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# Long-Term Storage of Meteoric Water in the Variscan Basement: Stable Isotope Constraints (δD–δ18O)

Loïc Bouat, Pierre Strzerzynski, Véronique Gardien

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Loïc Bouat, Pierre Strzerzynski, Véronique Gardien. Long-Term Storage of Meteoric Water in the Variscan Basement: Stable Isotope Constraints (δD–δ18O). Terra Nova, 2025, 10.1111/ter.70021. hal-05387770

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# Long-Term Storage of Meteoric Water in the Variscan Basement: Stable Isotope Constraints (δD-δ¹⁸O)

Loïc Bouat¹ | Pierre Strzerzynski² | Véronique Gardien³

¹BRGM, Orléans, France | ²Laboratoire de Planétologie et Géodynamique, UMR6112, CNRS, Le Mans Université, Le Mans, France | ³Laboratoire de Géologie de Lyon, UMR5276, CNRS, Université Lyon 1, Villeurbanne Cedex, France

**Correspondence:** Loïc Bouat (l.bouat@brgm.fr)

**Received:** 11 June 2025 | **Revised:** 30 October 2025 | **Accepted:** 6 November 2025

**Funding:** This work forms part of the first author's Ph.D., funded by the French CNRS program INSU-Tellus. The authors thank Patrick Jame and Erik Bonjour (ISA, Lyon, France) and T. Rigaudier (CRGP, Nancy, France) for providing the water and quartz isotope analyses.

**Keywords:** cavity ring-down spectroscopy | fluid inclusions | fluid mixing | paleo-meteoric waters | unconformity-related mineral deposits | δD-δ¹⁸O stable isotopes

## ABSTRACT

F-Ba-Pb-Zn (±Ag) deposits near the unconformity between the Armorican and Aquitaine basins formed by mixing surface waters and brines with fluid ascending from the basement. To better constrain the source of this fluid and the conditions of mixing, we analyzed the hydrogen (δD<sub>water</sub>) and oxygen (δ¹⁸O<sub>water</sub>) isotopic composition of water trapped in mineral fluid inclusions and the oxygen (δ¹⁸O<sub>quartz</sub>) isotope composition. Results indicate that isotopic equilibrium was not reached during fluid mixing. The isotopic signature of basement fluid is similar/identical to Variscan meteoric water that was trapped and stored in the basement and released at the onset of the Bay of Biscay opening.

## 1 | Introduction

Among numerous analytical methods, the stable isotopic composition of hydrogen (δD) and oxygen (δ¹⁸O) in water can help determine the origin of fluids (Sheppard 1986). In mineral deposits, this is used for water trapped in fluid inclusions if enough fluid is released for analysis. For several decades, δD has been directly measured using the crush-leach method and a dual-inlet mass spectrometer (Ohmoto and Rye 1974). δ¹⁸O of water is calculated relative to δ¹⁸O of quartz by considering isotopic equilibrium at a specific temperature (Baatartsogt et al. 2007; Cathelineau et al. 2012; Munoz et al. 1999). Because equilibrium is not always reached, and temperature estimates can be inaccurate, this method is not always reliable (de Graaf et al. 2020a). New methods developed in the early 2000s, such as continuous-flow isotope ratio mass spectrometry (Dublyansky and Spötl 2009) and cavity ring-down spectroscopy of the fluid (Rigaudier et al. 2012), enable direct analysis of a small amount

(3–10 µL) of water. These techniques have been effectively used to study climate change (Arienko et al. 2013), the isotopic evolution of ocean water (Lécuyer et al. 2020), and paleo-elevation of mountain ranges (Melis et al. 2023). In mineral deposits, they have been applied to F-Ba-Pb-Zn (de Graaf et al. 2020b), Mo and base metal mineralization (Stoltnow et al. 2022), and quartz-stibnite deposits (Sośnicka et al. 2022).

The presence of Sulfur and base metals (Pb, Zn and Cu) in unconformity-related mineralization deposits results from (i) transportation by brines originating from evaporitic sediments and (ii) mixing with seawater and/or meteoric water, which promotes precipitation (Aquilina et al. 2011; Baatartsogt et al. 2007; Boiron et al. 2002, 2010; Bons et al. 2014; Cathelineau et al. 2012; Essarraj et al. 2021; Leach et al. 2010; Munoz et al. 1999; Paradis et al. 2007; Schwinn et al. 2006). The presence of meteoric water is explained by two models based on residence time within the continental

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). Terra Nova published by John Wiley & Sons Ltd.
crust. The first model suggests a short residence time, with meteoric water being contemporaneous with mineral deposition (Munoz et al. 1999). The second model implies a long residence time, with paleo-meteoric water being trapped in the basement during emergence, stored within the crust and later released (Bons and Gomez-Rivas 2020; Staude et al. 2009).

To choose between these two models, we present the first δD and δ¹⁸O isotope data on fluid inclusions and δ¹⁸O isotopes in quartz from unconformity-related F-Ba(±Pb-Zn) deposits in Western Europe. We selected the Vendée coast mineral deposit, where hot fluids originating from the European basement mixed with surface and basin fluids (Bouat et al. 2025; Cathelineau et al. 2012). This helps to define the isotopic signature of fluid circulating in the continental crust and may reveal isotopic mineral-fluid disequilibrium.

## 2 | Geological Setting of the Vendée Coast Unconformity-Related F-Ba(±Pb-Zn) Deposits

The Vendée Coast (Figure 1) is situated in western France, where the Aquitaine Basin overlaps the Armorican Massif, a remnant of the Variscan Belt (Ballèvre et al. 2009). Basement outcrops such as orthogneiss, paragneiss, and micaschists were deformed during orogenic collapse and faulted during late exhumation (Figure 2; Goujou et al. 1992; Bouat et al. 2023). After peneplanation and erosion, the Aquitaine Basin series, consisting of detrital (SU1-unit), dolomitic (SU2-unit) and limestone deposits with varying amounts of clay (SU3 to SU5

units), was deposited as a marine incursion from Hettangian to Callovian times (Figure 1; Goujou et al. 1992).

The Vendée Coast unconformity is characterized by alteration of the basement and mineral deposition (Figure 3a; Bouat et al. 2023; Cathelineau et al. 2012; Strzerzynski et al. 2020). The paragenetic sequence includes fluorite, barite, and quartz disseminated in the SU1-unit and filling karst features, fractures, and normal faults (Figures 2 and 3b; Bouat et al. 2025). Previous microthermometric data indicate that the paragenetic sequence is associated with three successive fluids: (Fluid 1) low-temperature, high-salinity brines from buried evaporites; (Fluid 2) high-temperature, low-salinity fluids from the basement; and (Fluid 3) surface seawater (Figure 2; Bouat et al. 2025). Here, mineralization related to fluid circulation occurred in an N-S extensional environment approximately 145 My ago (Bouat et al. 2023).

## 3 | Methods

### 3.1 | Oxygen Isotopic Compositions on Quartz

BrF₅ was used to extract oxygen by fluorination from quartz samples. Ten mg of each sample was dried for 3 h at 110°C, then loaded into a nickel reaction vessel and degassed under vacuum at room temperature for 2 h before reacting with BrF₅ at 550°C for 12 h. A heated graphite rod converted the released O₂ into CO₂ for mass spectrometric measurement. Isotopic measurements were conducted on a Thermo-Fisher MAT253

<figure><img src="image_4.png" /><figcaption>**FIGURE 1** | Geological context of the Vendée Coast (modified from Bouat et al. 2023). (a) Simplified geological map, (b) sedimentary succession above the unconformity, and (c) synthetic sketch of the brittle evolution of the Vendée Coast between the late Variscan collapse and the Bay of Biscay rifting. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**FIGURE 2** | (a) Paragenesis sequence of the Vendée Coast mineralization. In purple, the paragenetic position of Qz5 quartz samples. (b) Fluid mixing model of the Vendée Coast mineralization from microthermometric data on fluid inclusions implying three different sources of fluid: First, a basinal brine derived from Aquitaine basin evaporite (Fluid 1), second: A hot low salinity fluid coming from the basement (Fluid 2) and third, the incursion of Upper Jurassic-Early Cretaceous seawater (Fluid 3) (modified after Bouat et al. 2025). The field in purple delineates the salinity versus homogenization temperature field of the quartz samples (Qz5) selected for isotope analyses. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

Mass Spectrometer at the CRPG, Nancy (France). All values are reported in δ notation per mil (‰) relative to VSMOW. All sample measurements were adjusted to the CO₂ internal reference material CAN-GAS (δ¹⁸O = -29.51‰) calibrated with the international standards IAEA CO-1, IAEA CO-8 and NBS 19. Reproducibility (2s) was calculated on 8 aliquots of internal reference material (quartz NL15 = 18‰) and was better than 0.4‰.

## 3.2 | Stable Isotopes (δD-δ¹⁸O) Analysis of Water From Fluid Inclusions

Sample preparation involves crushing and sieving to select 10–30 g of 0.5–2 mm large quartz grains that are rich in fluid inclusions. The samples were first cleaned with 27% concentrated nitric acid, for 2 h, then rinsed with distilled water. To remove any remaining rinsing water around the quartz grains, the samples were then placed overnight in an oven at 80°C. Following the procedures outlined in Rigaudier et al. (2012), Gardien et al. (2016) and Melis et al. (2023), the fluids were extracted by crushing the quartz in a vacuum line at the LGL (Lyon). The samples were heated to 600°C, which exceeds the homogenization temperature of the fluid inclusions determined by microthermometry. The volatiles from the fluid inclusions were trapped in two steps: first, using a liquid nitrogen cold trap (–196°C) to capture CO₂ and H₂O; second,

using an ethanol-nitrogen mixture (–78.5°C) to trap only H₂O. A transparent Teflon (cxTeflon PFA) sample holder was used to collect 3–10 µL of water.

Hydrogen and O stable isotope ratios were measured simultaneously at ISA (Lyon) using an ‘Off-Axis Integrated Cavity Output Spectrometer’ (OA-ICOS) IWA-45EP (Los Gatos Research, Mountain View, California) by repeatedly injecting 1 µL of water into the laser spectrometer. Tests on water samples with known composition showed good measurement accuracy. For a minimum water volume of 6 µL, measurement accuracy was ±5.5‰ for H and ±0.40‰ for O. The measurement precision drops to ±8 for H and ±1 for O because fewer replicas were injected. Calibration was performed with the standard WICO2 (δD = –41.9‰ and δ¹⁸O = –5.11‰; Wassenaar et al. 2018) and the internal standards 3C (δD = –97.3‰ and δ¹⁸O = –13.39‰) and 4C (δD = –51.6‰ and δ¹⁸O = –7.94‰).

# 4 | Results

## 4.1 | Sampling

Quartz samples were collected along the Vendée Coast (Figure 1) from karst and fractures (Figure 3). We focused on the most favourable quartz population, Qtz5-quartz, for the crush-leach method (Figure 2). It consists of large centimetre-size euhedral crystals with abundant fluid inclusions (Figure 3). The fluid temperature of the sample, as determined by thermometry, ranges from 191°C to 275°C (Table 1), and its salinity is consistently below 10 wt% eq. NaCl (Bouat et al. 2025).

## 4.2 | Oxygen Isotope Composition (δ¹⁸O) of Quartz

Six samples from the St-Nicolas, Mine des Sards, Cayola Bay and Moines Bay areas (Figure 1; Table 1) were used to determine the δ¹⁸O of Qtz5-quartz. The value ranges from +18.58 ± 0.02‰ in sample MS11 (Mine des Sards) to +23.42 ± 0.02‰ in sample QCAY (Cayola Bay) (Figure 2; Table 1). The δ¹⁸Oquartz values of the quartz samples from Moines Bay and St-Nicolas are +21.1 ± 0.5‰ (AM2 sample), +21.60 ± 0.03‰ (‘20’ sample), and +21.39 ± 0.02‰ (QSN sample), which are similar to the value of +20.20 ± 0.12‰ of sample BC1b from Cayola Bay (Figure 4; Table 1).

## 4.3 | Hydrogen and Oxygen Isotope Composition (δD-δ¹⁸O) of Water

Analyses were conducted on water extracted from fluid inclusions of five quartz samples. Only two samples from Cayola Bay and Mine des Sards contained enough water to determine δD and δ¹⁸O. The δDfluid and δ¹⁸Ofluid of sample BC1b (3 µL) are –57.7‰ ± 5‰ and –68.2‰ ± 5‰, respectively, and –7.8‰ ± 0.5‰ and –10.2‰ ± 0.5‰ for sample SARDS (5 µL) (Table 1; Figure 4). Since only 1 µL of water was extracted from the other three samples (AM2, 20, QSN), caution should be exercised when interpreting the δD-δ¹⁸O results (Table 1; Figure 4).
<figure><img src="image_7.png" /><figcaption>**FIGURE 3** | Structural context and petrology of the Vendée Coast mineralization. (a) Field image of the unconformity cut by NW-SE normal fault and (b) upper view of quartz and pyrite veins crossing the silicified cover (SU2-unit). (c) Microscope images of a vein of quartz (Qz5) and galena hosted in the silicified cover (Qz2). (d) Isolated biphasic fluid inclusion hosted in quartz (Qz5) with microthermometry data (Bouat et al. 2025). For the lithology units, see Figure 1b, and for the mineral acronym, see Figure 2a. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

# 5 | Discussion

## 5.1 | Evidence for Fluid Mixing During Quartz Crystallization

The maximum isotopic equilibrium temperature calculated is 75°C, assuming isotopic equilibrium and using δ¹⁸Oquartz and δ¹⁸Ofluid along with the quartz-water isotope fractionation coefficient (Figure 4b; Zheng 1993). This temperature indicates that isotopic equilibrium between quartz and fluid in the fluid inclusions has not been reached because it differs from the homogenization temperatures measured in fluid inclusions of the same quartz samples ([140°C–395°C], Bouat et al. 2025). Disequilibrium could be caused by (1) quartz cryptocrystalline texture, which leads to a high δ¹⁸O (Tanner et al. 2013), (2) diffuse post-trapping equilibration (Naden et al. 2003), and (3) a change in quartz growth rate with a kinetic effect due to fluid mixing (Naden et al. 2003; Qiao et al. 2022; Schwinn et al. 2006; Tanner et al. 2013, 2015). The first two possibilities are ruled out because there is no evidence for cryptocrystalline quartz, and most Qz5-quartz homogenization temperatures are below the minimum diffusion temperature (Bouat et al. 2025). Since fluid mixing is known to occur (Bouat et al. 2025; Cathelineau et al. 2012), quartz may precipitate faster than isotopic balance can be established, resulting in a lack of isotopic equilibrium. Therefore, when calculating δ¹⁸Ofluid using δ¹⁸Oquartz and assuming isotopic

equilibrium, certain precautions should be taken when mixing fluids of different temperatures.

## 5.2 | Carboniferous Meteoric Water Source

Quartz precipitates when evaporitic brines (Fluid 1), basement fluids (Fluid 2), and seawater (Fluid 3) are mixed, based on previous results from the paragenetic sequence and microthermometry data (Figure 2; Bouat et al. 2025). The mixing of these fluids should be evident in the δD-δ¹⁸O signatures of water trapped in the fluid inclusions (Figure 4).

Overall, the δDfluid and δ¹⁸Ofluid values are more depleted than those of evaporite brines ([+18; +6] ‰, Horita 2005) and seawater (SMOW [0; 0] ‰). Since δDfluid and δ¹⁸Ofluid are close to the GMWL, we dismiss a significant contribution from metamorphic, magmatic, and juvenile waters (Figure 4). The most likely explanation for δDfluid and δ¹⁸Ofluid is a mixture of evaporitic brines and/or seawater with water of meteoric origin. Because the δD-δ¹⁸O signatures of BC1b and SARDS samples are more depleted (Figure 4) than meteoric water ([-22; -4] ‰, Pouech et al. 2014), this source can be ruled out. In the French Variscan Belt, meteoric water trapped during the late Carboniferous has an average δD-δ¹⁸O signature of -96‰ ± 8‰ δD and -13.3‰ ± 1.1‰ δ¹⁸O (Dusséaux
<table><caption>TABLE 1 | δD versus δ¹⁸O values of fluid inclusions and quartz. The greyed volume value corresponds to the non-suitable volume of water extracted for δD-δ¹⁸O isotopic analysis.</caption><thead><tr><th rowspan="3">Zone</th><th rowspan="3">Sample</th><th rowspan="3">Host-rock</th><th colspan="2">Temperature</th><th colspan="4">δ¹⁸O<sub>water</sub> calculated (vSMOW%)</th></tr><tr><th colspan="2">From Bouat et al. (2025)</th><th colspan="4">After Zheng (1993) equation</th></tr><tr><th>(wt% eq. NaCl)</th><th>(°C)</th><th>V (µL)</th><th>δD<sub>fluid</sub> (vSMOW%)</th><th>δ¹⁸O<sub>fluid</sub> (vSMOW%)</th><th>δ¹⁸O<sub>quartz</sub> (vSMOW%)</th><th>T=75°C</th><th>T=175°C</th><th>T=275°C</th><th>T=375°C</th></tr></thead><tbody><tr><td>Moines Bay</td><td>AM2</td><td>Basement</td><td>6.7 ± 1.4</td><td>249.2 ± 39.4</td><td>1</td><td>-64 ± 5</td><td>-4.3 ± 0.5</td><td>21.06 ± 0.50</td><td>-3.91</td><td>+7.69</td><td>+13.15</td><td>+16.05</td></tr><tr><td>Cayola Bay</td><td>QCAY</td><td>SU2</td><td>6.4 ± 1.0</td><td>231.2 ± 45.2</td><td></td><td></td><td></td><td>23.42 ± 0.08</td><td>-1.55</td><td>+10.05</td><td>+15.51</td><td>+18.41</td></tr><tr><td></td><td>BC1b</td><td>SU2</td><td></td><td></td><td>3</td><td>-68 ± 5</td><td>-5.5 ± 0.5</td><td>20.20 ± 0.12</td><td>-4.77</td><td>+6.83</td><td>+12.29</td><td>+15.19</td></tr><tr><td>Mine des Sards</td><td>MS11</td><td>SU2</td><td>9.2 ± 3.8</td><td>257.7 ± 70.7</td><td></td><td></td><td></td><td>18.58 ± 0.02</td><td>-6.39</td><td>+5.21</td><td>+10.66</td><td>+13.56</td></tr><tr><td></td><td>SARDS</td><td>SU2</td><td></td><td></td><td>5</td><td>-58 ± 5</td><td>-7.8 ± 0.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>St-Nicolas</td><td>20</td><td>SU2</td><td>3.6 ± 2.6</td><td>273.1 ± 56.3</td><td>1</td><td>-85 ± 5</td><td>-8.0 ± 0.5</td><td>21.60 ± 0.03</td><td>-3.37</td><td>+8.22</td><td>+13.68</td><td>+16.58</td></tr><tr><td></td><td>QSN</td><td>SU2</td><td>4.6 ± 0.2</td><td>191.7 ± 24.5</td><td>1</td><td>-68 ± 5</td><td>-10.2 ± 0.5</td><td>21.39 ± 0.02</td><td>-3.58</td><td>+8.01</td><td>+13.47</td><td>+16.37</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>FIGURE 4 | (a) δD-δ¹⁸O diagram with values of fluids extracted from fluid inclusions in the Vendée Coast deposits (purple symbols) compared with different water compositions (grey dash-line areas). Fluid 1 (yellow-grey) and Fluid 3 (blue-grey) are reported as isotopic signatures of the evaporation of seawater for brines derived from basinal evaporite and the SMOW for contemporaneous seawater respectively as defined in Bouat et al. (2025). b. δ¹⁸O diagram of δ¹⁸O<sub>water</sub>, calculated from δ¹⁸O<sub>quartz</sub> at 75°C, 175°C, 275°C and 375°C (shaded grey bars and boxes in (a) and (b)). [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

et al. 2021; Figure 4). Mixing with a small amount of seawater and/or evaporitic brines could account for the δD<sub>fluid</sub> and δ¹⁸O<sub>fluid</sub> observed in the Mine des Sards sample. In this context, the slight rightward shift of the GMWL in the isotopic signature of the Cayola Bay sample can be explained by a minor contribution of metamorphic and/or magmatic waters (Figure 4). Consequently, the source of the low salinity and hot crustal Fluid 2 involved in the mineral deposition of Vendée may have been meteoric water trapped in the basement during the Carboniferous period.

## 5.3 | Long-Lived Storage and Release of Meteoric Water

The long-term storage of meteoric water in the continental crust, previously suggested for the Schwarzwald massif (Bons
et al. 2014) and Morvan mineral deposits (Lenoir et al. 2023), requires several specific conditions.

Firstly, meteoric water must be trapped in the continental crust (Figure 5). Such downward infiltration has been described to reach depths of up to 10 km in the French Variscan Belt during the Carboniferous (Dusséaux et al. 2021). It is promoted by common features observed during late orogenic collapse, such as normal faulting, a high geothermal gradient and hydraulic heads (Dusséaux et al. 2019; Quilichini et al. 2015).

Secondly, the continental crust must retain meteoric water for an extended period. Meteoric water resided in the crust from 300 to 150 Ma (Figure 5). The gradual return to a stable geothermal gradient, the erosion of relief features that removed potential hydraulic heads, and the absence of deformations allowed the continental crust to develop into a stable state, facilitating this storage (Bons and Gomez-Rivas 2020).

Thirdly, trapped water must rise to the unconformity to mix with brines during mineral deposition (Figure 5). This upwelling marked the beginning of oblique rifting in the Bay of Biscay around 150 Ma, when Western Europe experienced widespread diffuse N-S deflection (Asti et al. 2022; Bouat et al. 2023). Accordingly, fault-valve processes (Cox 1995; Sibson 2019; Sibson et al. 1988) and possible warming of the Western European lithosphere due to extension can account for fluid movement along faults.

## 5.4 | Maintaining the Meteoric Signature

To preserve the meteoric signature of Variscan meteoric water, no significant isotope exchange with the host rock should have occurred before, during and after the trapping of fluids in Qz5-quartz. This means that meteoric water should have been stored below 300°C–350°C (Bons and Gomez-Rivas 2020; Farver and Yund 1991; Naden et al. 2003).

At the time of trapping, the mean fluid temperatures have been constrained by microthermometry at 231°C ± 45°C and 257°C ± 70°C, for the Cayola Bay and Mine des Sards areas, respectively (Table 1; Bouat et al. 2025). In this context, no significant isotope exchange may have occurred during trapping.

Before and after the trapping phase, the temperature of basement rock near the unconformity never exceeded 100°C since around 300 My ago (Figure 5; Barbarand et al. 2020; François et al. 2020). This indicates that Variscan meteoric water may have been trapped without significant isotopic exchange during that period at depths of 1–5 km, considering a geothermal gradient of 30°C–50°C/km. At 320 My ago, the Variscan crust experienced a metamorphic peak exceeding 500°C in a context of ductile extension (Tartèse et al. 2012; Turrillot et al. 2011). During this time, Variscan meteoric water equilibrated with white micas (Dusséaux et al. 2022) and quartz. Based on this, the meteoric water trapped in Qz5-quartz likely fell after the Carboniferous metamorphic peak and percolated into the Variscan crust, which was undergoing post-orogenic brittle extension.

## 6 | Conclusion

The contribution of paleo-meteoric waters trapped in the basement is indicated by the δD-δ¹⁸O of fluid inclusions in unconformity-related F-Ba(±Pb-Zn) deposits of Vendée. The start of the Biscay Bay rift triggered the release of this paleo-meteoric water, which had been confined during an earlier orogeny and remained below 300°C–350°C for at least 100 My (Figure 5). Despite the high trapping temperature, microthermometry and isotopes reveal that the δ¹⁸Oquartz and the fluid trapped in the fluid inclusion are out of equilibrium. We suggest that this disequilibrium results from the kinetic effect of quartz precipitation caused by fluid mixing.

<figure><img src="image_9.png" /><figcaption>**FIGURE 5** | Schematic diagram illustrating meteoric water deep percolation during the Late Variscan orogeny along brittle detachments in the Permian, then passive trapping in the basement during the Permo-Triassic peneplanation and the Jurassic transgression and finally expulsion to the unconformity with the sedimentary cover during the Biscay Bay rifting. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
## Acknowledgements

This work forms part of the first author's Ph.D., funded by the French CNRS program INSU-TelluS. The authors thank Patrick Jame and Erik Bonjour (ISA, Lyon, France) and T. Rigaudier (CRGP, Nancy, France) for providing the water and quartz isotope analyses. They are also grateful to one anonymous reviewer, the handling editor, and editor Klaus Mezger for their fruitful comments, which greatly enhanced this manuscript.

## Conflicts of Interest

The authors declare no conflicts of interest.

## Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

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