# Constraining the evaporative loss of zinc during impact processes using terrestrial impact glasses

Zheng-Yu Long, Frédéric Moynier, Marine Paquet, James M. D. Day, Linru Fang, Tu-Han Luu, Dimitri Rigoussen, Kun-Feng Qiu, Jun Deng, Christian Koeberl

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Zheng-Yu Long, Frédéric Moynier, Marine Paquet, James M. D. Day, Linru Fang, et al.. Constraining the evaporative loss of zinc during impact processes using terrestrial impact glasses. Earth and Planetary Science Letters, 2024, 646, 10.1016/j.epsl.2024.118979. insu-05081555

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Distributed under a Creative Commons Attribution 4.0 International License
1 Constraining the evaporative loss of zinc during impact
processes using terrestrial impact glasses
2
3

4 Zheng-Yu Long<sup>1,2,*</sup>, Frédéric Moynier<sup>1,*</sup>, Marine Paquet<sup>1,3</sup>, James M. D. Day<sup>4</sup>, Linru

5 Fang<sup>1</sup>, Tu-Han Luu<sup>1</sup>, Dimitri Rigoussen<sup>1</sup>, Kun-Feng Qiu<sup>2</sup>, Jun Deng<sup>2</sup>, Christian Koeberl<sup>5</sup>

6 ¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 75005 Paris, France

7 ²Frontiers Science Center for Deep-time Digital Earth, State Key Laboratory of Geological

8 Processes and Mineral Resources, School of Earth Sciences and Resources, China University

9 of Geosciences, Beijing, China

10 ³Centre de Recherches Pétrographiques et Géochimiques de Nancy, CNRS, Université de

11 Lorraine 15 Rue Notre Dame des Pauvres 54500 Vandoeuvre-lès-Nancy, France

12 ⁴Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA,

13 92093-0244, USA

14 ⁵Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna,

15 Austria

16 *Corresponding author: Zheng-Yu Long (long@ipgp.fr); Frédéric Moynier (moynier@ipgp.fr)

18 ABSTRACT

19 Evaporation can fractionate elements and their isotopes between the condensed and gas
phases. The fractionation of zinc isotopes during impact-induced evaporation can be used to
20 effectively determine the extent of volatile loss. A robust understanding of the Zn isotope
21 system in assessing the volatile loss, however, relies on well-constrained empirical isotopic
22 fractionation factors (α) during evaporation under a range of pressure and temperature
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conditions. In this study, Zn isotopic data for well-documented impact glasses from six sites (Darwin, Australia; Zhamanshin, Kazakhstan; El'gygytgyn, Russia; Boltysh, Ukraine; Lonar, India; and Ries, Germany) are reported to investigate the extent of Zn isotopic fractionation under conditions of impact-induced evaporation on Earth. Our findings suggest that the initial Zn isotopic composition in terrestrial impact glasses is comparable to that of continental crustal rocks, but this composition becomes progressively heavier as more Zn is lost from the impact melt, reaching a maximum δ<sup>66</sup>Zn value of +1.1 ‰. The investigated samples show a statistically significant negative correlation between δ<sup>66</sup>Zn values and Zn contents, especially those from the Darwin crater (R<sup>2</sup> = 0.90). These samples define an α value of 0.99971 ± 0.00005 (1SE). This α value is consistent with those previously estimated for melt glasses and fused sands (α = 0.9997 to 0.9998) from the Trinity nuclear detonation site, slightly higher than the value estimated from tektites (α ~0.998), and notably higher than that theoretically expected for evaporation into a vacuum (α = 0.985 to 0.993). This result highlights the limited fractionation of Zn isotopes during terrestrial impact processes. Moreover, the modelling suggests that the range of α values from 0.9997 to 0.9998 aligns with the observed compositions in lunar mare basalts and products from nuclear detonation, supporting α values close to but not exactly unity for Zn isotopic fractionation during various high-energy impact events. Utilizing the modelled fractionation factor (α = 0.9997), it is possible to reproduce the Zn concentration and isotopic composition of the lunar mare basalts, indicating a loss of about 98 % of the Moon's initial Zn inventory. Terrestrial impact glasses demonstrate that, under natural impact conditions, stable Zn isotopes can undergo evaporative fractionation to a degree comparable to lunar mare basalts and melted fallout glass and fused sands from nuclear
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detonation, suggesting an important contribution from impact to the volatile depletion of
terrestrial planets.

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**Keywords: Zn isotopes; Impact glass; Impact; Volatile depletion; Moon**

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# 1 INTRODUCTION

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The inventory of volatile elements in planetary bodies plays a critical role in the evolution of planets and their potential for harboring life (Hirschmann, 2006). In the inner Solar System, moderately volatile elements, such as Zn, are depleted compared to the bulk solar composition estimated from carbonaceous Ivuna-type (CI) chondrites and Ryugu samples (e.g., Lodders and Fegley, 2010; Palme et al., 2014; Braukmüller et al., 2019; Yokoyama et al., 2022). To understand planetary formation and evolution processes, it is essential to constrain when and how volatile elements and compounds (e.g., H₂O, CO₂, CH₄, SO₂, and NH₃) were added to or lost from planetary bodies (Albarède, 2009; Marty, 2012; Morbidelli et al., 2000). In particular, melting and evaporation processes, such as those that occurred during the Moon-forming event (Kato et al., 2015; Norris and Wood, 2017; Paniello et al., 2012; Righter, 2019; Wang and Jacobsen, 2016), are likely to have played a crucial role in controlling the fractionation of volatile elements and compounds on planetary bodies (Dhaliwal et al., 2018; Hin et al., 2017; Pringle et al., 2014; Young, 2000).

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Element volatility is usually defined by the 50 % nebular condensation temperature (TC₅₀) under solar nebular conditions with assumed pressures of 10⁻³ to 10⁻⁵ bar (Lodders, 2003; Wood et al., 2019). The abundance and isotopic composition of moderately volatile elements with 650 K < TC₅₀ < 1300 K at 10⁻⁴ bar (TC₅₀ of 652 K for Cd, 726 K for Zn, 1006 K for K, and 1037 K for Cu, respectively; the temperatures are relevant to the evaporation of silicate melts; Lodders, 2003) have been used to constrain planetary volatile inventories (Day and Moynier, 2014; Hu et al., 2022; Norris and Wood, 2017; Paquet et al., 2023; Steenstra et al., 2019; Tartèse et al., 2021; Wang et al., 2024). Zinc isotopes (⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn, and ⁷⁰Zn),
in particular, offer a reliable means to study kinetic or equilibrium isotopic fractionation effects during evaporation (Day et al., 2017a; Kamber and Schoenberg, 2020; Moynier et al., 2009; Paniello et al., 2012). For instance, previous work has shown that the isotopic fractionation during the formation of the Moon is associated with a substantial loss of volatile elements, including Zn (Day and Moynier, 2014; Kato et al., 2015; Paniello et al., 2012). Vaporization leads to the fractionation of elements and their isotopes between condensed phases (liquid or solid) and the gas phase, following equilibrium or kinetic pathways or between the two regimes. During kinetic distillation, lighter Zn isotopes preferentially enter the vapor phase during evaporative loss, leaving the solid or liquid phase enriched in isotopically heavy Zn (Jiang et al., 2019; Kato et al., 2015; Moynier et al., 2009; Paniello et al., 2012). This isotopic behavior can be used to quantify the loss of volatile elements, if the Zn isotopic fractionation factors (α), which define the degree of isotopic fractionation between vapor and condensed phases, are known (Day et al., 2017a; Kamber and Schoenberg, 2020; Sossi et al., 2020; Wimpenny et al., 2019). Previous studies have used anthropogenic glasses formed at various distances from the nuclear detonation at the Trinity test site (New Mexico, USA) to estimate the fractionation factor of Zn isotopes and other volatile elements, such as K and Cu (Chen et al., 2019; Day et al., 2017a, 2020a; Wimpenny et al., 2019). However, the lack of well-defined isotopic fractionation factors during impact events is a limitation to quantitatively assess isotopic fractionation and volatile element loss. It requires a deeper understanding of the behavior of Zn isotopes during impact-driven evaporation.

Impact melt glasses, including tektites, provide a unique opportunity to study the evaporation processes in natural systems. These materials form through meteoroid collisions
with Earth's atmosphere, which lead to extreme heating, high pressure, reduction, and potentially localized atmospheric blow-off (Koeberl, 2014). Chemical and isotopic analyses suggest that the precursor material for impact glasses and tektites primarily originates from the upper terrestrial continental crust, with limited or no extraterrestrial impactor signature detected (Koeberl, 2014, 1986; Magna et al., 2021, 2017). Some of the glasses exhibit depletions in moderately volatile metals (e.g., Zn, Cu) relative to their precursor lithologies (Jiang et al., 2019; Koeberl, 2014, 1986; Magna et al., 2011; Meisel et al., 1990; Moynier et al., 2009; Žák et al., 2016). Impact glasses are formed by the shock melting of the uppermost target rocks due to high shock pressures (up to hundreds of gigapascals) and temperatures (Koeberl, 2014). Shock-melting generates super-heated liquids that solidify into impact glasses. Thus, impact glasses represent excellent natural analogs for comparison with the scale and temperatures associated with planetary-scale volatile processes. In this study, we present Zn isotope data for various types of impact glass samples from six different sites (i.e., Darwin, Australia; Zhamanshin, Kazakhstan; El'gygytgyn, Russia; Boltysh, Ukraine; Lonar, India; and Ries, Germany), representing a variety of rock types and glass compositions, to investigate the isotopic behavior of Zn during impact-related evaporative loss.

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## 2 MATERIALS AND METHODS

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Impact glasses (n = 16) investigated in this study were sampled from six different impact craters (Darwin, Zhamanshin, El'gygytgyn, Boltysh, Lonar, and Ries). These samples have been well characterized in previous studies (Table S1). The impact glasses are vitrophyric, though they vary in appearance and texture.
Darwin glass samples used here are notably compact, with fewer vesicles compared to other samples, and exhibit flow structures that are generally less pronounced than those seen in other impact glasses. This might indicate that they formed under somewhat higher pressures and temperatures than other studied impact glasses (Howard, 2008; Meisel et al., 1990). Moreover, the fragments of Darwin glass constitute a distal impact ejecta layer, which extends beyond a distance equivalent to five times the radius of the source crater, reaching up to a maximum of 20 km from Darwin Crater (Howard et al., 2013). This is a typical characteristic of high-velocity impact melt ejecta (Koeberl, 1986). Impacted targets for the studied samples are igneous crustal rocks (Boltysh), acid volcanic rocks (El'gygytgyn), sedimentary rocks (Zhamanshin, Ries), metamorphosed sediments (Darwin), or mafic rocks (Lonar). Detailed sample information can be found in Table S1. Major and trace element compositions of the investigated impact glasses are presented in Table S2.

For each sample, ~50 mg of homogenized bulk powder was digested using a mixture of 16 mol/L HNO₃ (500 µL) and 27 mol/L HF (2 mL) and heated on hotplates at 120°C in PTFE beakers for 48 hours. After evaporating the HNO₃:HF mixture, samples were subsequently treated with 6 mol/L HCl and heated to remove the remaining fluoride complexes. Samples were then evaporated to dryness. Chemical purification was conducted at the Institut de Physique du Globe de Paris (IPGP). The Zn concentrations and isotopic compositions of the impact glass samples were determined at IPGP using a Thermo Scientific Neptune Plus Multi-Collector Inductively-Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). More details on sample pretreatment, chemical purification, and analytical procedures were described in earlier works (e.g. Moynier and Le Borgne, 2015; van Kooten and Moynier, 2019). Isotope ratios were
measured using the standard-sample-standard bracketing method with JMC-Lyon as the bracketing standard. Its correspondence with other Zn and widely available rock standards (e.g., IRMM-3702) is presented in Moynier et al. (2017). Zinc isotope composition is expressed as the δ<sup>x</sup>Zn notation:

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

where X = 66 and 68. Terrestrial basalt standard BHVO-2 from the United States Geological Survey (USGS) was used to monitor the precision and reproducibility of the chemical separation and isotopic analyses, yielding δ<sup>66</sup>Zn values of 0.25 ± 0.03 ‰ (2SD, n = 3). This value is consistent with the recommended value of 0.28 ± 0.04 ‰ (Moynier et al., 2017).

## 3 RESULTS

The investigated impact glasses exhibit a range of Zn concentrations (from 2.24 to 120 µg/g) and δ<sup>66</sup>Zn values (from 0.16 to 1.1 ‰; n = 16; Fig. 1; Table 1). The δ<sup>66</sup>Zn and δ<sup>68</sup>Zn values in the samples follow the expected mass-dependent fractionation slope of ~2 (Fig. 2), indicating that the Zn isotopic variations observed in our samples reflect mass-dependent isotope fractionation. Overall, most of these investigated impact glasses show a decrease in their Zn content compared to their respective target rocks (summarized in Table S3). The majority of samples show heavier Zn isotopic compositions relative to the Bulk Silicate Earth (BSE), the latter having a relatively homogeneous Zn isotopic composition with an average δ<sup>66</sup>Zn of 0.16 ± 0.06 ‰ (Beunon et al., 2020; Doucet et al., 2016; Moynier et al., 2017; Sossi et al., 2018; Wang et al., 2017), and to crustal samples that have a δ<sup>66</sup>Zn of 0.28 ± 0.05 ‰
(Chen et al., 2013). Samples from the Darwin crater (n = 4) have extremely low Zn contents (2.24 to 10.5 µg/g) and display the most variable δ⁶⁶Zn, with values ranging from 0.63 to 1.12 ‰, which are significantly higher than the range of BSE and crustal rocks. In contrast, the Zhamanshin samples (n = 2) have Zn contents of 40 and 70 µg/g and δ⁶⁶Zn of 0.16 and 0.17 ‰ similar to BSE. The El'gygytgyn samples (n = 3) have Zn contents ranging from 40 to 97.3 µg/g Zn and δ⁶⁶Zn from 0.35 to 0.48 ‰. The Boltysh Samples (n = 3) exhibit Zn contents between 35 and 97.3 µg/g Zn and δ⁶⁶Zn ranging from 0.16 to 0.50 ‰. The Lonar samples (n = 2) have higher Zn concentrations ranging between 105 to 120 µg/g Zn and δ⁶⁶Zn ranging from 0.20 to 0.25 ‰. The Ries samples (n = 2) have Zn contents between 47.4 and 55 µg/g Zn and δ⁶⁶Zn ranging from 0.32 to 0.39 ‰.

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## 4 DISCUSSION

### 4.1 Zinc loss and isotope fractionation in the investigated impact glasses

The Zn isotopic composition in the investigated impact glasses shows a negative covariation with Zn content, where the highest δ⁶⁶Zn value corresponds to the lowest Zn content (Fig. 1). This correlation is mostly pronounced in the Darwin glasses (R² = 0.90) with ~0.6 ‰ variations in δ⁶⁶Zn. Given that impact glasses formed during high temperature-pressure events, such a variation is most likely related to Zn loss and isotopic fractionation due to evaporation, as suggested by previous studies on lunar mare basalts (Herzog et al., 2009; Kato et al., 2015; Paniello et al., 2012), impact melt sheets (Kamber and Schoenberg, 2020), tektites (Jiang et al., 2019; Moynier et al., 2009; Rodovská et al., 2017; Wimpenny et al., 2019), and the melted fallout glass and fused sands from nuclear detonation (Day et al., 2017a, 2020a;
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184 In general, the Zn contents and δ⁶⁶Zn values (Fig. 1) among the investigated samples
185 appear to follow a Rayleigh distillation law: R = R₀ × fZn α⁻¹, where R is the ⁶⁶Zn/⁶⁴Zn ratio in
186 the samples that experienced evaporation, R₀ is the ⁶⁶Zn/⁶⁴Zn ratio in the initial material, and
187 fZn represents the fraction of retained Zn associated with the Zn loss due to evaporation. The
188 initial Zn content data from the target rocks of six impact craters have been summarized in
189 Table S3. Since the Zn isotope composition of these target rocks is unreported, the BSE is used
190 to approximately represent the initial δ⁶⁶Zn values. The calculated results are shown in Figure
191 3. Our samples demonstrate a clear trend where a greater loss of Zn (i.e., a smaller fZn)
192 corresponds to a heavier Zn isotope composition in impact glasses. The Darwin glasses
193 demonstrate a progressively increasing loss proportion of Zn (> 80%; Table S4), which
194 corresponds to the most significant variations in Zn isotopic composition. This suggests a
195 significant modification in the chemical composition of these samples due to the impact
196 processes. Among the samples from Boltysh, two of them display similar Zn loss ratios but
197 with δ⁶⁶Zn values ranging from 0.16 ± 0.09 ‰ (sample 721) to 0.32 ± 0.05 ‰ (sample 627.8).
198 This variation may be attributed to inheritance from their target granites, given that Zn isotopes
199 are generally fractionated to less than 0.2 ‰ during igneous processes (Chen et al., 2013; Day
200 et al., 2022; Sossi et al., 2018). In comparison, another sample from Boltysh (717) shows a
201 positive shift in δ⁶⁶Zn to 0.50 ‰ and significantly lower Zn content (34.9 µg/g), indicating
202 evaporative loss. Discernible Zn isotope fractionation, similar to that of the Boltysh samples,
203 has been observed in impact glasses from El'gygytgyn, also linked to evaporation. Although
204 the samples from Lonar and Ries exhibit limited Zn loss and isotope fractionation within their
respective datasets, compared to the Lonar samples, the impact glass samples from Ries experienced a greater loss of Zn, accompanied by a slightly heavier Zn isotope composition. Exceptions are the two Zhamanshin glasses that display a variation in major element and Zn contents (Fig. S2), although their Zn isotope compositions are similar (Fig. 1). One possibility is that the lithology of their target rocks is diverse, and the variable Zn contents are likely inherited from their target sedimentary rocks, which would be unrelated to the impact event. Also, one of the samples is a Si-rich zhamanshinite, and the other one is a Si-poor zhamanshinite (Fig. S2).

## 4.2 Calculation of the empirical fractionation factor

Isotopic fractionation at equilibrium is governed by the ratio of the equilibrium partial pressures of the two isotopes, which approaches unity at high temperatures. In contrast, for pure kinetic evaporation, theoretical calculations into a vacuum for different Zn compounds are determined by taking the square root of the ratio of the mass numbers of the light and heavy isotopologues of the vaporizing species (e.g., 0.985 for elemental Zn, 0.993 for ZnCl₂, and 0.990 for ZnS). Isotopic fractionation caused by kinetic evaporation is significantly greater than that occurring during equilibrium evaporation. However, the impact melt glasses on Earth, formed from asteroid impacts, such as tektites (Jiang et al., 2019; Moynier et al., 2009; Rodovská et al., 2017; Wimpenny et al., 2019), melt sheets (Kamber and Schoenberg, 2020), and nuclear detonation products (Day et al., 2017a, 2020a; Wimpenny et al., 2019) collectively exhibit reduced Zn isotope fractionation factors between vapor and liquid, with values higher than 0.998. Wimpenny et al. (2019) conducted experiments on silica-rich compositions,
equivalent to the upper continental-crustal source rocks of terrestrial impact melts, at temperatures from 1600 to 2200 °C in an Ar atmosphere. Their experiments suggested a Zn isotope fractionation factor range of 0.9988 < α < 0.9998 for impact-related evaporation.

Our results show that the slope in δ⁶⁸Zn versus δ⁶⁶Zn space (Fig. 2) for all investigated glasses (n = 16) is 2.048 ± 0.080 (1SE) with an intercept of -0.004 ± 0.041 (1SE). For a smaller sample group from Darwin glasses, the slope is slightly lower at 1.943 ± 0.220 (1SE) with an intercept of 0.078 ± 0.190 (1SE) (n = 4). These slopes are indicative of the mass fractionation law that governs isotope fractionation. Theoretically, an equilibrium process would result in a slope of 1.971, while kinetic fractionation tends to yield a slope of 1.942 in δ⁶⁸Zn versus δ⁶⁶Zn space. Given the high uncertainty in the slope measured here, the results suggest that both equilibrium and kinetic fractionation processes could control the Zn isotopic composition of impact glasses. In this study, we use the Darwin glasses to empirically calculate Zn isotope fractionation factors during impact-related evaporation (Fig. 3). A Rayleigh distillation model was produced for the Darwin impact event (starting compositions of Zn content and δ⁶⁶Zn in Table S3). The result shows that investigated Darwin samples collectively follow a Rayleigh distillation law, characterized by a strong relationship between Zn isotopic fractionation and Zn loss. A best-fit α factor for the Darwin impact glasses corresponds to a value of 0.99971 ± 0.00005 (SE; Fig. 3). Samples from other craters also follow this Rayleigh fractionation trend, except for an outlier from Zhamanshin (ZH-6014).

The α value calculated from impact glasses is slightly higher than those obtained from tektites (α = ~0.998; Fig. 4A) (Jiang et al., 2019; Moynier et al., 2009; Rodovská et al., 2017; Wimpenny et al., 2019). Moreover, the calculated value is marginally below that obtained from
reported impact melt sheets ($\alpha$ = 0.9998) (Kamber and Schoenberg, 2020) and sedimentary rock layers of the K-Pg boundary that experienced a more limited fractionation of Zn isotopes ($\alpha$ = 0.9999) (Mathur et al., 2021) (Fig. 4A). Intriguingly, the fractionation factors for evaporation estimated from melted fallout glass and fused sands (rhyolite soil and arkosic soil) from the Trinity test site (Day et al., 2017a, 2020a; Wimpenny et al., 2019) show a small range of variation from 0.9997 to 0.9998 (Fig. 4B). Our result is in line with the experimental data, suggesting that at natural impact conditions, stable Zn isotopes can experience evaporative fractionation to a degree comparable to nuclear detonation materials.

## 4.3 Zinc loss and isotopic fractionation during terrestrial impact processes

The accuracy of our calculations, to some extent, depends on the selection of starting materials. As the compositions of target rocks may be variable, it complicates the extraction of accurate fractionation factors from samples. In the following, we employed the Monte Carlo simulation to model the Zn isotopic fractionation in terrestrial rocks during impact-related evaporation. The model equation follows: $\delta^{66}\text{Zn} = (1000 + \delta^{66}\text{Zn}_0) \times f^{\alpha-1} - 1000$, where $\delta^{66}\text{Zn}_0$ represents the initial Zn isotopic composition and $f$ is the evaporative loss ratio of Zn. The initial Zn isotopic composition ($0.16 \pm 0.03\‰$ based on BSE) is assigned randomly following a normal distribution (mean $\pm \sigma_{SD}$). The simulation incorporates different scenarios that reflect isotopic fractionation under different conditions characterized by $\alpha$ values ranging from 0.98500 to 0.9999 ($\pm 0.00005$). Each scenario was simulated across 10,000 trials to ensure robust statistical outcomes. In each trial, $f$ is set as a random constant between 0 and 1. This simulation removed biases that could arise from changes in the Zn contents of starting materials
(pre-impact precursor lithologies). Four scenarios with α values of 0.99900 ± 0.00005, 0.99970 ± 0.00005, 0.99980 ± 0.00005, and 0.99990 ± 0.00005 reproduced the compositions observed in the investigated impact glasses and previously reported samples, and these simulations are considered successful (Fig. 4).

The results from Monte Carlo simulations indicate that using an α value of 0.99970 ± 0.00005, our simulation replicates the fractionation trend observed in impact glasses. It confirms the calculation result based on four Darwin glasses (α = 0.99971). Furthermore, in comparison to the previously recommended α value ranging from 0.997 to 0.998 for tektites, a slightly higher α of 0.99900 ± 0.00005 accurately reproduces the reported compositions of tektites, establishing a well-defined factor for these materials. For K-Pg sedimentary rocks, which exhibit very small isotopic fractionation, an α value close to unity (0.99990 ± 0.00005) is estimated. All modelled values of α (closer to unity) for natural materials are markedly higher than that expected for evaporation into a vacuum in an ideal kinetic scenario. Zinc isotopic fractionation is sensitive to the physical and chemical conditions under which evaporation occurs (Kato et al., 2015; Paniello et al., 2012; Sossi et al., 2020). The relatively small fractionation of the Zn isotopes is likely influenced either by the kinetics of vaporization (Hashimoto, 1990) or by the total pressure and average molar mass of the gas medium into which the evaporation took place (Richter et al., 2011).

The modelled α value for natural materials is higher than the experimentally determined α of 0.9959 at 1 bar in a slow-flowing gas (Sossi et al., 2020). Experimental results suggested that reducing conditions would increase the α values (Sossi et al., 2020), which was attributed to limited Zn diffusion. Moynier et al. (2009) also proposed a diffusion-limited evaporation
model to explain the limited fractionation of Zn isotopes in tektites ($\alpha$ > 0.998). This occurs due to the slow elemental diffusion through the silicate melt relative to the rate of evaporative loss, which depresses the isotopic fractionation factor. Whereas, if evaporation rates significantly exceed diffusion rates in the melt, isotopic fractionation in the condensed phase does not occur. Sossi et al. (2020) predicted that fractionation factors approaching values of ~0.9997 are achievable in an H₂-rich gas. However, the process is different in the case of impact glasses and tektites given the lower H pressure of the Earth's atmosphere. The Cl content in the upper continental crust is ~300 µg/g and the Zn content is ~67 µg/g (Rudnick and Gao, 2014) with a Zn/Cl molar ratio of ~0.22. Under Earth's atmospheric conditions at 2000 K and impact pressures significantly exceeding 1 bar, vaporized Zn might form complexes with Cl (e.g., in some volcanic gases; Henley and Seward, 2018; Renggli et al., 2017). This diverges from the simplified assumption in the modelling by Sossi et al. (2020), which treats Zn as a monatomic gas.

In general, the $\alpha$ values are controlled by an array of interlinked factors, such as vapor separation processes from the melt (Jiang et al., 2019), variations in the P-T-fO₂ conditions (Jiang et al., 2019; Sossi et al., 2020, 2019; Wimpenny et al., 2019), Zn diffusion length-scales (Moynier et al., 2009), and complex vapor-liquid interactions (Day et al., 2017a; Day and Moynier, 2014), as well as post-impact processes that can dilute impact ejecta. Given these complexities, the discrepancies observed among different materials likely stem from more intricate processes that cannot be adequately explained by a single physical factor. Nevertheless, our study highlights a common scenario characterized by limited fractionation of Zn isotopes (0.9990 < $\alpha$ < 0.9999; Fig. 4A) during terrestrial impact processes. Our result supports the
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earlier hypothesis that actual fractionation factors do not align with ideal estimates based on isotopologues (Day and Moynier, 2014).

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## 4.4 Implications for evaporative loss of zinc from the Moon

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The Moon is volatile-depleted, as demonstrated by studies of moderately volatile element abundances (Albarède et al., 2015) and Zn, Cu, Cl, S, Rb, and K isotopic compositions of lunar rocks (Boyce et al., 2015; Day et al., 2017b; Day and Moynier, 2014; Gargano et al., 2022; Kato et al., 2015; Moynier et al., 2006; Paniello et al., 2012; Pringle and Moynier, 2017; Sharp et al., 2010; Wang and Jacobsen, 2016). Based on the assumption that Apollo samples and lunar meteorites are representative of the bulk Moon (Tartèse et al., 2021), the isotopic fractionation factor inferred for the Moon is typically derived from lunar mare basalts (Day et al., 2020b, 2019; Kato et al., 2015; Paniello et al., 2012; Wang and Jacobsen, 2016). Nuclear detonation materials also have been considered as analogs to investigate the isotopic fractionation factors that could be applied to planetary evaporation processes (Day et al., 2020a, 2017a; Wimpenny et al., 2019). Previous research on K isotopes of lunar mare basalts proposed that the loss of K happened under high total pressures of the post-impact atmosphere surrounding the Earth, drastically decreasing K isotopic fractionation (Wang and Jacobsen, 2016). This high pressure would change the K isotopic fractionation factor from the ideal kinetic value of 0.9753, expected in a vacuum, to observed ~0.9998 (Wang and Jacobsen, 2016). A similar fractionation factor has also been proposed for Zn (Day et al., 2020b; Kato et al., 2015; Paniello et al., 2012) to explain their abundances in mare basalts. The Monte-Carlo modelling suggests that the α ranging from 0.9997 to 0.9998 (Fig. 4B) can account for the

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observed compositions in lunar mare basalts (with an evaporative loss of ~98% of Zn; Fig. 5) and products from the first nuclear detonation at the Trinity test site, as well as investigated impact glasses. This supports a high-α (limited fractionation) model for various planetary evaporation events occurring under high temperature-pressure conditions. Nevertheless, continued volatile depletion could occur during the lunar magma ocean stage (Kato et al., 2015).

Thus, following an initial depletion, processes such as degassing from a magma ocean, condensation of vapor, and late accretion likely played crucial roles in the heterogeneous distribution of volatiles within the Moon (Day et al., 2017a, 2017b; Dhaliwal et al., 2018; Gargano et al., 2022) as well as other planetary bodies (Dhaliwal et al., 2024).

## 5 CONCLUSIONS

This study reports a substantial Zn isotopic variation measured in terrestrial impact glasses from six different impact structures on Earth, with $\delta^{66}\text{Zn}$ ranging from 0.16 to 1.1 ‰. The investigated samples show a clear negative correlation between the Zn content and $\delta^{66}\text{Zn}$ value. This result indicates Zn loss and isotopic fractionation due to impact-driven evaporation. The Darwin samples define a Zn isotopic fractionation factor $\alpha$ of 0.99971 ± 0.00005 (1SE), consistent with the estimates from melted fallout glass and fused sands from the Trinity test sites. The Monte-Carlo modelling highlights the limited fractionation of Zn isotopes (0.9990 < $\alpha$ < 0.9999) during terrestrial impact processes, which supports that actual fractionation factors do not align with ideal estimates based on isotopologues. Utilizing the modelled $\alpha$ factor of 0.9997, we show that impact-driven evaporation processes explain the Zn abundance and isotopic composition of the lunar crust, with an extremely high evaporative loss of ~98 % of
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its initial Zn. This study provides support for an impact origin of the volatile depletion in the
Moon and other airless planetary bodies.

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ACKNOWLEDGMENTS

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We deeply thank the journal editor and three anonymous reviewers for their thorough comments that have greatly improved the quality of the paper. F.M. acknowledges funding from the ERC under the European Community's H2020 framework program/ERC grant agreement No. 101001282 (METAL). J.M.D.D. acknowledges support from NASA Solar System Workings (80NSSC19K0932). Z.Y.L acknowledges the financial support of the China Scholarship Council (202206400044). We thank Baptiste Debret for his assistance with Zn isotope analyses.

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DATA AVAILABILITY

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All data supporting the findings of this study are available in the Supplementary
Materials.

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621 Tanaka, S., Tatsumi, E., Terui, F., Tsuda, Y., Tsukizaki, R., Wada, K., Watanabe, S.,

622 Yamada, M., Yamada, T., Yamamoto, Y., Yano, H., Yokota, Y., Yoshihara, K.,
623 Yoshikawa, M., Yoshikawa, K., Furuya, S., Hataakeda, K., Hayashi, T., Hitomi, Y.,
624 Kumagai, K., Miyazaki, A., Nakato, A., Nishimura, M., Soejima, H., Suzuki, A., Yada,
625 T., Yamamoto, D., Yogata, K., Yoshitake, M., Tachibana, S., Yurimoto, H., 2022.
626 Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous
627 meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850

628 Young, E.D., 2000. Assessing the implications of K isotope cosmochemistry for evaporation
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631 Žák, K., Skála, R., Řanda, Z., Mizera, J., Heissig, K., Ackerman, L., Ďurišová, J., Jonášová, Š.,
632 Kameník, J., Magna, T., 2016. Chemistry of Tertiary sediments in the surroundings of
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635 Zhang, J., Dauphas, N., Davis, A.M., Leya, I., Fedkin, A., 2012. The proto-Earth as a significant
636 source of lunar material. Nat. Geosci. 5, 251-255. https://doi.org/10.1038/ngeo1429

637

638

FIGURE CAPTIONS

639 **Figure 1.** A plot of δ<sup>66</sup>Zn values versus Zn concentrations for samples from six impact craters analyzed in this study. The shadow area shows the range of δ<sup>66</sup>Zn for the BSE with an average
640
641 δ<sup>66</sup>Zn of 0.16 ± 0.06 ‰ (Sossi et al., 2018). The linear fit result for the Darwin glasses shows
642 a R<sup>2</sup> value of 0.90.

643
644 **Figure 2.** A plot of δ<sup>68</sup>Zn versus δ<sup>66</sup>Zn for impact glass samples. The variation of δ<sup>66</sup>Zn and
δ⁶⁸Zn follows the expected mass-dependent fractionation slope of ~2.

**Figure 3.** A plot of Zn isotope data (R/R₀) versus retained Zn fraction (fZn) associated with the evaporative loss of Zn. The isotopic fractionation factor (α) associated with evaporative loss of Zn is calculated using the following function: R = R₀ × f⁻¹. R is the ⁶⁶Zn/⁶⁴Zn ratio in the samples that experienced evaporation and R₀ is the ⁶⁶Zn/⁶⁴Zn ratio in the initial material. The initial Zn concentration and isotopic composition of impact glasses are presented in Table S3.

**Figure 4.** A plot of Zn isotopic fractionation (Δ⁶⁶Zn) versus retained Zn fraction (fZn) associated with the evaporative loss of Zn. Small colored dots in the background represent the result of the Monte Carlo simulation. Details on modelling are presented in the main text. The dashed lines represent the Zn isotopic fractionation factors (α), which are calculated from reported Zn isotope data using the following function: R = R₀ × f⁻¹, where R represents the Zn isotopic ratio of a volatile depleted sample, R₀ is the initial Zn isotope ratio and f is the fraction of remained Zn after evaporation. Lunar mare basalts with negative δ⁶⁶Zn values due to secondary condensation effects were excluded from the dataset (Day et al., 2020b; Kato et al., 2015). The initial Zn concentration and isotopic composition of different samples are presented in Table S3.

The calculated results for Δ⁶⁶Zn and fZn values are presented in Table S4. Literature data: lunar mare basalts (Day et al., 2020b; Gargano et al., 2022; Herzog et al., 2009; Kato et al., 2015; Kooten et al., 2020; Paniello et al., 2012); tektites (Jiang et al., 2019; Moynier et al., 2009; Rodovská et al., 2017; Wimpenny et al., 2019); impact melt sheet (Kamber and Schoenberg, 2020); K-Pg sedimentary rocks (Mathur et al., 2021); phyolite and arkosic soils (Wimpenny et
al., 2019); nuclear fallout glass (Wimpenny et al., 2019); trinitites (Day et al., 2017a).

668

**Figure 5.** Modelling the evolution of δ<sup>66</sup>Zn values and Zn contents with increasing evaporative Zn loss. Given the likely cogenetic origin of the Earth and Moon (Mougel et al., 2018; Touboul et al., 2007; Wiechert et al., 2001; Zhang et al., 2012), the modelling assumed that Zn abundance in the initial material was similar to that of the Earth's mantle (50 µg/g; McDonough and Sun, 1995) and the δ<sup>66</sup>Zn value of the pre-impact system was akin to that of the bulk silicate Earth (BSE) (Beunon et al., 2020; Doucet et al., 2016; Moynier et al., 2017; Sossi et al., 2018; Wang et al., 2017). The bulk silicate Moon (BSM) is assumed to have a relatively homogeneous δ<sup>66</sup>Zn value of 1.4 ± 0.5 ‰ (2SD), as inferred from pristine mare basalts (Day et al., 2020b; Herzog et al., 2009; Kato et al., 2015; Paniello et al., 2012). Five fractionation factors are considered, including the theoretical α factors for evaporation of Zn in a vacuum as elemental Zn (0.985), ZnS (0.990), and ZnCl<sub>2</sub> (0.993) and the modelled empirical α factors for natural materials ranging from 0.999 to 0.9997 (see Fig. 4). Using the α factor of 0.9997 can well reproduce the Zn concentrations and δ<sup>66</sup>Zn values similar to those measured in lunar rocks (0.5 to 2 µg/g; Paniello et al., 2012; Kato et al., 2015; δ<sup>66</sup>Zn up to 1.4 ‰; Day et al., 2020b; Gargano et al., 2022; Herzog et al., 2009; Kato et al., 2015; Kooten et al., 2020; Paniello et al., 2012). The modelling indicates that to account for the high δ<sup>66</sup>Zn and low Zn concentration (0.5 to 2 µg/g; Paniello et al., 2012; Kato et al., 2015) of the BSM, the Moon likely lost ~98 % of its initial Zn content during evaporation (Fig. 5).

669

670

671

672

673

674

675

676

677

678

679

680

681

682

683

684

685

686
<figure><img src="image_2.png" /></figure>

Darwin

El'ggygytgyn

Boltysh

Lonar

Ries

Zhamanshin

BSE
δ⁶⁸Zn (‰)

3.0

2.5

2.0

1.5

1.0

0.5

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

0.2

0.4

0.6

Darwin glass:

Slope: 1.943 ± 0.220 (SE)

Intercept: 0.078 ± 0.190 (SE)

All samples:

Slope: 2.048 ± 0.080 (SE)

Intercept: -0.004 ± 0.041 (SE)

δ⁶⁶Zn (‰)

Darwin

El'gygytgyn

Boltysh

Lonar

Ries

Zhamanshin

1.2

1.4

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /></figure>

Darwin glass:

$$ \text{Slope} = 0.293 \pm 0.051 \text{ (SE)} $$

$$ \alpha = 0.99971 \pm 0.00005 \text{ (SE)} $$

$$ R^2 = 0.94 $$
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

$\alpha = 0.985$ to $0.993$

$\alpha = 0.999$

$\alpha = 0.9997$

$\alpha = 0.9998$

BSE
<table><caption>**Table 1.** Zinc content (µg/g) and isotopic compositions (‰) of investigated samples</caption><thead><tr><th>Number</th><th>Site</th><th>Type</th><th>Zn</th><th>δ<sup>66</sup>Zn</th><th>2SD</th><th>δ<sup>68</sup>Zn</th><th>2SD</th><th>N</th></tr></thead><tbody><tr><td colspan="9"><em>Samples</em></td></tr><tr><td>Zh-62-3b</td><td>Zhamanshin</td><td>Impact glass (Zhamanshinite)</td><td>69.8</td><td>0.17</td><td>0.11</td><td>0.32</td><td>0.24</td><td>4</td></tr><tr><td>ZH-6014</td><td>Zhamanshin</td><td>Impact glass (Zhamanshinite)</td><td>39.9</td><td>0.16</td><td>0.06</td><td>0.32</td><td>0.16</td><td>3</td></tr><tr><td>DG8701</td><td>Darwin</td><td>Darwin glass</td><td>9.48</td><td>0.63</td><td>0.11</td><td>1.24</td><td>0.24</td><td>3</td></tr><tr><td>DG8705</td><td>Darwin</td><td>Darwin glass</td><td>2.24</td><td>1.1</td><td>0.15</td><td>2.25</td><td>0.32</td><td>3</td></tr><tr><td>DG87010</td><td>Darwin</td><td>Darwin glass</td><td>10.5</td><td>0.73</td><td>0.01</td><td>1.59</td><td>0.08</td><td>2</td></tr><tr><td>DG87011</td><td>Darwin</td><td>Darwin glass</td><td>4.99</td><td>0.90</td><td>0.03</td><td>1.80</td><td>0.08</td><td>3</td></tr><tr><td>Subo18-36</td><td>Ries</td><td>Suevite</td><td>47.4</td><td>0.32</td><td>0.11</td><td>0.60</td><td>0.32</td><td>2</td></tr><tr><td>Subo18-4</td><td>Ries</td><td>Melt pod in suevite</td><td>54.9</td><td>0.39</td><td>0.03</td><td>0.74</td><td>0.09</td><td>3</td></tr><tr><td>E985-101</td><td>El'gygytgyn</td><td>Impact glass bomb</td><td>49.6</td><td>0.48</td><td>0.08</td><td>1.04</td><td>0.20</td><td>2</td></tr><tr><td>E9</td><td>El'gygytgyn</td><td>Impact glass</td><td>39.9</td><td>0.45</td><td>0.01</td><td>1.14</td><td>0.02</td><td>3</td></tr><tr><td>G4</td><td>El'gygytgyn</td><td>Pumice-like impact glass</td><td>97.3</td><td>0.35</td><td>0.09</td><td>0.81</td><td>0.13</td><td>3</td></tr><tr><td>13-1-3</td><td>Lonar</td><td>Impact glass</td><td>105</td><td>0.20</td><td>0.04</td><td>0.37</td><td>0.13</td><td>3</td></tr><tr><td>L-21</td><td>Lonar</td><td>Impact glass</td><td>120</td><td>0.25</td><td>0.11</td><td>0.47</td><td>0.19</td><td>3</td></tr><tr><td>627.8</td><td>Boltysh</td><td>Impact glass</td><td>94.8</td><td>0.32</td><td>0.05</td><td>0.57</td><td>0.11</td><td>2</td></tr><tr><td>717</td><td>Boltysh</td><td>Impact glass</td><td>34.9</td><td>0.50</td><td>0.13</td><td>0.93</td><td>0.30</td><td>2</td></tr><tr><td>721</td><td>Boltysh</td><td>Impact glass</td><td>97.3</td><td>0.16</td><td>0.09</td><td>0.35</td><td>0.18</td><td>4</td></tr><tr><td colspan="9"><em>Standard</em></td></tr><tr><td>BHVO2</td><td></td><td></td><td></td><td>0.25</td><td>0.03</td><td>0.46</td><td>0.06</td><td>3</td></tr></tbody></table>

Note: the uncertainty is reported as 2SD (two standard deviation) of repeated sample analyses.
N represents the number of analyses.