# Mantle helium in Southern Quebec groundwater: A possible fossil record of the New England hotspot

Pauline Méjean, Daniele L. Pinti, Takanori Kagoshima, Emilie Roulleau, Laura Demarets, André Poirier, Naoto Takahata, Yuji Sano, Marie Larocque

► To cite this version:

Pauline Méjean, Daniele L. Pinti, Takanori Kagoshima, Emilie Roulleau, Laura Demarets, et al. Mantle helium in Southern Quebec groundwater: A possible fossil record of the New England hotspot. Earth and Planetary Science Letters, 2020, 545, 10.1016/j.epsl.2020.116352. insu-03661438

HAL Id: insu-03661438

https://insu.hal.science/insu-03661438v1

Submitted on 5 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
1 MANTLE HELIUM IN SOUTHERN QUEBEC GROUNDWATER: A POSSIBLE FOSSIL

2 RECORD OF THE NEW ENGLAND HOTSPOT

3 Pauline MÉJEAN¹, Daniele L. PINTI², Takanori KAGOSHIMA³, Emilie ROULLEAU⁴, Laura

4 DEMARETS²,⁵, André POIRIER², Naoto TAKAHATA³, Yuji SANO², Marie LAROCQUE²

5 1 - Laboratoire des Geosciences Environnement Toulouse (GET) and Observatoire Midi-

6 Pyrénées (OMP), 14 avenue Edouard Belin, 31400 Toulouse, France

7 2 - GEOTOP and Département des sciences de la Terre et de l'atmosphère, Université du Québec

8 à Montréal, CP888, Succ. Centre-Ville, Montréal, QC, H3C 3P8, Canada

9 3 - Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564,

10 Japan

11 4 - STRATAGEM974, 62 boulevard du Chaudron, 97490 Ste Clotilde, La Réunion, France

12 5 - Centre Universitaire de Formation en Environnement et développement durable (CUFE),

13 Université de Sherbrooke, Longueuil, QC, J4K 0A8, Canada

14
15 **Keywords:** noble gases; helium isotopes; groundwater; magma aging; Monteregian Hills; New

16 England hotspot.

17
18

19
20 Abstract

The Monteregian Hills are an alignment of magmatic intrusions of Cretaceous age located in the St. Lawrence Lowlands, Quebec, Canada. Their origin is controversial and numerous studies have failed to decipher between a hotspot trail or sub-continental magmatism related to the opening of the North Atlantic Ocean. Here, we show that 17.7±9.6% of the helium of the modern to Holocene-aged groundwater from the regional aquifer is of mantle origin, with a ³He/⁴He (R) of up to 1.42 times the atmospheric ratio (Ra). It suggests that a fossil Monteregian Hills magmatic signal, diluted by local radiogenic helium and preserved in the Monteregian Hills intrusions, is leached locally by flowing modern or sub-modern groundwater. Helium isotopic measurements by pyrolysis in Monteregian Hills bulk rocks and clinopyroxene separates show R/Ra values of up to 4.96, suggesting that fossil mantle helium has been partially preserved in these rocks and their mineral phases. Monte Carlo simulations of a magma aging model shows that the initial ³He/⁴He ratio in these Cretaceous intrusions could have been between 21±10Ra and 33±28Ra (2σ), favoring the hypothesis that the Monteregian Hills are the product of the passage of the North American plate over the New England hotspot. This study raises the prospect of using modern groundwater as an archive of mantle He over a hundreds of millions of years timescale.

36
## 1. Introduction

The Cretaceous Monteregian Hills Igneous Province is a WNW-ESE alignment of nine alkaline intrusions outcropping across the St. Lawrence Lowlands, Quebec, Eastern Canada (Fig. 1 and 2b). The magmatic source of these intrusions has been the subject of debate for nearly 40 years. Crough (1981) and later Sleep (1990) have suggested that the Monteregian Hills are the result of the passage of the North American plate over the New England hotspot (also called the Great Meteor hotspot), a long-lived hotspot presently overridden by the Mid-Atlantic Ridge, south of the Azores. However, numerous geochemical and isotopic investigations of Monteregian Hills rocks have failed to unequivocally indicate the presence of a plume source (e.g., McHone, 1996; Roulleau et al., 2012; Roulleau and Stevenson, 2013; Chen and Simonetti, 2015).

The helium isotope ratio, $^{3}$He/$^{4}$He (R), can be used to trace the sources of continental magmatism. Indeed, the mantle is enriched in primordial isotope $^{3}$He over radiogenic isotope $^{4}$He when compared to the atmosphere (Ra = 1.382 x 10$^{-6}$: Sano et al., 2013). In continental settings, groundwater may contain helium derived from the mantle in regions of active transtensional deformation, even in regions where there is no substantial magmatism (e.g., Kennedy and van Soest, 1997). The processes that control the release of mantle-derived volatiles to the subsurface are likely sub-continental magma intrusions and underplating (Torgersen, 1993), with episodic advective transport in the crust (Castro, 2004). The occurrence of mantle helium should be identifiable in crustal environments, which are generally dominated by radiogenic $^{4}$He, produced through the radioactive decay of U and Th contained in rocks, over $^{3}$He, which can be produced only in minor amounts by neutron reactions with lithium (Tolstikhin et al., 1996). The resulting typical crustal $^{3}$He/$^{4}$He ratio is 0.01-0.02Ra (Tolstikhin et al., 1996), distinctly lower than the ratio
expected for mantle-related fluids. The helium isotopic ratio of the convective mantle, as sampled in the mid-ocean ridge basalts (MORBs), is 8±1Ra (e.g., Allègre et al., 1995), while sub-continental lithospheric mantle (SCLM) has a lower ratio, 6.1±0.9Ra (Gautheron and Moreira, 2002).

The non-degassed lower mantle, the expected source of mantle plumes and their related products (Oceanic Island Basalts or OIBs), should have preserved a primordial, solar-type $^{3}\text{He}/^{4}\text{He}$ ratio, with R/Ra values greater than the MORBs value of 8±1 (e.g., Trieloff et al., 2000). However, measurements from OIBs show highly variable $^{3}\text{He}/^{4}\text{He}$ ratios. Some have higher $^{3}\text{He}/^{4}\text{He}$ than MORB values, up to 30Ra in Loihi seamount (Hawaii) (Kurz et al., 1982) and greater than 40Ra in NW Iceland (e.g., Harðardóttir et al., 2018), but several others have lower values than MORBs, such at Tristan de Cunha and Gough Islands (5-7Ra; Kurz et al., 1982). The highest R/Ra value reported so far is 49.5, measured in picrites from Baffin Island (Stuart et al., 2003). The reason for such high variability is still a matter of debate, and several hypotheses have been proposed, including: 1) ubiquitous air contamination during basalt eruption; 2) the addition of radiogenic helium from subducted plate components; 3) mixing between plume-derived material and material derived from the upper mantle; and 4) isotopic heterogeneity within the plume itself (e.g., Kurz et al., 1982; Graham, 2002).

Over the past 25 years, several studies have identified fossil mantle helium in groundwater from the eastern North American craton. In New Hampshire, Torgersen et al. (1995) identified the presence of helium with a substantial mantle fraction (≈ 12% of the total; $^{3}\text{He}/^{4}\text{He}$ = 1.2Ra) in modern groundwater from the Mirror Lake Basin aquifer, where there has been no volcanism since the Jurassic to Early Cretaceous (190-95 Ma ago). Torgersen et al. (1995) attributed this mantle helium to either the passage of the New England hotspot, which created the nearby White
Mountains magmatic province (Eby, 1985) or to magmatic episodes related to the closure of the Iapetus ocean (before 370 Ma). Castro et al. (2009) measured He and Ne in deep groundwater of the Michigan Basin revealing a primordial solar-like contribution. However, the absence of magmatism and of low-velocity seismic anomalies that would have indicated a fossil hotspot track in the region suggested that this primordial signature could be accounted for by a shallow refractory mantle reservoir underneath the Michigan Basin (Castro et al., 2009). Pinti et al. (2011) identified small amounts of mantle helium (2% of the total) in hundreds of millions of years-old brines located in the deeper sedimentary aquifers of the St. Lawrence Lowlands (BEC location in Fig. 1). Saby et al. (2016) also found mantle helium in the Ordovician-age regional aquifer of the Nicolet St-François watershed, St. Lawrence Lowlands (8% of the total; NSF location in Fig. 1).

Here, three new sets of helium isotopic data from groundwater and rocks collected from several locations in the St. Lawrence Lowlands are reported (Figs. 2a, 2b). The first set was measured in groundwater collected from sedimentary aquifers of the Vaudreuil-Soulanges watershed (Table 1), located west of Montreal, close to the Oka carbonatite complex (Fig. 2a). The second set of helium data was measured in groundwater circulating in the magmatic intrusions of the Monteregian Hills (Table 1 and Fig. 2b). The third was measured in Monteregian Hills bulk rocks and clinopyroxene separates, collected as part of a previous study (Roulleau and Stevenson, 2013; Fig. 2b). Measured ³He/⁴He ratios in both groundwater and rocks suggest the presence of a fossil helium magmatic signal diluted by local radiogenic helium, preserved in the Monteregian intrusions and leached locally by flowing modern to Quaternary-age groundwater. The initial helium signature of the magmatic source, before radiogenic dilution, is estimated here using Monte Carlo simulations of a magma aging model (Torgersen and Jenkins, 1982). The results bring new
and independent evidence of the past presence of a primitive plume beneath the region, which generated the Monteregian intrusions.

105

## 2. Geological background

### 2.1 Hydrology of sampled waters

The first set of groundwater samples was collected 10 km south of the Oka complex, in the Vaudreuil-Soulanges watershed (Table 1 and Fig. 2a). With a surface area of 814 km², Vaudreuil-Soulanges is the westernmost watershed of the St. Lawrence Lowlands basin (Fig. 2a). It consists of Cambrian fluvio-marine quartzitic sandstones of the Potsdam Group and Early Ordovician shelf-carbonates and dolostones of the Beekmantown Group unconformably overlying the granite-gneiss-anorthosite basement of the Mesoproterozoic (1250-980 Ma) Grenville Orogeny (Larocque et al., 2015). This sedimentary sequence constitutes the bedrock aquifer sampled in this study, and is covered by a 120 m-thick sequence of Quaternary glacial till and clays from the marine transgression episode of the Champlain Sea, which occurred between 12 and 9 ka ago.

The sedimentary sequence of the Vaudreuil-Soulanges watershed is disturbed by two main magmatic episodes: (1) the intrusion of the syeno-granitic body of Mont Rigaud, with a U-Pb age of $564_{8}^{+10} Ma$ (Malka et al., 2000); and 2) K-rich alnoïtes intruding the sedimentary cover in the central-eastern part of the watershed at the location of Île Cadieux, with ages ranging 108 to 113 Ma (Fig. 2a; Eby, 1987; Chen and Simonetti, 2013). Although only the latter intrusion outcrops in the watershed, a regional low-resolution aeromagnetic survey of the St. Lawrence Lowlands has shown that hundreds of smaller intrusions lie under the sedimentary cover, with a high magnetic
alignment corresponding to the northern region of the Vaudreuil-Soulanges watershed (see maps at http://www.mrnfp.gouv.qc.ca/produits-services/mines.jsp). Several of these intrusions outcrop in numerous quarries, or reach the surface, by crosscutting the Ordovician sedimentary sequences of the St. Lawrence Lowlands (e.g., Harnois et al., 1990; Harnois and Mineau, 1991).

Recharge of the Cambrian-Ordovician bedrock aquifer is mainly from high-permeability Quaternary eskers and sand hills, which crosscut the low-permeability glacial till and the Champlain Sea clays. Less recharge is from the higher topography zone, corresponding to the Mont Rigaud syenitic intrusion. Regional groundwater flows from these topographic highs to the Ottawa River in the northern part of the area, and S-SE, in the direction of St. Lawrence River and Lake Saint-Francois in the southern part of the basin (Fig. 2a). Méjean (2016) reported modelled ³H-³He groundwater ages to be from 8.5±0.6 a to 57±4 a, with several samples showing ³H of 0.8 Tritium Units (TU; 1 TU is defined as the ratio of 1 tritium atom to 10¹⁸ hydrogen atoms), which corresponds to the environmental background, suggesting the presence of older water masses. This fossil water component was dated using the U-Th/⁴He method to be older than 45 ka (Méjean, 2016).

The second set of groundwater samples was collected from wells tapping directly into the fractured intrusive rocks of the Monteregian Hills at St. Bruno, St. Hilaire, Rougemont, Yamaska, and Brome (Fig. 2b and Table 1). Little is known about groundwater circulation in the Monteregian Hills. Geophysics have revealed the presence of several buried dykes around these hills, but the consequences for groundwater circulation are still unknown. A recent study by Beaudry et al. (2018) revealed that the Monteregian Hills contain confined to semi-confined aquifers, with water ages between modern and 3-12 ka. Groundwater from the Monteregian Hills belongs to two
chemical groups (Beaudry et al., 2018): 1) Group “A1”, which consists of modern recharge water associated with the presence of tritium, low salinity (Total Dissolved Salinity (TDS) of 168 mg/L), and a Ca-HCO₃ water type with relatively low pH (6.5) and a composition controlled by carbonate dissolution; and 2) group “A2”, which consists of evolved Na-HCO₃ composition water with high pH (7.8) and slightly saline (TDS of 368 mg/L) due to mixing with brackish water of the Champlain Sea marine invasion (Beaudry et al., 2018).

## 2.2 The debated origin of the Monteregian Hills

The Monteregian Hills alkaline province consists of plutons, dikes, and sills that were emplaced along faults associated with the Ottawa-Bonnechere graben (Fig. 2). The intrusions were emplaced into the Cambro-Ordovician sediments of the St. Lawrence Lowlands and the folded and thrusted Lower Paleozoic sequence of the Appalachian orogen (Figs. 1 and 2b). Carbonatite and associated alkaline rocks occur at the Oka complex, just north of the Vaudreuil-Soulanges watershed (Fig. 2a). Further east, the plutons are dominated by ultramafic and mafic rocks (gabbros) with fewer syenites. Toward the eastern end of the province, silica-saturated mafic and felsic rocks become dominant components of the plutons. Geochronological data from the Oka carbonatite complex by Chen and Simonetti (2013) show bimodal U-Pb ages centered on 113 Ma for alnöitic intrusions and 135 Ma for ijolite rocks. Measured ⁴⁰Ar/³⁹Ar ages of the other Monteregian complexes fall within the restricted range of 124±1 Ma (Foland et al., 1986).

The alkaline character of the Monteregian Hills and the Sr (0.7032–0.7040) and Nd (0.5125–0.5127) isotopic composition similar to those found in some OIBs supported a plume-related mantle source (Foland et al., 1986). McHone (1996) showed that there is no evidence of
age progression within and among the different magmatic provinces supposedly created by the
New England hotspot (Monteregian Hills in Quebec, White Mountains in New England, and the
New England seamounts off the east coast of the US), as would be expected of a hotspot track,
arguing for the involvement of heterogeneous SCLM melts rather than plume material. A decrease
in the buoyancy flux of the New England hotspot from the New England seamounts to the
Monteregian Hills was estimated by Sleep (1990), who explained that the lack of an obvious track
may be due to either the failure of the plume to penetrate the Canadian Shield. Roulleau et al.
observed an inverse relationship between $^{206}$Pb/$^{204}$Pb and $N_2$//$^{36}$Ar ratios, interpreted as the
mixing between a plume source with a recycled component (HIMU, High $\mu = 238$U/$^{204}$Pb) and an
ambiguous mantle source that could be either a non-HIMU-type plume source (such as that feeding
Loihi; Dixon and Clague, 2001) or the depleted mantle. Roulleau and Stevenson (2013) provided
new isotopic data (Nd-Sr-Hf-Pb) suggesting a dominant lithospheric mantle source for the
Monteregian Hills, but contaminated by rising asthenospheric mantle that progressively
thermomechanically eroded the lithosphere, hence, producing a metasomatized lithospheric mantle
with HIMU and EMI reservoirs. Finally, some other authors have ascribed the Monteregian
magmatism to the melting of the lithospheric mantle through NW-SE Iapetus-related faults
reactivated at the time of the North Atlantic Ocean opening (e.g., Faure et al., 1996).

185

## 3. Materials and analytical methods

### 3.1 *Groundwater sampling*

A total of 24 groundwater samples were collected from open bedrock municipal wells,
domestic wells, and piezometers. Sixteen were sampled in the Cambrian-Ordovician fractured
bedrock aquifer of the Vaudreuil-Soulanges watershed in the summer of 2014, at depths ranging between 20 m and 96 m. A second set of eight groundwater samples was collected in the summer of 2016, from fractured aquifers of the Monteregian Hills: Mount St. Bruno, Mount St. Hilaire, Mount Rougemont, Mount Yamaska and Mount Brome (Fig. 2b and Table 1). After purging the well, water was collected in 3/8-inch diameter refrigeration-type copper tubes, which were then cold-sealed following the method described in Vautour et al. (2015). Helium isotopic ratios of groundwater samples collected in 2014 were measured at the University of Tokyo, using a VG Helix SFT and compared to the Helium Standard of Japan (HESJ; Matsuda et al., 2002), with a 2σ precision of ±1%. The <sup>4</sup>He and <sup>20</sup>Ne concentrations were measured on a Pfeiffer Prisma™ 80 quadrupole mass spectrometer connected to the purification line (see Vautour et al., 2015 for details).

The eight samples collected in 2016 were analyzed at the GEOTOP Noble Gas Laboratory. After extraction and purification on one Ti-sponge and two ST-707 getters, helium was cryogenically separated and released at 40K into a HELIX-MC (Thermo®) mass spectrometer, and <sup>3</sup>He/<sup>4</sup>He ratios were measured by peak jumping on the axial CDD (<sup>3</sup>He) and Faraday (<sup>4</sup>He) detectors, with a precision of ca. 3% on the <sup>3</sup>He and less than 0.01% on <sup>4</sup>He, calibrated against an air pipette. Helium and neon elemental composition was measured in a Pfeiffer Prisma™ C-200 quadrupole mass spectrometer connected to the purification line, following the procedures described in Roulleau et al. (2012).
209 3.2 Bulk rocks and mineral separates

210 A total of nine rock samples (Figs. 2a, b) were collected from the Oka complex (OC; n=1),
211 Île Cadieux (Aln, n=1), Mount Royal (MR; n=1), Mount St. Bruno (MSB, n=1), Mount Rougemont
212 (MR, n=2), Mount Brome (MB, n=1), Mount St. Hilaire (MSH, n=1), and Mount Yamaska (MY,
213 n=1). Pyroxene is among the first minerals to crystallize in the magma chamber, together with
214 olivine, and thus documents the least evolved melt available. Monteregian Hills gabbros contain
215 very small and few olivine crystals, so clinopyroxene remains the main phase to analyze (Sasada
216 et al., 1997; Roulleau et al., 2012). Clinopyroxene grains were thus hand-picked from the Mount
217 St. Bruno sample (SMB3-R; Table 2). Helium was extracted and analyzed at the University of
218 Tokyo using a two-stage analytical process. Whole-rock samples and mineral separates (~0.5 g)
219 were wrapped in 10 µm-thick aluminum foil and stored inside the glass part of an induction furnace
220 under vacuum to degas weakly-bound atmospheric gases. Individual samples were then moved into
221 a Mo-Ta furnace and pyrolyzed at 1800°C for 30 min (Kagoshima et al. 2012). The gas was purified,
222 cryogenically separated, and He contents were determined on a Pfeiffer QMS Prisma™ quadrupole
223 mass spectrometer connected to the vacuum purification line. The 3He/4He ratios were measured
224 using the HELIX-SFT mass spectrometer (R-OC and R-Aln) and the VG5400 mass spectrometer
225 (R-MR15, R-MSB3, R-MB15, R-MRg1, R-MRg9, R-MSH1, R-MY3) following the procedures
226 described above.

227 The rock samples collected in the Vaudreuil-Soulanges and the Monteregian Hills areas
228 were analyzed by Bureau Veritas® laboratories in Vancouver for trace elements, including U and
229 Th, reported in Table 2. Samples were finely crushed and mixed with LiBO₂/Li₂B₄O₇ flux.
230 Crucibles were fused in a furnace. The cooled bead was dissolved in ACS grade nitric acid and
231
trace elements were analyzed using two different instruments: Elan 9000 ICP-MS and Spectro
232
233 Ciros Vision ICP-ES. Loss on ignition (LOI) was determined by igniting a sample split then
234 measuring the weight loss. The detection limits for U and Th by ICP-MS are 0.1 and 0.2 ppm
235 respectively.
236

237 4. Results

The atmosphere-normalized ³He/⁴He ratios (R/Ra) of the groundwater samples range from 238 0.15±0.01 to 3.32±0.08, pointing to the presence of both ³He-enriched and ⁴He-enriched sources 239 (Table 1). Dissolved ⁴He concentrations vary over three orders of magnitude. The lowest helium 240 concentration is 6.64 x 10⁻⁸ cm³STP/gH₂O (VS104), which is slightly higher than the value expected 241 from solubility equilibrium with the atmosphere (Air Saturated Water or ASW) at 9.8°C, of 4.7 x 242 10⁻⁸ cm³STP/gH₂O, which represents helium contained in freshwater at recharge. Two samples show 243 very high ⁴He concentrations: Mount St. Hilaire (WMSH3), with ⁴He of 0.0786 cm³STP/gH₂O, and 244 Mount Brome (WMB8), with ⁴He of 1.27 x 10⁻⁴ cm³STP/gH₂O. These high ⁴He concentrations are 245 also accompanied by high ²⁰Ne concentrations. In the case of WMSH3, the high ⁴He/²⁰Ne of 49.80 246 excludes the presence of excess air and the contemporary He-Ne enrichment could be related to 247 bubbling and a bi-phase (gas-water) mixture not sampled at the solubility equilibrium, as observed 248 in a few wells in the St. Lawrence Lowlands (Vautour et al., 2015). The ⁴He concentrations have 249 been corrected assuming a ²⁰Ne content at solubility equilibrium of 9.3 x 10⁻⁷ and 1.65 x 10⁻⁸ 250 cm³STP/gH₂O for WSMH3 and WMB8, respectively. Sample WMSH3 also shows the lowest and 251 most radiogenic ³He/⁴He ratio, of 0.15±0.01, which is caused by the high U and Th contents in this 252
Monteregian intrusion. Mount St. Hilaire is indeed known for its high natural radioactivity, which produces high $^{222}$Rn concentrations in groundwater (234 Bq/L; Pinti et al., 2014).

The $^{3}$He/$^{4}$He ratios measured in the bulk rocks range from 0.06±0.02 (R-Aln) to 1.36±0.11Ra (R-Mrg9). Higher values were obtained in the clinopyroxene separated from sample R-MSB3, with R/Ra = 1.23±0.10 and 4.96±0.01. The total amount of $^{4}$He measured ranges from 7.90 x 10⁻⁷ cm³STP/g$_{rock}$ to 2.28 x 10⁻⁵ cm³STP/g$_{rock}$ (Table 2). The measured $^{3}$He/$^{4}$He ratios in mineral separates is similar to, or higher than, those measured by Sasada et al. (1997) in Oka intrusion diopside, of 0.25±0.07 and 2.55±0.25Ra (Table 2).

## 5. Discussion

### 5.1 Helium sources in Vaudreuil-Soulanges and Monteregian Hills groundwater

Helium dissolved in groundwater can have multiple sources. Each helium component must be correctly identified in order to determine the mantle-derived one, if any. In groundwater, the helium components are:

$$ ^{3}\text{He}_{\text{tot}} = ^{3}\text{He}_{\text{eq}} + ^{3}\text{He}_{\text{tri}} + ^{3}\text{He}_{\text{ea}} + ^{3}\text{He}_{\text{rad}} + ^{3}\text{He}_{\text{mtl}} \quad (1) \text{ and} $$

$$ ^{4}\text{He}_{\text{tot}} = ^{4}\text{He}_{\text{eq}} + ^{4}\text{He}_{\text{ea}} + ^{4}\text{He}_{\text{rad}} + ^{4}\text{He}_{\text{mtl}} \quad (2), $$

where the suffix “eq” indicates atmospheric helium dissolved in groundwater during recharge at solubility equilibrium (ASW); “ea” is the excess-air helium resulting from air bubbles entering the water table at recharge and dissolving into groundwater (Heaton and Vogel, 1981); “tri” is the $^{3}$He
produced by the decay of atmospheric tritium dissolved in water, commonly referred to as tritiogenic helium; “mtl” is the mantle helium; and “rad” is the radiogenic helium produced in the aquifer. The latter can produce ³He from neutron reactions on lithium (⁶Li(n,α)³H(β)³He) and ⁴He are the α-particles supplied through the decay of ²³⁵,²³⁸U and ²³²Th and their decay products (Andrews and Kay, 1982). Radiogenic and mantle helium are commonly grouped using the term “terrigenic”.

There is another potential source of ³He, which is often ignored: cosmogenic ³He produced by spallation reactions between cosmic ray neutrons and the major elements of the rock. Cosmogenic ³He is produced in rocks in the uppermost 1 m of Earth's surface and could potentially be transferred to groundwater. However, calculations from Marty et al. (1993) of potential cosmogenic ³He in groundwater of the Paris Basin have shown that cosmogenic production in rocks is very limited as a potential source, and cannot account for significant ³He anomalies in groundwater. Sediments from Vaudreuil-Soulanges watershed outcropped in Ordovician time (430 Ma ago), but were then rapidly buried for several hundred million years, so that any cosmogenic ³He produced is expected to have been completely diluted by continuous radiogenic ⁴He production, and thus not detectable in our samples.

A convenient graphical approach to separate the different helium components is to plot the ³He/⁴He ratio corrected for the excess-air component ([(³He<sub>tot</sub>⁻³He<sub>ea</sub>)]/[⁴He<sub>tot</sub>⁻⁴He<sub>ea</sub>]) against the ratio of ASW helium (He<sub>eq</sub>) to total helium, corrected for the excess-air component (⁴He<sub>eq</sub>/⁴He<sub>tot</sub>⁻⁴He<sub>ea</sub>) (Weise and Moser, 1987). In our samples, the excess air component is calculated as the difference between the expected ASW value of ²⁰Ne at the well temperature and the measured one.
In this diagram, which is commonly referred to as a “Weise plot” (Fig. 4a), helium of 293 purely atmospheric origin (ASW composition) plots on the coordinates ([³He<sub>tot</sub>⁻³He<sub>ea</sub>]/[⁴He<sub>tot</sub>⁻⁴He<sub>ea</sub>]) 294 or R<sub>eq</sub> = 1.36 x 10⁻⁶ (0.983Ra; Benson and Krause, 1980) and (⁴He<sub>eq</sub>/⁴He<sub>tot</sub>⁻⁴He<sub>ea</sub>) = 1 respectively 295 (Fig. 4a). If only tritiogenic ³He<sub>tri</sub> is added to an initial atmospheric composition, then the sample 296 composition will plot parallel to the right Y-axis, starting from the initial ASW composition (Fig. 297 4a). If radiogenic ³He, produced through neutron reactions with ⁶Li, and ⁴He, produced through the 298 α-decay of ²³⁵,²³⁸U and ²³²Th, are added, the initial ASW composition will shift closer to the bottom- 299 left corner of the plot (R<sub>rad</sub>; Fig. 4a). In the St. Lawrence Lowlands fractured bedrock, the crustal 300 R<sub>rad</sub> is calculated to be 1.66 x 10⁻⁶ (0.012Ra), based on Li, U, and Th contents measured from 301 rocks of the region (Pinti et al., 2011). Finally, if mantle-derived helium is present, sample 302 composition will plot parallel to the Y-axis, starting from a radiogenic crustal composition (R<sub>terr</sub>; 303 Fig. 4a). 304

On the Weise plot, mixing between helium components appears as straight lines governed 305 by the equation (Weise and Moser, 1987; Fig. 4a): 306

$$ \underbrace{\frac{(^3He_{tot} - ^3He_{ea})}{(^4He_{tot} - ^4He_{ea})}}_{Y} = \underbrace{\left( R_{eq} - R_{terr} + \frac{^3He_{tri}}{^4He_{eq}} \right)}_{m} \cdot \underbrace{\frac{^4He_{eq}}{^4He_{tot} - ^4He_{ea}}}_{\bar{X}} + \underbrace{R_{terr}}_{b} \quad (3), \quad 307$$

where R<sub>terr</sub> is the ³He/⁴He of the terrigenic source, which corresponds to either crustal helium, 308 mantle-derived helium, or both. 309

Groundwater samples from Vaudreuil-Soulanges and the Monteregian Hills align along 310 mixing lines representing the complete decay of 6.7TU, 14.5TU, 18.5TU, 40TU, 61TU, and 311 285TU, and a terrigenic end-member, R<sub>terr</sub>, with values between 0.16Ra and 1.48Ra (³He/⁴He values 312
between 2.2 x 10⁻⁷ and 2.05 x 10⁻⁶; Fig. 4b). These ratios are higher than those expected from the local production of ³He from ⁶Li and of ⁴He from ²³⁸,²³⁵U and ²³²Th in St. Lawrence Lowlands sedimentary rocks or in rocks from the Grenville basement, of 0.45 to 1.66 x 10⁻⁸ (Pinti et al., 2011). The difference between the expected crustal Rₚₚ and the maximum calculated Rₜₜ from the Weise plot (i.e., the intercept of the mixing line between samples WSB1, WSB2, and WY6; Fig. 4b) suggests the addition of 18.4±6.7% of mantle-derived helium (labelled “fₘₜ” in Fig. 4b). The total uncertainty on the value comes mainly from the error on the intercept of the regression. Uncertainty derived from the variability of the mantle end-member helium isotopic ratio (8±1) and the possible variability on the crustal ratio (0.01 to 0.1Ra; Tolstikhin et al., 1996) produces less than 2% error. It is worth noting that here a ³He/⁴He mantle end-member ratio of 8±1 is assumed for sake of consistency with previous estimates from Pinti et al. (2011) and Saby et al. (2016). If the closer mixing line between samples VS-105-2, VS-108-2, and VS-108-2 and the sample from Mount Rougemont (WR-5) is taken, the intercept will give a Rₜₜ of 1.86 x 10⁻⁶ (or 1.35Ra; Fig. 4b). This is equivalent to a mantle fraction of 16.7±6.9%. This value is, within uncertainty, similar to the previous estimate.

These two fₘₜ values represent the highest mantle helium fractions ever reported for St. Lawrence Lowlands groundwater, higher than the 2% and 8% calculated by Pinti et al. (2011) and Saby et al. (2016), respectively. However, the calculated mantle He fractions could be conservative minimal estimates. The complete decay of ¹TU in a gram of water corresponds to 2.49 x 10⁻¹⁵ cm³STP/gₚₚ of ³He. Assuming an initial ⁴Heₐ₈w of 4.72 x 10⁻⁸ cm³STP/gₚₚ, corresponding to the dissolution of atmospheric helium at 9.8°C, which is the average temperature of the groundwater in the region (Vautour et al., 2015), the corresponding ³Heₐ₈w would be 6.42 x 10⁻¹⁴ cm³STP/gₚₚ.
Adding 285TU of tritium would produce a tritiogenic ³He/⁴He ratio of 11.86Ra. This value is not unreasonable for aquifers that contain a mixture of modern and older water, but is actually more than double the highest tritiogenic ³He/⁴He ratio measured among the 125 groundwater samples from Bécancour and Nicolet-St. François watersheds (internal compilation of unpublished data), and equal to 4.59±0.03Ra (equivalent to the decay of 94.5TU). If the maximum amount of tritium preserved in groundwater of the St. Lawrence Lowlands aquifers is 94.5TU, then the mixing line passing through, e.g., sample WSB1 (mixing line reported as an example in Fig. 4a), will have an intercept (R<sub>terr</sub>) at ³He/⁴He = 4.28 x 10<sup>-6</sup>, which is equivalent to the addition of 38.6% of mantle helium. For the purpose of this study and the following modeling, the average value of the two f<sub>mtl</sub> estimates above (18.4±6.7% and 16.7±6.9%), i.e. 17.7±9.6% (equivalent to a ³He/⁴He ratio of 1.42±0.14Ra), is assumed here (orange star in Fig. 4b).

346

347

## 5.2 Origin of terrigenic helium in Vaudreuil-Soulanges and Monteregian Hills groundwater

The ³He in excess of crustal production (Fig. 4b) can be explained by the presence of a mantle helium component in groundwater of the Vaudreuil-Soulanges watershed and in the Monteregian Hills local aquifers. The preservation of this mantle component in groundwater could be explained by 1) mantle He concentrated at the base of the crust and migrating through crustal-rooted faults; 2) a fossil mantle source preserved in very old brines having ages contemporary with the emplacement of the Monteregian Hills intrusions; or 3) a fossil mantle signal preserved in Monteregian Hills magmatic intrusions, but released in recent groundwater by water-rock processes, such as mineral dissolution or diffusive exchange.

348

349

350

351

352

353

354

355
The first hypothesis can be discarded. For example, Lee et al. (2019) have shown that mantle He can reach shallow groundwater through active strike-slip faults. Fluids are thought to enhance pore fluid pressures in association with fault weakening caused by seismicity. This will facilitate the upward migration of deep-seated helium. Although Quebec is the second more seismic area in Canada, seismicity seems to be caused by residual friction provoked along fault planes by the crustal isostatic rebound following the last deglaciation (Lamontagne, 2002). Fission-track and U-Th/He thermochronology of the Cambrian St-Lawrence Lowlands Rift Zone showed that fault activity in the region continued until the Mesozoic and terminated in the Cretaceous (Tremblay et al., 2013). Consequently, it is difficult to assume that present-day micro-seismicity has weakened these old tectonic structures to facilitate the upward migration of mantle helium in the region.

If the second hypothesis is valid, then old groundwater of at least the age of the Monteregian Hills intrusions (124±1 Ma) should have been preserved in the study area. Pinti et al. (2011) found brines with possible Mesozoic ages in the Cambro-Ordovician aquifers of the Potsdam Group of the St. Lawrence Lowlands supergroup in the Bécancour area (Fig. 1). However, these brines have little mantle-derived ³He (2% of the total), are highly saline Ca-Na-Cl type (up to 315,000 mg/L) waters, and are very radiogenic (⁸⁷Sr/⁸⁶Sr of 0.71214), characteristic of Precambrian basement brines. Groundwater from Vaudreuil-Soulanges and from Monteregian Hills aquifers are of Ca-HCO₃ or Na-HCO₃ types, with very low salinity (168-710 mg/L), mainly derived from exchange with glacio-marine porewater from the Champlain Sea clays (Larocque et al., 2015; Beaudry et al., 2018). The presence of tritium in several Vaudreuil-Soulanges groundwater samples suggests modern water, possibly mixed with Holocene to Quaternary glacial water containing predominantly radiogenic ⁴He, as suggested by Méjean (2016). Similar ages, from modern to 13
ka old, have been proposed for Monteregian Hills groundwater (Beaudry et al., 2018). Therefore, the occurrence of Mesozoic brines in the Vaudreuil-Soulanges and Monteregian Hills aquifers appears to be unlikely.

The third alternative and most plausible explanation, which will be explored in detail in the following sections, is the presence of a fossil mantle source, diluted over time through the addition of local radiogenic ⁴He (the so-called “magma aging model”; Torgersen and Jenkins, 1982), preserved in magmatic intrusions, and recently released into groundwater through leaching.

## 5.3 Impact of in situ helium production on the “magma aging” model

The ³He production in the crust is controlled by the thermal neutron capture of ⁶Li in the reaction ⁶Li(n,α)³H(β⁻)³He (Andrews and Kay, 1982), while ⁴He production is regulated by the α-decay of ²³⁸U, ²³⁵U, and ²³²Th. In this study, the nucleogenic production rate of ³He was calculated based on the Li abundances reported by Pinti et al. (2011) and Gold (1966) for the main Monteregian Hills lithologies. These vary from 11 ppm for a pyroxenite to 28 ppm for a gabbro (representing the lithologies of samples R-MSB3, R-MB15, R-MRg1, R-MRg9, R-MSH10, and R-MY3), 37 ppm in diorite (sample R-MR-15), and 15 ppm in carbonatite and in alnoïte. U and Th concentrations have been measured in bulk rocks and range from 3.3 to 12.4 ppm and from 9.4 to 40.9 ppm respectively (Table 2; data from this study and Roulleau and Stevenson, 2013). Li, U, and Th concentrations were not measurable in mineral separates.

The ³He production rate for Monteregian Hills bulk rocks was calculated using the modified equation of Castro (2004):
$$J_{3\text{He}} = (2.69 \times 10^{-4}[\text{U}] + 6.40 \times 10^{-5}[\text{Th}]) \times [\text{Li}] \times 10^{-23} \times 22414 \quad (4),$$

where $J^{3}\text{He}$ is the $^{3}\text{He}$ production rate in cm$^{3}$STP/g$_{rock}$/yr; [U], [Th] and [Li] are the U, Th and Li concentrations in ppm. The $^{3}\text{He}$ production rate varies from 0.63 x 10$^{-20}$ cm$^{3}$STP/g$_{rock}$/yr in the Oka carbonatite to 2.89 x 10$^{-20}$ cm$^{3}$STP/g$_{rock}$/yr in Mont Brome gabbro (Table 2).

The $^{4}\text{He}$ production rate ($J_{4\text{He}}$ in cm$^{3}$STP/g$_{rock}$/yr) is calculated following the equation (Torgersen et al., 1995):

$$J_{4\text{He}} = (0.2355 \times 10^{-12}) \times [\text{U}] \times \left(1 + 0.123 \left(\frac{[\text{Th}]}{[\text{U}]} - 4\right)\right) \quad (5),$$

where [U] and [Th] are the concentrations of U and Th in the rock in ppm (Table 2). The $^{4}\text{He}$ production rate varies from 0.63 x 10$^{-13}$ cm$^{3}$STP/g$_{rock}$/yr in the Oka carbonatite and Mount Royal gabbro to 2.07 x 10$^{-12}$ cm$^{3}$STP/grock/yr in Mount Brome gabbro (Table 2). The resulting $^{3}\text{He}/^{4}\text{He}$ derived by the *in situ* production of helium isotopes in rocks ranges from 0.005 to 0.0013 (Table 2), well below the values measured in fluids, confirming that the groundwater contains a fraction of terrigenic helium derived from a mantle reservoir (Table 1).

## 5.4. Monte Carlo simulation of a helium magma aging model

To test whether the high terrigenic $^{3}\text{He}/^{4}\text{He}$ ratios measured in groundwater (Table 1 and Fig. 4b) could reflect the presence of a fossil mantle source, diluted over time by the addition of *in situ*-produced radiogenic $^{4}\text{He}$, a “magma aging” model is developed here (Torgersen and Jenkins, 1982). This model describes the evolution of an initial magmatic $^{3}\text{He}/^{4}\text{He}$ ratio in a magma body since its crystallization, by the addition of radiogenic $^{4}\text{He}$ produced *in situ* by $^{235}\text{U}$, $^{238}\text{U}$, and $^{232}\text{Th}$ α-decay (Torgersen and Jenkins, 1982). Consequently, the U and Th amounts in the magma source
419 determine the rate of change of the helium concentration of the rock, while the initial concentration of helium in an emplaced magma determines the change in the isotope ratio of helium (R<sub>mtl</sub>/Ra). 420
421 The model does not take the nucleogenic production of <sup>3</sup>He from Li into account, which is considered here to be negligible. 422

423 The initial <sup>3</sup>He/<sup>4</sup>He ratio in the magma source (R<sub>mtl</sub>/Ra) can be calculated following the equation: 424

$$ \left(\frac{R_{\text{mtl}}}{\text{Ra}}\right)_{\text{initial}} = \frac{\left[\left(\frac{R_{\text{mtl}}}{\text{Ra}}\right)_{\text{final}} \times ({}^{4}\text{He}_{\text{initial}} + J_{4\text{He}} \times t)\right]}{{}^{4}\text{He}_{\text{initial}}} \quad (6), \quad 425 $$

426 where (R<sub>mtl</sub>/Ra)<sub>final</sub> is the <sup>3</sup>He/<sup>4</sup>He ratio measured in modern groundwater and normalized to the air value; <sup>4</sup>He<sub>initial</sub> is the amount of <sup>4</sup>He in the magma source at t=0; J<sub>He</sub> is the radiogenic <sup>4</sup>He production rate as calculated from eqn. (5); and t is the time since the magma emplacement. 427 428 429

430 From the results plotted on the Weise diagram, an average (R<sub>mtl</sub>/Ra)<sub>final</sub> value of 1.42±0.14 Ra is assumed, equivalent to a mantle fraction f<sub>mtl</sub> of 17.7±9.6% (orange star in Fig. 4b). Time, t, is assumed to be equal to the average age of alnoïte in Vaudreuil-Soulanges (110.5±3.5Ma), which ranges from 108±1Ma and 113±1Ma (Eby, 1985; Chen and Simonetti, 2013) and of the 431 432 433 434 Monteregian Hills magmatic bodies (124±1 Ma; Foland et al., 1986). The J<sub>4He</sub> for the alnoïte is 8.41 435 x 10<sup>-13</sup> cm<sup>3</sup>STP/g<sub>rock</sub>/yr. An uncertainty of 20% is assumed, based on the variability of trace 436 elements measured in alnoïte from Île Cadieux by Harnois and Mineau (1991). The J<sub>4He</sub> calculated 437 for the Monteregian Hills is highly variable, depending on the U and Th contents in different 438 lithologies, from gabbro to syenite (Table 2). Here, an average J<sub>4He</sub> of 11.78±5.01 x 10<sup>-13</sup> 439 cm<sup>3</sup>STP/g<sub>rock</sub>/yr is assumed. 440
A critical parameter to determine is the ⁴He initial. In the magma aging model to explain mantle
helium anomalies in groundwater of the Eastern America margin, Torgersen et al. (1995) suggested
the value of 6.7 x 10⁻⁶ cm³STP/g rock. This value would best represent a gas-rich magma that has
retained significant volatiles. The highest ⁴He concentration in OIBs glasses (database from
Graham et al., 2002; n=107) is of 6.24 x 10⁻⁶ cm³STP/g rock, very close to the theoretical value of
Torgersen et al. (1995). Torgersen et al. (1995) considered this concentration to be a good proxy
for a hot spot source. Indeed plume-derived melts should represent the non-degassed lower mantle,
which is expected to have preserved more volatiles than the upper degassed mantle, in the canonical
view of an Earth' mantle layered reservoir based on noble gas systematics (e.g., Allègre et al.,
1995). However, OIBs are constantly depleted in helium compared to the MORBs (e.g., Moreira
and Kurz, 2013). This contradiction is known as the “mantle helium paradox” (e.g., Hopp and
Trieloff, 2008). There are several models to explain this paradox (e.g., Hopp and Trieloff, 2008;
Gonnermann and Mukhopadhyay, 2007) but basically it is assumed that plume melts undergone a
non-equilibrium degassing elemental fractionation during their ascent to the surface (Gonnermann
and Mukhopadhyay, 2007). This model is based on the effect of CO₂ and H₂O contents in the melts
which can drastically reduce the solubility of noble gases. OIB melts would be enriched in CO₂
compared to MORB ones, decreasing substantially the solubility of He which will tend to be largely
degassed. Gonnermann and Mukhopadhyay (2007) constrained the He content in the pre-
degassed melt to be 3 x 10⁻⁴ cm³/g rock, suggesting that more than 99 % of helium is lost during non-
equilibrium magmatic degassing. This ⁴He content is 2 orders of magnitude higher than that
expected for the MORB magma source (3±2 x 10⁻⁶ cm³STP/g rock; Moreira and Kurz, 2013).
All parameters of Eqn. (6) with assumed or measured uncertainties are summarized in Table 3. An alternative way to calculate $ ^{4}\text{He}_{\text{initial}} $ is from rock analyses (Table 2). However, this is limited by the diffusion and loss of He from the rocks and the distribution of U and Th in the rock matrix and its effect on the dilution of the mantle signal, which is likely preserved in U and Th-poor minerals, such as in clinopyroxene.

The initial $ ^{3}\text{He}/^{4}\text{He} $ ratio expected in the fossil gas-rich magma source in the region ($R_{\text{mtl}}/R_{\text{initial}}$; Eqn. (6)) has been evaluated, together with its total uncertainty, using a Monte Carlo simulation. Monte Carlo simulations make use of the uncertainty domains for all given variables (Table 3) in a specific model, here the magma aging model explicated in Eqn. (6). By running the algorithm multiple times –a hundred thousand times here – with random sampling of those variables, individual output data can be generated. The probability distribution of the outputs is usually reported as a Gaussian curve, and the average and standard deviation thereof can be extrapolated.

Monte Carlo simulations were carried out using the NIST Uncertainty Machine (https://uncertainty.nist.gov/), which is a web-based software application to evaluate the measurement uncertainty associated with an output quantity defined by a measurement model of the form $y = f(x1,...,xn)$ (Lafarge and Possolo, 2015). Input parameters (Table 3) are introduced online, the number of iterations is selected, and the measurement model (Eqn. 6) is compiled in an “R” script.

Two simulations were carried out, representing two extreme cases (Table 3). The first one considers the progressive aging of a magma source equal to the alnöite, which emplaced between 107 and 114 Ma ago. The second one considers the progressive aging of an average magma source
of gabbro-sienitic composition, equal to the intrusions of the Monteregian Hills and emplaced 483
124±1Ma ago. A ⁴Heₐₗₐₐ₄ content of 6.7±(10%) x 10⁻⁶ cm³STP/gₐₗₐ₄ is assumed, equivalent to the 484
least degassed OIBs. We could consider an even more degassed OIBs source, however, for ⁴Heₐₗₐ₄ 485
contents ≤ 10⁻⁷ cm³STP/gₐₗₐ₄, the initial helium isotopic ratios would be rapidly diluted by *in situ* 486
radiogenic ⁴He production on a timescale of 10Ma or even less, completely masking any mantle 487
helium record in present-day volcanics. 488

The outputs of the Monte Carlo simulations have been represented in two different ways. 489
The first, reported in Fig. 5 is as the resulting Gaussian curves for alnöite and for the Monteregian 490
Hills intrusions (MH) simulations. The first simulation for the alnöite resulted in a R<sub>mtl</sub>/Ra<sub>initial</sub> of 491
21±10 (2σ), while that for the gabbro-sienitic MH resulted in a R<sub>mtl</sub>/Ra<sub>initial</sub> of 33±28 (2σ). The 492
larger uncertainty for the second simulation is due to the larger error assumed on the J<sub>4He</sub> (Eqn. 5 493
and Table 2) parameter. The output probability distributions for the two simulations have been 494
compared with the frequency histogram of the measured ³He/⁴He ratios in 401 samples of OIB 495
glasses (Fig. 5). The latter data was extracted from the global dataset of noble gas abundances and 496
isotopic ratios in Volcanic Systems – USGS-NoGaDat, compiled by Abedini et al. (2006). Present- 497
day plume-sourced helium in OIB glass phase has an average R/Ra of 15.8±8.6 (1σ) (Fig. 5). 498

Results of the Monte Carlo simulations have also been reported in a clearer way (Fig. 6), 499
where the progressive “aging” of the initial helium isotopic signature of the emplaced magma to 500
the present-day measured value is reported as a curve, together with its uncertainty envelope 501
calculated from the Monte Carlo simulations. For the two simulations, up to 96% of the results 502
show (R<sub>mtl</sub>/Ra)<sub>initial</sub> higher than that expected for a lithospheric magma source, either the convective 503
upper mantle (R=8±1Ra; Allègre et al., 1995) or the SCLM (R/Ra=6.1±0.9; Gautheron and 504
Moreira, 2002) (Fig. 6a). In this simulation, the mantle ³He/⁴He ratio is assumed to have been constant in the last 120Ma, which is what is expected from numerical simulations of the helium isotopic evolution in mantle reservoirs (e.g., Porcelli and Elliot, 2008).

These two simulations suggest that a mantle plume was possibly the source of the fossil helium presently found in the groundwater of the southern Quebec, which may be related to the New England hotspot.

If the previous Monte Carlo simulations were carried out using an undegassed OIB source ⁴Heinicte content of 3.14±(10%) x 10⁻⁴ cm³STP/grock (Gonnermann and Mukhopadhyay, 2007), then the resulting (Rmtl/Ra)initial will be very low from 1.8±0.2Ra (alnoïte) to 2.4±0.4Ra (Monteregian Hills). These values are even lower than those expected for a MORB or the SCLM-type magma source. Further, it would be very difficult to explain how the plume-generated melts beneath the Monteregian Hills could have preserved the pristine helium content when globally, OIBs and continental basalts show constantly different degrees of degassing (e.g., Moreira and Kurz, 2013; Kurz et al., 2009). It is worth noting that these two last simulations cannot represent a local MORB-type or SCLM source, because expected ⁴He content in MORBs are 2 orders of magnitude lower than those used for these simulations (Moreira and Kurz, 2013).

521

## 6. Conclusions

This study highlights the possibility of long-term lithospheric storage of primordial noble gases from past magmatic events. The occurrence of mantle-derived helium in the eastern North America craton, first suggested by Torgersen et al. (1995) and then argued by this study, should be further investigated. Analyses of groundwater around intrusions related to the New England
527 hotspot, such as the White Mountains, should be carried out to confirm the preservation of this
528 mantle memory. Such analyses would also better constrain the hydrogeology of fractured igneous
529 intrusions, which is still poorly documented and understood. The processes leading to the storage
530 of this mantle memory and to its release into groundwater remain to be identified. Recently, Méjean
531 et al. (2017) showed that the release of helium into groundwater could be episodic and related to
532 tectonic events that increase the local fracturing of the rocks. During these events, the release of
533 mantle helium preserved in high-temperature magmatic mineral phases, such as olivine or
534 clinopyroxene, could be substantial. It is then diluted over time by the progressive recharge of the
535 aquifer by freshwater (containing atmospheric helium) and the local accumulation of radiogenic
536 helium. This study suggests that groundwater could preserve a record of mantle helium on a
537 timescale of hundreds of millions of years, and may even provide a more faithful record of the
538 primary signature compared to the magmatic rocks from which the He is ultimately derived.

539

540 Acknowledgments

541 The authors wish to thank Ray Burgess (Un. Manchester), an anonymous reviewer and handling
542 editor F. Moynier for their careful reviews which greatly improved this manuscript. Thoughtful
543 discussions and suggestions with R. Stevenson (GEOTOP) and members of ERC ARCHEIS
544 (Toulouse) were appreciated. D. Graham (OSU) is thanked for giving access to his large noble gas
545 database on MORBs and OIBs. The authors thank the Quebec Ministry of Environment, the
546 Quebec Research Fund ("Fonds de recherche du Quebec - Nature et Technologies"), the Bécancour
547 River Watershed Organization ("organisme de bassin versant COBAVER-VS "), and the
548 municipalities that contributed funding to this research (MRC of Vaudreuil-Soulanges). PM and
549 LD fellowships were funded by a NSERC Discovery Grant (RGPIN-2015-05378) and a FRQNT

550 project (2010-HO-136990) to DLP.
551 REFERENCES

552 Abedini, A.A., Hurwitz, S., Evans, W.C., 2006. USGS-NoGaDat - A global dataset of noble gas
553 concentrations and their isotopic ratios in volcanic systems, Data Series, Reston, VA.

554 Allègre, C.J., Moreira, M., Staudacher, T., 1995. ⁴He/³He dispersion and mantle convection.
555 Geophys. Res. Lett. 22, 2325-2328.

556 Andrews, J.N., Kay, R.L.F., 1982. Natural production of tritium in permeable rocks. Nature 298,
557 361-363.

558 Beaudry, C., Lefebvre, R., Rivard, C., Cloutier, V., 2018. Conceptual model of regional
559 groundwater flow based on hydrogeochemistry (Montérégie Est, Québec, Canada). Canadian
560 Water Resour. J. 43, 152-172.

561 Benson, B.B., Krause Jr., D., 1980. Isotopic fractionation of helium during solution: A probe for
562 the liquid state. J. Sol. Chem. 9, 895-909.

563 Castro, M.C., 2004. Helium sources in passive margin aquifers—new evidence for a significant
564 mantle ³He source in aquifers with unexpectedly low in situ ³He/⁴He production. Earth Planet.
565 Sci. Lett. 222, 897-913.

566 Castro, M.C., Ma, L., Hall, C.M., 2009. A primordial, solar He–Ne signature in crustal fluids of a
567 stable continental region. Earth Planet. Sci. Lett. 279, 174-184.
568 Chen, W., Simonetti, A., 2013. In-situ determination of major and trace elements in calcite and
569 apatite, and U-Pb ages of apatite from the Oka carbonatite complex: Insights into a complex
570 crystallization history. Chem. Geol. 353, 151-172.

571 Chen, W., Simonetti, A., 2015. Isotopic (Pb, Sr, Nd, C, O) evidence for plume-related sampling of
572 an ancient, depleted mantle reservoir. Lithos 216-217, 81-92.

573 Crough, S., 1981. Mesozoic hotspot epeirogeny in eastern North America. Geology 9, 2-6.

574 Dixon, J.E., Clague, D.A., 2001. Volatiles in basaltic glasses from Loihi Seamount, Hawaii:
575 evidence for a relatively dry plume component. J. Petrol. 42, 627-654.

576 Eby, G.N., 1985. Age relations, chemistry, and petrogenesis of mafic alkaline dikes from the
577 Monteregian Hills and younger White Mountain igneous provinces. Can. J. Earth Sci. 22, 1103-
578 1111.

579 Eby, G.N., 1987. The Monteregian Hills and White Mountain alkaline igneous provinces, eastern
580 North America. Geol. Soc. London, Spec. Publ. 30, 433-446.

581 Faure, S., Tremblay, A., Angelier, J., 1996. State of intraplate stress and tectonism of northeastern
582 America since Cretaceous times, with particular emphasis on the New England-Quebec igneous
583 province. Tectonophysics 255, 111-134.

584 Foland, K., Gilbert, L.A., Sebring, C.A., Jiang-Feng, C., 1986. ⁴⁰Ar/³⁹Ar ages for plutons of the
585 Monteregian Hills, Quebec: Evidence for a single episode of Cretaceous magmatism. Geol. Soc.
586 Am. Bull. 97, 966-974.
587 Gautheron, C., Moreira, M., 2002. Helium signature of the subcontinental lithospheric mantle.

588 Earth Planet. Sci. Lett. 199, 39-47.

589 Gold, D.P., 1966. The minerals of the Oka carbonatite and alkaline complex, Oka, Quebec.

590 Mineral. Soc. of India IMA Volume, 109-125.

591 Gonnermann, H.M., Mukhopadhyay, S., 2007. Non-equilibrium degassing and a primordial source

592 for helium in ocean-island volcanism. Nature 449, 1037-1040.

593 Graham, D.W., 2002. Noble gas isotope geochemistry of mid-ocean ridge and ocean island basalts:

594 Characterization of mantle source reservoirs. Rev. Min. Geochem. 47, 247-317.

595 Harnois, L., Mineau, R., Morency, M., 1990. Rare-earth element geochemistry of alnoitic

596 Cretaceous rocks and ultramafic xenoliths from Île Bizard (Québec, Canada). Chem. Geol. 85,

597 135-145.

598 Harnois, L., Mineau, R., 1991. Geochemistry of the Île Cadieux monticellite alnöite, Quebec,

599 Canada. Can. J. Earth Sci. 28, 1050-1057.

600 Harðardóttir, S., Halldórsson, S.A., Hilton, D.R., 2018. Spatial distribution of helium isotopes in

601 Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and

602 evolution of the Icelandic mantle plume. Chem. Geol. 480, 12-27.

603 Heaton, T., Vogel, J., 1981. "Excess air" in groundwater. J. Hydrol. 50, 201-216.
604 Hopp, J., Trieloff, M., 2008. Helium deficit in high-³He/⁴He parent magmas: Predegassing
605 fractionation, not a “helium paradox”. Geochem. Geophys. Geosyst. 9, Q03009,
606 doi:10.1029/2007GC001833.

607 Kagoshima, T., Takahata, N., Jung, J., Amakawa, H., Kumagai, H., Sano, Y., 2012. Estimation of
608 sulfur, fluorine, chlorine and bromine fluxes at Mid Ocean Ridges using a new experimental
609 crushing and extraction method. Geochem. J. 46, e21-e26.

610 Kennedy, B.M., van Soest, M.C., 2007. Flow of mantle fluids through the ductile lower crust:
611 Helium isotope trends. Science 318, 1433-1436.

612 Kurz, M.D., Jenkins, W.J., Hart, S.R., 1982. Helium isotopic systematics of oceanic islands and
613 mantle heterogeneity. Nature 297, 43-47.

614 Kurz, M.D., Curtice, J., Fornari, D., Geist, D., Moreira, M., 2009. Primitive neon from the center
615 of the Galápagos hotspot. Earth Planet. Sci. Lett. 286, 23-34.

616 Lafarge, T., Possolo, A., 2015. The NIST Uncertainty Machine. NCSLI Measure J Meas. Sci. 10,
617 20-27.

618 Lamontagne, M., 2002. An overview of some significant eastern Canadian earthquakes and their
619 impacts on the geological environment, buildings, and the public. Nat. Hazards 26, 55-67.

620 Larocque, M., Meyzonnat, G., Barbecot, F., Pinti, D., Gagné, S., Barnetche, D., Ouellet, M.,
621 Graveline, M., 2015. Projet de connaissance des eaux souterraines de la zone de Vaudreuil-
622 Soulanges. Rapport final présenté au Ministère du Développement durable, de l'Environnement,
623 de la Faune et des Parcs (203pp.) (in French).
624 Lee, H., Kim, H., Kagoshima, T., Park, J.-O., Takahata, N., Sano, Y., 2019. Mantle degassing along strike-slip faults in the Southeastern Korean Peninsula. Sci. Rep. 9, 15334.

626 Malka, E., Stevenson, R.K., David, J., 2000. Sm-Nd geochemistry and U-Pb geochronology of the Mont Rigaud stock, Quebec, Canada: a late magmatic event associated with the formation of the Iapetus rift. J. Geol. 108, 569-583.

629 Marty, B., Torgersen, T., Meynier, V., O'Nions, R.K., Marsily, G., 1993. Helium isotope fluxes and groundwater ages in the Dogger Aquifer, Paris Basin. Water Resour. Res. 29, 1025-1035.

631 Matsuda, J., Matsumoto, T., Sumino, H., Nagao, K., Yamamoto, J., Miura, Y., Kaneoka, I., Takahata, N., Sano, Y., 2002. The ³He/⁴He ratio of the new internal He Standard of Japan (HESJ). Geochem. J. 36, 191-195.

634 McHone, J.G., 1996. Broad-terrane Jurassic flood basalts across northeastern North America. Geology 24, 319-322.

636 Méjean, P., 2016. Étude de la circulation des eaux souterraines des basses Terres du Saint-Laurent: approche par les isotopes de l'hélium et de l'uranium. PhD thesis, Université du Québec à Montréal, Montréal, p. 203 (in French).

639 Méjean, P., Pinti, D.L., Ghaleb, B., Larocque, M., 2017. Fracturing-induced release of radiogenic ⁴He and ²³⁴U into groundwater during the last deglaciation: An alternative source to crustal helium fluxes in periglacial aquifers. Water Resour. Res. 53, 5677-5689.
642 Moreira M.A., Kurz M.D., 2013. Noble gases as tracers of mantle processes and magmatic degassing, in: Burnard P. (Ed.) The noble gases as geochemical tracers. Advances in Isotope Geochemistry. Springer Berlin Heidelberg, pp. 371-391.

645 Pinti, D.L., Béland-Otis, C., Tremblay, A., Castro, M.C., Hall, C.M., Marcil, J.-S., Lavoie, J.-Y., Lapointe, R., 2011. Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes. Geochim. Cosmochim. Acta 75, 4228-4243.

649 Pinti, D.L., Retailleau, S., Barnetche, D., Moreira, F., Moritz, A.M., Larocque, M., Gélinas, Y., Lefebvre, R., Hélie, J.-F., Valadez, A., 2014. ²²Rn activity in groundwater of the St. Lawrence Lowlands, Quebec, eastern Canada: Relation with local geology and health hazard. J. Environm. Radioact. 136, 206-217.

653 Porcelli, D., Elliott, T., 2008. The evolution of He Isotopes in the convecting mantle and the preservation of high ³He/⁴He ratios. Earth Planet. Sci. Lett. 269, 175-185.

655 Roulleau, E., Pinti, D.L., Stevenson, R.K., Takahata, N., Sano, Y., Pitre, F., 2012. N, Ar and Pb isotopic co-variations in magmatic minerals: Discriminating fractionation processes from magmatic sources in Monteregian Hills, Québec, Canada. Chem. Geol. 326-327, 123-131.

658 Roulleau, E., Stevenson, R., 2013. Geochemical and isotopic (Nd-Sr-Hf-Pb) evidence for a lithospheric mantle source in the formation of the alkaline Monteregian Province (Quebec). Can. J. Earth Sci. 50, 650-666.
661 Saby, M., Larocque, M., Pinti, D.L., Barbecot, F., Sano, Y., Castro, M.C., 2016. Linking
662 groundwater quality to residence times and regional geology in the St. Lawrence Lowlands,
663 southern Quebec, Canada. Appl. Geochem. 65, 1-13.

664 Sano, Y., Marty, B., Burnard, P., 2013. Noble gases in the atmosphere, in: Burnard, P. (Ed.), The
665 Noble Gases as Geochemical Tracers. Springer Berlin Heidelberg, pp. 17-31.

666 Sasada, T., Hiyagon, H., Bell, K., Ebihara, M., 1997. Mantle-derived noble gases in carbonatites.
667 Geochim. Cosmochim. Acta 61, 4219-4228.

668 Sleep, N.H., 1990. Monteregian hotspot track: A long-lived mantle plume. J. Geophys. Res. Solid
669 Earth 95, 21983-21990.

670 Stuart, F.M., Lass-Evans, S., Fitton, J.G., Ellam, R.M., 2003. High ³He/⁴He ratios in picritic basalts
671 from Baffin Island and the role of a mixed reservoir in mantle plumes. Nature 424, 57-59.

672 Tolstikhin, I.N., Lehmann, B.E., Loosli, H.H., Gautschi, A., 1996. Helium and argon isotopes in
673 rocks, minerals, and related groundwaters: A case study in northern Switzerland. Geochim.
674 Cosmochim. Acta 60, 1497-1514.

675 Torgersen, T., 1993. Defining the role of magmatism in extensional tectonics: Helium 3 fluxes in
676 extensional basins. J. Geophys. Res.: Solid Earth 98, 16257-16269.

677 Torgersen, T., Jenkins, W., 1982. Helium isotopes in geothermal systems: Iceland, the geysers, raft
678 river and steamboat springs. Geochim. Cosmochim. Acta 46, 739-748.
679 Torgersen, T., Drenkard, S., Stute, M., Schlosser, P., Shapiro, A., 1995. Mantle helium in ground waters of eastern North America: Time and space constraints on sources. Geology 23, 675-678.

681 Tremblay, A., Roden-Tice, M.K., Brandt, J.A., Megan, T.W., 2013. Mesozoic fault reactivation along the St. Lawrence rift system, eastern Canada: Thermochronologic evidence from apatite fission-track dating. GSA Bull. 125, 794-810.

684 Trieloff, M., Kunz, J., Clague, D.A., Harrison, D., Allègre, C.J., 2000. The nature of pristine noble gases in mantle plumes. Science 288, 1036-1038.

686 Vautour, G., Pinti, D.L., Méjean, P., Saby, M., Meyzonnat, G., Larocque, M., Castro, M.C., Hall, C.M., Boucher, C., Roulleau, E., Barbecot, F., Takahata, N., Sano, Y., 2015. ³H/³He, ¹⁴C and (U–Th)/He groundwater ages in the St. Lawrence Lowlands, Quebec, Eastern Canada. Chem. Geol. 413, 94-106.

690 Weise, S., Moser, H., 1987. Groundwater dating with helium isotopes, in: Isotope techniques in water resources development. International Atomic Energy Agency (IAEA), pp. 105-126.

692
693
694 **Figure Captions**

695 **Figure 1.** Map of the St. Lawrence Lowlands, including the study area of the Vaudreuil-Soulanges watershed (VS) and the intrusions of the Cretaceous Monteregian Hills (MH). The Bécancour watershed (BEC) and the Nicolet and lower Saint-François River watershed (NSF) are also indicated. The percentage of mantle helium ($f_{mtl}$%) measured in each watershed is provided.

699 **Figure 2.** a) Map of the sampled groundwater and rocks in the VS watershed, with outcrops of the Cretaceous Oka carbonatite complex and the Île Cadieux alnoïte indicated, together with the Grenvillian Mount Rigaud. b) Map of the sampled groundwater and rocks of the Monteregian Hills.

702 **Figure 3.** Distribution of R/Ra values measured in bulk rocks and mineral separates from Monteregian Hills intrusions by Sasada et al. (1997) and in this study. The range of mantle-derived $ ^{3}He/$$^{4}He $ measured in groundwater of the St. Lawrence Lowlands basin is also reported for comparison.

706 **Figure 4.** (a) Theoretical Weise-type diagram showing the main helium sources, namely the atmosphere, the crust and the mantle, and how the helium isotopic and elemental compositions vary by adding tritiogenic He (light blue arrow), crustal He (yellow arrow), and mantle He (orange arrow) to an atmospheric helium source. A mixing line between a hypothetical tritiogenic helium end-member and sample WSB1 is reported. The intercept of this mixing line with the Y-axis gives the fraction of mantle He in the mixture. (b) Weise diagram of the measured helium ratios. The black line represents mixing between water at recharge ($R_{eq}$) with water having accumulated crustal radiogenic helium ($R_{rad}$). Dashed lines through samples represent mixing between modern water of
different initial tritium contents mixed with water containing terrigenic helium (mantle and crustal helium).

**Figure 5.** Probability distributions of the initial ³He/⁴He ratio (R<sub>mtl</sub>/Ra; Eqn. 6) of the emplaced magma source (alnoïte in the Vadreuil-Soulanges (VS) area and gabbro-sienitic for the Monteregian Hills (MH)), obtained from the Monte Carlo method. These distributions are plotted against the frequency distribution of the R/Ra measured in 401 OIB glasses (data from Abedini et al., 2006). SD = standard deviation (1σ); MAD= mean absolute deviation.

**Figure 6.** Evolution curves (and uncertainty envelope) of the (R<sub>mtl</sub>/Ra) as a function of time elapsed since the emplacement of the alnöite intrusions (a) in the VS region 110±3.5 Ma; and (b) in the sampled Monteregian Hills (124 Ma), as calculated by Monte Carlo simulations.
<figure><img src="image_2.png" /><figcaption>Figure 1
EPSL-D-19-01096R1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2 a,b
EPSL-D-19-01096R1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3
EPSL-D-19-01096R1</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4 a,b</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5
EPSL-D-19-01096R1</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Figure 6 a,b</figcaption></figure>
<table><caption>**Table 1.** Helium and neon elemental and isotopic data of sampled groundwater.</caption><thead><tr><th>Sample</th><th>Type of well</th><th>Location</th><th>Latitude<br>UTM NAD83</th><th>Longitude<br>UTM NAD83</th><th><sup>4</sup>He<br>cm<sup>3</sup>STP/g x 10<sup>-8</sup></th><th>±<br>cm<sup>3</sup>STP/g x 10<sup>-7</sup></th><th><sup>20</sup>Ne<br>R/Ra</th><th>±</th><th>±<br><sup>4</sup>He/<sup>20</sup>Ne</th><th>±</th><th>ΔNe%<br>(ea)</th></tr></thead><tbody><tr><td>VS102</td><td>Municipal</td><td>Vaudreuil-Soulanges</td><td>-74.31279</td><td>45.44997</td><td>9.28</td><td>4.64</td><td>2.87</td><td>0.14</td><td>1.02</td><td>0.01</td><td>0.32</td><td>0.16</td><td>54.48</td></tr><tr><td>VS103</td><td>Municipal</td><td>Vaudreuil-Soulanges</td><td>-74.31123</td><td>45.48339</td><td>116.10</td><td>5.81</td><td>2.11</td><td>0.10</td><td>0.58</td><td>0.01</td><td>5.51</td><td>0.39</td><td>12.83</td></tr><tr><td>VS104*</td><td>Municipal</td><td>Vaudreuil-Soulanges</td><td>-74.28452</td><td>45.42295</td><td>6.64</td><td>0.33</td><td>1.72</td><td>0.09</td><td>1.20</td><td>0.02</td><td>0.39</td><td>0.03</td><td>-8.02</td></tr><tr><td>VS105</td><td>Observation</td><td>Vaudreuil-Soulanges</td><td>-74.181861</td><td>45.381763</td><td>98.81</td><td>0.44</td><td>2.83</td><td>0.14</td><td>2.23</td><td>0.03</td><td>0.31</td><td>0.02</td><td>51.34</td></tr><tr><td>VS106</td><td>Observation</td><td>Vaudreuil-Soulanges</td><td>-74.349212</td><td>45.401767</td><td>19.46</td><td>0.97</td><td>1.95</td><td>0.10</td><td>1.19</td><td>0.01</td><td>1.00</td><td>0.07</td><td>4.28</td></tr><tr><td>VS107</td><td>Observation</td><td>Vaudreuil-Soulanges</td><td>-74.208624</td><td>45.296467</td><td>7.46</td><td>0.37</td><td>2.28</td><td>0.11</td><td>1.35</td><td>0.02</td><td>0.33</td><td>0.02</td><td>21.93</td></tr><tr><td>VS108</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.32428</td><td>45.3881</td><td>166.64</td><td>8.33</td><td>2.79</td><td>0.14</td><td>1.38</td><td>0.02</td><td>5.97</td><td>0.42</td><td>49.20</td></tr><tr><td>VS110</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.25092</td><td>45.47927</td><td>9.26</td><td>0.46</td><td>2.17</td><td>0.11</td><td>0.78</td><td>0.01</td><td>0.43</td><td>0.03</td><td>16.04</td></tr><tr><td>VS111*</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.38018</td><td>45.35587</td><td>139.09</td><td>6.95</td><td>1.75</td><td>0.09</td><td>0.19</td><td>0.01</td><td>7.96</td><td>0.56</td><td>-6.42</td></tr><tr><td>VS112</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.2761</td><td>45.40286</td><td>10.019</td><td>0.51</td><td>2.16</td><td>0.11</td><td>2.33</td><td>0.03</td><td>0.47</td><td>0.03</td><td>15.51</td></tr><tr><td>VS113*</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.23643</td><td>45.42119</td><td>183.57</td><td>9.18</td><td>1.53</td><td>0.09</td><td>0.41</td><td>0.01</td><td>11.96</td><td>0.94</td><td>-18.18</td></tr><tr><td>VS114*</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.22512</td><td>45.44675</td><td>1323</td><td>68.01</td><td>1.79</td><td>0.09</td><td>0.68</td><td>0.01</td><td>74.25</td><td>5.32</td><td>-4.28</td></tr><tr><td>VS115</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.38136</td><td>45.27199</td><td>100</td><td>5.03</td><td>2.24</td><td>0.11</td><td>0.18</td><td>0.004</td><td>4.48</td><td>0.32</td><td>19.79</td></tr><tr><td>VS116</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.26511</td><td>45.35669</td><td>39.39</td><td>1.97</td><td>2.54</td><td>0.13</td><td>0.53</td><td>0.01</td><td>1.55</td><td>0.11</td><td>35.83</td></tr><tr><td>VS28</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.33418</td><td>45.41088</td><td>15.19</td><td>0.76</td><td>3.01</td><td>0.15</td><td>0.71</td><td>0.01</td><td>0.50</td><td>0.04</td><td>60.96</td></tr><tr><td>VS29</td><td>Private</td><td>Vaudreuil-Soulanges</td><td>-74.30626</td><td>45.42946</td><td>139.59</td><td>9.41</td><td>2.1</td><td>0.07</td><td>0.28</td><td>0.01</td><td>6.65</td><td>0.50</td><td>12.30</td></tr><tr><td>WSB1*</td><td>Private</td><td>MH, St-Bruno</td><td>-73.3241974</td><td>45.5431257</td><td>30.43</td><td>0.29</td><td>1.76</td><td>0.02</td><td>3.32</td><td>0.08</td><td>1.73</td><td>0.02</td><td>-5.88</td></tr><tr><td>WSB2*</td><td>Private</td><td>MH, St-Bruno</td><td>-73.3249577</td><td>45.5441935</td><td>108.20</td><td>1.01</td><td>0.67</td><td>0.01</td><td>1.99</td><td>0.05</td><td>16.18</td><td>0.21</td><td>-64.17</td></tr><tr><td>WMSH3</td><td>Private</td><td>MH, St-Hilaire</td><td>-73.156288</td><td>45.537288</td><td>7857827</td><td>73675</td><td>15780</td><td>147</td><td>0.15</td><td>0.01</td><td>49.80</td><td>0.66</td><td>-</td></tr><tr><td>WMSH3 (corrected)</td><td></td><td></td><td></td><td></td><td>93.12</td><td>0.87</td><td>1.87</td><td>0.02</td><td></td><td></td><td></td><td></td><td>-</td></tr><tr><td>WR4*</td><td>Private</td><td>MH, Rougemont</td><td>-73.080321</td><td>45.4633378</td><td>35.73</td><td>0.33</td><td>0.52</td><td>0.01</td><td>1.00</td><td>0.03</td><td>6.81</td><td>0.09</td><td>-72.19</td></tr><tr><td>WR5</td><td>Private</td><td>MH, Rougemont</td><td>-73.0336287</td><td>45.4605797</td><td>17.39</td><td>0.16</td><td>2.02</td><td>0.02</td><td>1.81</td><td>0.05</td><td>0.86</td><td>0.01</td><td>8.02</td></tr><tr><td>WY6</td><td>Private</td><td>MH, Yamaska</td><td>-72.88344</td><td>45.46796</td><td>38.16</td><td>0.36</td><td>2.63</td><td>0.02</td><td>2.46</td><td>0.06</td><td>1.46</td><td>0.02</td><td>40.64</td></tr><tr><td>WY7*</td><td>Private</td><td>MH, Yamaska</td><td>-72.86829</td><td>45.46526</td><td>15.72</td><td>0.15</td><td>0.54</td><td>0.01</td><td>1.26</td><td>0.07</td><td>2.92</td><td>0.04</td><td>-71.12</td></tr><tr><td>WMB8</td><td>Private</td><td>MH, Brome</td><td>-72.694374</td><td>45.262202</td><td>12685</td><td>119</td><td>1435</td><td>13</td><td>0.88</td><td>0.03</td><td>0.88</td><td>0.01</td><td>-</td></tr><tr><td>WMB8 (corrected)</td><td></td><td></td><td></td><td></td><td>1.65</td><td>0.94</td><td>1.88</td><td>0.02</td><td></td><td></td><td></td><td></td><td>-</td></tr></tbody></table>

VS = Vaudreuil Soulanges watershed; MH = Monteregian Hills

*$^{1}$He was corrected for degassing when $^{20}$Ne is lower than the atmospheric value (solubility of Ne at 9.8°C is 1.87 x 10$^{-7}$ cm$^{3}$STP/g)

ΔNe (%), given as $[(Ne]_{measured}/[Ne]_{atmospheric equilibrium} - 1) \times 100$, and represents the total amount of excess air
<table><caption>**Table 2.** Measured helium isotopic composition, calculated helium ratios and He production rates in</caption><thead><tr><th>Sample</th><th>Location</th><th>Rock type</th><th>Phase</th><th>R/Ra</th><th>±</th><th>[<sup>4</sup>He]</th><th>±</th><th>cm<sup>3</sup>STP/g x 10<sup>-5</sup></th></tr></thead><tbody><tr><td>R-Aln</td><td>Ile Cadieux</td><td>Alnöite</td><td>Bulk</td><td>0.06</td><td>0.02</td><td>0.70</td><td>0.07</td><td></td></tr><tr><td>R-OC</td><td>Oka Complex</td><td>Carbonatite</td><td>Bulk</td><td>0.60</td><td>0.01</td><td>1.28</td><td>0.12</td><td></td></tr><tr><td>OKDI*</td><td>Oka Complex</td><td>Carbonatite</td><td>Diopside</td><td>0.25</td><td>0.07</td><td>0.82</td><td>0.08</td><td></td></tr><tr><td>OKDI*</td><td>Oka Complex</td><td>Carbonatite</td><td>Diopside</td><td>2.55</td><td>0.25</td><td>0.13</td><td>0.01</td><td></td></tr><tr><td>OKCA*</td><td>Oka Complex</td><td>Carbonatite</td><td>Calcite</td><td>0.31</td><td>0.09</td><td>1.16</td><td>0.12</td><td></td></tr><tr><td>R-MR15</td><td>Mont Royal</td><td>Essexite</td><td>Bulk</td><td>0.10</td><td>0.03</td><td>0.95</td><td>0.09</td><td></td></tr><tr><td>R-MSB3</td><td>St. Bruno</td><td>Felsic gabbro</td><td>Bulk</td><td>0.62</td><td>0.06</td><td>0.55</td><td>0.05</td><td></td></tr><tr><td>R-MSB3</td><td>St. Bruno</td><td>Felsic gabbro</td><td>Clx-pyroxene</td><td>1.23</td><td>0.10</td><td>0.48</td><td>0.04</td><td></td></tr><tr><td>R-MSB3</td><td>St. Bruno</td><td>Felsic gabbro</td><td>Clx-pyroxene</td><td>4.96</td><td>0.01</td><td>0.07</td><td>0.01</td><td></td></tr><tr><td>R-MB15</td><td>Mont Brome</td><td>Gabbro</td><td>Bulk</td><td>0.43</td><td>0.03</td><td>0.41</td><td>0.04</td><td></td></tr><tr><td>R-MRg1</td><td>Mont Rougemont</td><td>Banded gabbro</td><td>Bulk</td><td>0.67</td><td>0.13</td><td>0.07</td><td>0.01</td><td></td></tr><tr><td>R-MRg9</td><td>Mont Rougemont</td><td>Gabbro</td><td>Bulk</td><td>1.36</td><td>0.11</td><td>0.41</td><td>0.04</td><td></td></tr><tr><td>R-MSH10</td><td>Mont St. Hilaire</td><td>Gabbro</td><td>Bulk</td><td>0.09</td><td>0.01</td><td>2.28</td><td>0.22</td><td></td></tr><tr><td>R-MY3</td><td>Mont Yamaska</td><td>Gabbro</td><td>Bulk</td><td>0.13</td><td>0.01</td><td>1.92</td><td>0.19</td><td></td></tr></tbody></table>

* Data from Sasada et al. (1997)

**Rin/Ra is the resulting helium isotopic ratio from in situ production of <sup>3</sup>He and <sup>4</sup>He.
1. Monteregian Hills' rocks and minerals.

<table><thead><tr><th>[U]<br>ppm</th><th>[Th]<br>ppm</th><th>[Li]<br>ppm</th><th>J<sup>3</sup>He ±<br>cm<sup>3</sup>STP/g<sub>rock</sub>/y<br>r x 10<sup>-20</sup></th><th>J<sup>4</sup>He ±<br>cm<sup>3</sup>STP/g<sub>rock</sub>/y<br>r x 10<sup>-13</sup></th><th>R<sub>in</sub>/Ra<sup>**</sup></th></tr></thead><tbody><tr><td>3.3</td><td>15.4</td><td>15</td><td>1.78 0.36</td><td>8.41 1.68</td><td>0.015</td></tr><tr><td>9.9</td><td>40.9</td><td>15</td><td>0.63 0.13</td><td>23.56 4.71</td><td>0.002</td></tr><tr><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>4.5</td><td>9.6</td><td>37</td><td>1.58 0.32</td><td>8.48 1.7</td><td>0.013</td></tr><tr><td>3.9</td><td>9.4</td><td>28</td><td>1.04 0.21</td><td>7.35 1.5</td><td>0.010</td></tr><tr><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>12.4</td><td>20.1</td><td>28</td><td>2.89 0.58</td><td>20.54 4.1</td><td>0.010</td></tr><tr><td>4.5</td><td>8.8</td><td>28</td><td>1.11 0.22</td><td>7.89 1.58</td><td>0.010</td></tr><tr><td>4.1</td><td>9.4</td><td>28</td><td>1.07 0.21</td><td>7.59 1.5</td><td>0.010</td></tr><tr><td>10.0</td><td>19.6</td><td>28</td><td>2.47 0.50</td><td>17.64 3.5</td><td>0.010</td></tr><tr><td>6.6</td><td>17.7</td><td>28</td><td>1.82 0.36</td><td>13.0 2.6</td><td>0.010</td></tr></tbody></table>
<table><caption>**Table 3.** Summary of input and output parameters for Monte Carlo simulations.</caption><thead><tr><th>Input parameters</th><th>units</th><th>Alnoite</th><th>±</th><th>MH</th><th>±</th></tr></thead><tbody><tr><td>J<sub>4</sub>He</td><td>x 10<sup>-13</sup> ccSTP/g<sub>rock</sub>/yr</td><td>8.41</td><td>1.68</td><td>11.78</td><td>5.01</td></tr><tr><td>(R<sub>mtl</sub>/Ra)<sub>final</sub></td><td></td><td>1.42</td><td>0.14</td><td>1.42</td><td>0.14</td></tr><tr><td><sup>4</sup>He initial</td><td>x 10<sup>-6</sup> ccSTP/g</td><td>6.7</td><td>0.7</td><td>6.7</td><td>0.7</td></tr><tr><td>Age since intrusion</td><td>Ma</td><td>110.5</td><td>3.5</td><td>124</td><td>1</td></tr><tr><td colspan="6">Output parameter :</td></tr><tr><td>(R<sub>mtl</sub>/Ra)<sub>initial</sub></td><td>Average*</td><td>21</td><td>5</td><td>33</td><td>14</td></tr><tr><td>(R<sub>mtl</sub>/Ra)<sub>initial</sub></td><td>Median**</td><td>21</td><td>5</td><td>32</td><td>10</td></tr></tbody></table>

Monte Carlo simulation results after 1 x 10⁵ iterations.

* Uncertainty on the average value is standard deviation (1σ)

** Uncertainty on the median is the mean absolute deviation (MAD)