# Helium isotope evidence for cryptic magmatism along
Central France

Carolina Dantas Cardoso, Emma Russier, Bernard Marty, Raphaël Pik, David Bekaert, Alan Seltzer, Michael Broadley, David Byrne, Thomas Rigaudier, Yves Géraud, et al.

► To cite this version:

Carolina Dantas Cardoso, Emma Russier, Bernard Marty, Raphaël Pik, David Bekaert, et al. Helium isotope evidence for cryptic magmatism along Central France. Chemical Geology, 2023, 634, pp.121588. 10.1016/j.chemgeo.2023.121588. hal-04194111

HAL Id: hal-04194111

https://hal.univ-lorraine.fr/hal-04194111v1

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Distributed under a Creative Commons Attribution 4.0 International License
1 **TITLE**

2 Helium isotope evidence for cryptic magmatism along Central France

3 **AUTHORS**

4 Carolina Dantas Cardoso<sup>a,*</sup>, Emma Russier<sup>b,c</sup>, Bernard Marty<sup>a</sup>, Raphaël Pik<sup>a</sup>,

5 David V. Bekaert<sup>a,d</sup>, Alan Seltzer<sup>d</sup>, Michael W. Broadley<sup>a</sup>, David Byrne<sup>a</sup>, Thomas

6 Rigaudier<sup>a</sup>, Yves Géraud<sup>b</sup>, Alexandre Tarantola<sup>b</sup>, Benoît Hauville<sup>c</sup>

7 **AFFILIATIONS**

8 <sup>a</sup>Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France

9 <sup>b</sup>Université de Lorraine, CNRS, GeoRessources, F-54000 Nancy, France

10 <sup>c</sup>45-8 Energy, Metz 57000, France

11 <sup>d</sup>Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic

12 Institution, Woods Hole, Massachusetts 02543, USA.

13 **ABSTRACT**

14 Determining the source(s) of helium in regions of the crust where ³He anomalies occur
15 can be challenging when no surface manifestations of magmatic activity or a clear
16 active extension regime are present. This is the case of the Paris basin (France), where
17 geothermal fluids and oils ³He/⁴He have been previously shown to range from 0.02 R<sub>a</sub>
18 up to 0.14 R<sub>a</sub> (Marty et al., 1993; Pinti & Marty, 1995, 1998), whereas natural gases to
19 the south of the Paris Basin (near the Massif central) show unambiguous evidence for
20 mantle helium contribution, with ³He/⁴He up to 6.4 R<sub>a</sub> (Bräuer et al., 2017). However,
21 higher ³He/⁴He than the crustal endmember (~ 0.01 R<sub>a</sub>), as observed in the Paris Basin,
22 could be a sign of either (i) a weak contribution of mantle He (enriched in primordial
23 ³He) or (ii) the contribution of ³He naturally produced by nuclear reactions of lithium,
24 for which commercially exploitable concentrations have been reported in rare-metal-
25 rich granites. One way to distinguish between these two possibilities is to combine He
26 isotope systematics with other noble gas tracers of mantle inputs, such as xenon
27 isotopes. Here, we report the isotope compositions of helium and xenon, measured by
28 static and dynamic mass spectrometry, respectively, in gas samples collected in the
29 Nièvre County, between the Massif Central and the Paris Basin. ³He/⁴He ratios range
30 between 0.16 and 0.22 Ra. Combined with previous studies, our data are consistent with
a southward increase of He isotope ratios along major fault systems from the Paris Basin (~ 0.02 R<sub>a</sub>) towards the Massif Central (6.4 R<sub>a</sub>). This geographical trend, paired with high precision Xe isotopes exhibiting an excess of mantle <sup>129</sup>Xe, shows a clear mantle input that is most likely restricted to the major fault systems in Central France. The occurrence of mantle-derived helium and xenon in Central France highlights the presence of cryptic magmatism in areas with no apparent volcanic manifestations.

**Keywords:** noble gases; helium isotopes; xenon isotopes; mantle input; CO<sub>2</sub> reservoir

# 1 Introduction

On Earth, the two helium isotopes, <sup>3</sup>He and <sup>4</sup>He, are found in the atmosphere, crust, and mantle in different proportions. <sup>3</sup>He is mainly of primordial origin, occurring in higher proportions in the mantle, where it has been able to be preserved since Earth's accretion (Craig & Lupton, 1976). <sup>4</sup>He on the other hand is primarily of radiogenic origin and is concentrated in the continental crust (Ballentine & Burnard, 2002). The production of <sup>4</sup>He primarily occurs by α-decay of <sup>235,238</sup>U and <sup>232</sup>Th, and is thus dependent on the concentration of these two parent elements (Ballentine & Burnard, 2002). However, production of <sup>3</sup>He can also occur in the crust produced through the thermal neutron capture on <sup>6</sup>Li by the reaction <sup>6</sup>Li (n,α) <sup>3</sup>H (β⁻) → <sup>3</sup>He (Morrison & Pine, 1955).

The <sup>3</sup>He/<sup>4</sup>He ratio is thus an essential tool for determining the origin of different gas reservoirs. In some regions of the crust, the interpretation of <sup>3</sup>He/<sup>4</sup>He can be ambiguous, such as in the Paris Basin (France), where no magmatic manifestations at the surface, or evidence for modern tectonic extension are present, yet helium isotope ratios show higher <sup>3</sup>He/<sup>4</sup>He than expected for the continental crust (up to 0.14 Ra; Marty et al., 1993; Pinti & Marty, 1995, 1998). Helium isotopes have been investigated in groundwater and crude oil from the Paris Basin and in gas manifestations in the Massif Central, respectively (Bräuer et al., 2017; Marty et al., 1993; Pinti & Marty, 1995, 1998). While there is unequivocal evidence for a mantle contribution in the Massif Central, with <sup>3</sup>He/<sup>4</sup>He ratios up to 6.4 R<sub>a</sub> (where R<sub>a</sub> is the atmospheric <sup>3</sup>He/<sup>4</sup>He - 1.39 x 10<sup>-6</sup>; Graham, 2002), a mantle contribution in the Paris Basin is less evident. Helium isotope ratios (0.02 - 0.11 R<sub>a</sub>, reaching 0.14 R<sub>a</sub> for basement-hosted groundwater) are close to the crustal end-member (0.001 - 0.02 Ra) as computed from average U+Th/Li abundances in related lithologies of the Paris Basin (Pinti & Marty, 1998). Because <sup>3</sup>He/<sup>4</sup>He ratios of geothermal fluids and oils are higher than values predicted for a purely
crustal origin, Pinti and Marty (1998) proposed a mantle contribution to gases hosted by basinal fluids. In agreement with this possibility, it was observed that the ³He/⁴He ratios increase with depth in the basin, suggesting injection of mantle-derived helium from the basement underlying the sedimentary units, and progressive dilution of this signal by radiogenic, ⁴He-rich helium produced in the sedimentary pile (Marty et al., 1993). Helium isotope ratios also tend to be higher towards the southern margin of the basin (0.09 Ra in groundwater and 0.14 Ra at the basement; Pinti and Marty, 1998). This trend is possibly related to the North-South major fault system ("Sillon Houiller") in Central France, and to the addition of mantle-derived ³He from the nearby Rhine rift magmatism at the eastern border of the Paris Basin in Lorraine (Marty et al., 2003). Alternatively, ³He/⁴He ratios higher than expected for the purely crustal end-member could reflect local enrichments of lithium-derived ³He, with He isotope variations potentially tracking the occurrence of Li-rich lithologies of economical relevance.

Recently, a CO₂-rich gas reservoir with He concentration reaching 0.04% has been discovered while conducting helium exploration in the Nièvre County, between the Massif Central to the south and the southern border of the Paris Basin to the north (Fig. 1). Although not reaching commercial concentrations for helium (> 0.3%), its helium content is about 100 times higher than the atmosphere (~ 0.0005%; Glueckauf, 1946). This gas accumulation has the potential to shed light into the nature of the intermediate ³He/⁴He ratios, between typical mantle values and the crustal range, observed in the Paris Basin (e.g., Pinti and Marty, 1998). The geochemical description of this reservoir is still at an early stage, and so our paper focuses on the use of isotope geochemistry to identify the source(s) of the Nièvre gas.

In the present study, we sampled bulk gas samples using two distinct approaches: copper tubes (dedicated to He isotope measurements) and Giggenbach bottles (dedicated to Ar-Ne-Xe isotope measurements). The samples consist in free gases from two CO₂-rich wells (SP4 and SP7) and bubbling springs from the surrounding areas – Gelin, Fonts-Bouillants, Fontaine des Vertus, Pougues-les-Eaux, Fontaines Salées, and Bourbon-Lancy (Fig. 1). Most springs were cold (12-14°C; Boineau & Maisonneuve, 1972; Risler, 1974), apart from Bourbon-Lancy, which has a temperature of 52°C (Batard et al., 1982). To identify the different sources of helium, we analysed ³He/⁴He and ⁴He/²⁰Ne on gas collected from all sites. We also analysed the major gas
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composition, stable isotopes of CO₂ (δ¹³C) and N₂ (δ¹⁵N), and Ar, Ne, and Xe isotopes for the SP4 and SP7 well gas samples.

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## 2 Geological setting

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The study area is located between the southern border of Paris Basin and the north of the Massif Central and Limagne basin, west of the Morvan massif (Fig. 1). The Massif Central and Morvan belong to the Hercynian belt. The basement of the basin is made up of Hercynian granites, migmatites, gneisses and mica schists that outcrop in the Decize horst (Clozier and Turland, 1982; Roger et al., 2010). The study zone is also likely to contain uranium-rich carboniferous and permian deposits, mostly alternating clay and sandstone, that formed during the dismantling of the Variscan belt under the Mesozoic and Cenozoic cover (Beccabetto et al., 2015; Farjanel, 1989; Mercuzot et al., 2021).

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The area underwent magmatic and volcanic activity during the Hercynian period and Paleogene (Baptiste, 2016; Farjanel, 1989). The emplacement of the Decize and Montmarault granitic intrusion (321 ± 2 Ma U-Th-Pb dating on monazite; Joly, 2007) and rare metal-enriched granites in the Morvan and Massif Central took place as the belt began dismantling (Gloaguen et al., 2018; Roger et al., 2010). The most recent volcanic activity occurred during the Paleogene, at ~90 km to the South of the Paris Basin, in "Chaîne des Puys" related to the Alpine orogeny and the Cenozoic extension regime (Barruol & Granet, 2002). The origin of volcanism in the area is still debated (Nehlig et al., 2003). On the one hand, it could reflect the surface expression of a mantle plume (Coisy & Nicolas, 1978; Froidevaux et al., 1974), as potentially suggested by seismic tomography (Mazabraud et al., 2005), originating from the asthenosphere related to the activity of a hot spot (Froidevaux et al., 1974) or to a passive rift (Merle & Michon, 2001). On the other hand, volcanism may result from crustal extension and melting of the lithospheric material (Gautheron et al., 2005). One last possibility is that a deep plume led to the formation of shallower plumes in the Sub Continental Lithospheric Mantle (SCLM) and/or in the old oceanic lithosphere crust (~400 Ma subduction during the Hercynian orogeny) (Buikin et al., 2005; Gautheron et al., 2005). Noble gas data from the Massif Central, namely Xe and Ne isotopes, indicate a MORB-like source, discarding the influence of a mantle plume (Moreira et al., 2018).

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Seismicity is not restricted to the Massif Central and seismic events have been recorded along the major structures of the area (Marche and Sillon Houiller faults).
Most faults appear to be related to the complex geodynamic history of the area (Variscan subduction, collision, and exhumation) and are deeply embedded in the basement, such as the Sillon Houiller and Loire faults (Baptiste, 2016; Regorda et al., 2020; Roger et al., 2010; Vanderhaeghe et al., 2020). The Magnetic Paris Anomaly follows the same N-S orientation as the Loire fault in the North of Massif Central. Even if its origin is debated, this anomaly could be linked to a deep major structure, such as a main strike-slip fault or an oceanic suture covered by the Paris Basin sediments (Baptiste, 2016). Variscan faults oriented N20-30 have been reactivated into shear and then normal faults during the formation of the Carbonifereous pull-out and Permian hemi-graben (Beccabetto et al., 2015; Farjanel, 1989). They were also reactivated by extension and compression at the end of the Jurassic (E-W) during the opening of the Tethys, amid the Cretaceous (SW-NE extension and N-S compression) and the W-E extension in the Tertiary (Guillocheau et al., 2000; Jolivet et al., 2021; Manatschal et al., 2021; Roger et al., 2010). Gas-rich springs and drillings are located along faulted areas with mainly N-S directions. SP4, SP7, Gelin, Fonts-Bouillants, and Fontaine des Vertus follow a single fault, the Saint-Parize-le-Châtel fault while Pougues-les-Eaux is located in an area affected by multiple normal faults. Bourbon-Lancy and Fontaines Salées are situated along Morvan bounding fault.

# 3 Methods

## 3.1 Bulk gas compositions

Gases occur either as bubbles in springs (Gelin, Fonts-Bouillants, Fontaine des Vertus, Pougues-les-Eaux, Bourbon-Lancy and Fontaines Salées) or as free CO₂-rich gas in two drill holes (SP4 and SP7). Samples were collected during three field campaigns and/or well testing operations, in August-September, 2021, February and May-June, 2022 (Table 1). SP4 and SP7 drill holes are rooted in the Triassic sediments and cross the Saint-Parize-le-Châtel fault. Gases were sampled in evacuated vials or Tedlar Bags after purging the tubes connecting them to the wellheads. The springs near SP4 (Gelin, Fonts-Bouillants, Pougues-les-Eaux, and Fontaine des Vertus) were sampled before and after the well test to detect potential variations in helium isotopes. Bubbling gases from springs were collected through funnel and Teflon tubing in previously evacuated vials (Pasquet et al., 2021) or Tedlar bags (Battani et al., 2010). Finally, the gas composition was measured by gas chromatography (GC) (Supplementary material; Pasquet et al., 2021).
162 3.2 *Noble gases*

163 3.2.1 <sup>3</sup>He/<sup>4</sup>He and <sup>4</sup>He/<sup>2</sup>Ne

164 The analyses of <sup>4</sup>He/<sup>20</sup>Ne and <sup>3</sup>He/<sup>4</sup>He ratios of gases sampled in copper tubes
165 were performed, respectively, on a MKS Microvision 2 quadrupole mass spectrometer
166 (QMS) and a split flight tube noble gas mass spectrometer (Helix SFT - Thermo Fisher
167 Scientific) at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) noble
gas analytical facility (Mabry et al., 2013). The standard deviation of the <sup>3</sup>He/<sup>4</sup>He ratios
168 ranged from 1.4 to 3.7 % based on the replicate analysis of standard He (atmospheric).
169 Results are corrected for blanks, which contributed < 0.1 % of the total He abundances
170 (Table 2).
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172 3.2.2 *Ar, Ne, and Xe isotopes*

173 Gases were collected in 1.5 L Giggenbach bottles (Giggenbach & Goguel, 1988;
174 Bekaert et al. 2019) containing 4N NaOH solution in which CO2 and other reactive
175 species (e.g., S compounds) can be trapped. This method permits the concentration of
176 inert gases (mainly N2 and noble gases and possible traces of CH4, CO, H2, etc) in the
177 vacuum headspace above the solution. In the laboratory, the gaseous fraction was
178 equilibrated under vacuum with an evacuated metal bottle fitted with two metal valves
179 (Nupro SS4H®), where the gas could be stored for long periods of time without
180 diffusive leakage or exchange with atmospheric gases. Aliquots of gas were expanded
181 in a vacuum line and the abundances of He and Ar as well as Ar isotopic ratios were
182 measured after purification via static mass spectrometry on a GV Helix MC mass
183 spectrometer. Two aliquots of the SP4 well gas were also sampled from the metal
184 bottles and analysed for all noble gases (Table 3). Gases were purified using hot
185 (600°C) and cold (25°) Ti-getters and, after cryogenic separation (Broadley et al.,
186 2022), noble gas abundances and isotopic ratios were analysed with a Helix MC+ mass
187 spectrometer at the Centre de Recherches Pétrographiques et Géochimiques (CRPG)
188 noble gas analytical facility (Bekaert et al., 2019; Broadley et al., 2020).

189 At last, argon and xenon isotope ratios were measured in the Seltzer Lab at
190 Woods Hole Oceanographic Institution via dynamic isotope-ratio mass spectrometry
191 (Bekaert et al., 2023; Seltzer & Bekaert, 2022) for one sample of the SP7 well gas
192 collected in a large Giggenbach bottle (1.5 L) following the sampling procedure detailed
193 by Bekaert et al. (2019). With a dynamic mass spectrometer, often called an “isotope
194 ratio mass spectrometer,” the rapid switching between analysis of sample and reference
gas streams reduces the effect of instrumental drift (McKinney et al., 1950) and allows
for many comparable measurements of purified sample gas and standard gas of known
isotopic composition over the course of an analysis, hence ensuring accurate and
unbiased isotopic ratio determination at the sub-per mil level. Results are in permil (‰)
and notated as δ following δR = [(Rsample/Ratmosphere) - 1] × 1000; where R is a
Xe isotope ratio. Raw measurements were corrected for diffusive transport fractionation
(DTF; light isotope enrichment due to physical subsurface fractionation; Bekaert et al.,
2023) using δ¹²⁸Xe/¹³⁰Xe, yielding a mass-dependent correction of 0.504 ± 0.157 ‰ u⁻¹.
Our reported uncertainties on Xe isotope data reflect both analytical and DTF-correction
sources of uncertainty (Table 3).

## 3.3 Stable isotopes

The carbon and nitrogen isotopic compositions of CO₂ and N₂, respectively, were
measured with a Thermo Scientific MAT 253 Mass Spectrometer at the Centre de
Recherches Pétrographiques et Géochimiques (CRPG) stable isotopes analytical
facility. Sampling was performed using Giggenbach bottles and evacuated glass bottles.

### 3.3.1 δ¹³C

For the δ¹³C measurements of the CO₂, CO₂ and H₂O were trapped into liquid
nitrogen and then cryogenically separated. All values are reported in the delta notation
in permil (‰) relative to PDB, with error below 0.2 ‰ (Table 2).

### 3.3.2 δ¹⁵N

Nitrogen isotope measurements were done using the non-condensable fraction of
the Giggenbach bottles, which consists mainly of N₂, CO, and noble gases since CO₂
has been removed. Both CO and N₂ gases were transferred in a sealed pyrex tube
containing CuO and Cu₂O grains to be oxidized into N₂ and CO₂ at 450 °C for 1 hour.
Prior to being used, the CuO and Cu₂O grains have been heated few minutes under
vacuum at 350°C and pumped overnight at room temperature to remove adsorbed gases.
The produced CO₂ was then cryogenically trapped and N₂ was collecting using a
Toepler pump. All values are reported using the delta notation, in permil (‰) relative to
air, with error ~0.5 ‰ calculated from the reproducibility of duplicates (Table 2).
# 4 Results

## 4.1 Wells

During the main flow of the SP4 well test, which lasted approximately 10h, samples were collected for $ ^{3}\text{He}/^{4}\text{He} $ and $ ^{4}\text{He}/^{20}\text{Ne} $ measurements every hour. Sample names follow the format “site name-sampling order + copper tube number” and duplicate samples have an “M1” at the end; sampling order is “100” when before well test, “200” during, and “300” if sampled after the well test (Table 1). For the SP7 well, only one sample was collected for noble gas measurements. SP4 and SP7 are CO$_2$-dominated, consisting of ~ 94% CO$_2$ and 6% N$_2$ and only traces of CH$_4$ (< 0.6%); helium contents range from 394 to 426 ppm vol. The $ \delta^{13}C $ value of the SP4 sample is -10.1 ‰, which is compatible with a primarily organic source of CO$_2$ or non-marine carbonates (Table 2).

Throughout the SP4 well test, no significant change on helium abundance or isotopic ratios was detected. $ ^{3}\text{He}/^{4}\text{He} $ ratios range from 0.17 to 0.21 (± 0.01) R$_{a}$, with no temporal variation (Table 2). $ ^{4}\text{He}/^{20}\text{Ne} $ ratios are consistently > 2000, however, measurements are not precise due to the low content of $ ^{20}\text{Ne} $ relative to $ ^{4}\text{He} $, with the former showing values very close to the baseline. During the SP7 well test, we only collected one sample, which shows similar results with a $ ^{3}\text{He}/^{4}\text{He} $ ratio of 0.18 (± 0.01) R$_{a}$ and $ ^{4}\text{He}/^{20}\text{Ne} $ > 2000.

The $ ^{40}\text{Ar}/^{36}\text{Ar} $ ratios, measured for two aliquots of the SP4 well gas, are 2866 ± 5 and 2574 ± 11, respectively, with the $ ^{38}\text{Ar}/^{36}\text{Ar} $ ratios being indistinguishable from atmosphere (Table 3). The $ ^{20}\text{Ne}/^{22}\text{Ne} $ (9.40 ± 0.05) and the $ ^{21}\text{Ne}/^{22}\text{Ne} $ ratios (0.0621 ± 0.0011) deviate slightly from the atmospheric Ne value, indicating contribution of crustal (and possibly mantle) neon (see section 5.2). Analyses of SP7 show $ ^{40}\text{Ar}/^{36}\text{Ar} $ (696.6 ± 0.2) above the air value, but still lower than the SP4 well, indicating possible atmospheric contamination during drilling operations. Xenon isotopic ratios of SP7 by dynamic mass spectrometry (Table 3) indicate contribution of fissionogenic and mantle Xe (see section 5.2).

## 4.2 Springs

Fontaines Salées and Bourbon-Lancy are N$_2$-dominated (93 % and 97 %, respectively) while Fontaine des Vertus is comprised of 63 % CO$_2$ and 39 % N$_2$ (Table 1). The remaining springs are CO$_2$-dominated ranging from 95 to 97 %. For all springs, only traces of CH$_4$ are identified (< 0.6%). Helium contents range from 245 to
257 426 ppm vol for the CO₂-dominated sites, and from 2874 to 45794 ppm vol for Fontaine des Vertus, Fontaines Salées, and Bourbon-Lancy.
258

259 The springs near SP4 (Gelin, Fonts-Bouillants, and Fontaine des Vertus) present
260 $ ^{3}\text{He}/^{4}\text{He} $ ratios similar to those of SP4, ranging from 0.16 to 0.21 (± 0.01) R<sub>a</sub>, as well as
261 the Pougues-les-Eaux spring which is 25 km north from SP4 (0.22 ± 0.01 R<sub>a</sub>) (Table 2).
262 Fontaines Salées and Bourbon-Lancy which are located 80 km NE and 50 km SE of
263 SP4, have distinct $ ^{3}\text{He}/^{4}\text{He} $ signatures (0.04 ± 0.00 and 0.03 ± 0.00 R<sub>a</sub>, respectively) that
264 are close to the purely radiogenic end-member. $ ^{3}\text{He}/^{4}\text{He} $ variations do not correlate with
265 gas compositions, with similar $ ^{3}\text{He}/^{4}\text{He} $ for springs near SP4 (including Fontaine des
266 Vertus) despite much higher helium and N<sub>2</sub> contents.
267

267 δ¹³C of CO₂ values show little variation among the springs, ranging from -11.8 to
268 -10 ‰ (vs. PDB), except for Fontaines Salées (-20 ‰), which suggests either different
269 sources of CO₂ or different processing in the crust. Weinlich (2005) reported
270 comparable values of -13.9 ‰ for a N₂-dominated spring at Pougues-les-Eaux (74 %
271 N₂) and of -20.4 ‰ at Saint Père (94 % N₂), near Fontaines Salées. The Pougues-les-
272 Eaux spring in the present study is dominated by CO₂ (95 vol. %) and has a very similar
273 δ¹³C (-10.3 ‰) to the one described in Weinlich (2005). Moreover, the springs with
274 lower CO₂ contents tend to present lighter δ¹³C values (Fig. S1) that could indicate
275 chemical processes during transport in the crust due to solution of CO₂ in the water
276 phase and/or carbonate precipitation, concentrating helium and N₂ in the gas phase.
277 δ¹⁵N results for the springs from both groups display similar values (+6.1 to +8.4 ‰)
278 compatible with a primarily crustal source.
279

5 Discussion

280 5.1 Source of helium

281 In European basins of various ages, $ ^{3}\text{He}/^{4}\text{He} $ values range between 0.01 and 3.9
282 R<sub>a</sub>, reaching ~0.1 R<sub>a</sub> in the western Alps (Marty et al., 1992; Oxburgh et al., 1986).
283 Oxburgh et al. (1986) suggested that this wide range in Neogene basins was related to
284 their tectonic setting. Helium isotope compositions in the Pannonian basin reach values
285 up to 3.9 R<sub>a</sub> consistent with an extension regimen and clear mantle input while the
286 Molasse and Po basins show a range of $ ^{3}\text{He}/^{4}\text{He} $ between 0.02 and 0.09 R<sub>a</sub> (Marty et al.,
287 1992; Oxburgh et al., 1986), which, for the Molasse basin, Oxburgh et al. (1986)
288 interpret as being of crustal origin despite the apparent small mantle contribution (up to
289 0.9%). In the Paris Basin, $ ^{3}\text{He}/^{4}\text{He} $ ratios are close to the range of values for the Molasse
and Po basins (0.02 to 0.11 R<sub>a</sub>), reaching 0.14 R<sub>a</sub> at the basement in Couy (Pinti & Marty, 1998). These values are higher than the theoretical crustal ³He/⁴He ratio (0.001-0.02 R<sub>a</sub>), based on the U-Th and Li values of the underlying sedimentary rocks. These elevated ³He/⁴He values were therefore considered to be evidence for the input of mantle material with a higher than crustal ³He/⁴He (Pinti & Marty, 1998). However, with only small ³He/⁴He variations, up to 0.1 R<sub>a</sub>, it is difficult to fully discriminate this component from variation in the crustal ³He/⁴He ratio. In addition, these areas show no evidence of igneous manifestations, and there is the possibility that ³He could be produced in the crust without mantle input (see section 5.3.1).

In the ³He/⁴He versus ⁴He/²⁰Ne plot (Fig. 2), we identify two groups of samples: one of them contains Fontaines Salées and Bourbon-Lancy, and the other one contains SP4, SP7, surrounding springs, and Pougues-les-Eaux. Considering the typical values found in the various basins in Europe, the ³He/⁴He signature found at Fontaines Salées and Bourbon-Lancy is within the European crustal range (0.02 – 0.1 R<sub>a</sub>). The remaining samples show significantly higher ³He/⁴He (0.16 – 0.22 R<sub>a</sub>). However, in any case, the main source of helium in the basin is from the continental crust.

Using U, Th, and Li contents of lithologies from the Paris Basin (Pinti & Marty, 1998) (since rock chemistry data is not available for our target area), we calculated the theoretical ³He/⁴He signature to be between 0.001 and 0.016 R<sub>a</sub>, based on radiogenic (⁴He) and nucleogenic (³He) production equations (Eq. 1; Andrews, 1985). For the SP4 sample group that includes SP4, SP7, nearby springs, and Pougues-les-Eaux, we compute that the higher ³He/⁴He ratios can either be accounted for by, (i) either nucleogenic ³He production from the presence of high amounts of lithium in the host rocks, (ii) or by a 2.5 % mantle input to the original crustal signature. In the next sections, we discuss both possibilities for the ³He/⁴He signature we find for the SP4 group.

$$ (\text{Eq. 1}) \quad P_3/P_4 = f(R, Na, Mg, Al, Si, Ca)F_{Li}/(3.2108 \times 10^6 + 7.7633 \times 10^5 R) $$

Where P<sub>3</sub>/P<sub>4</sub> is the theoretical ³He/⁴He based on the ³He and ⁴He production rates, f(...) is the neutron production rate based on the composition, F<sub>Li</sub> is the fraction of neutrons that are captured by ⁶Li in the rock matrix, and R is the Th/U ratio.
### 5.1.1 Nucleogenic $^3$He production

As previously stated, one potential production pathway to increase the $^3$He/$^4$He from crustal values to that measured in the SP4, SP7, surrounding springs, and Pougues-les-Eaux samples is through the localised production of nucleogenic helium ($^3$He) via thermal neutron capture on $^6$Li via the reaction $^6$Li(n,α)$^3$H (β⁻) → $^3$He (Morrison & Pine, 1955). To raise the $^3$He/$^4$He signatures of the SP4 sample group (Fig. 2, Table 2) from an assumed original crustal signature (0.001 – 0.016 R$_{a}$) we calculate that it would require Li concentrations in the basement and/or host rocks to be on the order of 600 ppm (Andrews, 1985; Eq. 1). This calculation assumes that all the excess $^3$He within the samples is produced from the thermal neutron capture by $^6$Li and that the basement / host rock contains typical crustal U-Th concentrations (1.8 and 7.2 ppm, respectively; Krauskopf and Bird, 1967). Although this amount of Li is atypical in view of the average continental crust value (30 ppm Li), small proximal basins such as the ones in the Massif Central have the potential to accumulate lithium (Sun et al., 2022).

One of the major concentrated sources of lithium in the crust is surface and subsurface brines. Near the study region (~ 100 km), in the Massif Central, a geothermal brine with 81 ppm Li has been identified in the Limagne basin (Fonroche geothermie, 2019) but values higher than that have not yet been reported. Brines from the area were previously described with much lower lithium contents (e.g., Millot et al., 2007; 7 ppm). In the Paris Basin, lithium concentration is low, in brines associated with Triassic salt deposits (up to 60 ppm; Fontes & Matray, 1993a; Millot et al., 2011) and oil fields (4.8 ppm; Fontes & Matray, 1993b b).

Apart from brines and geothermal water, the main *hard-rocks* that are sources of lithium are rare-metal-rich granites such as the Beauvoir, Montebras, and Chavence granites, near our study area, LCT pegmatites, and greisen deposits produced by high-temperature hydrothermal alteration of the aforementioned types (Gourcerol et al., 2019). The lithium mineralizations in the Massif Central are usually located near important ductile faults (Gloaguen et al., 2018).

Considering the various sources of lithium with production potential (Gloaguen et al., 2018) in the Massif Central, near the study area, it is possible to consider that a Li-rich granitic basement produces significant quantities of helium. The closest lithium sources are the Chavence (56 km SE) and Beauvoir granites (72 km SW), the latter
containing lithium concentrations between 367 and 5,220 ppm in its upper units (Raimbault et al., 1995). Taking the U, Th, and Li contents from the Beauvoir granite (16.2 - 52.4, 1.15 - 5.63, and 367 - 5,220 ppm, respectively; Raimbault et al., 1995), the theoretical ³He/⁴He ratios would range between 0.12 and 1.66 Rₐ. In that case, helium could be produced in the basement, before migrating upwards to the basin and being stored in the sedimentary layers, acting as a reservoir. Another possibility is that, as a proximal basin, detrital material from the Li-rich rocks accumulates in the basin and ³He is produced in the sedimentary layers. The Chavence granite is located near Bourbon-Lancy (~ 9 km) where we find a crustal signature for helium isotopes; however, at Bourbon-Lancy, the spring is directly on top of the basement and not in a sedimentary basin. To observe the same signatures as for the SP4 sample group at Bourbon-Lancy, a sedimentary basin that accumulates and traps the produced gas is required, as otherwise degassing from a Li-rich source would only occur locally and relatively far from the spring. There are also non-conventional sources of lithium that have recently been brought to light and that cannot be discarded, such as coal (e.g., Qin et al., 2015). Considering the regional geology, Stephanian-Permian basins in the Massif Central region have coal levels (Mercuzot et al., 2021) that could potentially provide the required amounts of lithium.

In the context of local production, Martel et al. (1990) found ³He/⁴He ratios (0.025 Rₐ) higher than the expected theoretical value (0.0033 Rₐ) for the Carnmenellis granite-hosted water (England). They explain this observation by presenting a model of preferential release of ³He from the rocks, relative to ⁴He. However, this model only works if some conditions are satisfied to retain ⁴He within the minerals – i.e., U and Th get concentrated in accessory phases and the host rock has remained at low temperature since formation (hence limiting He loss through diffusion). In our current study, testing this model would require detailed mineralogical characterization, but currently the composition of underlying basement and host rock is not well known Furthermore, the Paris Basin has a relatively high heat flux (60 to 120 mW/m²; Dentzer et al., 2016; Torgersen, 1993) which makes this model unlikely.

### 5.1.2 Mantle contribution

The alternative explanation for ³He/⁴He ratios above typical crustal values is a contribution of mantle-derived ³He. This could originate from an old magmatic source
in the crust, put in place around the time of basin formation, or from more recent
magma intrusions.

Méjean et al. (2020) proposed a model in which a fossil magmatic signal is diluted over time by radiogenic ⁴He production in the crust. This model is appropriate to explain the mantle input observed in the Southern Quebec groundwater since a clear alignment of Cretaceous magmatic intrusions is present (Monteregian Hills Igneous Province). For the Paris Basin, however, no evidence of magma emplacement in such scale during its formation is found.

Pinti and Marty (1998) describe two possibilities for the minor mantle signature in the Paris Basin observed in He isotopes – either from Permo-Carboniferous intrusions in Couy (lamprophyre dykes) or from a deep-seated magmatic source. The former could explain localized variations in the ³He/⁴He ratios but is unlikely to explain the signatures observed across a broad area. Furthermore, if the mantle input was coming from magmatic bodies that have already crystallized, helium would need to be liberated from minerals. For such a process to happen, certain temperatures must be reached to release He from the mineral; in the case of olivine crystals, helium release from fluid inclusions seems to start at ~ 500°C (Tolstikhin et al., 2010) while closure temperatures are between 143 and 244°C (Shuster et al., 2004; Wang et al., 2015). In this case, diffusion is the primary process, and will not only depend on the current isotherms but also on the age of the crystallized body. Furthermore, cooling rates may play a role in controlling the availability of He since, if the heat flux has been continuously high in the region and the minerals crystallized long ago, the majority of the helium would likely have been lost from the magmatic body. A deep-seated magmatic source could explain the vertical ³He/⁴He gradient observed (Marty et al., 1993; Pinti & Marty, 1998) from the basement-hosted groundwater with 0.12-0.14 R_a, going through the Triassic aquifer with 0.08 R_a, and terminating at the Middle Jurassic waters at 0.02 R_a.

In Fig. 3, we observe a geographical trend with ³He/⁴He decreasing from the South (Massif Central) towards the north (Paris Basin) while the SP4 sample group shows intermediate ³He/⁴He ratios. Considering that this trend coincides with the N-S orientation of the fault systems, it could point to a localized weakening of the crust that facilitates mantle input by degassing of a melt generated by decompression and partial melting. The horizontal transport of gases with a mantle signature from a relatively young magmatic system through faults is unlikely since the most recent active volcanic
system in the region is located in the Chaîne des Puys (4040 BCE), 90 km south of our study area in the Massif Central (2.9-3.2 Ra; Bräuer et al., 2017). Also, no evidence for transport of gas via groundwater is observed in the dataset: transport through large distances would entail gas-water interaction and the $^{4}$He/$^{20}$Ne ratios are consistently above 2000 and far from the air-saturated water value (~ 0.28). For the eastern segment of the Paris Basin, further from our study area in the Lorraine region, Marty et al. (2003) proposed that the elevated signatures (0.4 R$_{a}$) might originate from nearby rifting in the Rhine graben (1.7 Ra; Griesshaber et al., 1992), which is up to 120 km away from their studied sites. Similarly, for the Drôme County, in the subalpine massifs (SE France), $^{3}$He/$^{4}$He ratios have a wide range and can reach relatively high values (0.02 to 3.64 Ra; Marty et al., 1992) while being ~ 70 km away from the nearest volcanic systems (Ardèche and Auvergne). In both studies, $^{4}$He/$^{20}$Ne and $^{3}$He/$^{4}$He ratios of groundwater samples plot along a mixing line between ASW and a crust/mantle component and thus water could work as the transport medium of the magmatic $^{3}$He/$^{4}$He component.

The input of mantle $^{3}$He in some of our sample sites may therefore come directly from the mantle underlying the continental crust. In the Pannonian basin and Rhine graben, the crustal thickness is low, around 25 km and heat flow is on the order of 80 mW/m$^{2}$ (Čermák, 1982). Similarly, the current crustal thickness of the Paris Basin varies between 29 and 41 km (Lefort & Agarwal, 1996; Prijac et al., 2000) while the heat flux is between 60 and 120 mW/m$^{2}$ (Dentzer et al., 2016; Torgersen, 1993). However, in the Morvan, the crustal thickness is lower at between 27 and 29 km (Lefort & Agarwal, 1996) despite the typical crustal $^{3}$He/$^{4}$He signatures (up to 0.04 R$_{a}$; current study). This suggests that crustal thickness and diffusive transport of mantle material through the crust is unlikely to be the dominant source of mantle $^{3}$He within these systems. Therefore, Marty et al. (1993) interpret the anomalous $^{3}$He/$^{4}$He signatures as the result of degassing of partial melts at depth, which explains the vertical $^{3}$He flux.

Although the commonly known processes by which mantle helium enters the crust are via magmatic intrusions, degassing of magma, or diffusion, in areas without recent volcanism and under either extension or compression regimes, mantle input can also occur via advective flow controlled by faults due to improved permeability (Kennedy & van Soest, 2007; Kulongoski et al., 2005, 2013). In the model proposed by Kennedy & van Soest (2007), a vertical fault splay must penetrate the ductile lower
crust and reach the mantle acting as a channel for fluid flow. To satisfy this model, strain localization must occur to generate vertical faults in depth; Kennedy & van Soest (2007) propose this process is possible with increased dextral shear strain. The areas where mantle input via advective flow have been described (Kennedy & van Soest, 2007; Kulongoski et al., 2005, 2013) are seismically active regions, however, which is not the case for the Paris Basin. Slow deforming areas can present seismicity such as in the Armorican Massif or the Massif Central, west and south of the Paris Basin, respectively; the main difference between them, in a tectonic context, is the presence of a thick unfaulted Meso-Cenozoic sedimentary pile that acts as a "patch" and protects the basement rocks in the Paris Basin from brittle movement by improving the cohesion and decreasing the thickness of the breakable portion of the upper crust (Petit et al., 2019). Our study area, nonetheless, is at the border of the basin and contains a series of faults. Further South, closer to the Massif Central, earthquakes with magnitudes up to M4.4 have been recorded (Mazabraud et al., 2005). The closest to the study area (~ 40 km south) was an M4.1 in 1977, near Vieure (46.52°N, 2.93°E; Mazabraud et al., 2005). To test if mantle input occurs via advective flow controlled by faults, a detailed structural and geophysical study of the region is necessary.

In any case, the anomalous ³He/⁴He signatures – above typical crustal values for this part of the crust – seem to be focused along the N-S fault systems with an apparent decreasing gradient starting at the Massif Central volcanic area up to the Paris Basin. This trend, coupled with the vertical ³He/⁴He gradient observed at the Paris Basin ³He/⁴He (Marty et al., 1993; Pinti & Marty, 1998), seem to point to a localized and limited mantle input along these faults, which is endorsed by the ¹²⁹Xe excess observed at SP7 (see section 5.2).

## 5.2 Ar, Ne, and Xe isotope constraints

The Ne isotopic signature of SP4 is dominated by a nucleogenic production within the continental crust, however, the data point lies slightly above the typical crust-air mixing line (Fig 4), which could be due to either (i) simple two component mixing between air and a Ne endmember between modern crustal values and Archean terrains, or (ii) a limited contribution of mantle Ne to crustal and atmospheric Ne. The fact that there is only a single data point does not allow us to draw mixing line(s) and therefore precludes a definitive conclusion about a possible contribution of mantle Ne.
The argon isotopic composition of SP4 gas (Table 3) indicates either atmospheric or mantle origin for the ³⁸Ar/³⁶Ar ratio and a significant contribution of radiogenic ⁴⁰Ar produced by the decay of ⁴⁰K over long periods of time (T₁/₂ = 1.25 Ga). Indeed, the ⁴⁰Ar/³⁶Ar ratios of 2866 are one order of magnitude higher than the atmospheric value of 298.6. High ⁴⁰Ar/³⁶Ar values are observed in both crustal and mantle rocks and gases, so it is not possible a priori to decipher the origin of such radiogenic Ar, but the continental environment of the gas reservoir and the low ³He/⁴He ratio compared to typical mantle He values suggests a strong crustal contribution.

At the level of ultra-high precision provided by dynamic mass spectrometry, the isotopic composition of SP7 xenon is shown to deviate from that of atmospheric Xe. High precision Xe analysis from SP7 (Fig 5) shows a small yet distinguishable ¹²⁹Xe excess (relative to the atmosphere). ¹²⁹Xe excess is linked to ¹²⁹I decay either by ²³⁸U spontaneous fission, cosmogenic origin, or trapped in the mantle reservoir from production in the first ~ 100 Myr of the Earth formation (Moreira, 2013; Mukhopadhyay & Parai, 2019). Although ¹²⁹Xe can be produced in the crust by ¹²⁹I decay from ²³⁸U spontaneous fission, considering the ¹³⁶Xe/¹³⁰Xe is very low (corrected absolute ratio of 2.207) and the upper limit for ¹²⁹Xe/¹³⁶Xe of ²³⁸U-fissiogenic Xe is 0.001 (Eikenberg et al., 1993), the ¹²⁹Xe/¹³⁰Xe originated from fissiogenic ²³⁸U would be minimal (< 1%).

Cosmogenic ¹²⁹I is concentrated in sediments enriched in carbon (Santos et al., 2007); Holland et al. (2013) attributed the ¹²⁹Xe observed in the Timmins mine fluids to cosmogenic ¹²⁹I due to the age of the host rocks/fluids (Archean), presence of carbon-rich sediments, and absence of any evidence for mantle input since the ³He/⁴He ratios were within typical crustal ranges (up to 0.01 R_a). As discussed in section 5.1.1, Stephanian-Permian basins in the Massif Central region have coal levels (Mercuzot et al., 2021), however, the geology of the basin of our current study is not yet well established. Thus, in our study area, considering the helium isotope signature above typical crustal values (up to 0.22 R_a), ¹²⁹Xe excess is linked to mantle input.

Notably, heavy Xe isotopes are clearly enriched relative to the lighter ones (e.g., ¹³⁰Xe). Such enrichment is likely to result from the contribution of fissiogenic Xe due to the spontaneous fission of long-lived ²³⁸U (Mukhopadhyay & Parai, 2019). Thus, the isotopic composition of xenon confirms that SP7 hosts gas components derived from both the mantle and crust.
### 5.3 CO₂<sup>δ</sup>He and stable isotope constraints

The overall δ<sup>13</sup>C of CO₂ signatures are outside the MORB range (-5 ± 2 ‰) and within range of non-marine carbonates, graphite, and marine plants (Favara et al., 2002). However, for the SP4 group, with exception of Fontaine des Vertus, CO₂<sup>³</sup>He is close to the MORB values (~ 1 to 6 x 10<sup>9</sup>; Marty & Zimmermann, 1999), ranging from 8 x 10<sup>9</sup> to 1 x 10<sup>10</sup>. In a CO₂<sup>³</sup>He versus δ<sup>13</sup>C plot (Sano and Marty, 1995; Fig. 6), the CO₂-dominated samples show a mixing between three components, namely mid-ocean ridge basalts (MORB), organic sediments, and marine limestones (or slab carbonates). Considering that <sup>3</sup>He is mainly of mantle origin, the end-member contributions would range from 15 % to 30 % MORB, 40 % to 55 % limestones, and 25% to 30 % sediments contribution to the source of CO₂. Secondary processes can still play an important role in the CO₂ and δ<sup>13</sup>C values (Supplementary material); Gilfillan et al (2009) propose that CO₂<sup>³</sup>He variations in different gas fields are due to CO₂ loss either from dissolution in groundwater or carbonate precipitation, and that the δ<sup>13</sup>C will vary according to the water pH in a negative correlation (Randazzo et al., 2021).

In the Massif Central, δ<sup>13</sup>C varies between -6.3 ‰ and -3.6 ‰ while CO₂<sup>³</sup>He ranges from 3.9 x 10<sup>9</sup> to 1 x 10<sup>13</sup> (Bräuer et al., 2017), which falls between upper mantle and marine limestone in Fig. 6. This range can be interpreted as representing the composition of the typical regional mantle end-member. In this case, we might observe, for the SP4 group, the occurrence of a mantle-like component associated with CO₂ dissolution in water leading to more depleted δ<sup>13</sup>C or a small contribution from organic sediments, and at Fontaines Salées and Bourbon-Lancy, an organic source. Alternatively, CO₂ in the SP4 group samples might originate from non-marine carbonates or graphite which can range, respectively, from ~ -18 ‰ to 0 ‰ and ~ -40 ‰ to 0 ‰ (Favara et al., 2002) that can still be associated with mixing with a MORB component. The history of CO₂ transport might be more complex for Fontaines Salées and Bourbon-Lancy and could reflect secondary processes.

CO₂ reservoirs worldwide are generally interpreted as being magmatic in origin, with δ<sup>13</sup>C ranging between - 5.7 to - 2.7 ‰ (Gilfillan et al., 2009); compared to them our dataset is atypical. However, in such reservoirs, water is the storage medium of CO₂ (Dubacq et al., 2012; Gilfillan et al., 2008) while in our study we did not identify water bodies at depth and no water manifested during the well tests. Nonetheless, in areas with no evidence of volcanism, CO₂ flow in the surface has already been reported for
548 seismically active areas (e.g. Li Vigni et al., 2022), indicating that fault systems can
549 lead to complex histories of CO₂ transport and entrapment.

550 For all samples, the δ¹⁵N values show little variation, ranging from +6.1 to +8.4
551 ‰. This range is typical of continental or subduction-related gases and differs from that
552 of MORB (around -5 ‰ in the convecting mantle; Marty & Dauphas, 2003). Mantle
553 plumes also show lower δ¹⁵N values, around 0 ± 3 ‰ (Labidi et al., 2020; Marty &
554 Dauphas, 2003). In the Massif Central, δ¹⁵N values around +2 ‰ (Bräuer et al., 2017)
555 could potentially indicate the presence of a mantle plume or, alternatively, mixing
556 between MORB-like and continental-like components. The near-constant δ¹⁵N values of
557 the present samples point to a common nitrogen source despite large variations in the N₂
558 contents of the surveyed springs. Such variable N₂ contents might be relative to
559 secondary physical/chemical processes in the crust since higher contents of N₂ and
560 helium are associated with more depleted δ¹³C values (Fig S1).

561

562 6 Conclusion

563 The main source of helium in the study area is the continental crust. ³He/⁴He ratios are independent of gas composition, given that ³He/⁴He is roughly the same for 564 springs near SP4, including Fontaine des Vertus, while its helium and N₂ contents are 565 much higher than for the other springs of the group. Local helium production alone is 566 unlikely since the ³He/⁴He ratios (0.17-0.22 Rₐ) are in the same range across a 40 km 567 area (Pougues-les-Eaux and Couy). To verify if ³He and ⁴He are produced and stored in 568 the reservoir layers or if they are transported from a crustal flux, the U-Th-Li contents 569 of the host rocks of the sites investigated is required, and a better understanding of the 570 basin geometry and sources is necessary. 571

572 The decreasing ³He/⁴He gradient starting at the Massif Central towards the Paris
573 Basin and the vertical ³He/⁴He gradient previously described in the literature at the Paris
574 Basin aquifers (Marty et al., 1993; Pinti & Marty, 1995, 1998) point to a small mantle
575 contribution limited to the N-S fault system, since laterally, few kilometres (50-80) east
576 at Bourbon-Lancy and Fontaines Salées, ³He/⁴He ratios show typical crustal values (≤
577 0.04 Rₐ). The ¹²⁹Xe excess observed at SP7 confirms that excess ³He relative to the
578 crustal end-member composition is derived from the mantle. This indicates that either a
deep-seated magmatic source is present, or deep fault systems reach the mantle and input ³He via an improved permeability system (Kennedy & van Soest, 2007).

Considering that the δ¹³C of CO₂ of our surveyed springs are distinct from the regional mantle value (Massif Central), CO₂ is probably mainly of crustal origin. However, mixing between crustal or biogenic components and mantle is expected, especially since CO₂ would most likely be the carrier phase of the mantle helium component and CO₂/³He ratios (between 8 x 10⁹ and 1 x 10¹⁰) are close to the MORB value and indicate a ~ 15% mantle contribution, in agreement with what is observed in He and Xe isotopes. Secondary processes in the crust are also possible given the low solubility of CO₂ in water, and potential carbonate precipitation can also explain the decrease of CO₂ contents associated with elevated concentrations of N₂ and helium at Fontaine des Vertus, Bourbon-Lancy, and Fontaines Salées. δ¹⁵N results, for both sample groups, are comparable and point to a common crustal source.

## ACKNOWLEDGEMENTS

We thank A. Caracusi for the insightful discussions. We are also thankful to L. Zimmermann and B. Tibari for helping with the analyses. We gratefully thank the two anonymous reviewers of this article and the Editor for his handling of our manuscript. This work was partially supported by the French PIA project "Lorraine Université d'Excellence", reference ANR-15-IDEX-04-LUE. This is CRPG contribution # ?

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**Fig 1**

Geological map of the study area with helium isotope results (in R_a) and literature data. a) Structural map with major structures inherited from the Variscan orogeny (Baptiste, 2016). b) Simplified geological map (1/1,000,000 BRGM French geological map) with main geological structures of the area (Au: Aumance basin, De: Decize horst and Ma: Machine graben), location of the well SP4, SP7, springs and the lithium deposits of the area (Ch: Chanvence and Bv: Beauvoir; Gloaguen et al., 2018), and distribution of literature helium isotope data (Bräuer et al., 2017; Marty et al., 1993; Pinti & Marty, 1998). c) Focus on the Saint-Parize-le-Châtel area with location of the springs and wells. d) Simplified cross section line of the study area, with location on the map "b" marked by the orange line.

**Fig 2**

$^{4}$He/$^{20}$Ne versus $^{3}$He/$^{4}$He plot. Samples plot in two groups, one with signatures close to a crustal source and the other with either a 2.5% input of mantle or with nucleogenic $^{3}$He production consistent with the presence of 600 ppm Li. Lithium contents for the lower and upper limits of the European continental crust range are based on average values of U-Th for the continental crust (1.8 and 7.2 ppm, respectively; Krauskopf and Bird, 1967). $^{4}$He/$^{20}$Ne values for SP4 are not precise due to $^{20}$Ne abundances indistinguishable from baseline measurement. Massif Central and Paris Basin data are from the literature (Bräuer et al., 2017; Marty et al., 1993; Pinti & Marty, 1998).

**Fig 3**

Variation of helium isotope ratios with latitude. There is an apparent trend with decreasing $^{3}$He/$^{4}$He ratios from south, starting at the Massif Central, to north, ending in the Paris Basin. Helium contents are higher for our dataset. Massif Central and Paris Basin data are from the literature (Bräuer et al., 2017; Marty et al., 1993; Pinti & Marty, 1998).

**Fig 4**

Three isotope diagram of neon. Mixing between air and solar, MORB (Sarda et al., 1988), Archean terrains (Lippmann-Pipke et al., 2011), crustal (Kennedy et al., 1990) end-members are displayed. SP4 plots between the crustal and Archean mixing lines with air and errors shown in 1 sigma level.

**Fig 5**

Xenon isotopic composition of SP7 gas corrected for physical fractionation. Data is expressed in δ (‰) of Xe ratios relative to atmospheric composition and normalised to $^{130}$Xe. Error bars are 2 sigma. The SP7 gas has a clear $^{129}$Xe excess that indicates mantle input.

**Fig 6**

$CO_2/$^{3}$He versus δ$^{13}$C plot (Sano and Marty, 1995). The dataset shows mixing between the three components and plots close to the MORB value while Massif Central data (Bräuer et al., 2017) only shows mixing between MORB and marine limestone. The grey field represents the MORB range from Marty & Zimmermann (1999).
<figure><img src="image_2.png" /></figure>

## Geological units

### Sedimentary cover

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

### Basement

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

### Structural elements

SPC
- Saint-Parize-le-Châtel
- Faults
- Expected faults
- Main faults
- Cross section line

### Springs and wells

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /></figure>

⁴He/²⁰Ne
<figure><img src="image_7.png" /></figure>

■ Fonts-Bouillants ■ Gelin ■ Fontaine des Vertus ◦ SP4

■ Pougues-les-Eaux × Fontaines Salées × Bourbon-Lancy

▲ Massif Central + Paris basin basement ◦ Paris basin
<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /></figure>

Fonts-Bouillants

Gelin

Pouques-les-Eaux

SP4

Massif Central
<table><caption>Table 1 List of sampled sites with coordinates, sampling dates, and average of main gas phase results from GC measurements. *A second sampling campaign occurred in May-June/2022 for δ¹⁵N analyses.</caption><thead><tr><th>Site</th><th>type</th><th>Date</th><th>map ref</th><th>LAT (°N)</th><th>LONG (°E)</th><th>CO<sub>2</sub> (vol.%)</th><th>N<sub>2</sub> (vol.%)</th><th>He (ppmV)</th></tr></thead><tbody><tr><td>SP4</td><td>well</td><td>Aug-Sept/2022</td><td>1</td><td>46.85979</td><td>3.19336</td><td>93</td><td>5.6</td><td>394</td></tr><tr><td>SP7</td><td>well</td><td>Feb/2022</td><td>2</td><td>47.82861</td><td>4.18083</td><td>94</td><td>5.6</td><td>426</td></tr><tr><td>Gelin</td><td>spring</td><td>Aug-Sept/2022</td><td>3</td><td>46.86035</td><td>3.19268</td><td>91</td><td>6.1</td><td>416</td></tr><tr><td>Fonts-Bouillants</td><td>spring</td><td>Aug-Sept/2022</td><td>4</td><td>46.86008</td><td>3.19284</td><td>92</td><td>5.7</td><td>393</td></tr><tr><td>Fontaine des Vertus</td><td>spring</td><td>Aug-Sept/2022*</td><td>5</td><td>46.82025</td><td>3.17123</td><td>60</td><td>36.3</td><td>2874</td></tr><tr><td>Pougues-les-Eaux</td><td>spring</td><td>Aug-Sept/2022*</td><td>6</td><td>47.07635</td><td>3.09240</td><td>94</td><td>4</td><td>245</td></tr><tr><td>Fontaines Salées</td><td>spring</td><td>Aug-Sept/2022</td><td>7</td><td>47.44900</td><td>3.77680</td><td>0.9</td><td>92.5</td><td>45794</td></tr><tr><td>Bourbon-Lancy</td><td>spring</td><td>May-June/2022</td><td>8</td><td>46.61786</td><td>3.76974</td><td>2.6</td><td>91.3</td><td>20644</td></tr></tbody></table>
Table 2 $ ^{3}\text{He}/^{4}\text{He} $, $ ^{4}\text{He}/^{20}\text{Ne} $, $ CO_{2}/^{3}\text{He} $, and stable isotopes results. $ ^{3}\text{He}/^{4}\text{He} $ are normalized by the atmospheric $ ^{3}\text{He}/^{4}\text{He} $ ratio ($ R_{a} $ – 1.39 x 10$^{-6}$; Graham, 2002).

*Average $ \delta^{15}\text{N} $ values from multiple measurements; SP4 value from two measurements performed on the same day (6.3 and 8.3 ‰) and Gelin from three measurements on different days (7.9, 8.5, and 8.9 ‰).

<table><thead><tr><th>Site</th><th>Sample</th><th><sup>4</sup>He/<sup>20</sup>Ne</th><th><sup>3</sup>He/<sup>4</sup>He (R<sub>a</sub>)</th><th>CO<sub>2</sub>/<sup>3</sup>He</th><th>δ<sup>13</sup>C (‰ vs PDB)</th><th>δ<sup>15</sup>N (‰ vs Air)</th></tr></thead><tbody><tr><td rowspan="6">SP4</td><td>SP4-100 CTN6</td><td>-</td><td>-</td><td>-</td><td>-10.1</td><td>7.3*</td></tr><tr><td>SP4-200 CTN3</td><td>62080 ± 4724</td><td>0.21 ± 0.01</td><td>8.16E+09</td><td>-</td><td>-</td></tr><tr><td>SP4-200 CTN3 M1</td><td>≤ 1656612</td><td>0.19 ± 0.01</td><td>8.96E+09</td><td>-</td><td>-</td></tr><tr><td>SP4-200 CTN4</td><td>35291 ± 3003</td><td>0.21 ± 0.00</td><td>7.99E+09</td><td>-</td><td>-</td></tr><tr><td>SP4-200 CTN9</td><td>66651 ± 5465</td><td>0.17 ± 0.01</td><td>1.00E+10</td><td>-</td><td>-</td></tr><tr><td>SP4-200 CTN11 M1</td><td>1975 ± 55</td><td>0.20 ± 0.01</td><td>8.71E+09</td><td>-</td><td>-</td></tr><tr><td rowspan="2">SP7<br>Gelin</td><td>SP7</td><td>180114 ± 34276</td><td>0.18 ± 0.01</td><td>8.66E+09</td><td>-</td><td>-</td></tr><tr><td>GEL-100 CTN1</td><td>9509 ± 354</td><td>0.19 ± 0.00</td><td>8.37E+09</td><td>-10.0</td><td>8.4*</td></tr><tr><td rowspan="2">Fontes-Bouillants</td><td>GEL-300 CTN1 M1</td><td>2222 ± 72</td><td>0.17 ± 0.01</td><td>9.09E+09</td><td>-</td><td>-</td></tr><tr><td>FBO-100 CTN1 M1</td><td>776 ± 22</td><td>0.21 ± 0.00</td><td>8.00E+09</td><td>-10.0</td><td>-</td></tr><tr><td rowspan="2">Fontaine des Vertus</td><td>FBO-300 CTN1 M1</td><td>7525 ± 281</td><td>0.21 ± 0.01</td><td>8.18E+09</td><td>-</td><td>-</td></tr><tr><td>VER-100 CTN1</td><td>10240 ± 336</td><td>0.16 ± 0.01</td><td>9.26E+08</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Pougues-les-Eaux</td><td>VER-200 CTN1 M1</td><td>9042 ± 476</td><td>0.17 ± 0.00</td><td>9.06E+08</td><td>-11.8</td><td>7.5</td></tr><tr><td>POU-100 CTN1 M1</td><td>1170 ± 35</td><td>0.22 ± 0.01</td><td>1.22E+10</td><td>-10.3</td><td>6.1</td></tr><tr><td>Fontaines Salées</td><td>FON-100 CTN1 M1</td><td>2587 ± 66</td><td>0.04 ± 0.00</td><td>-</td><td>-20.0</td><td>7.1</td></tr><tr><td>Bourbon-Lancy</td><td>B-LANCY A</td><td>960 ± 20</td><td>0.03 ± 0.01</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>
<table><caption>Table 3 Ne, Ar, and Xe isotope results. Xe ratios are reported in the delta notation relative to the atmospheric isotope composition. “corr” refers to data corrected for physical fractionation. Errors shown at 1 sigma.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2"><sup>20</sup>Ne/<sup>22</sup>Ne</th><th rowspan="2"><sup>21</sup>Ne/<sup>22</sup>Ne</th><th rowspan="2"><sup>40</sup>Ar/<sup>36</sup>Ar</th><th rowspan="2"><sup>38</sup>Ar/<sup>36</sup>Ar</th><th colspan="6">δ vs Air, ‰</th></tr><tr><th><sup>128</sup>Xe/<sup>130</sup>Xe</th><th><sup>129</sup>Xe/<sup>130</sup>Xe</th><th><sup>131</sup>Xe/<sup>130</sup>Xe</th><th><sup>132</sup>Xe/<sup>130</sup>Xe</th><th><sup>134</sup>Xe/<sup>130</sup>Xe</th><th><sup>136</sup>Xe/<sup>130</sup>Xe</th></tr></thead><tbody><tr><td>SP4-200-GB1</td><td>9.40 ± 0.05</td><td>0.0621 ± 0.0011</td><td>2866 ± 5</td><td>0.189 ± 0.001</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SP4-100-GB1</td><td>-</td><td>-</td><td>2574 ± 11</td><td>0.179 ± 0.001</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SP7 (raw)</td><td>-</td><td>-</td><td>696.6 ± 0.2</td><td>0.18840 ± 0.00002</td><td>0.996 ± 0.311</td><td>1.865 ± 0.145</td><td>-0.042 ± 0.179</td><td>1.463 ± 0.145</td><td>6.554 ± 0.167</td><td>9.464 ± 0.195</td></tr><tr><td>SP7 (corr)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0 ± 0.311</td><td>1.371 ± 0.212</td><td>0.445 ± 0.235</td><td>2.430 ± 0.335</td><td>8.462 ± 0.619</td><td>12.286 ± 0.902</td></tr></tbody></table>