($T_c$ = 726 K) than Cu ($T_c$ = 1037 K; Lodders 2003). Because of the difference in volatility, the range of isotope fractionation induced by evaporation-condensation processes on Zn is generally larger than that of Cu (Luck et al. 2005; Moynier et al. 2007). In the plot of δ⁶⁶Zn against δ⁶⁵Cu (Fig. 3), group IICD and IIIAB show loose negative trends, while group IAB shows a resolvable positive trend, which is opposite to that observed by Luck et al. (2005). This discrepancy might be due to the limited number of samples analyzed in Luck et al. (2005) compared with the present study. It is also possible that local heterogeneities in composition may render the difference in the δ⁶⁶Zn – δ⁶⁵Cu trend, however, Zn (this study) and Cu (Bishop et al. 2012) isotope measurements were conducted from the same parent chip, indicating the positive trend by our data should be reliable. The positive trend in the IAB group is likely the result of evaporation, as Cu and Zn are both moderately volatile elements; therefore, the Zn and Cu isotopes should be affected at the same time. It is also noted that the variations of Zn isotopes in this group are larger than Cu, which is consistent with elemental difference in volatility. Isotopic fractionation during metal/silicate fractionation is also a possibility and has been shown to occur for elements such as Si (Georg et al. 2007). There are presently no experiments of Zn isotopic fractionation during metal/silicate partitioning and it is therefore not possible to exclude metal/silicate partitioning as a source of isotopic fractionation. However, it has been shown (Savage et al. 2013) that Cu isotopes are not fractionated to a large extent (<0.1 permil/amu) by metal/silicate fractionation and therefore the correlation between Zn and Cu isotopes suggests a similar origin for the isotopic variability, which may not be metal/silicate fractionation.
## The Silicate-Bearing Groups IAB and IIE

Compared with the asteroid-core magmatic groups, the silicate-bearing iron meteorites are isotopically more fractionated in Zn. Although the silicate-bearing irons experienced melting during their history, magmatic process is not likely the mechanism for the large isotopic variations, as it has been known that Zn isotopic fractionation during fractional crystallization is rather limited (Ben Othman et al. 2006; Chen et al. 2013). Until now, the origin of silicate-bearing groups is still under debate, but different theories agree that impact occurs at one stage of their formation (Wasson and Wang 1986; Benedix et al. 2000; Wasson and Kalllemeyn 2002; Yang et al. 2008). It is reasonable that severe impacts contribute to volatile loss accompanied by isotopic mass-dependent fractionation, leaving the heavier isotopes in the residue (Scott and Wasson 1975; Wasson and Wang 1986). Our data show that IAB and IIE members are the isotopically heaviest samples in Zn. Furthermore, the positive δ⁶⁶Zn – δ⁶⁵Cu trend of IAB (Fig. 3) and negative correlation between Zn concentration and δ⁶⁶Zn of IAB and IIE (Fig. 2) also support the mass-dependent evaporation with depletion in Zn and preferential loss of lighter isotopes to the vapor phase.

The IIE group can be divided into two subgroups: the old IIE (4.1 Ga) and the young IIE (3.7 Ga) due to their difference in Ar-Ar ages (Niemeyer 1980; Snyder et al. 2001), which were possibly formed due to a localized impact that reset the ages and removed volatile elements such as Mn from the silicate melt (McDermott et al. 2012). It has been found that these two subgroups have distinct nitrogen isotopic compositions (Mathew et al. 2000). The Zn isotope compositions of the young and old IIE samples also show the difference. The young group (of which Watson is a representative) has lower Zn abundance than that of the old group (0.8 ppm in Watson compared to 1.1 ppm and 1.05 in Weekeroo Station and Arlington, respectively). Watson (δ⁶⁶Zn = 5.61) is also significantly enriched in heavy isotopes compared with the members of the old group (δ⁶⁶Zn = 2.08 for Weekeroo Station; δ⁶⁶Zn = 2.19 for Arlington). This observation in Zn agrees well with another moderately volatile element Ge (T_c = 825 K). The IIE group shows the largest variations in Ge isotopic compositions among iron meteorites, and the young group samples also have lower concentration of Ge than old group, together with distinct heavier Ge isotopes (Luais 2007).

In summary, the silicate-bearing groups seem to be more isotopically fractionated, most likely due to their different formation processes (volatilization by impacts) compared with the fractionally crystallized

groups primarily yielded by high-temperature fractional-crystallization processes.

## The Fractionally Crystallized Iron Groups

The magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid-sized bodies; therefore, they generally have low lithophile element abundances. The variations in Zn isotopic composition of fractionally crystallized irons are smaller than those of silicate-bearing groups, and there is no distinct difference between groups. However, group IIIAB seems to be the exception with high Zn concentration and two members being the isotopically lightest irons analyzed so far. However, this could be questioned as the data for the isotopically lightest samples (Casas Grandes and Charcas) were analyzed in a different study (from Luck et al. 2005).

Within the same fractionally crystallized group, the elemental variations are mainly controlled by fractional crystallization during cooling, which can be reflected on scattering diagrams of elemental concentrations against Ni, Au, Ir, etc. (Wasson 1999; Walker 2012). Au is an incompatible element with concentration increasing during crystal-liquid fractionation in iron meteorites. As total ranges of Au are usually larger than Ni within the iron meteorite group, but the relative analytical uncertainties are even lower (Wasson and Kalllemeyn 2002), Au has been widely used as an indicator of fractional crystallization (Wasson 1999; Wasson and Huber 2006). In a δ⁶⁶Zn versus Au content plot (Fig. 4), there is no within-group correlation for these groups, indicating that fractional crystallization of metallic liquid does not fractionate Zn isotopes.

The IIIAB group has one member (Gnowangerup) with very high Zn concentration (34 ppm) compared with the rest of its group members (1–10 ppm), and yet its isotope compositions (δ⁶⁶Zn = 0.46‰) are not that distinct. Meanwhile, the group IID member Wallapai has significantly less Zn (0.4 ppm; δ⁶⁶Zn = 1.08‰) compared with the rest of its group (average 2 ppm), but does not show significant isotopic differences from other IID irons (0.97 < δ⁶⁶Zn < 1.56‰). Due to strong fractionation during evaporation-condensation processes (Albarède 2004; Paniello et al. 2012a), the Zn isotopic composition of IVA members (Gibeon and Yanhuitlan) can aid in better understanding the origin of volatile depletion. The IVA irons are among the oldest iron meteorites and differentiated materials known in the solar system (Blichert-Toft et al. 2010; Kruijer et al. 2013) and are drastically depleted in moderately volatile elements (Wasson and Richardson 2001). The continuity of the chemical trends in the IVA group indicates that
<figure><img src="image_5.png" /><figcaption>Fig. 4. δ<sup>66</sup>Zn versus Au. The absence of correlation suggests that fractional crystallization of metallic liquid does not fractionate Zn isotopes.</figcaption></figure>

IVA irons come from a single metallic body that fractionally crystallized (Wasson and Richardson 2001). Our data show that the IVA samples are depleted in Zn (0.17 and 0.66 ppm), while their isotope compositions are undifferentiated (δ<sup>66</sup>Zn = 0.06 and 0.07) compared with carbonaceous chondrites (0 < δ<sup>66</sup>Zn < 0.50; Luck et al. 2005; Barrat et al. 2012). Similarly, despite the depletion of Cu, the Cu isotopic compositions of the IVA iron meteorites (−1.83‰ < δ<sup>65</sup>Cu < −0.35‰; Bishop et al. 2012) also fall in the range of carbonaceous chondrites (−1.51‰ < δ<sup>65</sup>Cu < 0.07‰; Luck et al. 2003; Barrat et al. 2012). It seems premature to propose an isotopically light precursor such as L/LL group chondrites (Clayton and Mayeda 1996; Wang et al. 2004) for IVA iron meteorites, as most of the chondrites have heavier Zn and Cu isotope compositions than IVA irons (Luck et al. 2003, 2005). Therefore, volatile depletion in the IVA parent body is probably not due to partial kinetic evaporation induced by impact; should it have been the case, we would see an enrichment in the heavier isotopes. Our results rather favor a parent body of the IVA group members already depleted in the volatile element via an incomplete condensation of volatile elements.

It is important to note that the origins and histories
of volatile loss of many iron meteorite groups are still
under debate, and the conclusions obtained by different
methods and from different authors can be antithetic
(Haack and McCoy 2003; Goldstein et al. 2009). Our
work provides new constraints from an isotopic
perspective on this issue; however, we give no
unequivocal answer for iron meteorite petrogenesis and
further work is still required.

CONCLUSIONS

We measured Zn isotopic compositions of seven IAB, four IIC, three IIB, six IID, three IIE, one IIF, three IIIB, one IIIE, one IIIF, two IVA, and one ungrouped (Redfields) iron meteorites. The range of isotopic composition among the groups is large, ranging from the isotopically heaviest sample Watson (δ<sup>66</sup>Zn = 5.61‰) to the lightest Casas Grandes (δ<sup>66</sup>Zn = -0.59‰ as measured by Luck et al. 2005). All of the samples support mass-dependent fractionation by forming a linear trend on the three-isotope plot (Fig. 1). The negative correlation between Zn concentration and δ<sup>66</sup>Zn of the silicate-bearing groups IAB + IIE (Fig. 2) is consistent with mass-dependent evaporation with preferential loss of lighter isotopes. The positive trend of IAB and IIE on the δ<sup>66</sup>Zn-δ<sup>65</sup>Cu plot is also expected as Cu and Zn are both moderately volatile elements (Fig. 3).

Compared with the silicate-bearing groups,
fractionally crystallized iron meteorites show smaller
isotopic variations within group, which is consistent
with previous observations that Zn isotopic
fractionation during magmatic differentiation is limited
(Ben Othman et al. 2006; Chen et al. 2013). Our data
for the two IVA samples suggest that volatile depletion
in the IVA parent body is not likely the result of
evaporation and favor an incomplete condensation
hypothesis instead. Overall, investigating the Zn isotopic
composition of iron meteorites has afforded important
insights, leading to a better understanding of the origin
and evolution of these meteorites.

*Acknowledgments*—The authors greatly appreciate Dr. Julien Foriel for his help in the laboratory at Washington University. We also are indebted to Franz Brandstätter (Naturhistorisches Museum, Vienna), Alex Bevan (Western Australian Museum, Perth), Timothy McCoy (US National Museum of Natural History, Smithsonian Institution, Washington DC), Caroline Smith (The Natural History Museum, London), Philipp Heck and James Holstein (The Field Museum, Chicago), Meenakshi Wadhwa (Arizona State University, Tempe), Rhiannon Mayne (Oscar E. Monnig Meteorite Collection, TCU) for their generous donations of meteorite samples for this work and their confidence in our analytical and scientific capabilities. The editor Timothy Jull and Henning Haack and two anonymous reviewers are acknowledged for considerable constructive comments.

This work was partially supported by NASA
Cosmochemistry Fund # NNX12AH70G and a Chaire
of Excellence from the Sorbonne Paris Cité to FM and
the support of the Office of Undergraduate Research at
Washington University.
*Editorial Handling*—A. J. Timothy Jull

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