# Neon Isotopes in Individual Vesicles of Icelandic Basaltic Glasses: Insights into the Origin of Light Volatile Elements on Earth

Romain Sauvalle, Manuel Moreira, Bruno Scaillet

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Romain Sauvalle, Manuel Moreira, Bruno Scaillet. Neon Isotopes in Individual Vesicles of Icelandic Basaltic Glasses: Insights into the Origin of Light Volatile Elements on Earth. Earth and Planetary Science Letters, 2026, 673, pp.119722. 10.1016/j.epsl.2025.119722. hal-05295785v2

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# Neon isotopes in individual vesicles of Icelandic basaltic glasses: Insights into the origin of light volatile elements on earth

Romain Sauvalle* (1), Manuel Moreira, Bruno Scaillet

Institut des Sciences de la Terre d'Orléans, UMR 7327 CNRS-BRGM – Université d'Orléans, France

## ARTICLE INFO

Editor: Dr R. Hickey-Vargas

**Keywords:**

Neon

OIB

Single vesicle

Solar wind

Origin of volatiles

## ABSTRACT

The origin of Earth's volatile elements remains a topic of debate due to the diverse sources within the solar system. Noble gases, owing to their chemical inertness, serve as powerful tracers for investigating planetary formation and mantle evolution. Noble gas isotopic studies suggest the existence of a preserved, relatively undegassed mantle reservoir, which is tapped by deep-rooted mantle plumes, such as those beneath the Galápagos, Hawaii, and Iceland. We present the first measurements of neon isotopic ratios in individual vesicles from a subglacially erupted basalt from Iceland. This method allows for the measurement of noble gas isotopes free from atmospheric contamination. Through statistical analyses of neon isotopic ratios in individual vesicles, we derive the isotopic composition for Ne of the Iceland mantle to be 12.63 ± 0.14 for the ²⁰Ne/²²Ne ratio. This value is in excellent agreement with the Galápagos mantle source and the signature expected for implanted solar wind, and lower than that of the value expected for the dissolution of a primordial atmosphere in the magma ocean. These results suggest a dominant role for solar wind implantation in accounting for Earth's mantle neon isotopic signature, and therefore as the source of light volatile elements. However, neither the implantation nor the nebular dissolution model is entirely satisfactory to explain all observations. Both models are not necessarily mutually exclusive, and more work should be conducted to tackle this long-standing issue.

## 1. Introduction

The origin of light volatile elements is a key question in Earth sciences that remains to be clearly answered. Earth formed in a region of the solar system where the protoplanetary disk was blown off probably before the condensation of water ice (~ 5 Myr after Calcium-Aluminum Inclusion or CAIs for the inner Solar System; e.g. Sung et al., 2009). Consequently, terrestrial planets are strongly depleted in highly volatile elements (e.g. water, nitrogen, carbon & noble gases, Lodders, 2003) compared to gas giants and carbonaceous chondrites that formed farther from the Sun. As volatile elements could not condense, this raises questions as to the mechanism(s) that could account for Earth's volatile inventory.

Noble gases, being chemically inert and comporting several isotopes either non-radiogenic or radiogenic, are especially valuable physical tracers for investigating planetary formation processes.

Neon (Ne) is particularly relevant to study the origin of light volatile elements. Unlike the heavier noble gases such as argon, krypton and xenon, neon is probably not recycled back into mantle by subduction

(Staudacher and Allègre, 1988; Moreira et al., 2003; Holland and Balentine, 2006). Ne has three isotopes: ²⁰Ne, ²¹Ne and ²²Ne. While ²¹Ne is mostly nucleogenic, ²⁰Ne and ²²Ne can be considered as primordial in the mantle due to their negligible production rates (Yatsevich and Honda, 1997). The existence of a primitive preserved reservoir (over 4.45 Ga) in the lower mantle has been inferred from noble gas systematics (Kurz et al., 1982; Allègre et al., 1983; Honda et al., 1991; Mukhopadhyay, 2012). Determining the ²⁰Ne/²²Ne ratio of the primitive mantellic reservoir can thus provide clues as to the source of light volatile elements on Earth, as this ratio varies significantly in different components of the Solar System (²⁰Ne/²²Ne = 8.5 (presolar or Ne-A); 1 (cosmogenic); 10.1–10.7 (Phase Q); 12.5–12.7 (implanted solar wind or Ne-B), 13.4 (solar), Black, 1972; Busemann et al., 2000; Heber et al., 2012).

The ²⁰Ne/²²Ne isotopic ratio in the mantle is solar-like (> 12; Sarda et al., 1988). Late volatile accretion by impacts (Albarède, 2009) cannot explain the solar neon in the mantle because this solar signature is distinct to that of the neon measured in the “Phase Q” contained in chondrites. Two potential components in the Solar System could account

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: romain.sauvalle@etu.univ-orleans.fr (R. Sauvalle).</footnote>
for this solar-like Ne in the mantle: (a) the solar nebula ($^{20}$Ne/$^{22}$Ne ~ 13.4; Heber et al., 2012) or (b) solar wind implantation on dust, so-called 'Ne-B' ($^{20}$Ne/$^{22}$Ne = 12.52 to 12.73, Black, 1972; Trieloff et al., 2000, 2002; Trieloff and Kunz, 2005; Raquin and Moreira, 2009; Moreira and Charnoz, 2016; Yamamoto et al., 2025). The depleted mantle, the source of mid-ocean ridge basalts (MORBs), is well characterized by a $^{20}$Ne/$^{22}$Ne ratio of ~ 12.5 (Moreira et al., 1998; Ballentine et al., 2005). This value has been determined from the analysis of basaltic glasses (or CO₂-rich wells), formed when lava rapidly quenches upon contact with seawater at depth, resulting in pillow lavas with a glassy outer rind. This glassy rim is gas-rich, as magmatic gases are trapped in the vesicles at eruption pressures at the glass transition temperatures.

The neon signature of the primitive lower mantle remains less well constrained for several reasons. This reservoir is only tapped by deeply rooted plumes and gas-rich samples are required to accurately measure noble gas isotopes, as they can be considered as ultra-trace elements in the mantle (~ $10^{-12}$ g·g⁻¹; Moreira, 2013). Hence, submarine or subglacially erupted lavas are required to ensure (1) the formation of glassy rim trapping the gas and (2) a high enough erupting pressure for the sample to be gas-rich. This significantly restricts the locations suitable for studying noble gas isotopic systematics in the lower mantle to a few key hotspots, namely, Hawaii, the Galápagos, and Iceland.

Additionally, the ubiquity of an atmospheric component contained in natural samples complicates the precise determination of the mantle-derived neon isotopic signature. Indeed, classical techniques, such as step-crushing or step-heating, yield a linear mixing array between the atmospheric neon ($^{20}$Ne/$^{22}$Ne = 9.8) and the mantle-derived component ($^{20}$Ne/$^{22}$Ne > 12). The highest value measured by step-analyses is therefore often considered as a lower limit for the mantle (as high as 13.04 ± 0.2, Yokochi and Marty, 2004; and 13.03 ± 0.04, Williams and Mukhopadhyay, 2019), as atmospheric contamination can never be completely ruled out.

However, the highest measured $^{20}$Ne/$^{22}$Ne ratio could result from kinetic fractionation of Ne isotopes during vesiculation, particularly under conditions of fast ascent and rapid quenching that could inhibits attainment of equilibrium for extrusive samples (Núñez-Guerrero et al., 2025). Atmospheric contamination and potential kinetic fractionation of a subset of vesicles significantly blur the interpretation of the highest

measured $^{20}$Ne/$^{22}$Ne isotopic ratio by step-crushing analyses.

Single vesicle analysis using laser ablation, pioneered by Burnard et al. (1997) and Burnard (1999), is a powerful tool to address these issues (Raquin et al., 2008). Coupling this technique with 3D-microtomography of the sample prevents contamination by trapped air located in cracks or open vesicles and facilitates localization of closed vesicles. Péron et al. (2016, 2017) demonstrated the potential of this method by measuring the $^{20}$Ne/$^{22}$Ne ratio in single vesicles from Galápagos basalts. In this study, we applied a similar approach to Icelandic subglacially erupted pillow lavas. These analyses are challenging owing to the relatively low eruptive pressure of the Miðfell lava flows (25-30 bar; Colin et al., 2015), which is about 8-10 times lower than that of the Galápagos samples studied by Péron et al. (2016, 2017), resulting in a lower gas content in individual vesicles. Yet, the precise determination of the $^{20}$Ne/$^{22}$Ne ratio was made possible by coupling statistical analysis with a new generation of high sensitivity mass spectrometers capable of partially resolving $^{40}$Ar⁺⁺ from $^{20}$Ne⁺ peaks.

## 2. Sample information & analytical techniques

### 2.1. Sample information and preparation

All the analyses were conducted on the glassy rims of a subglacially erupted Icelandic tholeiite, collected in 2024, hereafter referred to as RS-ISL-2024-6. The samples were collected in an old quarry on the eastern flank of the Miðfell volcano (64°10'29"N; 21°2'52"W, Fig. 1), within the Dagmalafell unit (Risku-Norja, 1985). Miðfell is part of the Hengill volcanic system, located in the Western Volcanic Zone (WVZ), one of Iceland's axial rift zones where recent volcanic activity is concentrated. While the precise age of the Miðfell unit is not known, previous studies suggest that it erupted during the last Pleistocene glaciation, approximately 12–14 ka ago (Saemundsson et al., 2010). The outcrop comprises exposed pillow-lavas with fresh glassy rims of varying thickness (Fig. 1). The glassy rim is notably vesicular and contains large Mg-rich olivine crystals (Fo₈₈) in line with the primitive character of the rock.

The collected samples are similar to the 'DICE10–11' (Harrison et al., 1999; Dixon et al., 2000; Trieloff et al., 2000; Mukhopadhyay, 2012;

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** a) Sample location of RS-ISL-2024-6, a subglacially erupted tholeiite from the Miðfell volcano. b) Photograph of a typical outcrop of the Dagmalafell unit where RS-ISL-2024-6 was sampled showing pillow-lava textures with glassy rims.</figcaption></figure>
Colin et al., 2015), MID-1 (Füri et al., 2010; Barry et al., 2014), 'DG2017' (Péron et al., 2021) and 'MiðfellRP09' (Parai, 2023) samples. These glassy rims have been extensively used in previous studies as they are remarkably unaltered (see SI) and relatively unaffected by degassing-driven noble gas fractionation, with $^{4}$He/$^{21}$Ne* and $^{4}$He/$^{40}$Ar* close to the theoretical production ratios in the mantle (2.22.10$^7$ - 2.79.10$^7$, Yatsevich and Honda, 1997; and 1.6 - 4.2, Staudacher et al., 1989; Graham, 2002, respectively).

Additionally, these lavas exhibit primordial signatures typical of magmatism associated with the Icelandic plume ($^{20}$Ne/$^{22}$Ne > 12.5; low, unradiogenic, $^{4}$He/$^{3}$He and $^{21}$Ne/$^{22}$Ne).

In the lab, glass chunks of RS-ISL-2024-6 were hand-picked and cleaned in a heated 0.2 mol·L$^{-1}$ oxalic acid solution, then rinsed with distilled water to remove any traces of weathering. Subsequent cleaning steps involved ethanol, acetone, a final bath in distilled water, and drying. Large glass fragments (1-3 g) were selected for step-crushing analyses, while smaller pieces were prepared for laser ablation. For the latter, the glass chunks were polished into rectangular wafers with flat surfaces to ensure proper laser focusing during analysis.

## 2.2. Microtomography acquisitions

X-ray microtomography was performed on the glass samples prior to laser-ablation analyses. This 3D imaging technique helps localize vesicles before targeting them with the laser. The high-resolution scans (6-micron voxels) also revealed cracks within the glass, enabling the user to verify that the targeted vesicles were not connected to the air before piercing. The X-ray microtomography acquisitions were all conducted at the Institut des Sciences de la Terre d'Orléans (France) using a Phoenix Nanotom M (180 kV X-ray tube with variable filament currents (40 - 170 μA) and operating voltage of up to 120 kV) using a tungsten target. The acquisition consists of a series of slices, corresponding to a specific section of the sample. These slices can be assembled and processed to reconstruct a complete 3D scan of the sample. We used a copper filter to limit beam hardening artefacts. An example of a single slice acquired during X-ray microtomography is shown in Fig. S1.

The 3D volume of the sample was reconstructed using the VGStudio software (Fig. S2). This allows for precise localization of the vesicles (due to the strong density difference between glass and crystals) and to assess the vesicularity of the sample. The vast majority of the vesicles are spherical while some are slightly elongated. Based on the acquisitions, we calculate a mean vesicularity of 7.3 ± 0.2% for the quenched sub-glacial lava from Miðfell.

## 2.3. Noble gas mass spectrometry

All the analyses were performed on a Noblesse® 5F 5 M mass spectrometer, capable of partially resolving $^{20}$Ne$^+$ peaks from that of $^{40}$Ar$^{++}$(and $^{1}$H$^{19}$F$^+$). Interferences between $^{22}$Ne$^+$ and $^{44}$(CO$_2)^{++}$ were corrected by measuring (CO$_2)^{+}$ at each analysis cycle while having precisely calibrated the (CO$_2)^{++}$/ (CO$_2)^{+}$ ionization ratio (0.012). Ne isotopes are analyzed in multicollection on the electron multipliers in pulse counting mode, while He isotopes are analyzed by peak switching, with $^{4}$He being analyzed on a faraday cup and $^{3}$He on an electron multiplier in pulse counting mode. Using the high-resolution mode, the HD and $^{3}$He peaks are completely resolved.

Two different standards were used for the analyses. The first was an air standard, prepared by diluting a 0.1 cc pipette of air into a 2 L bottle. The air standard is inadequate for helium analyses, for which a dedicated helium standard was used. The helium standard was homemade and consisted of 150 cc of fumarole gas sampled in the Pingvallavatn geothermal field and 150 cc of gas sampled from a CO$_2$ plant located 10 km southeast of the lake Pingvallavatn. This helium standard was calibrated against air and has a $^{3}$He/$^{4}$He isotopic ratio of 21.07 ± 0.15 R/Ra. Each 0.1 cc pipette used during the analyses contains (9.78 ± 0.72)·10$^{-8}$ cc of $^{4}$He. To account for potential time-dependent variability

in the measured standard composition, the standards measured closest in time to the samples were systematically used for correction. Analyses performed on the standards are presented in the Supplementary Materials (Figs. S3-S5).

## 2.4. Gas extraction techniques

Step-crushing and laser ablation analyses were conducted on RS-ISL-2024-6. For step-crushing, large pieces of the sample were loaded into a cylindrical crusher. The crusher was then baked at 100-150 °C under ultra-high vacuum for at least one night (or until the analytical blank was sufficiently low) before the experiment. A Pfeiffer Vacuum D-35,614 Asslar manometer was connected to the crusher to monitor the pressure released during each crushing step.

For laser ablation, smaller polished pieces of RS-ISL-2024-6 were placed in a laser cell equipped with a sapphire window. The cell was baked for at least two days at 120 °C and under ultra-high vacuum prior to the laser ablation analyses. After the baking period, it was necessary to wait for a few days to a few weeks for the Ne blank to be low enough to start the analyses. A diode-pumped air-cooled Q-switcher laser (Quantum Light Instruments Q1-E10-1053) with a wavelength of 263 nm, a beam diameter of 35 µm, ablation frequency of 10 Hz and an energy of 7 mJ per pulse was used to pierce the matrix. Closed vesicles were targeted based on the X-ray microtomography 3D acquisitions. A MKS Baratron® manometer is connected to the laser cell (0.1 Torr). When a vesicle is pierced, the release of gas trapped inside causes a sudden pressure spike, in contrast with the gradual pressure increase observed during the ablation of the glassy matrix (represented in Fig. 2). If no vesicle was pierced within a ~ 3-minute ablation timeframe, the cell was pumped for 1 min before resuming ablation. This is done to minimize the influence of dissolved gases in the matrix on the vesicle data. Multiple cycles of ablation and pumping were often required before successfully reaching a vesicle. The depths of pierced vesicles relative to the sample surface ranged from 20 to 250 µm.

## 2.5. Purification

After extraction, the gas is purified before being introduced into the mass spectrometer. Analytical blanks are critically important due to the relatively low gas content in the Miðfell samples (which can be attributed to the low eruption hydrostatic pressure of < 27 ± 1 bars Colin et al., 2015). This is especially significant for laser-ablation analyses, as the quantity of gas released by a single vesicle is very low (as low <

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

**Fig. 2.** Representation of the variation of pressure observed during a typical session of laser-ablation analysis aimed at piercing closed vesicles. From 0 to approximately 19 min, the variations in pressure are produced by ablation of the matrix/pumping cycles prior to reaching the vesicle. At minute 19, the vesicle is pierced resulting in a sudden pressure spike. The laser is then turned off and the purification process followed by the analysis using the mass spectrometer begins.
1.10⁻¹³ cc of ²²Ne).

For step-crushing analyses, the gas is first chemically purified using two Ti-getters first heated to 1073 K and then at room temperature. The remaining gas, predominantly noble gases, is then trapped using two cryogenic cold heads: one containing steel and the other activated charcoal. The steel trap is maintained at 40 K during the experiment to trap Ar, Kr, Xe (and the potentially residual CO₂, which is the major gas specie in the vesicles, Moore et al., 1977). The activated charcoal trap is initially kept at 7 K to trap He and Ne. He and Ne are then analyzed sequentially through thermal desorption from the charcoal trap: the temperature is first increased to 35 K to release He. After analysis of He, the temperature of the trap is set to 65 K to release Ne.

The purification process differs between step-crushing and laser-ablation analyses to minimize analytical blanks. For gas extracted via laser ablation, only the cryogenic separation step involving the cold traps is used, omitting the Ti-getters. This reduces the analytical blank by approximately ~65% (from 4.10⁻¹³ cc down to as low as 1.3.10⁻¹³ cc) and does not cause significant analytical issues, as the quantity of gas extracted from a single vesicle is very low.

Analytical blank analyses were conducted following the same procedure as the sample analyses. For laser-ablation experiments, blanks were particularly important to account for any potential matrix effect on the data. If the matrix was ablated for 2 min before reaching a vesicle, the blank analysis was performed under the same conditions (i.e., the matrix was ablated for 2 min) to ensure accurate correction of matrix effects. Examples of matrix blank compositions are provided in Table S3. The matrix was observed to have an atmospheric isotopic composition and low gas content, resulting in minimal matrix effects on vesicle data. This effect was further reduced by pumping the cell for ~1 min every ~3 min if no vesicle was pierced, as stated previously.

## 3. Results

Our dataset consists of 13 pierced individual vesicles and 16 crushing steps performed on glassy rims of RS-ISL-2024-6 (Miðflell, Iceland). The vesicles dataset is presented in Table 1 and the crushing steps in Table 2. The ²²Ne content of the vesicles (after blank correction) is well-correlated to vesicle volume (Fig. 3). The vesicles we analyzed (Iceland) contains much less ²²Ne for a given volume compared to that of Péron et al. (2016, 2017) correlated to the much lower eruptive pressure of our samples. This characteristic necessitated adaptations of the purification process during the laser ablation analyses to minimize the analytical blank and, in turn, increase the signal-to-noise ratio (see II. e)). For the reasons given previously, samples from this locality have been extensively used to study the Icelandic mantle source (Trieloff et al., 2000; Mukhopadhyay, 2012; Colin et al., 2015; Péron et al., 2021; Parai., 2023). Datasets from these step-crushing studies have been compiled and can be used as a direct comparison to ours (Fig. 4), as they

reflect a single locality (hereafter referred to as ‘literature’).

### 3.1. Helium isotopes

Helium isotope measurements conducted on RS-ISL-2024-6 show relatively good homogeneity for both the step-crushing and individual vesicle datasets. The mean ³He/⁴He ratio derived from the crushing steps (Table 2) is 18.3 ± 0.4 R/Ra (where Ra represents the atmospheric ³He/⁴He ratio). This unradiogenic ³He/⁴He ratio (compared to the atmosphere) is in good agreement with the literature (Trieloff et al., 2000; Mukhopadhyay, 2012; Colin et al., 2015; Péron et al., 2021; Parai., 2023). For the vesicles, the mean ³He/⁴He ratio is 17.3 ± 0.8 Ra (Table 1), being slightly more heterogeneous but in good agreement (within uncertainties) with the mean value obtained from step crushing.

### 3.2. Neon isotopes

Step-crushing data for both the literature dataset and our own crushing steps exhibit a linear array between atmospheric neon and a mantle-derived endmember (Fig. 4). The ²⁰Ne/²²Ne ratio of the vesicle dataset ranges between 12.01 ± 0.47 and 13.47 ± 0.44 and the ²¹Ne/²²Ne ratio between 0.0255 ± 0.0065 and 0.0451 ± 0.0061. Some vesicles exhibit high blank contributions for Ne (> 50%), associated with their very low gas content (in some cases < 1.10⁻¹³ cc ²²Ne), resulting in large error bars (Fig. 4). The individual vesicles analyzed are isotopically homogeneous for Ne (within error), particularly when considering vesicles with low blank contributions (Table 1). The homogeneity of the vesicle dataset suggests that they sample a single gas composition. This allows us to apply a statistical approach to evaluate the ²⁰Ne/²²Ne and ratio ²¹Ne/²²Ne of the vesicle dataset in its entirety, accounting for the uncertainties associated with each individual measurement.

### 3.3. Statistical analysis of the neon isotopic compositions of the vesicles

The dataset has been analyzed using both IsoplotR (Vermeesch, 2018) and the Gaussian summing technique (Péron et al., 2017). Both approaches provide weighted averages for the ²⁰Ne/²²Ne and ²¹Ne/²²Ne isotopic ratios. Using IsoplotR, the weighted average for ²⁰Ne/²²Ne is 12.65 ± 0.13 (2σ), with MSWD = 0.95 and p(χ²) = 0.50. These values (MSWD ≈ 1; p ≈ 0.5) indicate that the scatter among individual vesicles is fully consistent with the reported analytical uncertainties, and therefore the weighted mean can be considered statistically robust. For ²¹Ne/²²Ne, the weighted average obtained with IsoplotR is 0.0350 ± 0.0018 with MSWD = 1.4 and p(χ²) = 0.16. These suggest slightly greater dispersion than expected from uncertainties alone but remain within the range generally regarded as acceptable (MSWD ≈ 1, p > 0.05). Thus, the dataset is overall coherent, and the weighted mean can

<caption>**Table 1**</caption>

Laser ablation analyses of individual vesicles trapped in RS-ISL-2024-6 glass samples.

<table><thead><tr><th></th><th>Volume<br>(10<sup>7</sup> µm<sup>3</sup>)</th><th>Depth<br>(µm)</th><th>⁴He<br>(10<sup>-8</sup> cc STP)</th><th>³He/⁴He<br>(Ra<sup>b</sup>)</th><th>²²Ne<br>(10<sup>-14</sup> cc STP)</th><th>α<sup>b</sup></th><th>²⁰Ne/²²Ne</th><th>²¹Ne/²²Ne</th></tr></thead><tbody><tr><td>B1</td><td>4.8</td><td>260</td><td>2.0 ± 0.2</td><td>16.9 ± 0.3</td><td>25±0.7</td><td>62</td><td>12.46±0.34</td><td>0.0292±0.0042</td></tr><tr><td>B2</td><td>1.7</td><td>90</td><td>1.0 ± 0.1</td><td>16.9 ± 0.4</td><td>11±0.6</td><td>77</td><td>12.57±0.65</td><td>0.0256±0.0088</td></tr><tr><td>B3</td><td>10.6</td><td>78</td><td>4.2 ± 0.3</td><td>18.3 ± 0.2</td><td>55±0.9</td><td>41</td><td>12.43±0.16</td><td>0.0348±0.0023</td></tr><tr><td>B4</td><td>2.9</td><td>144</td><td>1.2 ± 0.1</td><td>18.6 ± 0.4</td><td>22±0.6</td><td>59</td><td>12.26±0.32</td><td>0.0399±0.0047</td></tr><tr><td>B5</td><td>3.0</td><td>20</td><td>1.4 ± 0.1</td><td>17.5 ± 0.4</td><td>16±0.4</td><td>40</td><td>12.68±0.29</td><td>0.0342±0.0040</td></tr><tr><td>B6</td><td>2.1</td><td>60</td><td>1.0 ± 0.1</td><td>18.6 ± 0.4</td><td>10±0.4</td><td>50</td><td>13.18±0.51</td><td>0.0457±0.0061</td></tr><tr><td>B7</td><td>9.7</td><td>180</td><td>3.5 ± 0.3</td><td>16.4 ± 0.3</td><td>52±0.8</td><td>21</td><td>12.73±0.14</td><td>0.0346±0.0020</td></tr><tr><td>B8</td><td>2.4</td><td>20</td><td>1.2 ± 0.1</td><td>16.0 ± 0.3</td><td>11±0.5</td><td>58</td><td>12.01±0.47</td><td>0.0304±0.0066</td></tr><tr><td>B9</td><td>3.3</td><td>40</td><td>1.6 ± 0.1</td><td>17.5 ± 0.3</td><td>14±0.5</td><td>55</td><td>13.47±0.44</td><td>0.0305±0.0053</td></tr><tr><td>B10</td><td>3.4</td><td>120</td><td>1.6 ± 0.1</td><td>16.7 ± 0.3</td><td>17±0.5</td><td>48</td><td>12.75±0.33</td><td>0.0284±0.0042</td></tr><tr><td>B11</td><td>1.9</td><td>40</td><td>1.2 ± 0.1</td><td>16.7 ± 0.3</td><td>9 ± 0.4</td><td>60</td><td>12.56±0.52</td><td>0.0255±0.0065</td></tr><tr><td>B12</td><td>8.2</td><td>30</td><td>4.1 ± 0.3</td><td>16.9 ± 0.2</td><td>37±0.7</td><td>27</td><td>12.81±0.18</td><td>0.0395±0.0025</td></tr><tr><td>B13</td><td>10.3</td><td>120</td><td>5.1 ± 0.4</td><td>17.8 ± 0.2</td><td>55±0.8</td><td>20</td><td>12.65±0.14</td><td>0.0362±0.0019</td></tr></tbody></table>

Notes: All errors are 1 sigma. a ³He/⁴He normalized to the atmospheric ratio (1.384.10⁻⁶). b Blank contribution (%) to the analysis for ²²Ne (prior correction).
<table><caption>**Table 2**
Step-crushing analyses of in RS-ISL-2024-6 glass samples.</caption><thead><tr><th></th><th><sup>4</sup>He<br>(10<sup>-8</sup> cc STP)</th><th><sup>3</sup>He/<sup>4</sup>He<br>(Ra<sup>b</sup>)</th><th><sup>22</sup>Ne<br>(10<sup>-13</sup> cc STP)</th><th>α<sup>b</sup></th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th></tr></thead><tbody><tr><td colspan="7"><strong>RS-ISL-2024-6, 1.0 g</strong></td></tr><tr><td>Step 1</td><td>18±1.3</td><td>18.5 ± 0.2</td><td>1350±15</td><td>1</td><td>9.90±0.02</td><td>0.0290±0.0004</td></tr><tr><td>Step 2</td><td>20±1.5</td><td>18.5 ± 0.2</td><td>22±0.3</td><td>14</td><td>12.55±0.08</td><td>0.0351±0.0011</td></tr><tr><td colspan="7"><strong>RS-ISL-2024-6, 3.0 g</strong></td></tr><tr><td>Step 1</td><td>28±2.1</td><td>18.3 ± 0.2</td><td>86±1.0</td><td>3</td><td>10.78±0.03</td><td>0.0316±0.0005</td></tr><tr><td>Step 2</td><td>28±2.1</td><td>18.6 ± 0.4</td><td>114±1.3</td><td>2</td><td>10.80±0.03</td><td>0.0310±0.0004</td></tr><tr><td>Step 3</td><td>13±1.0</td><td>18.4 ± 0.2</td><td>38±0.4</td><td>4</td><td>10.98±0.03</td><td>0.0312±0.0006</td></tr><tr><td>Step 4</td><td>16±1.2</td><td>18.2 ± 0.2</td><td>51±0.6</td><td>3</td><td>10.95±0.03</td><td>0.0310±0.0005</td></tr><tr><td>Step 5</td><td>32±2.4</td><td>17.9 ± 0.1</td><td>193±2.2</td><td>2</td><td>10.35±0.03</td><td>0.0301±0.0004</td></tr><tr><td>Step 6</td><td>33±2.4</td><td>18.6 ± 0.3</td><td>149±1.6</td><td>1</td><td>10.28±0.03</td><td>0.0301±0.0004</td></tr><tr><td>Step 7</td><td>13±1.0</td><td>18.3 ± 0.3</td><td>16±0.2</td><td>11</td><td>12.37±0.05</td><td>0.0337±0.0008</td></tr><tr><td>Step 8</td><td>15±1.1</td><td>18.2 ± 0.1</td><td>29±0.3</td><td>6</td><td>11.97±0.04</td><td>0.0334±0.0007</td></tr><tr><td>Step 9</td><td>8 ± 0.6</td><td>16.9 ± 0.1</td><td>13±0.1</td><td>12</td><td>12.20±0.06</td><td>0.0344±0.0008</td></tr><tr><td>Step 10</td><td>6 ± 0.5</td><td>18.5 ± 0.2</td><td>24±0.3</td><td>7</td><td>10.83±0.07</td><td>0.0307±0.0005</td></tr><tr><td>Step 11</td><td>13±1.0</td><td>18.6 ± 0.2</td><td>150±1.7</td><td>2</td><td>9.85±0.03</td><td>0.0303±0.0004</td></tr><tr><td>Step 12</td><td>6 ± 0.4</td><td>18.5 ± 0.2</td><td>7.5 ± 0.1</td><td>19</td><td>12.10±0.08</td><td>0.0332±0.0010</td></tr><tr><td>Step 13</td><td>6 ± 0.5</td><td>18.6 ± 0.2</td><td>141±1.6</td><td>1</td><td>9.92±0.04</td><td>0.0291±0.0004</td></tr><tr><td>Step 14</td><td>7 ± 0.5</td><td>18.5 ± 0.2</td><td>7.4 ± 0.1</td><td>16</td><td>12.07±0.07</td><td>0.0328±0.0009</td></tr></tbody></table>

Notes: All errors are 1 sigma. <sup>a</sup> $^{3}$He/$^{4}$He normalized to the atmospheric ratio. <sup>b</sup> Blank contribution (%) to the analysis for $^{22}$Ne (prior correction).

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Neon concentration in individual vesicles against their volume, both for the suglacially erupted lava from Iceland used for this study and the dredged pillow-lavas from the Galapagos used by Péron et al. (2017). The much steeper slope for the Galápagos samples results from a much higher eruptive pressure of ~240 bars based on the dredge depth (Geist et al., 2006) against 27 bars for RS-ISL-2024-6 (Colin et al., 2015).</figcaption></figure>

be considered reliable (even if a small degree of additional scatter cannot be excluded.). Applied to both the $^{20}$Ne/$^{22}$Ne and $^{21}$Ne/$^{22}$Ne ratios, this test identified no outliers, indicating that the vesicles' composition is homogeneous and record a single gas composition. The corresponding Isoplot diagrams are provided in the supplementary material (Figs. S6 and 7).

In addition to IsoplotR, we analyzed the dataset using the Gaussian summing technique as it preserves the full probability distribution of each vesicle (hence is more relevant for smaller datasets as presented here) and provides intuitive visualization of the weighted average (Fig. 5).

For this each vesicle's Ne isotopic composition is modeled as a Gaussian distribution:

$$f = \frac{1}{\sigma\sqrt{2\pi}}\exp\left(-\frac{(x - R)^2}{2\sigma^2}\right)$$

Where R is the measured isotopic ratio and σ the associated error, x

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Neon-three diagram where our crushing steps and the analyses performed on individual vesicles from RS-ISL-2024-6 (Miðflell) are presented. Crushing-step data from literature for this locality are also shown (from Trieloff et al., 2000; Mukhopadhyay, 2012; Colin et al., 2015; Péron et al., 2021; Parai, 2023). The MORB array, represented by the popping-rock 2IID43 (Moreira et al., 1998), is presented for reference. Vesicles are plotted in green with their associated error bars and distinction are made for vesicles with high blank contributions (> 50%) and lower blank contributions (higher opacity). The calculated mantle source using the vesicle dataset is represented as a red star. Different endmembers are also shown: Air, Ne-B (from Raquin and Moreira, 2009) and the Sun value (calculated from bulk Solar Wind by Heber et al., 2012). Errors are 1 sigma.</figcaption></figure>

being the interval considered for the isotopic ratio (9 to 14 for the $^{20}$Ne/$^{22}$Ne ratio for example). These individual Gaussian curves are then summed to generate a cumulative curve. The curves resulting in the cumulation of the modelled individual vesicles are gaussian-like (both for $^{20}$Ne/$^{22}$Ne and $^{21}$Ne/$^{22}$Ne). This cumulative curve is fitted to a Gaussian distribution, and the peak of this distribution corresponds to the mean, error-weighted isotopic ratio of the vesicle dataset (Fig. 5). The associated uncertainty of the mean isotopic ratio is computed as $\sigma/\sqrt{n}$, where n represents the number of vesicles. This approach assumes that all vesicles sample the same gas composition (which is consistent with the results of IsoplotR), ensuring that the error on the mean reflects the precision of the dataset as a whole. This method gives
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Gaussian curve (in red) for a) $^{20}$Ne/$^{22}$Ne and b) $^{21}$Ne/$^{22}$Ne obtained by fitting the green dotted cumulative curve for the vesicle dataset. Two individual vesicles are presented to show that a more accurately determined isotopic (B13) ratio results in a stronger contribution to the cumulative curve than a less accurately determined isotopic ratio (B6). a) The error-weighted average for $^{20}$Ne/$^{22}$Ne (corresponding to the highest density value on the gaussian fit) is 12.63 and the associated error is 0.14 (σ/√n, shaded area). Two potential endmembers are shown, representative of the two models evocated for the origin of solar Ne in the mantle: Ne-B (solar wind irradiation of protoplanetary dust, from Raquin and Moreira, 2009) and the Sun value (dissolution of a primordial atmosphere in the magma ocean, calculated from bulk Solar Wind by Heber et al., 2012). b) The error-weighted average for $^{21}$Ne/$^{22}$Ne (corresponding to the highest density value on the gaussian fit) is 0.0349 and the associated error is 0.0023 (σ/√n, shaded area). Because the $^{21}$Ne isotope is nucleogenic in the mantle, the $^{21}$Ne/$^{22}$Ne cannot be used to decipher between the two models, as $^{21}$Ne was produced over 4.45 Ga. The air $^{20}$Ne/$^{22}$Ne and $^{21}$Ne/$^{22}$Ne isotopic compositions are also presented for reference.</figcaption></figure>

greater weight to more accurately determined isotopic ratios (Fig. 5), while still fully incorporating less accurate, but valid, measurements.

The Gaussian summing results are 12.63 ± 0.14 (σ/√n) for $^{20}$Ne/$^{22}$Ne and 0.0349 ± 0.0023 (σ/√n) for $^{21}$Ne/$^{22}$Ne, values that are consistent with the highest step-crushing measurements (Fig. 4) and statistically indistinguishable from the means obtained using IsoplotR. The convergence of these two independent approaches demonstrates the robustness of the final values and confirms that they are not dependent on the choice of statistical method. For internal consistency with Péron et al. (2017), we use the Gaussian summing results in the remainder of this paper, although adopting the means obtained with IsoplotR would not alter the conclusions.

# 4. Discussion

## 4.1. Air contamination of the vesicles

Proper interpretation of our results for the Iceland plume mantle relies on the assumption that the analyzed vesicles are not affected by secondary atmospheric contamination. We consider this to be true for several reasons.

First, we specifically targeted “closed” vesicles that are free of secondary fractures using 3D microtomography. There are two main mechanisms which could introduce air into these vesicles: (1) contamination at the time of eruption, and (2) post-eruptive alteration or diffusion of Ne out of the vesicles (following the natural pressure gradient).

Regarding the alteration, rocks from Miðflell were selected for their remarkable preservation (see SI). We consider diffusion of Ne out of the vesicles to be unlikely. Indeed, experimental diffusion coefficients in basaltic glasses indicate very slow Ne diffusivity at low temperatures (e. g., Amalberti et al., 2016). Assuming linear extrapolation to 25 °C gives $D_{Ne} \approx 10^{-22} \text{ m}^2/\text{s}$ (see SI). This implies a diffusion time of ~176 million years to escape 1 mm of glass ($T \approx L/D^2$), far longer than the age of the rock (12-14 ka) or its surface residence time.

In addition, if diffusion was significant, He would be even more affected, as it diffuses orders of magnitude faster than Ne at the same temperature (e.g., Amalberti et al., 2016), even potentially fractionating the $^3$He/$^4$He isotopic composition of the gas (Shelby, 1971). However, the rocks remain He-rich ($> 10^{-6} \text{ cc } ^4\text{He}$), and the $^3$He/$^4$He isotopic compositions measured are consistent with other localities close to Miðflell (e.g., Füri et al., 2010), even considering analyses of thermal pools. This indicates that Ne escape by diffusion has been probably negligible.

This leaves atmospheric contamination contemporaneous with eruption as the only plausible mechanism for significant air addition to the closed vesicles, which would lower the measured $^{20}$Ne/$^{22}$Ne and $^{21}$Ne/$^{22}$Ne ratios. Several lines of evidence suggest that this was not the case: (i) Vesicle $^{22}$Ne content shows a tight positive correlation with vesicle volume (Fig. 3), similar to that observed in the Galápagos samples. Even a small addition of air would introduce heterogeneity, causing scatter in vesicle Ne concentrations. If air contamination had occurred within the magma, the smallest vesicles (nucleated last) should be the most affected resulting in heterogeneous Ne concentrations for the smallest vesicle volumes. Such an effect is not observed (Fig. 3) and would likely be visible given the small individual uncertainties on $^{22}$Ne abundances (1-5%) and vesicle volumes (1-3%). Any significant addition of air or loss of magmatic gas would have lowered the internal vesicle pressure, thereby altering the vesicle size-gas content relationship. No such effect is observed in our data (Fig. 3).

(ii) Such contamination would also be expected to generate significant dispersion in the $^3$He/$^{22}$Ne ratios, which is likewise not observed (Fig. 6). This observation is consistent with the absence of statistical outliers in the dataset, as revealed by IsoplotR analysis.

Second, significant pressure jumps are recorded when piercing closed vesicles (Fig. 2), whereas open vesicles that contain air show no such pressure change. This has also been documented in previous vesicle-by-vesicle noble gas studies (e.g., Raquin et al., 2008; Colin et al., 2015; Péron et al., 2016).

All these considerations lead us to believe that closed vesicles that are not affected by secondary fractures probably preserves the original composition of the magmatic gases, enabling inferences on the composition of the Icelandic mantle source.

## 4.2. The subduction of atmospheric neon into the mantle

The recycling of atmospheric Ar, Kr and Xe into the lower mantle via subduction is widely recognized over long timescales on Earth (e.g. Holland and Ballentine, 2006; Péron and Moreira, 2018; Bekaert et al.,
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Plot of the $^{20}$Ne/$^{22}$Ne isotopic ratio of our crushing steps and individual vesicles from RS-ISL-2024-6 (Miðflell) against their $^{3}$He/$^{22}$Ne elemental ratio. Crushing-step data from literature for this locality are also shown (from Trieroff et al., 2000; Mukhopadhyay, 2012; Colin et al., 2015; Péron et al., 2021; Parai, 2023). Vesicles are plotted in green with their associated error bars and distinction are made for vesicles with high blank contributions (> 50%) and lower blank contributions (higher opacity). We used the linear regression produced by crushing and our error-weighted average for $^{20}$Ne/$^{22}$Ne (calculated from the vesicle dataset) to yield a $^{3}$He/$^{22}$Ne of 2.35 ± 0.12 for Iceland (represented by the red star). Errors are 1 sigma.</figcaption></figure>

2021). However, this process is poorly efficient as most of the noble gases are expelled from the slab before and during arc-magma genesis (the so-called “subduction barrier”, Staudacher and Allègre, 1988; Moreira and Raquin, 2007). This is supported by the fact that mantle-derived samples still exhibit excesses of unradiogenic Ar, Kr, and Xe isotopes relative to the atmosphere (Moreira et al., 1998; Holland and Ballentine, 2006), despite current subduction fluxes being theoretically sufficient to overprint these excesses in the upper mantle within less than 100 Myr (Krantz et al., 2019).

The impact of this process for Ne is more controversial, as the unradiogenic Ne signature of the mantle is clearly solar, even in the depleted upper mantle. Evaluating the relevance of atmospheric neon subduction in OIB sources is crucial for accurately interpreting the neon isotopic composition of the lower mantle. Indeed, the error-weighted isotopic ratio of $^{20}$Ne/$^{22}$Ne = 12.63 ± 0.14, which we determined as representative of the lower mantle, could result from the addition of approximately 21% atmospheric neon ($^{20}$Ne/$^{22}$Ne = 9.8) via subducted material to a lower mantle component with a solar nebula signature ($^{20}$Ne/$^{22}$Ne = 13.4; Heber et al., 2012).

This process has been proposed by some authors to explain the discrepancy between their highest measured isotopic ratios from step-crushing experiments and a “true” solar value which would have been inherited from the primordial ingassing of the magma ocean by Earth’s early atmosphere (Yokochi and Marty, 2004; Mukhopadhyay, 2012; Williams and Mukhopadhyay, 2019).

Atmospheric noble gases in subducted slabs are primarily stored in marine sediments, altered oceanic crust, and serpentinites, with elemental abundance ratios similar to seawater (Moreira et al., 2003; Kendrick et al., 2011; Chavrit et al., 2016; Smye et al., 2017). Being chemically inert, noble gases are not chemically bound to mineral phases. Holland and Ballentine (2006) suggested that pore fluids act as the main carrier of noble gases in subducted slabs. However, subduction reduces porosity extremely efficiently, which results in the effective expulsion of the porewater from the slab (Connolly, 2010) at relatively shallow depths. Yet, noble gases may be incorporated into hydrous

minerals with ring-like Si-O structures (Jackson et al., 2013), such as amphibole, serpentines, and chlorite, which are major components of altered oceanic crust. These noble gases could be released from their host phase either by mineral breakdown or diffusion.

Recent studies suggest that Ne could be partially retained beyond 200 km depth in cold subduction zones (Smye et al., 2017; Kendrick et al., 2018; Wang et al., 2020). However, the retained fraction of Ne is significantly lower than that of Ar, Kr, and Xe, as noble gas diffusivity in the host mineral is inversely correlated with atomic radius (Wang et al., 2020). This is further supported by step-heating experiments, which show that nearly all Ne is lost from altered oceanic crust at temperatures exceeding 1100 °C, temperatures likely reached before the slab reaches the D" layer where the source mantle of ‘primordial’ OIBs could be located (Moreira et al., 2003).

The potential impact of subducted Ne on the preserved lower mantle's isotopic composition is further challenged by its high noble gas content. This pristine mantle is significantly less (> 99%, Allègre et al., 1987) degassed than the upper mantle resulting with an estimated $^{20}$Ne concentration in this reservoir that is approximately 400 times higher than that of the upper mantle (based on noble gas systematics and incompatible element ratios such as K and U, Allègre and Moreira, 2004). Given the high concentration of $^{20}$Ne in the preserved mantelic reservoir compared to serpentine (7.3 ± 9.6) $10^{-10}$ cc/g $^{20}$Ne for oceanic serpentinites, Kendrick et al., 2018) or marine sediments (Moreira et al., 2003; Chavrit et al., 2016; Smye et al., 2017; Krantz et al., 2019), the addition of subducted atmospheric Ne would likely have a negligible effect on the preserved primordial mantle Ne isotopic composition.

A first-order approximation of the influence of atmospheric Ne subduction on the bulk mantle composition can be made using the current $^{36}$Ar flux to the mantle. This flux is relatively well constrained at (3.2$^{+3.8}_{-1.2}$) $10^8$ cc/yr of $^{36}$Ar (Chavrit et al., 2016), amounting to (9.6$^{+11.2}_{-3.7}$) $10^{17}$ cc over 3 Ga, assuming a constant flux over time (and the a start of a global subduction regime 3 Ga ago, Péron and Moreira, 2018). If the $^{20}$Ne/$^{36}$Ar ratio of the subducted oceanic crust is similar to that of seawater (0.17, Staudacher and Allègre, 1988; Chavrit et al., 2016), the estimated $^{20}$Ne amount over 3 Ga would be (1.6$^{+1.9}_{-0.6}$) $10^{17}$ cc ($\Phi_{20\text{Ne}}$).

Marty (2012) estimated the bulk mantle $^{22}$Ne inventory, from which, assuming a solar-like $^{20}$Ne/$^{22}$Ne of 13.4 (Heber et al., 2012), we can derive the $^{20}$Ne budget of the bulk mantle to be (7.1 ± 3.9) $10^{18}$ cc.

Under the extreme assumption that 100% of the $^{20}$Ne in subducted slabs is recycled, using the upper bound of the $\Phi_{20\text{Ne}}$, and using the lower bound for bulk mantle $^{20}$Ne budget, the total $^{20}$Ne flux into the mantle via subduction would represent at most ~ 11% of the bulk mantle's inventory. This atmospheric fraction is probably overestimated by a factor of >100 given that most of the noble gases, and especially He and Ne (Bekaert et al., 2021), are expelled from the slab and returned to the atmosphere (Staudacher and Allègre, 1988; Moreira and Raquin, 2007).

If the bulk mantle originally had a solar $^{20}$Ne/$^{22}$Ne ratio of 13.4 (Heber et al., 2012), homogeneously incorporating 11% of atmospheric Ne would lower this ratio to ~ 13.04. This is inconsistent with the $^{20}$Ne/$^{22}$Ne estimate inferred for both the Iceland (12.63 ± 0.14, this study) and Galápagos mantle sources (12.65 ± 0.04, Péron et al., 2017). Furthermore, the mantle source considered here is particularly gas-rich compared to the rest of the mantle, especially the depleted upper mantle. Therefore, the effect of Ne subduction (if any) would be minimal compared to the rest of the mantle.

This conclusion applies only to Ne however. Applying the same approach to $^{130}$Xe, using the $^{130}$Xe flux from Chavrit et al. (2016) and the bulk mantle $^{130}$Xe budget from Marty (2012), would result in an estimated 350% increase of the current $^{130}$Xe bulk mantle's budget via subduction over 3 Ga.

While our estimate is not strictly quantitative, it underscores the minimal impact of Ne subduction on the gas-rich primordial reservoir that some OIB sample. We therefore conclude that subduction could not
have altered significantly the $^{20}$Ne/$^{22}$Ne of the preserved, primordial, mantelic reservoir and the value we infer from individual vesicle analyses probably reflects the $^{20}$Ne/$^{22}$Ne ratio of the source as it was 4.45 Ga ago.

### 4.3. The noble gas composition of the Iceland plume

The error-weighted $^{20}$Ne/$^{22}$Ne isotopic ratio of the vesicle dataset, 12.63 ± 0.14, is indistinguishable from the $^{20}$Ne/$^{22}$Ne ratio of 12.65 ± 0.04 determined for the Fernandina mantle source (Galápagos) by Péron et al. (2017) using the same approach. Both $^{20}$Ne/$^{22}$Ne and $^{21}$Ne/$^{22}$Ne error-weighted isotopic ratios (12.63 ± 0.14 and 0.0349 ± 0.0023, respectively) are consistent with the highest isotopic ratios obtained with step-crushing. Hence, we suggest the Iceland plume to have a composition close to our error-weighted Ne isotopic ratio of 12.63 ± 0.14 for $^{20}$Ne/$^{22}$Ne and 0.0349 ± 0.0023 for $^{21}$Ne/$^{22}$Ne (Fig. 4).

A clear linear relationship is observed between $^{3}$He/$^{22}$Ne and $^{20}$Ne/$^{22}$Ne when plotting both the compiled data and our own step-crushing data (Fig. 6). We utilized the linear regression of this trend, which necessarily passes through the air value, and combined it with the error-weighted $^{20}$Ne/$^{22}$Ne isotopic ratio determined from the vesicles (12.63 ± 0.14). This gives a $^{3}$He/$^{22}$Ne ratio for Iceland of 2.35 ± 0.12 (Fig. 6), in good agreement with previous measured values (e.g. Moreira et al., 2001; Mukhopadhyay, 2012; Colin et al., 2015). This value is unequivocally higher than the $^{3}$He/$^{22}$Ne ratio of ~1.8 reported by Péron et al. (2016) for the Galápagos.

A plausible explanation the higher $^{3}$He/$^{22}$Ne ratio in Iceland compared to the Galápagos is the incorporation of a MORB mantle component. Variations in the degree of mixing with MORB mantle material, where $^{3}$He/$^{22}$Ne can reach values as high as 10 (Tucker and Mukhopadhyay, 2014), could significantly affect the $^{3}$He/$^{22}$Ne ratio of the magma while having little effect on the $^{20}$Ne/$^{22}$Ne isotopic ratio (Péron et al., 2018). The plume-derived Icelandic material likely incorporated a greater fraction of MORB mantle material compared to the plume-derived Galápagos material. This is in keeping with Iceland's location on the active Mid-Atlantic Ridge, where interaction with MORB mantle material could be more pronounced.

### 4.4. Implications for the origin of light volatile elements

A $^{20}$Ne/$^{22}$Ne ratio of 12.63 ± 0.14 for the Icelandic mantle source is lower than the expected ratio resulting from the dissolution of a primordial atmosphere in the magma ocean (~13.4; Heber et al., 2012). This scenario for the origin of light volatile elements has been previously suggested based on the highest step-crushing values measured in the Kola Peninsula (13.0 ± 0.2; Yokochi and Marty, 2004) and the Discovery Ridge Segment on the South Atlantic Ocean ridge (13.03 ± 0.04; Williams and Mukhopadhyay, 2019) for example. The error-weighted $^{20}$Ne/$^{22}$Ne ratio of 12.63 ± 0.14 (determined using the vesicle dataset) is however in good agreement with the $^{20}$Ne/$^{22}$Ne ratio of 12.73 ± 0.02 (steady-state) expected from solar wind implantation coupled with sputtering of protoplanetary dust (Fig. 4, Ne-B; Raquin and Moreira, 2009; Moreira and Charnoz, 2016). The modeled Ne-B component is likely to vary and can reach values as high as 12.9-13.0 if the exposure time of parent bodies to solar wind is short. This could explain some of the anomalously high step-crushing values reported in other studies.

As explained above, subduction of atmospheric neon is considered as being negligible, and we do not expect atmospheric contamination of the vesicles. Therefore, solar wind irradiation of protoplanetary dust, coupled with sputtering (though it might not be required, see Yamamoto et al., 2025), is strongly supported for the origin for Earth's light volatile elements based on our results and that of Péron et al. (2017). This interpretation faces, however, two major challenges.

First, only few enstatite chondrites (ECs) exhibit solar-like Ne signatures. In addition, and among all the measurements corrected for cosmogenic neon performed on ECs, only some brecciated meteorites

yield $^{20}$Ne/$^{22}$Ne > 12.3 (Patzer and Schultz, 2001; Marty, 2022). Therefore, the vast majority of ECs present $^{20}$Ne/$^{22}$Ne isotopic composition far lower than the isotopic composition of Earth's mantle ($^{20}$Ne/$^{22}$Ne > 12.5). Taken at face value, this argues against solar wind irradiation of protoplanetary dust (coupled with sputtering) as the source of solar Ne in the mantle (as ECs are considered the main meteoritic source of Earth, Javoy et al., 2010).

Solar Ne with a modeled Ne-B-like composition ($^{20}$Ne/$^{22}$Ne = 12.6-12.9) is observed only in extraterrestrial samples that experienced prolonged solar wind irradiation, such as brecciated meteorites (Patzer and Schultz, 2001) or lunar regolith (Péron et al., 2018).

The higher particle flux of the young Sun (Sasaki, 1991) could have significantly enhanced the implantation of solar Ne in dust that formed close to the Sun in the early Solar System. This could result in objects dominated by an implanted solar wind component with an isotopic composition near the modeled Ne-B (~12.7; Moreira and Charnoz, 2016) over much shorter exposure times than what appears to be required under present-day solar flux. As a result, one possibility is that some ECs incorporating dust that formed very early and close to the Sun, (and participating to the formation of Earth) contained higher amounts of solar Ne with a Ne-B-like signature.

Trapped solar-like Ne is often released during the first steps of heating in ECs (Nakashima and Nakamura, 2006) and some carbonaceous chondrites (e.g. Gamblin et al., 2024), indicating that this component is particularly labile. This is consistent with the observation that solar-like Ne signatures are predominantly preserved in EC3 meteorites and not in higher petrologic types (EC4-7; Marty, 2022). Therefore, the apparent absence of the modeled Ne-B-like composition ($^{20}$Ne/$^{22}$Ne = 12.6-12.9) among ECs could be a result of secondary processes rather than being primordial. The Ne isotopic signature of most of these meteorites could have been progressively altered over the subsequent 4.5 billion years even by minimal thermal or aqueous alteration given the apparent lability of this component.

A second challenge is that invoking an exclusive origin of Ne from solar wind irradiation and sputtering of Earth's precursors does not fully account for the Ne isotopic signatures observed in terrestrial samples. Step-heating experiments conducted on sample EW9309-5D (from the Discovery Ridge segment, hereafter referred to as 5D) by Sarda et al. (2000) produced a $^{20}$Ne/$^{22}$Ne ratio as high as 13.04 ± 0.07 (1σ). This result was recently confirmed by Williams and Mukhopadhyay (2019) using step-crushing (maximum $^{20}$Ne/$^{22}$Ne of 13.03 ± 0.02), using sample 5D. Such values are clearly higher than the modeled Ne-B composition, even if steady-state had not been reached (Moreira and Charnoz, 2016).

The gas composition of sample 5D, however, is unusual. It exhibits highly fractionated noble gas elemental ratios that deviate substantially from theoretical mantle production ratios. For example, step-heating by Sarda et al. (2000) yielded $^{4}$He/$^{40}$Ar* ratios up to 54.5, far exceeding theoretical values (1.6-4.2; Graham, 2002). Similarly, step-crushing analyses by Williams and Mukhopadhyay (2019) produced $^{4}$He/$^{21}$Ne* ratios ranging from 5.5.10^7 to 8.3.10^7, again significantly outside the mantle range (2.2.10^7-2.8.10^7, Yatsevitch and Honda, 1997). These observations indicate ~60% He loss, assuming closed-system Rayleigh degassing and a Ne/He solubility ratio of 1/2 (Jambon et al., 1986). Such extensive degassing would also induce substantial Ne loss (~84%), potentially fractionating Ne isotopes.

Indeed, recent experiments carried by Núñez-Guerrero et al. (2025) demonstrate that kinetic fractionation of Ne isotopes during degassing is possible. They showed that vesicles can attain isotopic compositions near the maximum fractionation factor through extensive, out-of-equilibrium degassing. This mechanism could increase $^{20}$Ne/$^{22}$Ne ratios from an initial 12.63 to values as high as 13.25 (following Graham's Law; see SI). This process could account for the unusually high $^{20}$Ne/$^{22}$Ne values relative to our (and Péron et al., 2017) lower mantle error-weighted average, starting from a Ne isotopic composition close to Ne-B.
Caution is therefore warranted before ruling out an implanted solar wind contribution based on the step-analyses of sample 5D, given its unusual noble gas elemental composition. In addition, Discovery Ridge samples are the only ones to exhibit $^{20}$Ne/$^{22}$Ne ratios unequivocally higher than the modeled Ne signature of implanted solar wind. However, if the isotopic composition of sample 5D is indeed representative of the mantle below the ridge, then a role for nebular atmosphere dissolution in the magma ocean might be necessary to account for the mantle Ne signature (Williams and Mukhopadhyay, 2019).

This nebular dissolution model is challenged by the fact that a sufficiently massive planetary embryo ($> 0.2-0.3 M_{\text{Earth}}$; Jaupart et al., 2017) would be needed to gravitationally capture and retain an atmosphere to explain Earth's primitive mantle Ne inventory. A recent study by Pahlevan et al. (2025) highlights that on Mars, the mixing of outgassed chondritic volatiles with a nebular component could dramatically enhance the amount of nebular gas retained, by up to three orders of magnitude. If such a mechanism also operated on Earth, this hybrid scenario would significantly relax this constraint as the embryo mass required to capture a dense atmosphere would be much smaller.

Recent experimental work on Ne solubility in ultramafic melts (Cardoso et al., 2025) however further challenges the nebular gas dissolution scenario. Compared to earlier values obtained in basaltic melts (Jambon et al., 1986), Ne is significantly less soluble in ultramafic compositions, implying that embryos $> 0.8 M_{\text{E}}$ would be required to account for Earth's Ne inventory. The rapid formation of such massive embryos within the lifetime of the protoplanetary disk ($\sim 5$ Myr; Sung et al., 2009), which is especially short in the inner solar system, appears unlikely. This is particularly true if pebble accretion did not dominate terrestrial planet growth (Morbidelli et al., 2025). Moreover, this timescale of rapid accretion is inconsistent with the 30–100 Myr typically inferred for Earth's accretion (Albarède, 2009).

Ultimately, we stress that nebular capture-dissolution and solar-wind implantation models are not necessarily mutually exclusive to each other. Instead, the Earth's mantle Ne isotopic signature may represent mixed contribution of both processes, preserved in a primitive lower mantle that has remained isolated for over 4.45 Ga (Allègre et al., 1983; Mukhopadhyay, 2012; Parai, 2023). Though we suggest a greater contribution of the solar wind-irradiation process, precisely constraining the relative contribution of both mechanisms for Earth's light volatile inventory remains a target of high importance. It would bring key constraints on the timescale of accretion and the nature of Earth's building block. Further work on primitive plume-derived samples, together with isotopic characterization of extraterrestrial material, will be decisive in addressing the contentious issue of the origin of Earth's volatile elements.

## 5. Conclusions

We performed both step-crushing and individual vesicle analyses on RS-ISL-2024-6, a subglacially erupted tholeiite from Miðfell, Iceland. Our crushing-step data is consistent with the literature on noble gas systematics for this locality. We present the first measured $^{20}$Ne/$^{22}$Ne isotopic ratios in individual vesicles for Iceland (and the second for OIBs in general), overcoming the challenge posed by the lower subglacial eruptive pressure compared to submarine settings. Through statistical analysis, we obtained an error-weighted $^{20}$Ne/$^{22}$Ne isotopic ratio for the Icelandic mantle source of $12.63 \pm 0.14$ which is indistinguishable within error from the neon signature of implanted solar wind, and consistent with the Galápagos mantle source. This suggests that this mechanism played an important role for the origin of light volatile elements on Earth. Contribution from dissolved nebular neon in the magma ocean is however not excluded. Further work on plume-influenced samples, together with isotopic characterization of extraterrestrial material, will be decisive in addressing the contentious issue of the origin of Earth's volatile elements.

## CRediT authorship contribution statement

**Romain Sauvalle:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Manuel Moreira:** Writing – review & editing, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Bruno Scaillet:** Writing – review & editing, Supervision, 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.

## Acknowledgements

The authors thank P. Penhoud for their technical assistance with the microtomography acquisitions. The assistance of H. K. Torfadóttir, A. Hoskuldsson, J. Andújar, E. Núñez-Guerrero, and C. Cardoso on the field are gratefully acknowledged. E. Núñez-Guerrero, C. Cardoso, K.Milier and S. Péron are also thanked for the scientific discussions. We thank the anonymous reviewers for their constructive comments and suggestions, which helped improve the first version of the manuscript. We acknowledge the financial support of the European Research Council (Grant agreement No. 101096688[APATE][ERC-2022-ADG]) and from LabEx VOLTAIRE (ANR-10-LABX-100-01).

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119722.

## Data availability

All datasets generated during this study are publicly available from the Mendeley Data Repository at the following link: 10.17632/cy36wggdyh.2.

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