# Volatile loss history of the Moon from the copper isotopic compositions of mare basalts

Marine Paquet, Frederic Moynier, Paolo A. Sossi, Wei Dai, James M. D. Day

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Marine Paquet, Frederic Moynier, Paolo A. Sossi, Wei Dai, James M. D. Day. Volatile loss history of the Moon from the copper isotopic compositions of mare basalts. Earth and Planetary Science Letters, 2025, 656, pp.119250. 10.1016/j.epsl.2025.119250. insu-05080912

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# Volatile loss history of the Moon from the copper isotopic compositions of mare basalts

Marine Paquet<sup>a,b,*</sup> <sup>D</sup>, Frederic Moynier<sup>a</sup>, Paolo A. Sossi<sup>c, D</sup>, Wei Dai<sup>a</sup>, James M.D. Day<sup>d, D</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Editor: Fang-Zhen Teng

**Keywords:**

Moon

Copper isotopes

Volatile elements

Apollo samples

## ABSTRACT

The abundances and isotopic signatures of volatile elements provide critical information for understanding the delivery of water and other essential life-giving compounds to planets. It has been demonstrated that the Moon is depleted in moderately volatile elements (MVE), such as Zn, Cl, S, K and Rb, relative to the Earth. The isotopic compositions of these MVE in lunar rocks suggest loss of volatile elements during the formation of the Moon, as well as their modification during later differentiation and impact processes. Due to its moderately volatile and strongly chalcophile behaviour, copper (Cu) provides a distinct record of planetary accretion and differentiation processes relative to Cl, Rb, Zn or K. Here we present Cu isotopic compositions of Apollo 11, 12, 14 and 15 mare basalts and lunar basaltic meteorites, which range from $\delta^{65}\text{Cu}$ of +0.55±0.01 ‰ to +3.94±0.04 ‰ (per mil deviation of the $^{65}\text{Cu}/^{63}\text{Cu}$ from the NIST SRM 976 standard), independent of mare basalt Ti content. The $\delta^{65}\text{Cu}$ values of the basalts are negatively correlated with their Cu contents, which is interpreted as evidence for volatile loss upon mare basalt emplacement, plausibly related to the presence Cl- and S-bearing ligands in the vapour phase. This relationship can be used to determine the Cu isotopic composition of the lunar mantle to a $\delta^{65}\text{Cu}$ of +0.57 ± 0.15 ‰. The bulk silicate Moon (BSM) is 0.5 ‰ heavier than the bulk silicate Earth (+0.07 ± 0.10 ‰) or chondritic materials (from -1.45 ± 0.08 ‰ to 0.07 ± 0.06 ‰). Owing to the ineffectiveness of sulfide segregation and lunar core formation in inducing Cu isotopic fractionation, the isotopic difference between the Moon and the Earth is attributed to volatile loss during the Moon-forming event, which must have occurred at- or near-equilibrium.

## 1. Introduction

Unravelling the origin and distribution of moderately volatile elements (MVE) in and on the Moon is crucial for a firmer understanding of the formation of the Earth-Moon system, as well as accretional and differentiation processes, but also igneous and impact processes (e.g., *Paniello et al., 2012; Day and Moynier, 2014; Day et al., 2017*). Moderately volatile elements are defined as elements with 50 % condensation temperatures ($T_c$) ranging from 665 to 1135 K, under canonical nebular gas conditions, at a pressure of $10^{-4}$ bar (*Lodders, 2003*). It was first thought that depletions relative to Cl chondrites in MVE found in the carbonaceous chondrites and in small planetary bodies such as the Moon and Eucrite Parent Body (now identified as Vesta) were inherited from common nebular fractionation processes (*Anders, 1977*).

Later analytical development with the advent of multi-collector inductively-coupled plasma mass spectrometry allowed measurement of MVE stable isotope signatures in various planetary bodies, including the Moon, providing new constraints on its origin (e.g., *Herzog et al., 2009; Paniello et al., 2012, 2013*). Notably, consistent enrichment in the heavier isotopes of Zn in a variety of lunar mare basalts, together with the depletion in absolute abundances of Zn and moderately volatile elements relative to their terrestrial counterparts in general, suggests that the Moon suffered a whole-scale high-temperature vaporization event probably associated with the global melting of the Moon during and following the giant impact (*Paniello et al., 2012*). Similar Earth-Moon dichotomies have been observed for other MVE such as K (*Wang and Jacobsen, 2016; Tian et al., 2021*), Ga (*Kato and Moynier, 2017*), Cl (*Sharp et al., 2010; Stephant et al., 2019*), Rb (*Pringle and Moynier,*

<footnote><sup>a</sup> Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, F-75005, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, CRPG, F-54000, Nancy, France</footnote>

<footnote><sup>c</sup> Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, Zürich CH-8092, Switzerland</footnote>

<footnote><sup>d</sup> Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA</footnote>

<footnote>* Corresponding author.
E-mail address: marine.paquet@univ-lorraine.fr (M. Paquet).</footnote>
2017; Nie and Dauphas, 2019) and Sn (Wang et al., 2019). These iso-
topic signatures suggest that the depletion of these elements is
controlled by global-scale evaporation during or following (e.g., magma
ocean stage) the giant impact (e.g., Paniello et al., 2012; Day and
Moynier, 2014; Albarède et al., 2015; Boyce et al., 2015; Dhaliwal et al.,
2018; van Kooten et al., 2020; Tian et al., 2021).

Alternatively, it has been proposed that the Moon might have accreted from materials that were already volatile-depleted. For example, Wimpenny et al. (2022) suggested one way of explaining the heavier Ga isotopic composition of mare basalts relative to terrestrial basalts is through volatile-depleted feedstocks. Using the Rb-Sr isotope system, Borg et al. (2022) also proposed that the volatile depletion of the Moon was principally inherited from precursor materials rather than from planetary processes, although this model permits differences in the Rb/Sr ratio of the Earth and Moon at their time of formation. Depleted planetary feedstocks among small telluric bodies are observed in rocks from the basaltic asteroid Vesta (Dhaliwal et al., 2024). Alternatively, it has also been proposed that at least a fraction of the missing volatile elements (including Cu, Ga, Ge, Cd, Sn, Se and Pb) of the Moon now reside in the core (e.g., Steenstra et al., 2016). A recent hybrid model proposed that volatile elements in the lunar mantle can be attributed to a combination of volatile-depleted precursors – primitive upper mantle-like -, mixing and gas-melt equilibrium in the protolunar disk, followed by the segregation of the lunar core (Righter, 2019). Determining the importance of these potential volatile element depletion processes is important for a fuller understanding of how planets evolved to their present-day states.

Copper is a key element with which to investigate volatile-related versus core segregation processes because it is strongly chalcophile, weakly siderophile and moderately volatile (T~c~ of 1037 K). Mars, the Moon and Vesta have systematically heavier K isotopic compositions than the Bulk Silicate Earth (BSE), suggesting that volatile depletion proceeded to a greater extent the smaller the planetary body was, producing stronger K isotope fractionation by evaporation relative to their potential precursor materials (Tian et al., 2021). However, this observation does not hold for the highly volatile depleted angrite parent body (Hu et al., 2022). The Moon is more volatile-depleted than the Earth and Mars (e.g., Wolf and Anders, 1980; O'Neill, 1991; Jones and Palme, 2000; Lodders, 2003; Davis, 2006; Palme and O'Neill, 2014), and Cu is also moderately volatile (Lodders, 2003). It follows that similar enrichment in the heavier Cu isotope (⁶⁵Cu) should be expected for the Moon as is also observed for K.

Previous Cu isotopic data for lunar samples, including regolith or soil samples, mare basalts pyroclastic glass beads and "Rusty rock" 66095, span a range from δ⁶⁵Cu = -1 to +4.5 ‰ (with δ⁶⁵Cu the per mil deviation of the ⁶⁵Cu/⁶³Cu ratio from the NIST SRM976), with Cu abundances from 2.7 to 84 µg/g (Moynier et al., 2006; Herzog et al., 2009; Day et al., 2019). Lunar soil samples show systematically isotopically heavy Cu compositions (+0.3 to +4.5 ‰), which has previously been attributed to impact gardening and sputtering effects (Moynier et al., 2006; Herzog et al., 2009). The Apollo 16 "Rusty Rock" impact melt breccia 66095 is a volatile-rich sample, with a δ⁶⁵Cu of +0.72 ± 0.1 ‰ (Day et al., 2019), within the previously reported range for lunar mare basalts (Moynier et al., 2006; Herzog et al., 2009; Lindsay et al., 2011; Day et al., 2019). Until recently, there were limited Cu isotopic data available for lunar rocks, and therefore the estimate for the Bulk Silicate Moon (BSM) for Cu isotope composition was not well-constrained. The estimate of δ⁶⁵Cu of the lunar mantle of +0.5 ± 0.1 ‰ (n = 7) was previously derived from the average of the lunar mare basalts isotopic compositions (Herzog et al., 2009), which appears isotopically heavier than the Bulk Silicate Earth (+0.07 ± 0.10 ‰; Savage et al., 2015). To constrain the Cu isotopic composition of the Moon, we present new high precision Zn and Cu isotope data for twenty-five lunar samples collected by the Apollo missions (19) and meteorites (6), which represents a unique dataset that considerably expands Cu isotope compositions for the Moon, with the advantage that Zn isotopes have also been measured

for the same samples. Combined with previous measurements, this new
dataset allows a precise assessment of the Cu isotope composition of the
lunar mantle.

## 2. Materials and methods

2.1. Sample description

Lunar samples analyzed in this study include: two high-Ti mare basalts (Apollo 11), seventeen low-Ti mare basalts (Apollo, 12, 14, 15) – including the Al-rich mare basalt 14053, the pyroclastic glass bead clod 15426, and six lunar meteorites (Ramlat Fasad 532 (a gabbro), Northwest Africa [NWA] 6950 (a gabbro), NWA 12008, NWA 12384, NWA 13957 and NWA 14178 (lunar mare basalt breccias)).

## 2.2. Elemental concentration measurements

Before chemical purification, about 5 % of each acid-dissolved sample was separated and then diluted in 10 mL 0.5 mol/L HNO₃ for elemental concentration measurements with an Agilent 7900 ICP-QMS housed at the IPGP. Elemental concentrations were measured in low-resolution mode. Sample solutions were aspirated into a Scott spray chamber using a MicroMist micro-nebulizer (uptake rate = 0.2 mL/min). Elements with atomic masses between 23 (Na) and 75 (As) were measured using a collision-reaction cell supplied with He gas (5 mL/min) to remove polyatomic interferences. Scandium, In, and Re were used as internal standards to correct for signal drift and matrix effects. Count measurements were converted to solution concentrations by analyzing a mixture of certified standards with concentrations spanning the range of the samples. Data are reported in Table 1.

## 2.3. Copper purification

For each sample, between ~60 to 560 mg of homogenized powder were dissolved using a mixture of concentrated HNO3/HF and heated at 130 °C in PTFE beakers. Following evaporation of the HNO3/HF, samples were subsequently treated with 6 mol.L⁻¹ HCl and heated again to dissolve remaining fluoride complexes. Samples were then evaporated to dryness. Chemical purifications were conducted at the Institut de Physique du Globe de Paris, using the procedure described by Sossi et al. (2015) and Moynier et al. (2019, 2020) adapted from Maréchal et al. (1999) for Cu. All Apollo samples (except for 14072,70 and 14053,287) followed the protocol of Sossi et al. (2015) and were loaded in 1 mL of 6 mol.L⁻¹ HCl on home-made PTFE columns filled with 1 mL of AG1-X8 (200 – 400 mesh) resin. After passing ~7 mL of 6 mol.L⁻¹ HCl through the resin, the Cu was collected with 15 mL of 6 mol.L⁻¹ HCl. Lunar meteorites (together with 14072,70 and 14053,287) followed the protocol of Moynier et al. (2019, 2020) and were loaded in 1 mL of 7 mol.L⁻¹ HCl on home-made PTFE columns filled with 1.6 mL of AG-MP1 resin. After washing the resin with 8 mL of 7 mol.L⁻¹ HCl, the Cu was collected with 16 mL of 7 mol.L⁻¹ HCl. Both protocols were repeated two to three to ensure clean Cu fractions. The procedural blank represented < 1 ng of Cu, which is negligible relative to the amount of Cu in the sample mass analyzed for the lunar samples.

## 2.4. Copper measurements

Copper isotope compositions were determined using a Neptune Plus Multi-Collector Inductively-Coupled-Plasma Mass-Spectrometer (MC-ICP-MS) at IPGP running in low resolution mode. Because copper has only two stable isotopes (⁶³Cu (69.2 ‰) and ⁶⁵Cu (30.8 ‰)), it is difficult to separate mass-dependent fractionation occurring in nature from mass bias fractionation induced by the mass spectrometer using a double spike approach. Therefore, for Apollo samples (except 14072,70 and 14053,287), the instrumental mass bias correction was performed by coupled sample-standard bracketing and Ni doping as in Sossi et al.
<caption>**Table 1**</caption>

Isotope and abundance data for lunar mare basalts and meteorites. Published Zn isotope and abundance data for the same samples are from Paniello et al. (2012). The number of individual analyses including distinct samples or replicates analyses is indicated by *n*. The error reported for δ⁶⁵Cu is twice the standard deviation (2SD).

<table><thead><tr><th>Sample ID</th><th>Lithology</th><th>MgO<br>(wt.%)</th><th>TiO<sub>2</sub><br>(wt.%)</th><th>S (µg/g)</th><th>Total Mass<br>(g)</th><th>n</th><th>δ<sup>65</sup>Cu</th><th>SD</th><th>Cu<br>(µg/g)</th><th>f<sub>Cu</sub></th><th>In (f<sub>Cu</sub>)</th><th>Zn<br>(µg/g)</th><th>n</th><th>δ<sup>66</sup>Zn</th><th>2SD</th><th>δ<sup>67</sup>Zn</th><th>2SD</th><th>δ<sup>68</sup>Zn</th><th>2SD</th></tr></thead><tbody><tr><td colspan="20"><strong>High Ti mare basalt</strong></td></tr><tr><td>10003</td><td>Ilmenite Basalt</td><td>7.1</td><td>10.67</td><td>2700</td><td>0.3045</td><td>2</td><td>1.48</td><td>0.06</td><td>2.3</td><td>0.14</td><td>-1.95</td><td>1.4</td><td></td><td>1.47</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>10057</td><td>High-K Ilmenite Basalt</td><td>7.65</td><td>11.44</td><td>2280</td><td>0.2590</td><td>2</td><td>2.30</td><td>0.03</td><td>2.9</td><td>0.18</td><td>-1.71</td><td>5.4</td><td></td><td>1.83</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td colspan="20"><strong>Low Ti mare basalt</strong></td></tr><tr><td>12016</td><td>Ilmenite Basalt</td><td>12.65</td><td>4.02</td><td>1220</td><td>0.5580</td><td>2</td><td>2.48</td><td>0.02</td><td>0.4</td><td>0.03</td><td>-3.64</td><td>0.9</td><td></td><td>1.56</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Rpt</td><td></td><td></td><td>1220</td><td></td><td>2</td><td>1.85</td><td>0.05</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>12065</td><td>Pigeonite Basalt</td><td>7.82</td><td>3.34</td><td></td><td>0.4002</td><td>2</td><td>1.36</td><td>0.01</td><td>3.9</td><td>0.25</td><td>-1.41</td><td>1.1</td><td></td><td>1.52</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>12052</td><td>Pigeonite Basalt</td><td>8.07</td><td>3.35</td><td></td><td>0.4057</td><td>2</td><td>1.71</td><td>0.04</td><td>1.9</td><td>0.12</td><td>-2.12</td><td>1.1</td><td></td><td>1.34</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Rpt</td><td></td><td></td><td></td><td></td><td>2</td><td>1.35</td><td>0.04</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>12021</td><td>Pigeonite Basalt</td><td>7.38</td><td>3.45</td><td>781</td><td>0.5215</td><td>2</td><td>0.69</td><td>0.01</td><td>8.6</td><td>0.54</td><td>-0.62</td><td>1.4</td><td></td><td>1.15</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Rpt</td><td></td><td></td><td>781</td><td></td><td>2</td><td>0.61</td><td>0.02</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>12005</td><td>Ilmenite Basalt</td><td>19.97</td><td>2.76</td><td>560</td><td>0.4960</td><td>2</td><td>2.23</td><td>0.02</td><td>0.8</td><td>0.05</td><td>-3.04</td><td>0.8</td><td></td><td>-0.22</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>12002</td><td>Olivine Basalt</td><td>14.88</td><td>2.6</td><td>627</td><td>0.5215</td><td>2</td><td>0.86</td><td>0.15</td><td>4.0</td><td>0.25</td><td>-1.39</td><td>1.5</td><td></td><td>0.84</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>12012</td><td>Olivine Basalt</td><td>14.37</td><td>2.64</td><td>890</td><td>0.4750</td><td>2</td><td>2.38</td><td>0.01</td><td>1.8</td><td>0.11</td><td>-2.20</td><td>1.0</td><td></td><td>1.62</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>12040</td><td>Olivine Basalt</td><td>16.21</td><td>2.41</td><td></td><td>0.4870</td><td>2</td><td>3.97</td><td>0.04</td><td>0.7</td><td></td><td></td><td>0.8</td><td></td><td></td><td>1.58</td><td>0.05</td><td></td><td></td><td></td></tr><tr><td>14053</td><td>Mare basalt</td><td>8.97</td><td>2.91</td><td>700</td><td>0.1000</td><td>2</td><td>3.04</td><td>0.12</td><td>0.2</td><td>0.01</td><td>-4.58</td><td>0.6</td><td></td><td>1.57</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>14072, 70*</td><td>Olivine basalt</td><td>13.4</td><td>2.41</td><td></td><td>0.0576</td><td>1</td><td>1.98</td><td>-</td><td>11.0</td><td>0.69</td><td>-0.37</td><td>5.6</td><td>2</td><td>0.46</td><td>0.04</td><td>0.56</td><td>0.13</td><td>0.84</td><td>0.02</td></tr><tr><td>14053, 287*</td><td>Mare basalt</td><td>8.5</td><td>2.62</td><td></td><td>0.1047</td><td>1</td><td>1.04</td><td>-</td><td>11.2</td><td>0.70</td><td>-0.35</td><td>13.9</td><td>2</td><td>-0.45</td><td>0.05</td><td>-0.55</td><td>0.14</td><td>-0.85</td><td>0.05</td></tr><tr><td>15499</td><td>Pigeonite Normative</td><td>8.94</td><td>1.81</td><td>767</td><td>0.3031</td><td>2</td><td>1.37</td><td>0.02</td><td>3.3</td><td>0.21</td><td>-1.58</td><td>0.9</td><td></td><td>1.23</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>15058</td><td>Pigeonite Normative</td><td>9.56</td><td>1.6</td><td>960</td><td>0.2958</td><td>2</td><td>1.72</td><td>0.00</td><td>1.5</td><td>0.09</td><td>-2.37</td><td>1.7</td><td></td><td>1.41</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Rpt</td><td></td><td></td><td>960</td><td></td><td>2</td><td>1.82</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15555</td><td>Olivine Normative</td><td>11.19</td><td>2.26</td><td>752</td><td>0.5313</td><td>3</td><td>0.61</td><td>0.01</td><td>15.7</td><td>0.98</td><td>-0.02</td><td>1.1</td><td></td><td>1.56</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>15557</td><td>Olivine Normative</td><td>9.43</td><td>2.55</td><td>673</td><td>0.4453</td><td>2</td><td>0.85</td><td>0.08</td><td>15.8</td><td>0.99</td><td>-0.01</td><td>0.6</td><td></td><td>1.34</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>15065</td><td>Gabbroic Basalt</td><td>10.57</td><td>1.48</td><td>543</td><td>0.5251</td><td>2</td><td>1.87</td><td>0.06</td><td>1.2</td><td>0.08</td><td>-2.58</td><td>1.1</td><td></td><td>1.33</td><td>0.05</td><td></td><td></td><td></td><td></td></tr><tr><td>15426</td><td>Green Glass</td><td>17.5</td><td>0.42</td><td></td><td>0.0600</td><td>2</td><td>0.55</td><td>0.01</td><td>7.8</td><td>0.49</td><td>-0.72</td><td>53.0</td><td></td><td>-0.98</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td colspan="20"><strong>Meteorites</strong></td></tr><tr><td>Ramlat Fasad 532*</td><td>Lunar gabbro</td><td>6.13</td><td>1.95</td><td></td><td>0.1061</td><td>1</td><td>0.75</td><td>-</td><td>8.3</td><td>0.52</td><td>-0.66</td><td>3.8</td><td>2</td><td>1.45</td><td>0.07</td><td></td><td></td><td></td><td></td></tr><tr><td>NWA 6950*</td><td>Lunar gabbro</td><td>22.98</td><td>0.4</td><td></td><td>0.10232</td><td>4</td><td>0.70</td><td>0.02</td><td>3.1</td><td>0.19</td><td>-1.65</td><td></td><td></td><td>1.10</td><td>0.10</td><td></td><td></td><td></td><td></td></tr><tr><td>NWA 14178*</td><td>Basaltic Breccia</td><td>7.49</td><td>2.53</td><td></td><td>0.10701</td><td>4</td><td>0.88</td><td>0.06</td><td>8.1</td><td>0.51</td><td>-0.68</td><td></td><td></td><td>0.65</td><td>0.10</td><td></td><td></td><td></td><td></td></tr><tr><td>NWA12008*</td><td>Lunar mare basalt</td><td>8.12</td><td>3.02</td><td></td><td>0.07315</td><td>5</td><td>0.94</td><td>0.12</td><td>2.3</td><td>0.14</td><td>-1.95</td><td>3.60</td><td>2</td><td>1.03</td><td>0.06</td><td>1.55</td><td>0.04</td><td>2.01</td><td>0.07</td></tr><tr><td>NWA12384*</td><td>Lunar mare basalt breccia</td><td>15.82</td><td>1.53</td><td></td><td>0.11292</td><td>5</td><td>0.56</td><td>0.08</td><td>4.7</td><td>0.30</td><td>-1.22</td><td></td><td>1</td><td>-0.05</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NWA13957*</td><td>Lunar mare basalt breccia</td><td>8.15</td><td>0.30</td><td></td><td>0.10941</td><td>5</td><td>2.41</td><td>0.18</td><td>2.0</td><td>0.13</td><td>-2.06</td><td></td><td>1</td><td>5.96</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>BCR-2</td><td>Columbia River Basalt</td><td></td><td></td><td></td><td>0.0990</td><td>3</td><td>0.07</td><td>0.02</td><td></td><td></td><td></td><td></td><td>6</td><td>0.23</td><td>0.04</td><td>0.30</td><td>0.14</td><td>0.47</td><td>0.07</td></tr><tr><td>BHVO2*</td><td>Hawaiian Basalt</td><td></td><td></td><td></td><td></td><td>2</td><td>0.17</td><td>0.08</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>BHVO-2</td><td>Hawaiian Basalt</td><td></td><td></td><td></td><td></td><td>3</td><td>0.09</td><td>0.01</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote>*Zinc isotopes from this study; #Zinc isotopes from Dai et al. (under review). δ⁶⁶Zn and Zn abundances from Paniello et al., (2012) and S contents from Sossi et al. (2025) and references therein (Beaty and Albee, 1978; Compston et al., 1970; Gibson et al., 1975, 1977; Kaplan et al., 1970, 1976; Ress and Thode, 1972; Thode and Rees, 1972; Wänke et al., 1975; Willis et al., 1971, 1972).</footnote>
(2015). Each measurement consisted of 30 cycles at 4.134 s integration time, and each aliquot was analyzed two to four times depending on the amount of Cu available for each sample. The samples were analyzed in two sessions at concentrations between 10 and 50 ppb of Cu (roughly 2–4 V on $^{63}$Cu) on a Scott Double-Pass Cyclonic spray chamber, with an uptake of 100 µL.min$^{-1}$. Lunar meteorites (with 14072,70 and 14053, 287) were analyzed using sample-standard bracketing for instrumental mass bias correction as in Moynier et al. (2019, 2020). Each replicate was analyzed one to five times (25 cycles with an integration time of 8.4 s) depending on the amount of Cu available for each sample. The samples were analyzed in two sessions at concentrations between 20 and 100 ppb of Cu (roughly 1.5–8 V on $^{63}$Cu), on a cyclonic spray chamber with an uptake of 100 µL.min$^{-1}$. All reported errors are the standard deviation (SD) of these repeated measurements. Copper isotope compositions are reported in Table 1, as the deviation in parts per 1000 from the terrestrial standard NIST SRM 976.

$$ \delta^{65}\text{Cu} = \left[ \frac{\left( \frac{^{65}\text{Cu}}{^{63}\text{Cu}} \right)_{\text{Sample}}}{\left( \frac{^{65}\text{Cu}}{^{63}\text{Cu}} \right)_{\text{SRM976}}} - 1 \right] \times 10^3 \quad (1) $$

Uncertainties are given as the two times standard deviation (2SD) of replicate measurements. The reference geological materials BHVO-2 and BCR-2 give $\delta^{65}\text{Cu}$ values of +0.09 ± 0.02 (2SD, n = 3) and +0.07 ± 0.04 (2SD, n = 3), consistent with reported literature values (e.g., Moynier et al., 2017). Uncertainties in concentration data are between 4 and 15 % relative standard deviation as assessed through repeat analyses of the same solution.

### 3. Results

Zinc and copper isotope compositions and abundances are presented in Table 1, Figs. 1 and 2. The measured Cu abundances for lunar samples range from 0.2 to 15.8 µg/g, which are within but generally lower than the range of values reported in previous work for Apollo mare basalts (e. g., 2.7 to 22.3 µg/g; Moynier et al., 2006; Herzog et al., 2009; Day, 2018; Day et al., 2019; Gleißner et al., 2022). Low-Ti mare basalts from Apollo 12, 14, 15 and 16 span the larger range, from 1.5 to 15.8 µg/g, overlapping Apollo 11 and 17 samples from Herzog et al. (2009) and Gleißner et al. (2022), while high-Ti mare basalts (Apollo 11) exhibit uniformly low Cu abundances (2.3 to 2.9 µg/g), slightly lower than those reported in previous studies (Gleißner et al., 2022). It is worth noting that Cu contents of 15555 and 15557 in this study are a factor of ~2 higher (15.7 and 15.8 µg/g, respectively) than the values reported for the same samples in Gleißner et al. (2022) (6.41 and 7.96 µg/g, respectively): this difference could represent heterogeneity in the different analyzed fractions of these mare basalts. This heterogeneity is probably to be expected. Since Cu is chalcophile, it will be concentrated in sulfides or metal (Day and Paquet, 2021). Heterogeneous distribution of these phases will lead to heterogeneities simply as a function of the small sample sizes typically given to investigators for analysis (sub 5g).

<figure><img src="image_4.png" /><figcaption>Fig. 1. Variations of (a) MgO versus TiO$_2$, (b) MgO versus Cu, (c) TiO$_2$ versus $\delta^{65}\text{Cu}$ and (d) MgO versus $\delta^{65}\text{Cu}$ in lunar mare basalts. Literature data: Cu isotope compositions (Moynier et al., 2006; Herzog et al., 2009), and MgO and TiO$_2$ contents (see Table S1).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Comparison of δ⁶⁵Cu and δ⁶⁶Zn in the same samples of lunar mare basalts and meteorites. Zinc data from Moynier et al. (2006), Herzog et al. (2009), Paniello et al. (2012) and this study. Bulk Silicate Earth (BSE), Moon (BSM) and Vesta (BSV) estimates are from Savage et al. (2015), Sossi et al. (2018), Day et al. (2019, 2020, 2022) and Dhaliwal et al. (2024).</figcaption></figure>

Finally, the lunar meteorites exhibit similar Cu abundances to the rest of the sample set with Cu contents between 3.1 and 8.3 µg/g for the lunar gabbros (Ramlat Fasad 532 and NWA 6950) and between 2.0 and 8.1 µg/g (lunar mare basalt breccias).

Our new dataset considerably expands the range of δ⁶⁵Cu for lunar mare basalts with values from +0.61 ± 0.02 ‰ to +3.97 ± 0.04 ‰ (Apollo 12 low-Ti mare basalts), mostly heavier than Apollo 11 and 17 mare basalts reported by Herzog et al. (2009) (Fig. 1). All other mare basalts analyzed in this study show δ⁶⁵Cu within that range: Apollo 15 low-Ti mare basalts range from +0.61 to +1.87 ‰ and Apollo 11 high-Ti exhibit values from +1.48 to +2.30 ‰. Note that Al-rich mare basalt 14053 has a δ⁶⁵Cu +3.04 ± 0.14 ‰, and the pyroclastic glass bead 15426 exhibits a δ⁶⁵Cu of 0.55 ± 0.01 ‰, similar to other Apollo 11 and Apollo 17 mare basalts (+0.12 to +1.4 ‰; Herzog et al., 2009), and close to the endmember of the Apollo 12 samples with the lightest Cu isotopic signature reported in this study. Lunar meteorites show values from +0.56 to 2.41 ‰, within the range of the Apollo samples.

# 4. Discussion

## 4.1. Terrestrial weathering and alteration

All lunar meteorites considered in this study were recovered from a hot desert environment, and therefore may have undergone terrestrial alteration and weathering, potentially perturbing the Cu isotope compositions in these rocks from their primary igneous values. Meteorite NWA 12384 exhibits a thin coating of brown weathering products (Mn-rich), but has fresh grey interiors (Carpenter et al., 2019). Cohen et al. (2019) reported a positive Ce anomaly in NWA 12008, implying terrestrial contamination in an oxidizing environment. These two lunar meteorites plot within the range of the low-Ti mare basalts, suggesting that terrestrial weathering did not significantly modify their Cu isotopic compositions. Ramat Fasad 532 only exhibits limited signs of weathering (Gattacceca et al., 2022). Petrological observation of NWA 6950, NWA 13957 and NWA 14178 did not allow identification of secondary mineral phases that could indicate meaningful terrestrial alteration in a hot desert environment (Bao et al., 2020; Gattacceca et al., 2022). Overall, the lunar meteorites considered in this study appear to have experienced limited terrestrial weathering. Apollo samples were

collected on the Moon and so are free of terrestrial weathering effects.

## 4.2. Mare basalt signature inherited from mixing of chondritic reservoirs?

The Earth and the Moon have indistinguishable mass-independent isotope compositions for elements that exhibit varying geochemical behaviour, such as O, Ti, Si or Cr (e.g., Spicuzza et al., 2007; Zhang et al., 2012; Millet et al., 2016; Mougel et al., 2018). It is therefore tempting to assume that the stable Cu isotopic composition of the Moon is also equivalent to that of the Earth. However, irrespective of the bulk composition model assumed for the Earth's composition (e.g., 0.16:0.21:0.63 CI:CO:LL (Fitoussi and Bourdon, 2012) or 0.70:0.21:0.05:0.04 EH: H: CV:CI (Lodders, 2000)), given that Cu in all chondrites is isotopically lighter and more abundant than in the Moon (e.g., Luck et al., 2003, 2005; Barrat et al., 2012; Savage et al., 2015; Paquet et al., 2023a), it is not possible to reproduce the Moon's composition by a mixture of chondritic materials. Moreover, there is a negative correlation between elemental abundances and isotopic variations of moderately volatile elements in carbonaceous chondrites: lower concentrations are associated with an enrichment in the light isotopes (e.g., for Zn and Cu: Luck et al., 2005; Pringle et al., 2017), which is the opposite to the predictions of isotopic fractionation caused by evaporation. Therefore, it is impossible to explain the composition of the Moon by addition of chondritic materials or from simply involving different precursors than the Earth. The consistently heavy Cu isotopic signatures in lunar mare basalts are not, therefore, inherited from chondritic contamination or chondritic precursors.

## 4.3. Petrogenesis of lunar mare basalts and role of sulfur saturation

Lunar mare basalts, together with the pyroclastic glass 15426 and lunar meteorites, exhibit consistently heavy Cu isotope signatures (+0.55 to +3.97 ‰) relative to terrestrial basaltic rocks (e.g., 0.07 ± 0.10 ‰ for the BSE; Savage et al., 2015) (Fig. 2). To investigate whether Cu isotopes were fractionated during the eruption of lunar mare basalts, we compare their δ⁶⁵Cu values with petrological tracers of lava flow differentiation and outgassing. Coarse-grain lavas are associated with prolonged cooling timescales, while the presence of vesicles or vugs in basaltic rocks indicates saturation in a gas phase. As observed for Zn
isotopes, the comparison of δ⁶⁵Cu values showed no dependence on the presence of vesicles (Day et al., 2020). There is no correlation between TiO₂ or Cu contents with MgO, nor between the Cu isotopic composition of the lunar samples and TiO₂ or MgO contents (Fig. 1). No correlation is observed between petrological type (e.g., high-Ti versus low-Ti mare basalts), or S content and δ⁶⁵Cu signatures (Fig. S1), reinforcing the absence of a petrologic control on the Cu isotope compositions of these rocks, an observation analogous to the absence of a correlation between Zn isotope compositions and magmatic differentiation proxies that were used to draw a similar conclusion (Day et al., 2020). Sulfur abundances in Apollo 12 mare basalts positively correlate with the bulk composition, especially with TiO₂ content (Gibson et al., 1977), and the degree of near-surface fractionation experienced by specific mare basalt samples. Such a correlation between the S and TiO₂ contents is also observed in our dataset, with high-Ti mare basalts being the most S-rich samples (2280 – 2700 µg/g for TiO₂ contents ranging from 10.67 to 11.44 wt.%) and low-Ti mare basalts have the lowest S contents (542 and 1220 µg/g for TiO₂ contents ranging from 1.48 to 4.02 wt.%). However, in the mare basalts, there is no correlation between their S contents and their Cu contents, nor between their Cu isotopic compositions or their highly siderophile element abundances (Figs. S1 and S2). In terrestrial rocks, in particular for a differentiation suite from Kilauea Iki (Savage et al., 2015), no correlation is observed between the Cu isotope compositions and proxies of S-fractionation of magmas (e.g., MgO contents). Because the sulfur content does not correlate with the presence of bulk metallic iron in Apollo 12 mare basalts, Gibson et al. (1977) postulated that sulfur abundances measured in these samples are not greatly dependent on post-eruptive outgassing processes associated with sulfur loss from the melt, thus reflecting their source composition. The S contents of lunar mare basalts are too low (several hundred µg/g) to have saturated in FeS(I), even when accounting for the higher proportion of S²⁺/S⁶⁺ in comparison to terrestrial tholeiitic liquids (Steenstra et al., 2018). Even considering an impure S-bearing metal phase (and hence aFeS < 1, where a = activity), as shown to be in equilibrium with lunar composition silicate liquids at ΔIW = -1 (IW = the iron-wüstite buffer), the computed sulfur content at S-bearing metal saturation is of the order of 1500 ppm at 1400 °C (Brenan et al., 2019). Therefore, the high-Ti basalts, with roughly 2500 ppm S, may indeed be saturated in an Fe-rich S-bearing alloy (either at a higher fO₂) or have accumulated S-rich phases. On the other hand, all low-Ti basalts have S contents below 1220 ppm. Insofar as there is no systematic Cu isotopic difference observed between the low-Ti- and high-Ti mare basalts, alloy saturation appears not to have been responsible for the observed isotopic range. Indeed, Day and Paquet (2021) showed petrographic evidence for metal trapped in early crystallizing silicate and oxide phases, consistent with removal of these phases during fractional crystallization of both primitive and evolved mare basalts in reducing conditions during the ascent and eruption of the magmas. Therefore, while some lunar mare basalts may have been saturated in a metallic phase, this process had no discernible influence on their Cu isotopic compositions.

Overall, the Cu isotopic variations for lunar mare basalts, up to several per mil, is too large to have resulted primarily from crystal-liquid equilibrium processes at high temperatures (e.g., Liu et al., 2015; Savage et al., 2015; Huang et al., 2016), with no strong correlation with indicators of fractional crystallization such as MgO (Fig. 1). While these observations do not rule out minor degassing of Cu during lava flow emplacement processes, it appears that it is not the dominant process for δ⁶⁵Cu variations in mare basalts. Instead, the overwhelming evidence is that the δ⁶⁵Cu of mare basalts record their source composition and large-scale evaporative loss that fractionated Cu isotopes during planetary processes, as observed for Zn isotopes (e.g., Moynier et al., 2006; Herzog et al., 2009; Day et al., 2020).

### 4.4. Cause of Cu isotope fractionation in mare basalts: large-scale volatilization due to giant impact

The lunar mare basalts define a trend of increasing δ⁶⁵Cu with decreasing Cu content (Fig. 3). Although lunar mare basalts exhibit a large range of Cu isotopic signatures (+0.61 to +3.97 ‰), they show a narrow range of Zn isotopic compositions (δ⁶⁶Zn from +1.4 to +1.9 ‰, apart from Apollo 12 sample 12005 at -0.22 ‰), with the exception of the pyroclastic glass bead (-0.98 ‰) and NWA 13957 (+5.96 ‰). The light Zn isotopic signature of the pyroclastic glass bead is attributed to Zn evaporation during fire-fountaining with condensation of isotopically light Zn on the surfaces of the ejected beads (e.g., Moynier et al., 2006), lunar mare basalts exhibit a seemingly consistent δ⁶⁶Zn of ~+1.3 ± 0.2 ‰ (Fig. 2; e.g., Herzog et al., 2009; Paniello et al., 2012; Kato et al., 2015). The Cu isotopic compositions of mare basalts can be explained by volatile loss by evaporation, following a Rayleigh distillation law, with an empirical α_vap/liqMoon of 0.99945 (Fig. 3). This value is comparable to the best fit value of 0.9998 proposed by Day et al. (2020). However, it is significantly smaller (i.e., closer to unity) than the fractionation factor determined experimentally by Sossi et al. (2020), 0.9972, Ni et al. (2021), 0.9960, and Neuman et al. (2022), 0.9961 for Cu, which can be reconciled with the local partial pressure of Cu at the surface of the evaporating bead (pCu) relative to its equilibrium partial pressure (pCu<sub>eq</sub>). For these 1 bar furnace experiments, the ratio pCu/pCu<sub>eq</sub> is close to 0.993 (Sossi et al., 2020) or 0.74 in the experiments of Ni et al. (2021), with ratios closer to 1 tending to the equilibrium value of α, by definition. As such, and as noted by these authors, the fractionation factor closer to unity inferred from our lunar samples indicate higher values of pCu/ pCu<sub>eq</sub> than 0.993, meaning lunar volatile depletion occurred at near-equilibrium conditions (see also Wang and Jacobsen, 2016; Nie and Dauphas, 2019).

Complementary studies have shown that mare basalts have similar Zn, S and Cl isotope compositions, and generally enriched in heavy isotopes relative to the Earth, suggesting that the giant impact and/or lunar magma ocean degassing resulted in the enrichment in heavy isotopes of Zn, S and Cl in the mare source region (e.g., Gargano et al., 2022 and references therein), which reflect isotopic fractionation throughout lunar magma ocean crystallization and degassing, and late exhumation and crystallization of mare basalt lava flows. The similarities in the isotopic compositions of low-Ti and high-Ti argue against isotope fractionation during Lunar Magma Ocean crystallization due to variable extents of degassing and assimilation of late-stage melts (Gargano et al., 2020). In addition, Zn exhibits limited isotope fractionation during terrestrial tholeiitic basalt generation processes (e.g., partial melting [Δ⁶⁶Zn = 0.06 – 0.08 ‰] and fractional crystallization [Δ⁶⁶Zn ~ 0.07 ‰]; Chen et al., 2013; Wang et al., 2017; Sossi et al., 2018). Thus, despite variable Zn abundances due to fractional crystallization and assimilation and because mare basalts show a restricted range of isotope values, Gargano et al. (2022) suggested that mare basalts did not experience extensive post-eruptive volatile loss and/or the integrated effective isotopic fractionation factor associated with degassing was ~1, concluding that the lunar mantle inherited a heavy isotope enriched signature from a major vapour extraction event during the lunar formation (i.e., the giant impact event or magma ocean degassing) and remained mostly unaffected by processes during later mare volcanism. By contrast, the range of Cu isotopic compositions for lunar mare basalts determined herein show a relatively extended isotopic range, implying the mare basalts (or their sources) had experienced preferential loss of Cu to varying extents.

During vapour loss, the δ⁶⁶Zn of the residual magma increases and can be reproduced by evaporation of Zn as ZnCl₂ (or ZnF₂ if Cl is insufficient to bind Zn) on the Moon (e.g., Chou et al., 1975), while on Earth, Zn is evaporated as Zn²⁺ with a negligible fractionation between vapour and melt (e.g., Symonds et al., 1987). During evaporation of anhydrous (here taken to mean volatile-free) melts, Cu(g) and Zn(g) are the stable species (Sossi et al., 2019). For lunar analogue composition
<figure><img src="image_6.png" /><figcaption>Fig. 3. Values of δ$^{65}$Cu as a function of Cu content in lunar mare basalts and meteorites. Literature data from Moynier et al. (2006) and Herzog et al. (2009). Rayleigh distillation curves of α =0.99945 and different starting compositions are shown for comparison.</figcaption></figure>

gases, Renggli et al. (2017) showed that Zn(g) is the dominant Zn-gas species at all conditions, for calculations including a Gibbs Free En- ergy minimization at lunar conditions. Copper can also occur as Cu(g) at higher temperature and CuCl(g) below ~700 °C at IW-2 and 10⁻⁶ bar, meaning that Zn and Cu are not always transported as similar species (Renggli et al., 2017). The homogeneous equilibrium between Cu(g) and CuCl(g) can be written;

$$
\mathrm{Cu(g)} + \mathrm{Cl(g)} = \mathrm{CuCl(g)} \quad (1\mathrm{a})
$$

and is therefore dependent on the fCl of the gas phase. Similarly, CuS(g)
has also been predicted to be stable in terrestrial volcanic gases (e.g.,
Symonds et al., 1987; Aiuppa et al., 2004), and its formation can be
described as:

$$
2\text{Cu(g)} + \text{S}_2\text{(g)} = 2\text{CuS(g)} \quad (2)
$$

Because of the entropy of reactions (there are more gas molecules on the left-hand side with respect to on the right-hand side), both reactions favour the molecular Cu-bearing species (CuCl or CuS) down- temperature, with lnK(1) = 45,059/T -12.53 and lnK(2) = 20,223/T - 7.50. The equilibrium constants for the equivalent reactions involving Zn; Zn(g) + 2Cl(g) = ZnCl2(g) and 2Zn(g) + S2(g) = 2ZnS have lnK(1,Zn) = 77,393/T -26.73 and lnK(2,Zn) = -1132/T - 7.35.

At 1773K, the chloride species for Cu and Zn predominates over the
monatomic gas above 10⁻⁶ bar fCl or 10⁻⁴ bar fCl, respectively, as shown
in Fig. 4. The slope of log(pZnCl₂/pZn) as a function of fCl is two, steeper
than those of all other pairs, because pZnCl₂ is proportional to fCl²,
whereas pCuCl has a linear dependence on fCl. The partial pressures of
ZnS and CuS are both proportion to fS₂⁰.⁵, yielding a slope equivalent to
the exponent in all cases. Therefore, Cu may be more volatile than Zn for
fCl above ~10⁻⁶ bar up to ~10⁻² bar, providing a mechanism for the
evaporation of Cu whilst retaining Zn in the silicate liquid. Despite its
nominally volatile-poor nature, lunar volcanic gases are thought to be
composed primarily of H-bearing species (Rutherford et al., 2017;
Renggli et al., 2017; Varnam et al., 2024), in which case Cl is likely to
speciate as HCl according to the reaction H₂ + 2Cl = 2HCl, which
suppresses the formation of CuCl(g) and ZnCl₂(g) by lowering fCl.
Because neither the bulk compositions nor total pressures (perhaps
reaching 0.01 bar; Needham and Kring, 2017) of lunar vapour phases
during mare magmatism are precisely known, quantitative predictions
as to the prevalence of molecular species of Cu and Zn are uncertain.

<figure><img src="image_7.png" /><figcaption>Fig. 4. Equilibrium partial pressure ratios at 1773 K of MClₓ/M (green) and MS/M (blue) as a function of fCl or fS₂. M = Cu (solid lines) or Zn (dashed lines) and x = 1 (CuCl) or 2 (ZnCl₂), all expressed in log₁₀.</figcaption></figure>

There is no clear correlation between δ⁶⁵Cu and δ³⁷Cl (Fig. S3), though
this is based on a limited number of samples; more analyses would be
required to determine whether Cl played a role in enhancing the vola-
tility of Cu relative to Zn.

4.5. Estimation of the bulk Cu isotopic composition and content of the Moon

4.5.1. Bulk Cu isotopic composition of the Moon

The correlation between the Cu isotopic compositions and
abundances is observed in both samples from this study and from the literature (Herzog et al., 2009; Day et al., 2019). Building on the interpretation that vapour loss is likely responsible for the isotopic fractionation seen among mare basalts, we modeled the δ<sup>65</sup>Cu evolution of a residual magma during progressive evaporation. All mare basalt compositions can be reproduced by loss of Cu by evaporation following a Rayleigh distillation law, from an initial Cu content of ~16 µg/g (depletion of a factor ~10 with respect to terrestrial lavas: e.g., [Cu]<sub>BHVO.2</sub> = 129.3 µg/g; GeoRem) and δ<sup>65</sup>Cu of +0.57 ± 0.15 ‰ (with an empirical α<sub>vap/liquMoon</sub> of 0.99945; Sossi et al., 2020) (Figs. 3 and 5). We suggest that this value is the best estimate of the composition of the bulk silicate Moon.

With respect to the fractionation factor derived by Sossi et al. (2020) for evaporation in which isotopic fractionation is limited by mass transport in the vapour phase (and is proportional to [D<sub>ik</sub>/D<sub>jk</sub>]<sup>2/3</sup>, where D is the binary diffusion coefficient, i and j are the two evaporating isotopes and k is the background gas), the fractionation factor determined empirically here is much closer to unity. This suggests that either i) an open system is allowed and fractionation occurred at a steady-state in the diffusive limit in an atmosphere that was H<sub>2</sub>-rich (cf. Fig. 10 of Sossi et al., 2020) or ii) the value of p<sub>i</sub>/p<sub>i,eq</sub> approached unity and evaporation occurred at- or near-equilibrium. Physically, the latter condition is met as the total pressure increases, thereby limiting mass transport of the evaporating species, i, away from the surface (e.g., Young et al., 2019). Similarly, for the evaporation to have been diffusion-limited, relatively low values of D are required. Since D is proportional to 1/P (Chapman and Cowling, 1970), it also implies high pressures, above ~0.01 bar (see Fig. 8 of Sossi et al., 2020).

Authors in recent studies have reported limited isotopic fractionation between metal and silicate (Δ<sup>65</sup>Cu<sub>metal-silicate</sub> ~ 0.1–0.2 ‰; Savage et al., 2015; Xia et al., 2019), which, combined with the low mass of the lunar core (~1 % of its total mass) suggest a limited effect of the lunar core segregation on the isotopic composition of the lunar mantle, and hence permitting δ<sup>65</sup>Cu of +0.57 ± 0.15 ‰ to be adopted for the bulk Moon.

### 4.5.2. Bulk Cu content of the Moon

Previous attempts to estimate the Cu content of the lunar mantle gave values of ~2 µg/g of Cu (e.g., Ringwood and Kesson, 1977; O'Neill, 1991) following different methodologies (see Table 2 for details). While

<table><caption>**Table 2**
Estimates of the Cu concentration of the lunar mantle (µg/g).</caption><thead><tr><td>Cu (µg/g)</td><td>Refs.</td></tr></thead><tbody><tr><td>2.06</td><td>O'Neill (1991)</td></tr><tr><td>2.16</td><td>Ringwood and Kesson (1977)</td></tr><tr><td>7.22</td><td>Hauri et al. (2015)</td></tr><tr><td>11.0</td><td>Ni et al. (2019)</td></tr><tr><td>3.0</td><td>Righter (2019)</td></tr><tr><td>2.3</td><td>Gleißner et al. (2022); Sossi et al. (2025)</td></tr><tr><td>2.29 - 2.50<sup>a</sup></td><td>This study</td></tr></tbody></table>

<sup>a</sup>[Cu]BSE=25 µg/g (Sun et al., 2020).

Ringwood and Kesson (1977) used the ratio of lunar to terrestrial basalts and obtained a value of 2.16 µg/g of Cu, O'Neill's (1991) estimate (2.06 µg/g of Cu) is based on a mass balance calculation incorporating the giant impact that created the Moon, as well as further modifications such as volatilization, core formation and additional accretion of late-stage material. More recent studies gave a wider range of Cu contents for the lunar mantle from 0.66 to 11 µg/g (Hauri et al., 2015; Ni et al., 2019; Righter et al., 2019; Gleißner et al., 2022).

The approach here is to compare the mean abundances in terrestrial and lunar basalts, and then extrapolate this ratio to the lunar mantle abundance (e.g., Ringwood and Kesson, 1977; Wolf and Anders, 1980; see also Sossi et al., 2025). It is worth noting that this implicitly assumes that both types of basalts are derived from similar degrees of partial melting (i.e., Mid-Ocean Ridge Basalts – MORB and low Ti mare basalts). The BSE is estimated to 25 ± 5 µg/g of Cu (Sun et al., 2020), which is consistent with previous estimates based on peridotites, either from xenolith peridotites [20 µg/g of Cu (Palme and O'Neill, 2014)] or from extrapolating the Cu versus MgO correlation of post-archean massif peridotites [30 µg/g (McDonough, 2003; Wang and Becker, 2015)]. In addition, calculations based on partition experiments also suggest 20 µg/g of Cu for the BSE as a more realistic estimate (Corgne et al., 2008). Here, we adopt the value of 10 µg/g for the lunar melts, which corresponds to an average value of the Cu contents of low-Ti mare basalts whose δ<sup>65</sup>Cu is within uncertainty of the BSM value, and the value of 100 µg/g for primitive MORB (Jenner and O'Neill, 2012) to calculate that of the lunar mantle (see Table 3 for Cu abundances in mare basalts and MORB). Given the uncertainty in the lunar mare basalt value and

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Comparison of δ<sup>65</sup>Cu as a function of the fraction of remaining Cu. Initial δ<sup>65</sup>Cu is estimated to +0.57 ± 0.15 ‰ using the low Ti mare basalts, and corresponds to the value for the lunar mantle. The solid line is the linear regression of the dataset (R<sup>2</sup>=0.8455), and the dashed lines represent the 95 % confidence error envelope. Error bars: 2SD.</figcaption></figure>
<table><caption>**Table 3**
Abundances of Yb and Cu in mare basalts and MORB (µg/g).</caption><thead><tr><th>Elemental abundances (µg/g)</th><th>Moon - mare basalts</th><th>Earth - MORB</th></tr></thead><tbody><tr><td>Yb</td><td>3.5<sup>a</sup></td><td>3.2<sup>c</sup></td></tr><tr><td>Cu</td><td>10<sup>b</sup></td><td>100<sup>d</sup></td></tr></tbody></table>

a Sossi et al. (2025)

b This study

c Jenner et al. (2012)

the primitive Cu abundance, we expect the uncertainty on the Cu content of the lunar mantle to be close to 1 µg/g.

**4.5.2.1. First method of estimating the Cu content of the lunar mantle.**
Following this approach, the estimate for Cu in the BSE of 25 µg/g is used here to calculate the composition of the lunar mantle.

$$[Cu]_{\text{lunar mantle}} = \frac{[Cu]_{\text{mare basalt}}}{[Cu]_{\text{MORB}}} \times [Cu]_{\text{terrestrial mantle}} = 2.50 \, \mu\text{g/g} \quad (3)$$

**4.5.2.2. Second method of estimating the Cu content of the lunar mantle.**
In order to correct for differences in partial melting degrees between the Earth and the Moon, it is possible to normalize Cu to an element with similar incompatibility. The partition coefficient of Cu is ~0.1 to 0.2 (e. g., Fellows and Canil, 2012), which matches that of Yb, refractory lithophile element whose abundance in the lunar- and terrestrial mantles should have been unaffected by core formation or volatility (see Table 3 for Cu and Yb abundances in mare basalts and MORB).

Thus, the estimate for the lunar mantle corresponds to:

$$[Cu]_{\text{lunar mantle}} = \frac{(Cu/Yb)_{\text{mare basalt}}}{(Cu/Yb)_{\text{MORB}}} \times [Cu]_{\text{terrestrial mantle}} = 2.29 \, \mu\text{g/g} \quad (4)$$

Because Yb is lithophile and is hosted predominantly in clinopyroxene in the Earth's upper mantle relevant to the pressures under which MORB is generated, the implicit assumption made in applying such a normalisation is that Cu is also lithophile. Although the partition coefficient for Cu is ~10² – 10³ between sulfide- and silicate liquid (e.g., Kiseeva and Wood, 2013), the concentrations of Cu in MORB are consistent with its behaviour as a moderately incompatible element, implying that the role of sulfides, at the degrees of melting of ~10–15 %, was negligible (Fellows and Canil, 2012). On the other hand, Wang and Becker (2015) suggest the prevailing role of sulfides in maintaining the constancy of Cu/Ag ratios of peridotites. In either case, the correction remains empirical, and the similarity in the estimates derived from Eqs. (3) and (4) indicate minimal error was introduced by normalization to Yb.

<table><caption>**Table 4**
Moderately volatile element abundances in the Bulk Moon and Bulk Silicate Earth (µg/g).</caption><thead><tr><th rowspan="3">Elemental abundances<br>(µg/g)</th><th rowspan="3">BSE</th><th colspan="4">Bulk Moon</th><th colspan="4">Bulk Moon/BSE ratio</th></tr><tr><th colspan="2">Uncorrected</th><th colspan="2">Partial melting corrected</th><th colspan="2">Uncorrected</th><th colspan="2">Partial melting corrected</th></tr><tr><th>D<sub>metal-<br>silicate</sub>=25<sup>d</sup></th><th>D<sub>metal-<br>silicate</sub>=35<sup>e</sup></th><th>D<sub>metal-<br>silicate</sub>=25<sup>d</sup></th><th>D<sub>metal-<br>silicate</sub>=35<sup>e</sup></th><th>D<sub>metal-<br>silicate</sub>=25<sup>d</sup></th><th>D<sub>metal-<br>silicate</sub>=35<sup>e</sup></th><th>D<sub>metal-<br>silicate</sub>=25<sup>d</sup></th><th>D<sub>metal-<br>silicate</sub>=35<sup>e</sup></th></tr></thead><tbody><tr><td>Cu (this study) - 1 %<br>lunar core</td><td>25<sup>a</sup></td><td>3.10</td><td>3.35</td><td>2.83</td><td>3.06</td><td>0.124</td><td>0.134</td><td>0.113</td><td>0.123</td></tr><tr><td>Cu (this study) - 2.5 %<br>lunar core</td><td>25<sup>a</sup></td><td>4.00</td><td>4.63</td><td>3.66</td><td>4.23</td><td>0.160</td><td>0.185</td><td>0.146</td><td>0.169</td></tr><tr><th colspan="10" style="text-align: left;">Bulk Moon</th></tr><tr><td>K</td><td>260<sup>b</sup></td><td></td><td>36.9<sup>c</sup></td><td></td><td></td><td></td><td>0.142</td><td></td><td></td></tr><tr><td>Rb</td><td>0.605<sup>b</sup></td><td></td><td>0.605<sup>c</sup></td><td></td><td></td><td></td><td>0.215</td><td></td><td></td></tr></tbody></table>

a Sun et al (2020)

b Palme and O'Neill (2014)

c Taylor and Wieczorek (2014)

d Schmitt et al. (1989)

e Savage et al. (2015)

**4.5.2.3. Bulk Moon Cu estimate.** Accounting for the segregation of the lunar core, mass balance calculations using Ni abundances in the Moon constrain the lunar core to ~1 % of the total lunar mass (e.g., O'Neill, 1991; Sossi et al., 2025), if the lunar core formed at its present-day pressure. Alternatively, Steenstra et al. (2016) proposed that the lunar core comprises 2.5 wt.% of the mass of the Moon, for a S-rich core (> 8 wt.%) required to satisfy all the siderophile element (including Cu) depletions at mantle temperatures between solidus and liquidus in the absence of volatile-induced depletions.

Then, the Bulk Moon Cu abundance is given by the following equation:

$$[Cu]_{BM} = f_{LM}[Cu]_{LM} + f_{core}[Cu]_{core},$$

where BM corresponds to the Bulk Moon composition, LM to the lunar mantle and *f* is the mass fraction of each reservoir. Metal-silicate partition coefficients for Cu are between 25 and 35 (e.g., Schmitt et al., 1989; Savage et al., 2015; Steenstra et al., 2016), following the experimental parameterization from Corgne et al. (2008). The results of these calculations are given in Table 4, and give an estimate ranging from 2.83 to 3.35 µg/g, with an average of 3.09 ± 0.42 µg/g (2SD, n = 4) of Cu for the Bulk Moon with a 1 wt.% core mass, which is within uncertainty of that derived from Eqs. (3) and (4). Assuming an improbably high 2.5 wt.% core mass, [Cu]BM is 4.13 ± 0.81 µg/g (2SD, n = 4). These estimates are within the range of Cu contents previously reported for the Bulk Moon [2.0 µg/g (Righter et al., 2019) – 6.9 µg/g (Herzog et al., 2009)]. Note that the Bulk Moon estimate from Herzog et al. (2009) represents an average of individual mare basalt samples.

It is worth mentioning that lowering the core mass leads to higher required values for the metal-silicate partition coefficients (Steenstra et al., 2016). The Bulk Moon is significantly depleted in Cu (and other volatile elements) compared to the Bulk Silicate Earth (e.g., Ringwood and Kesson, 1977; Seifert and Ringwood, 1987; Day and Moynier, 2014; Taylor and Wieczorek, 2014). Such depletions have been interpreted as resulting only from core formation in the Moon (Steenstra et al., 2016), with the rationale being that a large volatilization event would only have resulted in the requirement of lower partition coefficients than those used in the model, thereby increasing the likelihood that these chemical signatures could have been produced through core segregation. However, metal-silicate experiments give positive Δ⁶⁵Cumetal-silicate (~0.1 – 0.2 ‰; where Δ⁶⁵Cumetal-silicate = δ⁶⁵Cumetal - δ⁶⁵Cusilicate), meaning that the metal phase is consistently enriched in the heavier Cu isotope (Savage et al., 2015; Xia et al., 2019). Therefore, the positive δ⁶⁵Cu values observed in lunar mare basalts cannot be explained by metal-silicate fractionation, because the isotopic fractionation is not in
the correct sense and too small to reproduce up to ~4‰ fractionation in the low-Ti mare basalts. In addition, as proposed for the Hadean Matte for the Earth, isotopic fractionation resulting from the segregation of sulfide-rich reservoir would also contribute in the enrichment in the heavier Cu isotope, as it has been showed that sulfides appear enriched in the light Cu isotope relative to the bulk starting material in partitioning experiments (Savage et al., 2015; Xia et al., 2019). However, it seems unlikely that the BSM, with even less S, would be saturated in a sulfide phase, since mare basalts are not saturated in sulfides (see discussion above). Moreover, investigating depletion factors for Cu and other moderately volatile elements, it appears that Cu is more depleted in the Moon than is Rb, regardless of the size of the lunar core considered, and similar- to or more depleted than K (Table 4), as predicted by the volatility sequence from vaporization of silicate melts (Sossi et al., 2019). The Cu isotopic composition of the Moon must also be, at least partly, inherited by significant devolatilization during the Moon-forming impact or the lunar magma ocean. Recent work on K isotopes showed that lunar mare basalts and meteorites have systematically heavier K isotopic compositions than the Bulk Silicate Earth (BSE), suggesting that volatile depletion proceeded to a greater extent, producing stronger K isotope fractionation by evaporation relative to their potential precursor materials (Tian et al., 2021), an interpretation supported by results of N-body simulations (Sossi et al., 2022). This hypothesis that volatile depletion and associated isotopic fractionation should be more pronounced on smaller bodies holds for Zn and Cu isotopic compositions for the BSE ([Cu]: Savage et al., 2015; [Zn]: Sossi et al., 2018), Mars ([Zn]: Paquet et al., 2023b) and the Moon ([Cu]: this study; [Zn]: Paniello et al., 2012). To resolve the relative contribution of sulfide sequestration and volatilization, Cu isotopic fractionation between metal and sulfides could be further tested in the future. The effect of sulfide sequestration is expected to be limited ($\Delta^{65}\text{Cu}_{\text{sulfide-silicate}} \sim -0.1 - 0.2 \text{‰}$; Xia et al., 2019) relative to global volatilization processes ($\Delta^{65}\text{Cu}_{\text{solid-gas}} \sim -5.5 \text{‰}$ for an $\alpha$ value of 0.99945 to reproduce the mare basalt dataset with the Rayleigh distillation curves described in this study).

## 5. Conclusions

We measured the Cu isotopic composition of lunar meteorites and Apollo samples to investigate the origin of volatile depletion history of the Moon. Copper isotope compositions of mare basalts and meteorites are heavier than the Bulk Silicate Earth. The Cu isotopic composition of the Moon is not inherited from its precursors as it is not possible to reproduce its composition with contribution from chondritic reservoirs given that all chondrites are isotopically lighter than the Moon. The mare basalts Cu isotopic compositions are consistent with a major vapour extraction event during the lunar formation rather than mare basalt volcanism. Our dataset allows assessment of the Cu isotope composition, representative of the Bulk Silicate Moon of $\text{\Delta}^{65}\text{Cu}$ of +0.57 ± 0.15 ‰, and a Cu content of 2.29 to 2.50 µg/g. The Cu abundance of the Bulk Moon can be corrected for the segregation of the lunar core to a range of 3.09 to 4.13 µg/g of Cu, for a core size of 1 to 2.5 wt.%. Despite recent studies suggesting that missing volatile elements of the Moon, such as Cu, are stored in the core, the positive $\Delta^{65}\text{Cu}_{\text{metal-silicate}}$ in isotope equilibration experiments cannot explain the extent nor the direction of the large isotopic fractionation observed in the BSM. Volatilization and sulfide sequestration would result in the enrichment in the heavier Cu isotope of the silicate melt. The relative contribution of these two processes is expected to be in favour of significant volatilization during the Moon-forming impact or the lunar magma ocean.

## Funding

This work was supported by the IPGP analytical platform PARI, Region Ile-de-France SESAME Grants no. 12,015,908, and DIM ACAV +, the ERC grant agreement No. 101,001,282 (METAL) (F.M.) the

UnivEarthS Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) (F.M.). J.M.D.D. acknowledges the support of the NASA Solar System Workings Program for participation in this work (80NSSC19K0932).

## Data and materials availability

All data are available in the main text or the supplementary materials.

Table S1. References for major element abundance data for lunar rocks.

## CRediT authorship contribution statement

**Marine Paquet:** Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Conceptualization. **Frederic Moynier:** Writing – review & editing, Validation, Supervision, Investigation, Conceptualization. **Paolo A. Sossi:** Writing – review & editing, Validation, Investigation, Formal analysis, Conceptualization. **Wei Dai:** Writing – review & editing, Formal analysis. **James M.D. Day:** Writing – review & editing, Validation, Investigation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119250.

## Data availability

Data will be made available on request.

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