# Tin isotopes indicative of liquid-vapour equilibration and separation in the Moon-forming disk

Xueying Wang, Caroline Fitoussi, Bernard Bourdon, Bruce Fegley, Sébastien Charnoz

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Xueying Wang, Caroline Fitoussi, Bernard Bourdon, Bruce Fegley, Sébastien Charnoz. Tin isotopes indicative of liquid-vapour equilibration and separation in the Moon-forming disk. Nature Geoscience, 2019, 12 (9), pp.707-711. 10.1038/s41561-019-0433-4. insu-02413699

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# Tin isotopes indicative of liquid-vapour equilibration and separation in the Moon-forming disk

Xueying Wang<sup>D1</sup>, Caroline Fitoussi<sup>D1*</sup>, Bernard Bourdon<sup>1</sup>, Bruce Fegley Jr<sup>2</sup> and Sébastien Charnoz<sup>3</sup>

The depletion in moderately volatile elements in the Moon relative to Earth and comparison of the isotope compositions of the Moon and Earth have placed important constraints on models of lunar formation. A liquid-vapour protolunar disk from a high-energy giant impact has been proposed to explain some of these constraints. Here we present high-precision tin isotope data for lunar rocks, measured by double-spike MC-ICP-MS (multicollector inductively coupled plasma spectrometry). The lunar rocks are enriched in light tin isotopes compared to the Earth (Δ<sup>124/116</sup>Sn<sub>Moon-Earth</sub> = -0.48 ± 0.15‰). On the basis of our data and constraints on tin speciation, we show that this tin isotope fractionation is inconsistent with volatile loss from a lunar magma ocean. Instead, we propose a scenario with vigorous mixing between the protolunar disk and the Earth in high-energy conditions during the impact, followed by liquid-vapour equilibration and phase separation at around 2,500 K while the disk was cooling. This scenario is consistent with the depletion in moderately volatile elements and isotope composition of the Moon.

Understanding how the Moon-forming impact is linked to the lunar and terrestrial chemical and isotope compositions has become a key question in planetary science. Recent lunar formation models have suggested that volatile element depletion in the Moon resulted from the vapour phase falling back to the Earth<sup>1-3</sup>. An important novelty of these models is that they predict the generation of a two-phase protolunar disk with high initial temperatures, and thus high vapour fraction<sup>4</sup>, as a consequence of a high-energy giant impact<sup>1,5</sup>. Such conditions in the disk might have favoured efficient mixing that led to the striking Earth-Moon isotope similarities (that is, Δ<sup>17</sup>O (refs. 6,7), ε<sup>182</sup>W (refs. 8,9) (before late accretion), and nucleosynthetic anomalies (for example Ti (ref. 10), Ca (refs. 11,12), Cr (ref. 13))) due to vigorous convection immediately following the impact, avoiding the need for an impactor that is unlike any known meteorite body<sup>9,14,15</sup>. It has then been argued that volatile loss in the protolunar disk took place under such high-temperature conditions<sup>1,2</sup>. Subsequent processes in the protolunar disk and after the Moon accreted (except for late accretion), such as condensation, evaporation or hydrodynamic escape of vapour above a lunar magma ocean, would have only modified stable isotope compositions in the form of mass-dependent fractionation<sup>16</sup>. Therefore, while isotopic anomalies offer strong constraints on the dynamics of the initial stages of the protolunar disk, during which all isotopic signatures might have been homogenized to the terrestrial value, the study of moderately volatile element isotope compositions should provide insights into the later stage conditions of the Moon history.

Investigation of mass-dependent fractionation of magnesium isotopes<sup>17</sup> and silicon isotopes<sup>14,18</sup> has shown identical Mg and Si isotope compositions for the Earth's mantle and the Moon, whereas rubidium, gallium, potassium and zinc isotope systematics have revealed that the Moon is enriched in heavy isotopes compared with the Earth<sup>19-25</sup>. This contrasting behaviour could reflect differences in volatilities between Mg and Si on the one hand and the more

volatile Rb, Ga, K and Zn on the other. The Zn and K observations have been interpreted as resulting from kinetic isotope fractionation. There is a large range in Zn isotope data in lunar samples reported for mare basalts, magnesium suite rocks, ferroanorthosites (FAN) and pyroclastic glasses. The negative δ<sup>66</sup>Zn observations reported for the pyroclastic glasses, FAN and some mare basalts can be interpreted as a result of secondary processes related to recondensation of volatilized Zn. In the case of mare basalts, leaching of this surficial Zn yields a δ<sup>66</sup>Zn value identical to that of uncontaminated samples (see below). Enrichment in heavy Zn isotopes in the magnesium suite rocks and some FAN is thought to reflect at least two stages of evaporation following a global event. These late-stage processes that led to a large dispersion in Zn isotopes are not further discussed in this study. Rather, we focused on the large-scale process reflected in the rather homogeneous Zn isotope composition of low-Ti and high-Ti basalts (δ<sup>66</sup>Zn = +1.4‰) as shown to be representing the bulk silicate Moon<sup>22-25</sup>: this will be the Zn isotope composition we will be referring to in the remainder of this study. More specifically, this Zn isotope signature was initially thought to reflect a global-scale kinetic isotope fractionation<sup>22</sup>, and the interpretations were later refined and called upon isotope fractionation during vapour loss from a lunar magma ocean<sup>25</sup>. In contrast with Zn isotopes characterized from a large data set, the difference in K isotopes in the Moon (from seven lunar rocks) relative to the Earth has been related to partial condensation of vapour in the protolunar disk<sup>21</sup>. To better understand the depletion of moderately volatile elements in the Moon, we sought new constraints from another moderately volatile element, tin, which has different geochemical characteristics from Zn and K and a distinct speciation in gas and condensed phases.

## Tin isotope composition of the Moon

In this study, the Sn isotope composition of lunar rocks (Fig. 1 and Supplementary Table 1) were measured for five low-Ti basalts, four

<footnote n="1">¹Laboratoire de Géologie de Lyon, ENS Lyon, CNRS UMR 5276, Université de Lyon, UCBL, Lyon, France. ²Planetary Chemistry Laboratory, McDonnell Center for the Space Sciences and Department of Earth and Planetary Sciences, Washington University, Saint Louis, MO, USA. ³Institut de Physique du Globe de Paris, Sorbonne Paris Cité, CNRS UMR 7154, Université Paris Diderot, Paris, France. *e-mail: caroline.fitoussi@ens-lyon.fr</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** | Sn isotope composition of lunar samples. Sn isotope data measured in low-Ti (diamonds) and high-Ti basalts (circles) and one crustal rock (square). Data are taken from Supplementary Table 1. The composition of the BSE determined by Wang et al.²⁸ is also indicated by the grey band. Samples 15555 and 74255 are marked by lighter shades to indicate that the lower Sn quantities for these samples resulted in larger uncertainties compared with the other samples. Error bars are 2σ.</figcaption></figure>

high-Ti basalts and one norite using a recently developed high-precision method²⁶. The mean δ¹²⁴Sn value (where ¹¹⁶Sn is the normalizing isotope) and Sn concentrations for the low-Ti basalts are -0.56 ± 0.06‰ (weighted 2σ) and 83 ± 8 ppb, respectively, while the high-Ti basalts are characterized by a mean Sn concentration of 93 ± 4 ppb and a mean δ¹²⁴Sn value of -0.54 ± 0.06‰ (weighted 2σ). There is no Sn isotope difference between the low-Ti basalts, representing up to 70% of lunar magma ocean crystallization, and the high-Ti basalts, representing melts of a mantle source that formed after 95% crystallization²⁷. In addition, one sample (norite) from the Mg suite has a δ¹²⁴Sn value within error of those two lunar reservoirs. Thus, according to these observations and arguments developed in the Supplementary Information, the estimated δ¹²⁴Sn value of the bulk silicate Moon (BSM) is δ¹²⁴Sn<sub>BSM</sub> = -0.56 ± 0.04‰ (weighted 2σ, Supplementary Table 1). In contrast with the Zn and K isotope observations²¹⁻²³, all the lunar rocks show an enrichment in light Sn isotopes compared with the bulk silicate Earth (BSE). The tin isotope composition of the BSE²⁸ has been estimated to be δ¹²⁴Sn<sub>BSE</sub> = -0.08 ± 0.11‰ (2σ). Thus, the offset in δ¹²⁴Sn between the BSM and the BSE is Δ¹²⁴Sn<sub>Moon-Earth</sub> = -0.48 ± 0.15‰ (2σ). A larger negative offset is obtained using the data of Badulovich et al.²⁹; however, their BSE value is not anchored on the same standard as that used in this study (further discussion in Supplementary Information). Using our new Sn concentration data in lunar rocks (Supplementary Table 1), the elemental depletion factor for Sn (Supplementary Fig. 4) in the Moon relative to BSE was shown to be 1/8, intermediate between that of K and Zn (ref. ³⁰), which is consistent with the condensation curve of Sn being intermediate between that of K and Zn over a wide range of pressures and temperatures relevant to the disk (Fig. 2). In what follows, we propose a single process to account for the elemental and isotope fractionations of K, Zn and Sn. Kinetic isotope fractionation during partial evaporation yields an enrichment in heavy isotopes in the source reservoir (the residual solid or liquid), and partial condensation yields

<figure><img src="image_3.png" /><figcaption>**Fig. 2** | *T*<sub>50%</sub> condensation curves for K, Sn and Zn as a function of total pressure and temperature in a disk wet BSE composition. The broken lines show the central plane pressure of the protolunar disk represented by the total vapour pressure of a melt of dry BSE composition (dotted line, this work) or of pure *SiO*₂ (dashed line, ref. ¹). The total pressure for the 'wet' BSE would be offset to the right of the dry BSE line. The exact conditions of the protolunar disk lie between these wet and dry conditions. The grey band shows the temperature range that is consistent with our model.</figcaption></figure>

an enrichment in light isotopes in the sink reservoir (the condensate)³¹. Therefore, a kinetic isotope fractionation cannot be invoked to explain the observations if the three isotope systems are to be explained by the same process, given that samples representative of the bulk lunar mantle are enriched in heavy K and Zn isotopes whereas the Sn isotopes show an enrichment in light isotopes.

## Equilibrium isotope fractionation during Moon formation

Here, we have explored an alternative approach for producing the observed elemental depletions in Sn, K and Zn and the three associated isotope compositions in the BSM by considering global processes inducing liquid-vapour equilibrium isotope fractionation. One can imagine such a process in two contexts: (1) liquid-vapour equilibrium took place when the Moon had a degassing lunar magma ocean²⁵, resulting in equilibrium isotope fractionation between liquid and gas³² while vapour was lost by hydrodynamic escape; (2) in the outer part of the cooling protolunar disk, vapour was lost by a mechanism leading to liquid-gas separation after having reached equilibrium conditions between liquid and vapour. Equilibrium isotope fractionation is a function of temperature and depends on the nature of species in each phase of interest.

To test these scenarios, we first need to determine the speciation of Sn, Zn and K in the liquid and the gas. For this purpose, we computed the speciation of the condensed phase using a thermodynamic approach³³. In the liquid, Sn<sub>metal</sub> is the dominant species in the *T*—*f*<sub>O₂</sub> domain relevant to BSE and BSM compositions, for temperatures above 2,000 °C (Supplementary Fig. 1). In the gas, tin is mainly present as SnO (g), regardless of oxygen fugacity³⁴. With the information about the speciation of Sn in the vapour and liquid (Supplementary Information), one can show that the Sn isotope fractionation between the vapour and the liquid phases results in an enrichment in light isotopes in the liquid. If we consider the interpretation of the Zn isotope data for the lunar mantle represented by mare basalts (discussion above), on the basis of new determinations of the isotope fractionation factors between Zn (g) and ZnO (s) (ref. ³⁵)
<figure><img src="image_4.png" /><figcaption>**Fig. 3** | Cartoon depicting the processes that could lead to volatile element depletion in the protolunar disk. The vapour is in thermodynamic equilibrium with the liquid that is concentrated in the midplane. The vapour is decoupled from the liquid and moves globally inward as indicated by the arrows. This process leads to volatile element depletion in the region beyond the Roche limit where the Moon ultimately accretes. The combined Zn, K and Sn isotope data provide constraints on the temperature of liquid-vapour separation.</figcaption></figure>

(which are the relevant species in the vapour and liquid), it is possible to infer qualitatively that, under equilibrium conditions, the condensed phase should be enriched in heavy Zn isotopes, in agreement with the Zn isotope observations<sup>22,23</sup>. In contrast with Zn, there are no available isotope fractionation factors for K in the literature; K (g) is the main K vapour species over K-bearing silicate melt<sup>26</sup>. To estimate the K isotope fractionation factor between the vapour and the silicate, we used the force constants for K<sub>2</sub>O and for K in alkali feldspar<sup>37</sup> (this clearly represents an approximation). This calculation shows that the condensed phase is enriched in heavy K isotopes (details in Supplementary Information). With these estimates of the isotope fractionation factors for Sn, Zn and K, it is possible to estimate the expected equilibrium isotope fractionation between vapour and melt. Overall, the isotope fractionation in one direction for K and Zn, and in the opposite direction for Sn, can be explained by an equilibrium process, and this stems directly from their respective speciation in the liquid (Sn, ZnO, K<sub>2</sub>O) and in the vapour phase (SnO, Zn, K).

Had the isotope fractionation occurred in the context of the first scenario (vapour loss above a lunar magma ocean), the speciation of the liquid phase in equilibrium with the vapour would have been modified compared with the protolunar disk setting. Indeed, under the (P, T, f<sub>O₂</sub>) conditions of lunar metal/silicate differentiation conditions, the major part of the tin budget will be in the magma ocean as SnO, as shown by studies of partitioning of siderophile elements in the Moon<sup>38</sup> corresponding to the P, T conditions of a lunar magma ocean. The lunar core rapidly segregated tin as zerovalent Sn (metal) in the liquid metal (lunar core), leaving SnO as the only tin species in the silicate part of the Moon (lunar mantle), since metallic tin is not partitioned into silicate melt<sup>39</sup>. Therefore, if Sn isotope fractionation had been associated with vapour loss in this context, SnO (g) would have equilibrated with SnO in the silicate part of the Moon, which could not have produced the observed lunar Sn isotope composition. Indeed, the Sn isotope fractionation factor between SnO (g) and SnO (l) is small (−0.02‰ at 2,000 K; Supplementary Information) and in the opposite sense of what is required. In addition, equilibrium between Sn<sub>metal</sub> in the lunar core and SnO in the lunar mantle should enrich heavier Sn isotopes in the mantle. In this case, the value of Sn isotope of the bulk Moon would be slightly lighter than the observed BSM value. However, quantitatively, this isotope fractionation of Sn during lunar core formation is negligible (Supplementary Information). By contrast, in the second scenario (vapour loss at

the protolunar disk stage), the speciation of tin should be considered in the context of vapour-liquid equilibrium between gas and liquid drops established under the physical (P, T) conditions of the cooling protolunar disk. Using our thermodynamic predictions, Sn(0) is expected to be the major species in the condensed phase (Supplementary Fig. 1) while SnO is expected to be the main species in the vapour<sup>34</sup> (Supplementary Information). We assume that liquid-gas equilibrium established in small enough moonlets, where no metal-silicate segregation was involved.

## Liquid-vapour equilibration model

Given the small magnitude of equilibrium fractionation factors of K, Zn and Sn at high temperature between liquid and vapour, we have considered a model where the equilibrium isotope fractionation is enhanced by repeated episodes of vapour-liquid equilibration followed by vapour removal from the system to the inner disk while the disk was cooling (Fig. 3). Various mechanisms of vapour-melt separation have been described<sup>1-3</sup>. For example, if the vapour and liquid phases are vertically stratified and have different effective viscosities, they will radially separate from each other because they viscously spread at different rates and lead to volatile element depletion in the region beyond the Roche limit. Such difference of viscosity between phases could, for example, stem from the liquid phase being gravitationally unstable, in equilibrium with a magneto-hydrodynamic (MHD) turbulent vapour<sup>40</sup>. In addition, gravitational interaction of the Earth with the proto-moons may prevent the volatile-rich elements from falling back to the protolunar Moon¹. A protracted Moon accretion could result from vigorous convection in the liquid-vapour mixture<sup>41</sup>. In the outer part of the cooling protolunar disk, different physical mechanisms may separate vapour from liquid: viscosity differences<sup>3</sup>, gravitational torque¹ or gas drag on big droplets<sup>2</sup>. Thus, the depletion in moderately volatile elements in the Moon could stem from equilibrium between droplets and subsequently removed vapour in the outer part of the protolunar disk.

We performed thermodynamic calculations over a range of P, T, f<sub>O₂</sub> conditions to compute the distribution of Sn, Zn and K between melt and vapour in the protolunar disk (Fig. 2). These results were used to model the Sn, Zn and K isotope compositions and elemental depletions of the liquid phase that is assumed to equilibrate with the vapour phase. By considering a multistage process of liquid-vapour equilibration after which the vapour is removed repeatedly, it was possible to match the observed mean Sn, Zn and K isotope fractionation
<figure><img src="image_5.png" /></figure>

**Fig. 4** | Evolution of $\Delta^{124}\text{Sn}_{\text{Moon-BSE}}$ as a function of Sn concentration in the condensate. The curves were calculated using a multistage vapour-liquid equilibration model for Sn isotopes. The tested initial starting temperatures are 3,000 K, 2,700 K and 2,400 K with five steps of decreasing temperature ($\Delta T = 150$ K), corresponding to the three model curves. The mass fraction of Sn in the condensate at each stage ($X_m^{Sn}$) is variable and ranges from 0.52 (at start) to 0.97 with 0.09 increments. The calculated $\delta^{124}\text{Sn}$ difference between Moon and BSE ($\Delta^{124}\text{Sn}_{\text{Moon-BSE}}$) versus Sn depletion in the Moon (normalized to initial Sn abundance in the condensed phase) is plotted, and the shaded area represents the observed values.

between the Moon (as represented by the low-Ti and high-Ti basalt composition) and the BSE. The model could reproduce jointly the $\delta^{124}\text{Sn}$, $\delta^{68}\text{Zn}$ and $\delta^{41}\text{K}$ in lunar rocks as well as the observed Sn (~1/8), Zn (~1/100) and K (1/5) depletion in the Moon relative to the BSE (Fig. 4 and Supplementary Information). Note that this model does not attempt to reproduce the large Zn isotope variations observed for example in FAN and pyroclastic green glasses that are caused by secondary processes occurring after the large-scale event. The conditions to reproduce the elemental and isotopic data were obtained for protolunar disks with initial liquid-vapour equilibrium temperatures of 2,400–2,700 K, followed by removal of the vapour, then another liquid-vapour equilibrium, while the system was cooling (Fig. 4 and Supplementary Information). This interpretation is also in qualitative agreement with Rb and Ga isotope observations that show an overall enrichment in heavy isotopes in lunar rocks relative to the BSE¹⁹,²⁰. As the Rb and Ga species in the vapour phase are Rb (g) and Ga₂O (g), respectively, both vapour species have a lower oxidation state than in the condensed phase (Rb₂O and Ga₂O₃, respectively)³⁴,⁴²,⁴³, which is again qualitatively consistent with the observed enrichment in heavy Rb and Ga isotopes in the condensed phase in equilibrium conditions.

Recent models attempting to explain the depletion of the Moon in volatile elements, where dynamic simulations of the impact have been coupled with thermodynamics calculations, resulted in temperatures of vapour-melt separation up to 3,500 K¹,². However, the liquid-vapour equilibrium range of temperatures obtained for our best fit to the observed elemental and isotope data for Sn, Zn and K is much lower. This difference is probably due to two factors. First, our calculations were done using activity coefficients from the FactSage or MELTS codes³³ instead of those from the MAGMA code¹ or GRAINS² used previously. Second, earlier studies¹,⁴⁴ used the vapour pressure of pure molten silica instead of the actual vapour pressure of the complex silicate melt in the protolunar disk for their P–T profile of the disk (see figures 2 and 3a in ref. ³⁶).

While a full understanding of the conditions of volatile depletion in the Moon awaits a deeper investigation of the protolunar disk and of the processes associated with liquid-vapour equilibrium, Sn isotopes provide new perspectives compared with K and Zn isotopes. The new Sn isotope constraints suggest that Sn, K and Zn isotope compositions of the BSM (as defined in the preceding) cannot be explained by a vapour loss in the context of the lunar magma ocean. By contrast, volatile element depletion in the Moon could result from vapour separation in the protoplanetary disk stage and further suggests a relatively cold temperature for this process (~2,500 K). These observations will require a better understanding of the processes of liquid-vapour separation during Moon formation.

## Online content

Any methods, additional references, Nature Research reporting summaries, source data, statements of code and data availability and associated accession codes are available at https://doi.org/10.1038/s41561-019-0433-4.

Received: 8 October 2018; Accepted: 23 July 2019;
Published online: 26 August 2019

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## Acknowledgements

We thank the CAPTEEM of NASA for providing the lunar rocks. This project was funded by ANR project ISOVOL and by LABEX LIO (Lyon Institute of Origins). B.F. was funded by grants AST-1517541 and AST-1412175 from the NSF Astronomy Program and NASA grant NNX17AC02G (XRP Program), and thanks N. S. Jacobson for useful discussions. We thank J. Creech for insightful comments that helped improve this manuscript.

## Author contributions

X.W. and C.F. developed the chemical and isotopic methods. X.W. performed Sn isotope and abundance analysis of all samples. C.F. and B.B. conceived the study, organized sample acquisition, contributed to the modelling and wrote significant parts of the manuscript. B.F. performed the calculations of condensation temperatures and the speciation in the condensed phase. S.C. carried out the equilibration timescale between liquid and vapour and associated modelling. All authors contributed to the interpretation of results and the writing of the manuscript.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** is available for this paper at https://doi.org/10.1038/s41561-019-0433-4.

**Reprints and permissions information** is available at www.nature.com/reprints.

**Correspondence and requests for materials** should be addressed to C.F.

**Publisher's note:** Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© The Author(s), under exclusive licence to Springer Nature Limited 2019
Methods

The samples were analysed using a method that has been described in detail elsewhere<sup>35</sup>. The samples were first ground in an agate mortar to avoid any Sn contamination. Using three ion-chromatography columns, Sn was separated from its matrix and potential isobaric interferences (for example, In, Cd and Te) with a final yield higher than 95%. The instrumental mass bias or any isotope fractionation produced during the chemical separation was corrected for with a <sup>117</sup>Sn-<sup>122</sup>Sn double spike.

The isotope measurements were done with an MC-ICP-MS Neptune Plus (Thermo Scientific), equipped with an Aridus II and jet cones, at the Laboratoire de Géologie de Lyon, ENS Lyon. The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 3161a standard was used as a primary standard for calculating δ<sup>124</sup>Sn values (normalized to <sup>116</sup>Sn). Each sample was bracketed with a mixture of NIST SRM 3161a standard and <sup>117</sup>Sn-<sup>122</sup>Sn double-spike solution. The <sup>116</sup>Sn and <sup>118</sup>Sn signals were corrected with an on-peak zero to remove molecular interferences formed by Ar<sub>3</sub> in the plasma source. In addition, <sup>116</sup>Sn was corrected for minor isobaric interference of <sup>116</sup>Cd. The Sn isotope compositions were obtained using a Matlab program for Sn double-spike inversion described previously<sup>45</sup>. The results are reported as δ<sup>124</sup>Sn (using <sup>116</sup>Sn, <sup>117</sup>Sn, <sup>122</sup>Sn and <sup>124</sup>Sn for the inversion) relative to the NIST SRM 3161a standard. Tin isotope compositions are reported as δ<sup>124</sup>Sn values in parts per thousand (%c):

$$ \delta^{124}\text{Sn} = \left( \left( \frac{^{124}\text{Sn}}{^{116}\text{Sn}} \right)_{\text{sample}} / \left( \frac{^{124}\text{Sn}}{^{116}\text{Sn}} \right)_{\text{NIST3161a}} - 1 \right) \times 1,000 $$

The notation δ<sup>124/118</sup>Sn indicates a normalization to the <sup>124</sup>Sn/<sup>118</sup>Sn ratio. The internal reproducibility obtained for the NIST SRM 3161a standard with a 10 ppb concentration was better than 0.06%c (2σ) on δ<sup>124</sup>Sn for a single session. Repeated Sn isotope measurements of the USGS (United States Geological Survey) standard reference andesite AGV-1 from multiple digestions, chemical purification and mass spectrometer measurements yielded a reproducibility of 0.11%c on δ<sup>124</sup>Sn (2σ; Supplementary Table 2). This value was adopted as the external reproducibility for our lunar samples with reproducibility better than this value. For low-level samples (15555 and 74255), the dilution of the run solution was adapted to ensure a minimum signal of 0.9 ± 0.1 V on the <sup>124</sup>Sn peak (0.9 ± 0.1 × 10<sup>-11</sup> A), which was the same intensity as for other runs (Supplementary Table 2 shows individual runs of these samples). This means that the signal/noise ratio was always the same for all sample acquisitions to minimize potential accuracy issues. However, the counting

statistics for these samples were less favourable, and this was further quantified using standards measured with the same number of cycles. To further assess the analysis of low-level samples (less than 20 ng), we have prepared a Sn-free matrix from the standard reference basalt BE-N by separating quantitatively Sn from the natural sample. The different fractions in the column separation were grouped to reconstruct the Sn-free matrix of BE-N. Two aliquots (#1 and #2) were prepared with 10 ng of NIST SRM 3161a added to aliquot #1, while 20 ng were added to aliquot #2. Both aliquots were processes for chemical separation and analysed by MC-ICP-MS using the same protocol as the lunar samples. The results are reported in Supplementary Table 2 and show full agreement with the NIST standard composition. This test is important to show the accuracy of our results as it yields δ<sup>124</sup>Sn and δ<sup>124/118</sup>Sn equal to the standard value within error. For reference, the results of standard runs bracketing the Sn-free BE-N matrix doped with 10 ng of Sn are reported in Supplementary Table 2 as well.

The data were also inverted using <sup>118</sup>Sn, <sup>117</sup>Sn, <sup>122</sup>Sn and <sup>124</sup>Sn (reported as δ<sup>124/118</sup>Sn) to check for data consistency, and no differences were observed outside of the quoted uncertainties (Supplementary Table 1 and Supplementary Fig. 9). This indicates that measurements were carried out without artifacts caused by isobaric interferences and that the role of spallation reactions or neutron capture on Sn isotopes was limited for our samples as any of those reactions would have produced discrepancies between the δ<sup>124/118</sup>Sn and δ<sup>124</sup>Sn values, since the cross sections for these reactions differ by factors of 2 or more between isotopes<sup>46,47</sup>. The Sn concentrations were determined by isotope dilution using the <sup>117</sup>Sn/<sup>116</sup>Sn ratio with a precision better than 4% (relative standard deviation).

Data availability

All data that support the findings of this study are available in the manuscript or the supplementary materials.

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