# CH4-He-H2-Rich Fluids Associated With the Intrusive Rocks of the Mid-Continent Rift System in Minnesota (USA)

V. Combaudon, O. Sissmann, J. Guélard, S. Noirez, H. Derluyn, E. Deville

► To cite this version:

V. Combaudon, O. Sissmann, J. Guélard, S. Noirez, H. Derluyn, et al. CH4-He-H2-Rich Fluids Associated With the Intrusive Rocks of the Mid-Continent Rift System in Minnesota (USA). Geochemistry, Geophysics, Geosystems, 2025, 26 (9), pp.e2025GC012236. 10.1029/2025GC012236. hal-05410021

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Distributed under a Creative Commons Attribution 4.0 International License
# Geochemistry, Geophysics, Geosystems®

## RESEARCH ARTICLE

10.1029/2025GC012236

**Key Points:**

* We characterized CH₄-He-H₂-rich fluids associated with the intrusive rocks of the Mid-Continent Rift system in Minnesota (USA)

* We suspect H₂ is being produced at depth and consumed under surface conditions to generate CH₄

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

V. Combaudon, combaudon.valentine@gmail.com

**Citation:**

Combaudon, V., Sissmann, O., Guélard, J., Noirez, S., Derluyn, H., & Deville, E. (2025). CH₄-He-H₂-rich fluids associated with the intrusive rocks of the Mid-Continent Rift system in Minnesota (USA). *Geochemistry, Geophysics, Geosystems*, 26, e2025GC012236. https://doi.org/10.1029/2025GC012236

Received 20 MAR 2025

Accepted 31 AUG 2025

**Author Contributions:**

**Conceptualization:** V. Combaudon, O. Sissmann, E. Deville

**Data curation:** V. Combaudon

**Formal analysis:** V. Combaudon, O. Sissmann, J. Guélard, S. Noirez

**Funding acquisition:** O. Sissmann, H. Derluyn, E. Deville

**Methodology:** V. Combaudon, O. Sissmann, E. Deville

**Project administration:** O. Sissmann, H. Derluyn, E. Deville

**Supervision:** O. Sissmann, J. Guélard, H. Derluyn, E. Deville

**Visualization:** V. Combaudon, O. Sissmann, E. Deville

**Writing – original draft:** V. Combaudon

## CH₄-He-H₂-Rich Fluids Associated With the Intrusive Rocks of the Mid-Continent Rift System in Minnesota (USA)

V. Combaudon<fnref n="1" />,²,<fnref n="3" /> Ⓓ, O. Sissmann<fnref n="1" /> Ⓓ, J. Guélard<fnref n="1" />, S. Noirez<fnref n="1" />, H. Derluyn² Ⓓ, and E. Deville<fnref n="1" />

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

**Abstract** Several studies have shown that H₂ and CH₄ emissions are recorded within Precambrian areas, where the origin of gases has yet to be determined. The Mid-Continent Rift (MCR) is an aborted rift aged from 1.1 Ga and composed of volcanic and (ultra)-mafic rocks. Wells presenting free or dissolved gas were sampled in NE-Minnesota along the MCR. High pH water (up to 11.4) and low Eh (down to -300 mV) with elevated concentrations (up to 756 µmolar) of formate and acetate are associated with gases consisting of He, CH₄, CO₂, heavier hydrocarbons, and up to 500 ppmv of H₂. The CH₄ presents different ¹³C and D isotopic values, suggesting that several abiotic and biotic pathways might be active in these shallow systems. The alkaline and reducing waters associated with the igneous rocks of the MCR suggest that H₂ could have been produced through water-rock interactions at deeper levels not reached by the shallow sampled wells. The associated high concentrations of gaseous and dissolved carbon compounds (VOA and methane) suggest that subsequent redox reactions have occurred in most of the rocks crossed by the wells, consuming a part of the H₂ as it was migrating toward the surface. Those results highlight potentially active H₂ production and consumption processes, providing keys for targeting source rocks in Precambrian environments. Those results suggest that direct H₂ detection in soil gas may not be the most effective exploration strategy. Searching for biogenic methane associated with deep He and N₂ may prove to be more effective.

**Plain Language Summary** The search for natural hydrogen (or geological hydrogen) has been blooming for the past years as this latter could be considered as an energy resource. We investigated the northern part of the Mid-Continent Rift in NE-Minnesota. This latter is a 1.1 Ga aborted rift, and its extension in Minnesota is characterized mainly by a multi-metric scale igneous complex. To assess NE-Minnesota natural hydrogen potential, we sampled groundwater and dissolved gases associated with the Mid-Continent Rift igneous rocks. The groundwater are reduced and highly alkaline and are associated with volatile organic acids (namely formate and acetate), He, CH₄, and H₂. We suspect redox reactions are occurring in the rocks, H₂ is being produced at depth, and consumed under surface conditions to generate CH₄.

## 1. Introduction

For a long time, natural hydrogen was only studied at the Mid-Oceanic Ridges (Campbell et al., 1988; Welhan & Craig, 1979) in lithospheric plate boundary areas where studies of the emissions were complex and their exploitation impossible. Methane was observed with this abiotic hydrogen, but in the continental domain, methane and heavier hydrocarbons were mostly considered biotic or thermogenic gases originating from microbial methanogenesis or thermogenic maturation of organic matter.

However, subsurface fluids in the continental crust, rich in methane and natural hydrogen have been progressively studied (Etiope & Sherwood Lollar, 2013; Guélard, 2016; Potter et al., 2004; Schoell, 1988; Sherwood et al., 1988, amongst many others). CH₄ and H₂-rich fluids are now considered a major “fuel” for microorganism development within the crust (H. M. Miller et al., 2018; Nelson et al., 2005), but also, when found in significant amounts, as a potential energy source (Gaucher, 2020; Milkov, 2022; Prinzhofer & Deville, 2015; Truche et al., 2020; Zgonnik, 2020). Methane origins have also been studied to better constrain CH₄ variations within Mars's atmosphere (Etiope, Ehlmann, & Schoell, 2013; Etiope, Tsikouras, et al., 2013).

As Precambrian rocks represent nearly 70% of the surface area of the continental crust (Goodwin, 1996) and are often not prone to hydrocarbon (HC) occurrences, other production pathways of gaseous sources of energy than microbial gas or thermogenic HC must be considered to explain the presence of these CH₄-H₂-rich fluids. During the last two decades, the scientific community has carried out a significant effort to better constrain abiotic

© 2025 The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. 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.

<footnote n="1">¹IFPEN, Rueil-Malmaison, France, ²University of Pau and the Adour Countries, E2S UPPA, CNRS, LFCR, Pau, France,</footnote>

<footnote n="3">³Now at Department of Geological Sciences, University of Colorado, Boulder, CO, USA</footnote>
**Writing – review & editing:** O. Sissmann, J. Guélard, S. Noirez, H. Derluyn, E. Deville

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Simplified geological map of NE-Minnesota with the location of the major geological complexes and studied wells (modified after Taranovic et al. (2015) and Jirsa et al. (2011)).</figcaption></figure>

reactions susceptible to producing hydrogen, methane, and other hydrocarbons observed within the Precambrian rocks of the continental crust (please see references below).

$CH_4-H_2$-rich gases have been found notably in the Canadian Shield, the Fennoscandian Shield, or the Kansas Precambrian basement (Coveney, 1987; Guélard et al., 2017; Newell et al., 2007; Sherwood et al., 1988; Sherwood-Lollar et al., 1993, 2006, 2014; Warr et al., 2019). Studies have shown that the gas content, the isotopic values, as well as the physicochemical parameters of associated waters have similarities with gases of volcanic and geothermal origins, such as gases sampled at the Mid-Oceanic ridges (Campbell et al., 1988; Welhan & Craig, 1979) or in continental ophiolitic and peridotite massifs (Abrajano et al., 1988; Deville & Prinzhofer, 2016; Etiope et al., 2011; Etiope, Ehlmann, & Schoell, 2013; Etiope, Tsikouras, et al., 2013; Morrill et al., 2013; Szponar et al., 2013; Vacquand et al., 2018, amongst many others). This led the authors to suggest that abiotic reactions such as metamorphism of carbonate-graphite-bearing rocks or gas-water-rock interactions (GWRI) like iron carbonates decomposition (McCollom, 2003) or serpentinization (Moody, 1976) could produce significant amounts of $H_2$, $CH_4$, and other hydrocarbons within the Precambrian basement rocks. However, in such contexts, gases are often the results of mixing or post-genetic alteration, and their final content and isotopic values do not necessarily reflect their initial origin, making the study of such gases complex. By coupling geology, major gas composition, carbon and deuterium isotopy as well as noble gas composition and isotopy, it is possible to better constrain the origin and mechanisms responsible for such gas generation.

Northeastern Minnesota rocks are part of the North American craton. Mesoproterozoic mafic and ultramafic rocks belonging to the Lake Superior part of the Mid-Continent rift (MCR) have intruded Paleoproterozoic and Archean rocks (Jirsa et al., 2011; J. Miller et al., 2002) (Figure 1). We spotted these rocks for potential hydrogen and abiotic methane generation as they present similarities with the Canadian and Fennoscandian shields rocks as well as the $H_2-CH_4$-generating rocks of the Mid-Oceanic Ridges, and the Kansas Precambrian rocks (Combaudon et al., 2024; Guélard et al., 2017). In this study, we present in situ physicochemical parameters of waters associated with rocks from the MCR and high-performance liquid chromatography (HPLC) analyses, as well as gas
chromatography (GC), GC combustion isotope ratio mass spectrometry (GC-C-IRMS), and noble gases spectrometry analyses performed on associated gases. This study aims to show that the Precambrian rocks of NE-Minnesota are prone to produce hydrogen and short-chain organic compounds such as methane or volatile organic acids (VOA) and represent a unique opportunity to better constrain abiotic or biotic gas synthesis as well as to better understand the unique role of Precambrian rocks in hydrogen and methane generation.

## 2. Geological Context

Minnesota rocks belong to the Canadian Shield and include rocks from the Archean to Cretaceous ages. A large majority of these rocks are Archean rocks divided into several sub-provinces, Paleoproterozoic rocks form the Penokean orogen and the Animikie basin, and Mesoproterozoic magmatic and volcanic rocks form the northern part of the MCR. The Cretaceous rock occurrences are very limited (Figure 1).

This study was conducted within the northeastern Mesoproterozoic intrusive rocks of the MCR. This latter was formed between 1115 and 1086 Ma during four main rifting episodes (J. Miller et al., 2002). The first stage (1115–1107 Ma) is mainly outcropping near Lake Nipigon (Ontario, Canada - Heaman et al., 2007) and corresponds to the earliest magmatism recorded with mafic to ultramafic rocks being formed. The latent stage (1107–1100 Ma) corresponds to a hiatus in the mafic magmatism due to extensive underplating of the crust by mantle melt, which caused partial melting of the crust and thus allowed for crystallization of felsic rocks (J. Miller & Vervoort, 1996). The main stage (1110–1094 Ma) is characterized by magma with a tholeiitic composition (J. Miller et al., 2002). Finally, the late stage (1094–1086 Ma) of the rifting is characterized by the interbedding of lavas and detrital sedimentary rocks, marked by a subsidence phase, and by the Grenville orogeny east of the MCR (Cannon, 1994).

The rocks of the studied area are divided into several volcanic and magmatic complexes and subsequent intrusions. The Tamarack intrusion, 80 km south of Duluth, is an ultramafic cumulate intruded in the Paleoproterozoic Animikie basin. The intrusion belongs to the first rifting stage and is not outcropping (Goldner, 2011; Taranovic et al., 2015). A recent study (Taranovic et al., 2015) divided it into two major ultramafic intrusive sequences: the Bowl intrusion in the south and the Dike area intrusion in the north (which hosts most mineralization). The Dike area is divided into the coarse-grained olivine-bearing intrusion (Fo<sub>89-82</sub>) and the fine-grained olivine-bearing intrusion (Fo<sub>84-82</sub>). They are made of peridotite, feldspathic peridotite, feldspathic pyroxenite, melatroctolite, and melagabbro. The Bowl peridotites comprise serpentinite, magnetite, and chlorite as secondary minerals, traducing potential hydrothermal alteration and serpentinization (Taranovic et al., 2015).

North of Duluth, the rocks of the MCR are divided into three complexes. The North Shore Volcanic Group (NSVG) is a co-magmatic volcanic and interflow sedimentary edifice 7–10 km thick. The emplacement of this edifice was dated between 1108 and 1107 Ma by U-Pb on zircons (Green et al., 2002). The Duluth Complex was then emplaced into the base of the NSVG between 1108 and 1098 Ma. It is the main magmatic edifice of the rift with semi-continuous ultramafic to felsic rocks that cover nearly 5,000 km² (J. Miller et al., 2002). The Duluth Complex consists of pre-rift Paleoproterozoic metasedimentary rocks (Virginia Formation and Biwabik Iron Formation), Archean granites, and greenstone terranes (Giants range batholith). The complex is divided into 4 main syn-rift series: the layered series, the anorthositic series, the felsic series, and the early gabbroic series, which are divided into intrusions. Peridotites from the Duluth Complex have been studied by Evans et al. (2017) and show evidence of altered olivine (Fa<sub>36-44</sub>), fayalite (Fa<sub>53-80</sub>), magnetite, and iron-rich clay minerals. Some miscellaneous hypabyssal intrusions such as the Brule-Hovland or Logan intrusions are synchronous to the Duluth Complex and can be found in all the MCR. The Duluth Complex is covered by the syn-rift Beaver Bay Complex, the younger and last magmatic complex of the area, emplaced between 1098 and 1095 Ma and divided into at least 13 intrusions of mafic and ultramafic rocks (J. Miller et al., 2002). One of them is the Sonju Lake intrusion, composed of mafic and ultramafic rocks, oxidized olivine-bearing gabbros, and serpentinized oxide dunites (Joslin, 2004). No post-rift Paleozoic or Mesozoic rocks are present in the studied area (Jirsa et al., 2011).

The studied wells included two wells that were drilled into the Tamarack complex (Tamarack well#1, and #2), two wells drilled into the NSVG rocks (DMK well#5 and #6, and D.MCK well#1), two wells drilled into the gabbro's of the Duluth Complex (DMK well#1 and BC well#1) and three drilled into the hypabyssal intrusions (DMK well#2, #3, and #4).
# 3. Materials and Methods

## 3.1. Surface Measurements

Samples (water and gas) were collected on non-artesian wells. Water was pumped and flushed for over a minute to yield a representative chemical composition, and to avoid measuring old accumulated and re-equilibrated water in the wellhead. For each well sampled, temperature, redox potential (Eh), conductivity, and pH were measured using a HI 98194 multiparameter from Hanna Instrument.

## 3.2. Gas Sampling

Tamarack well #1 was not tightly sealed, thus being exposed to atmospheric contamination. Tamarack well#2 on the other hand was correctly sealed, inducing a small gas flux at the opening of the well. Both gases were sampled in exetainers® (Labco©) and in steel containers that had been vacuumed previously. The gas pressure in Tamarack well#2 was above the atmospheric pressure; thus, the steel container was flushed for a few minutes before sampling.

All DMK wells and BC well #1 are private drinking water wells. DMK well#1 and DMK#1–2 are two samples from the same well collected at a one-day time interval. Gas, probably present as dissolved at depth, was exsolving as bubbles in the water when reaching the surface within most of the sampled wells. Thus, water was pumped from the aquifer and accumulated within a tank. A reversed funnel sealed by a rubber cork was placed below the water surface to let the gas accumulate and prevent any atmospheric contamination. This sampling technique has been described by Lévy et al. (2023). Accumulated gas was sampled using a double-sided needle to connect the headspace at the top of the funnel to vacuumed exetainers vials and inox tubes.

## 3.3. Gas Analysis

### 3.3.1. Gas Chromatography

Gas compositions of H₂, CH₄, CO₂, C₂H₆, O₂, and N₂ as trace components (i.e., ppmv quantification limit) were determined using a Gas Chromatograph Barrier Ionization Discharge (GC-BID) 2030 from Shimadzu equipped with a Restek RT-Q-BOND 30 m × 530 mmID column and operated under He gas carrier at IFPEN (Rueil-Malmaison, France). In addition, the chromatographic response was calibrated with external standards being a mixture of the studied gases at different concentrations. Each analysis was bracketed with blanks. The detection limit on internal standards is set at 10 ppmv with an uncertainty of 5%.

A second GC connected to a gas line was used for gas samples stored in steel containers. Gas compositions of He, H₂, CH₄, CO₂, C₂H₆, O₂, and N₂ as main components were determined by using a GC-7890B from Agilent equipped with a PoraPlotQ column at IFPEN and operated under an N₂ gas carrier for analyzing He and H₂, and He for the rest of the analyzed species (Rueil-Malmaison, France). This GC is used to measure higher species compositions (10 ppmv to %) than the GC-BID 2030 previously used. The chromatographic response was calibrated with external standards being a mixture of the studied gas at different concentrations. Each analysis was bracketed with blanks.

### 3.3.2. Noble Gas Spectrometry

Relative abundances of noble gases (⁴He, ²⁰Ne, ³⁶Ar, ⁸⁴Kr, ¹²⁹Xe, and ⁴⁰Ar/³⁶Ar) were determined using quadrupolar mass spectrometry. ³He/⁴He and ²⁰Ne/⁴He ratios were determined using a VG 5400 mass spectrometer with a magnetic sector. Noble gas spectrometry was performed at IFPEN (Rueil-Malmaison, France). The Analytical uncertainties for each gas and ratios are displayed in the results section (Table 3).

For each sample, the response of the spectrometer is calibrated by performing systematic analyses of a purified air dose (Calibrated Dose) for which the quantities of He, Ne, Ar, and Kr as well as the ⁴⁰Ar/³⁶Ar ratio are controlled weekly by an air standard analysis. The ⁴⁰Ar/³⁶Ar isotopic ratio is calibrated against the Ar partial pressure in the mass spectrometer whenever the source is tuned (approximately once a month) to correct for the nonlinearity of the mass discrimination factor. The ⁴⁰Ar/³⁶Ar is calculated based on the air reference of 298.6 ± 0.5 (Lee et al., 2006). All the analyses are performed with the Secondary Electron Multiplier comprising 30 successive measurements of the signals associated with *m/z* ratios 3, 4, 18, 20, 21, 22, 28, 36, 38, 40, 44, 82, 84, and 86. The drift of the signal is corrected to the time of introduction of the gas, and then the residual background noise
<caption>**Table 1**</caption>

Physico-Chemical Parameters of Water at the Wellhead for BC Well#1, DMK Well#1, Well#1-2, Well#2, Well#3, Well#4, Well#5, Well#6, and D.MCK Well#1

<table><thead><tr><th>Well name</th><th>Latitude</th><th>Longitude</th><th>pH</th><th>Eh (mV)</th><th>Cond (mS/cm)</th><th>T (°C)</th></tr></thead><tbody><tr><td colspan="7">Duluth Complex or Hypabyssal intrusions</td></tr><tr><td>BC well#1</td><td>48.042927</td><td>-90.472453</td><td>10.7</td><td>-298.4</td><td>2,566</td><td>6.7</td></tr><tr><td>DMK well#1</td><td>48.054779</td><td>-90.530676</td><td>11.4</td><td>-16</td><td>5,952</td><td>5.6</td></tr><tr><td>DMK well#1-2</td><td>48.054779</td><td>-90.530676</td><td>10.6</td><td>-241.8</td><td>6,827</td><td>6.5</td></tr><tr><td>DMK well#2</td><td>48.047316</td><td>-90.057707</td><td>10.6</td><td>-317.9</td><td>71</td><td>9.4</td></tr><tr><td>DMK well#3</td><td>47.841494</td><td>-89.968807</td><td>10.6</td><td>-219</td><td>22,594</td><td>10.4</td></tr><tr><td>DMK well#4</td><td>47.828946</td><td>-90.014867</td><td>9.4</td><td>-32.3</td><td>9,975</td><td>8.1</td></tr><tr><td colspan="7">North Shore Volcanic Group</td></tr><tr><td>DMK well#5</td><td>47.796196</td><td>-90.136688</td><td>9.2</td><td>-25.9</td><td>1,927</td><td>5.7</td></tr><tr><td>DMK well#6</td><td>47.768139</td><td>-90.268604</td><td>8.3</td><td>-20.8</td><td>2,305</td><td>9.4</td></tr><tr><td>D.MCK well#1</td><td>47.488058</td><td>-90.989913</td><td>10.6</td><td>129.5</td><td>102</td><td>8.0</td></tr></tbody></table>

measured before the introduction of the sample is subtracted. A procedural blank for the entire line is measured every week and does not exceed 1~2% of the signal for each noble gas of a Calibrated Dose (DC) and typical sample analyses. The mean blank is subtracted from the signal of the sample and its standard deviation is integrated into the uncertainty of the sample analysis. For ³He/⁴He ratio, 30 successive measurements are performed for each isotope. A statistic regression is performed to determine the intensity of the signal at the time of gas introduction into the source. The calibration of the ³He/⁴He ratio is achieved by repeated analyses of an air standard with a known composition of ⁴He 5.24 ppm and ³He/⁴He 1.39·10⁻⁶ (Ozima & Podosek, 2002).

### 3.3.3. GC-C-IRMS

The carbon isotopes of CO₂, CH₄, and C₂H₆ and the deuterium isotopes of CH₄ were measured using a MAT253 (Finnigan Mat-Thermo Fisher) triple collection mass spectrometer coupled to a gas chromatograph operating with He as a carrier gas at IFPEN (Rueil-Malmaison, France). Isotopes were calibrated using commercial international standards (AlphaGaz, procured from Air Liquid). The analysis uncertainties are displayed in the results section (Table 2). Each analysis was bracketed with blanks. Internal standards, with values determined separately on a dual-inlet Infra-Red Mass Spectrometry setup, were used to calibrate the samples' analysis, while external standards were injected as samples to validate the analytical method.

### 3.3.4. HPLC

Volatile organic acids, formate and acetate, were quantified on an Agilent HPLC equipped with an Aminex HPX-87H from Biorad, and a spectra system RI 150 refractometer detector, which measures the refractive index of an analyte relative to the solvent. The solvent phase was deionized water with 0.01 M sulfuric acid, with an output of 0.6 mL.min⁻¹. The column was maintained at 60°C during the analysis. The apparatus has a second detector, a Waters 2487 dual UV detector at 210 nm. The limit of detection of acetic acid and formic acid analyzed in HPLC varies with the detector. For formate, it is about 10 ppm (220 µmo/L) with the refractometry index detector and 1 ppm (22 µmol/L) with the UV detector. For acetic acid, it is about 20 ppm (330 µmol/L) with the refractometry index detector and 1 ppm (16 µmol/L) with the UV detector. There is one order of magnitude difference between the detectors. However, because no difference in VOA detection was observed between both detectors, and because linear response and standard reproducibility were better on the refractometry index detector, results from the refractometry index detector were reported over the UV measurements.

## 4. Results

### 4.1. Surface Measurements

Surface measurements of water sampled in private water wells are presented in Table 1 and Figure 2.
<caption>**Table 2**</caption>

Gas Composition in H₂, CH₄, CO₂, and C₂H₆ for Tamarack Well#1 and Well#2, DMK Well#1, Well#1–2, Well#2, Well#3, Well#4, Well#5, Well#6, BC Well#1, and D. MCK Well#1

<table><thead><tr><th>Well name</th><th>Container</th><th>H<sub>2</sub> (ppmv)</th><th>CH<sub>4</sub> (vol%)</th><th>CO<sub>2</sub> (ppmv)</th><th>C<sub>2</sub>H<sub>6</sub> (ppmv)</th><th>C<sub>3</sub>H<sub>8</sub> (ppmv)</th><th>iC<sub>4</sub> (ppmv)</th><th>nC<sub>4</sub> (ppmv)</th><th>He (ppmv)</th><th>O<sub>2</sub> (vol%)</th><th>N<sub>2</sub> (vol%)</th><th>C<sub>1</sub>/C<sub>2</sub></th><th>δD CH<sub>4</sub> (%)</th><th>σ (±)</th><th>δ<sup>13</sup>C CH<sub>4</sub> (%)</th><th>σ (±)</th></tr></thead><tbody><tr><td>Tamarack well#1</td><td>Exetainer</td><td>3</td><td>0.320</td><td>141</td><td>40</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>81</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td></tr><tr><td>Tamarack well#1</td><td>Exetainer</td><td>4</td><td>0.300</td><td>151,465</td><td>46,105</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>65</td><td>-277.22</td><td>0.89</td><td>-58.68</td><td>0.13</td></tr><tr><td>Tamarack well#2</td><td>Exetainer</td><td>1.6</td><td>10</td><td>2,497</td><td>1,050</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>93</td><td>-335.98</td><td>3.57</td><td>-45.35</td><td>0.13</td></tr><tr><td>Tamarack well#2</td><td>Inox tube</td><td>500</td><td>59</td><td>1,050</td><td>5,400</td><td>100</td><td>50</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>109</td><td>-327.78</td><td>3.57</td><td>-47.4</td><td>0.13</td></tr><tr><td>Tamarack well#2</td><td>Inox tube</td><td>400</td><td>60</td><td>2,160</td><td>5,500</td><td>110</td><td>40</td><td>10</td><td>12,200</td><td>6.4</td><td>5.6</td><td>108</td><td>-330.88</td><td>3.57</td><td>-47</td><td>0.13</td></tr><tr><td>BC well#1</td><td>Exetainer</td><td>10</td><td>9</td><td>56</td><td>683</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>136</td><td>-274.08</td><td>1.47</td><td>-30.03</td><td>0.13</td></tr><tr><td>DMK well#1</td><td>Exetainer</td><td>n/d</td><td>3</td><td>39</td><td>366</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>87</td><td>-159.05</td><td>3.94</td><td>-18.89</td><td>0.74</td></tr><tr><td>DMK well#1-2</td><td>Exetainer</td><td>4</td><td>39</td><td>7</td><td>1,224</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>22</td><td>-155.06</td><td>0.68</td><td>-18.93</td><td>0.34</td></tr><tr><td>DMK well#1-2</td><td>Inox tube</td><td>100</td><td>52</td><td>1,670</td><td>15,500</td><td>340</td><td>80</td><td>290</td><td>940</td><td>8.7</td><td>37.6</td><td>34</td><td>-144.91</td><td>0.68</td><td>-18.75</td><td>0.34</td></tr><tr><td>DMK well#2</td><td>Exetainer</td><td>n/d</td><td>3</td><td>60</td><td>112</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>274</td><td>-278.61</td><td>5.68</td><td>-30.95</td><td>0.52</td></tr><tr><td>DMK well#2</td><td>Inox tube</td><td>n/d</td><td>4</td><td>160</td><td>200</td><td>70</td><td>10</td><td>60</td><td>2,350</td><td>13.7</td><td>82.1</td><td>214</td><td>-273.81</td><td>5.68</td><td>-31.13</td><td>0.52</td></tr><tr><td>DMK well#3</td><td>Exetainer</td><td>n/d</td><td>0.002</td><td>648</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td></tr><tr><td>DMK well#3</td><td>Inox tube</td><td>470</td><td>0.004</td><td>500</td><td>0.3</td><td>0</td><td>0</td><td>0</td><td>40</td><td>20.2</td><td>79.7</td><td>131</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td><td>-</td></tr><tr><td>DMK well#4</td><td>-</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td><td>-</td></tr><tr><td>DMK well#5</td><td>Exetainer</td><td>n/d</td><td>n/d</td><td>898</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td><td>-</td></tr><tr><td>DMK well#6</td><td>Exetainer</td><td>n/d</td><td>0.008</td><td>301</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/d</td><td>n/a</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td><td>-</td></tr><tr><td>D.MCK well#1</td><td>-</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>n/a</td><td>-</td><td>n/a</td><td>-</td><td>-</td></tr></tbody></table>

Note. C₁/C₂ ratios are presented for Tamarack well#1 and well#2, DMK well#1, well#1–2, well#2, and well#3, as well as BC well#1. Isotopic ratios of δ¹³C of CH₄ per mil versus vPDB and δD of CH₄ per mil versus SMOW, with calculated standard deviation, are presented for Tamarack well#1 and well#2, DMK well#1, well#1–2, well#2, as well as BC well#1. n/a is for not analyzed; n/d is for under the detection limit. The type of containers associated with each sample is specified as it induces a difference in the sampling method.

Waters from DMK well#1, DMK well#1–2, DMK well#2, DMK well#3, and BC well#1 are reduced (Eh as low as –298.4 mV) and highly basic (pH ranges from 10.6 to 11.4). The conductivities range from 71 to 22,594 mS/cm. Temperatures range from 5.6 to 10.4°C. Waters from DMK well#4, DMK well#5, and DMK well#6 are slightly reduced (Eh ranges from –20.8 to –32.3 mV) and neutral to slightly basic (pH ranges from 8.3 to 9.4). Conductivities range from 1927 to 9975 mS/cm. In addition, measured temperatures are ranging from 5.7 to 9.4°C. Water from D.MCK well#1 has a pH of 10.6 and is oxidized (130 mV) with a conductivity of 102 mS/cm and a temperature of 8°C.

Waters from DMK well#1, well#2, well#3, well#4, and BC well#1, mostly dominated by higher pH and reduced Eh are associated with intrusive rocks of the Duluth Complex or other hypabyssal intrusions. The less reduced
<caption>**Table 3**</caption>

Relative Abundance of Noble Gases and Isotopic Ratios (⁴He, Calculated ³He, ²⁰Ne, ³⁶Ar, Calculated ⁴⁰Ar, ⁸⁴Kr, ¹²⁹Xe, ⁴⁰Ar/³⁶Ar, ³⁸Ar/³⁶Ar, ³He/³He, ²⁰Ne/³He) for Each Well, Air Normalized Values and Uncertainties

<table><thead><tr><th>Sample</th><th>Tamarack well 1</th><th>Tamarack well 2-1</th><th>Tamarack well 2-2</th><th>DMK well#1-2</th><th>DMK well#2</th><th>DMK well#2-2</th><th>DMK well#3</th></tr></thead><tbody><tr><td>Pressure (bar)</td><td>64</td><td>63.0</td><td>63.8</td><td>61.6</td><td>61.6</td><td>4.1</td><td>5</td></tr><tr><td>Date of analysis</td><td>22/3/23</td><td>8/3/23</td><td>9/3/23</td><td>12/4/23</td><td>3/5/23</td><td>3/5/23</td><td>4/5/23</td></tr><tr><td>Noble gases</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td></tr><tr><td>⁴He</td><td>9.7</td><td>12,858.0</td><td>13258.7</td><td>1440.4</td><td>2371.2</td><td>2594.1</td><td>81.4</td></tr><tr><td>²⁰Ne</td><td>16.252</td><td>5.446</td><td>5.247</td><td>6.046</td><td>13.206</td><td>14.264</td><td>17.408</td></tr><tr><td>³⁶Ar</td><td>27.788</td><td>10.485</td><td>10.022</td><td>11.655</td><td>25.591</td><td>29.763</td><td>31.669</td></tr><tr><td>⁴⁰Ar*</td><td>8211.48</td><td>3098.35</td><td>2961.59</td><td>3528.20</td><td>7562.23</td><td>8894.39</td><td>9358.28</td></tr><tr><td>⁸⁴Kr</td><td>5.84E-01</td><td>2.19E-01</td><td>1.91E-01</td><td>2.63E-01</td><td>4.19E-01</td><td>6.39E-01</td><td>6.11E-01</td></tr><tr><td>¹²⁹Xe</td><td>2.42E-02</td><td>1.17E-02</td><td>3.53E-03</td><td>1.05E-02</td><td>1.03E-02</td><td>3.10E-02</td><td>1.76E-02</td></tr><tr><td>⁴⁰Ar/³⁶Ar</td><td>295.5</td><td>295.5</td><td>295.5</td><td>302.7</td><td>295.5</td><td>298.8</td><td>295.5</td></tr><tr><td>³⁸Ar/³⁶Ar</td><td>0.018</td><td>0.019</td><td>0.019</td><td>0.019</td><td>0.019</td><td>0.019</td><td>0.019</td></tr><tr><td>³He/⁴He</td><td>1.00E-06</td><td>3.16E-08</td><td>2.74E-08</td><td>9.84E-08</td><td>1.23E-07</td><td>1.20E-07</td><td>1.82E-07</td></tr><tr><td>²⁰Ne/⁴He</td><td>1.6786</td><td>0.0004</td><td>0.0004</td><td>0.0042</td><td>0.0056</td><td>0.0055</td><td>0.2139</td></tr><tr><td>⁴He/²⁰Ne</td><td>0.596</td><td>2361.091</td><td>2526.710</td><td>238.222</td><td>179.552</td><td>181.865</td><td>4.676</td></tr><tr><td>⁴He/⁴⁰Ar</td><td>0.001</td><td>4.15</td><td>4.48</td><td>0.41</td><td>0.31</td><td>0.29</td><td>0.01</td></tr><tr><td>NG uncert.</td><td>+/-</td><td>+/-</td><td>+/-</td><td>+/-</td><td>+/-</td><td>+/-</td><td>+/-</td></tr><tr><td>⁴He</td><td>0.7</td><td>891.7</td><td>919.1</td><td>99.8</td><td>164.3</td><td>179.7</td><td>5.6</td></tr><tr><td>²⁰Ne</td><td>1.007</td><td>0.338</td><td>0.325</td><td>0.375</td><td>0.818</td><td>0.884</td><td>1.079</td></tr><tr><td>³⁶Ar</td><td>0.398</td><td>0.151</td><td>0.143</td><td>0.167</td><td>0.366</td><td>0.426</td><td>0.463</td></tr><tr><td>⁸⁴Kr</td><td>2.71E-02</td><td>1.02E-02</td><td>8.85E-03</td><td>1.22E-02</td><td>1.94E-02</td><td>2.97E-02</td><td>2.85E-02</td></tr><tr><td>¹²⁹Xe</td><td>2.24E-03</td><td>1.09E-03</td><td>3.12E-04</td><td>9.48E-04</td><td>9.59E-04</td><td>2.99E-03</td><td>1.65E-03</td></tr><tr><td>⁴⁰Ar/³⁶Ar</td><td>2.8</td><td>2.9</td><td>2.9</td><td>3.0</td><td>2.8</td><td>1.4</td><td>2.8</td></tr><tr><td>³⁸Ar/³⁶Ar</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td><td>0.0</td></tr><tr><td>³He/⁴He</td><td>6.31E-08</td><td>8.40E-09</td><td>1.44E-08</td><td>1.46E-08</td><td>2.15E-08</td><td>1.22E-08</td><td>4.01E-08</td></tr><tr><td>Normalized to Atm values</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td><td>ppm</td></tr><tr><td>⁴He</td><td>1.8</td><td>2453.8</td><td>2530.3</td><td>274.9</td><td>452.5</td><td>495.0</td><td>15.5</td></tr><tr><td>³He*</td><td>1.3</td><td>56.0</td><td>50.2</td><td>19.5</td><td>40.3</td><td>42.8</td><td>2.0</td></tr><tr><td>²⁰Ne</td><td>0.988</td><td>0.331</td><td>0.319</td><td>0.368</td><td>0.803</td><td>0.867</td><td>1.058</td></tr><tr><td>³⁶Ar</td><td>0.880</td><td>0.332</td><td>0.317</td><td>0.369</td><td>0.811</td><td>0.943</td><td>1.003</td></tr><tr><td>⁸⁴Kr</td><td>0.898</td><td>0.337</td><td>0.293</td><td>0.405</td><td>0.645</td><td>0.983</td><td>0.940</td></tr><tr><td>⁴⁰Ar/³⁶Ar</td><td>0.990</td><td>0.990</td><td>0.990</td><td>1.014</td><td>1.001</td><td>1.001</td><td>0.990</td></tr><tr><td>³He/⁴He</td><td>0.723</td><td>0.023</td><td>0.020</td><td>0.071</td><td>0.089</td><td>0.087</td><td>0.132</td></tr></tbody></table>

(higher Eh) waters from DMK well#5, well#6, and D.MCK well#1, on the other hand, are associated with volcanic rocks from the North Shore Volcanic Group (NSVG).

As Tamarack well#1 was not tightly sealed, it did not present any gas flux or accumulation. Conversely, Tamarack well#2 presented a gas accumulation at the opening and then a small gas flux. No water was present in either well.

## 4.2. Gas Analysis

### 4.2.1. Gas Composition

Tamarack Well#1 and Tamarack well#2, DMK well#1, DMK well#1-2, DMK well#2, DMK well#3, DMK well#5, and DMK well#6, and BC well#1 presented enough gas to be sampled. The gas, sampled at atmospheric
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Physico-chemical parameters (pH and redox state) of water for each well in function of the geology of the host rocks.</figcaption></figure>

pressure in exetainers and inox tubes, was analyzed by GC to characterize the composition and the C₁/C₂ ratio (Table 2, Figure 3). All samples were dominated by O₂ and N₂.

Tamarack wells present two different trends. Tamarack well#1 is dominated by CO₂-C₂H₆ with concentrations reaching 1.5% of CO₂ and 4.6% of C₂H₆ and associated with approximately 3,000 ppmv of CH₄ and less than 5 ppmv of H₂. Tamarack well#2 is dominated by CH₄-H₂ with concentrations reaching 60% of CH₄ and with up to 1.2% of He, 500 ppmv of H₂, but only 2,497 ppmv of CO₂, and 5,400 ppmv of C₂H₆.

Some samples, namely BC well#1 (9%), DMK well#1 (3%), well#1–2 (52%), and well#2 (4%) are CH₄ dominated. Associated CO₂ and C₂H₆ concentrations range from 7 to 1,670 ppmv and 112 ppm to 1.5%. H₂ is detected in BC well#1 and DMK well#1 and reaches, respectively, 10 and 100 ppmv. Helium also reaches 2,350 ppmv in DMK well#2.

DMK well#3 is H₂-CO₂ dominated with 470 and 500 ppmv, respectively. Helium also reaches 40 ppmv in DMK well#3. CH₄ values are under 50 ppmv and C₂H₆ is recorded under 1 ppmv. The balance is on N₂ and O₂. DMK well 6 is CO₂-dominated with 301 ppmv, while CH₄ only reaches 72 ppmv. No H₂ or C₂H₆ are recorded.

### 4.2.2. Methane Stable Isotopes

Isotopic values presented in this paragraph are values from Table 3. Tamarack well#1 is characterized by a δD value of −277‰ and a δ¹³C value of −59‰. δD values of Tamarack well#2 range between of −336 and −327‰, δ¹³C values range from −45 to −47.4‰. These two wells have the highest carbon and deuterium-depleted methane values in this study.

The most D and ¹³C enriched gases sampled are from DMK well#1 and well#1–2, which have δD values of −159 to −145‰ and a δ¹³C value of −19‰, and DMK well#1, which has a δD value of −159‰ and a δ¹³C value of −19‰. Variations of δ¹³C and δD of CH₄ are recorded from one site to another. DMK well#2 has deuterium values ranging from −279 to −274‰ and a carbon isotopic value of −31‰.

BC well#1 has a δD value of −274‰ and a δ¹³C value of −30‰. Compared to the Tamarack wells, DMK well#2 and BC well#1 have methane that is slightly enriched in ¹³C and D.

The δ¹³C and δD of CH₄ of the studied wells displayed a large range of values with ¹³C and D-depleted methane in Tamarack wells and ¹³C and D-enriched methane in DMK well#1, 1–2, 2, and BC well#1 compared to the Tamarack ones.

### 4.2.3. Noble Gas Composition and Isotopy

Noble gas analysis was performed on gas samples from Tamarack well#1, Tamarack well#2 (and its duplicate Tamarack well#2-2), DMK well#1–2, DMK well#2 (and its duplicate DMK well#2-2), and DMK well#3. The results are presented in Table 3. Such analyses were not performed on DMK well#1, DMK well#4, DMK well#5, DMK well#6, and BC well#1 as not enough gas could be successfully sampled to perform both qualitative GC and noble gas analysis.

For the Tamarack well#1, ²⁰Ne, ³⁶Ar, ⁸⁴Kr contents and ⁴⁰Ar/³⁶Ar have a typical atmospheric signature (Stute & Schlosser, 2000) (Table 3, Figure 4). The ⁴⁰Ar/³⁶Ar ratio of air is 298.6 (Lee et al., 2006). It is coherent with our field observations because the well was not sealed properly, and no gas flow was detected, explaining the large atmospheric contamination of the sample.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Gas composition in H₂, CH₄, CO₂, C₂H₆, and He acquired by gas chromatography for Tamarack well#1 and well#2, DMK well#1, well#1–2, well#2, well#3, well#5, well#6, and BC well#1 (\* is for gases in exetainers). The main gases, O₂ and N₂, are not represented in this figure.</figcaption></figure>

For the Tamarack well#2, Tamarack well#2-2, DMK well#1, DMK well#2, DMK well#2-2, and DMK well#3, the ²⁰Ne, ³⁶Ar, ⁸⁴Kr contents are similar to the atmospheric content with a small depletion in these gases compared to air (Table 3).

For the DMK well#1–2, ²⁰Ne, ³⁶Ar, ⁸⁴Kr contents show a degree of depletion correlated with the elemental mass of the noble gases (Kr > Ar > Ne) as ⁸⁴Kr is less depleted than ²⁰Ne and ³⁶Ar. This is coherent with an atmospheric equilibrated water. The ⁴⁰Ar/³⁶Ar ratio of each sample is close to the air value.

Tamarack well#2-2 shows enrichment in ⁴He of 1.28 (±0.09) and 1.32 (±0.09)% (Figure 4) in comparison with the atmospheric value of ⁴He of 5.2 ppmv (Glückauf & Paneth, 1946). DMK well#1–2, DMK well#2, DMK

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Relative abundance of the air-normalized isotope composition of noble gases (⁴He, ²⁰Ne, ³⁶Ar, ⁸⁴Kr, ⁴⁰Ar/³⁶Ar, and ³He/⁴He) for each studied well. Also plotted is the relative abundance isotope composition of noble gas for the air-equilibrated water. Noble gases above the air average composition (black dashed line) are enriched compared to air, while all noble gases below the air average composition are depleted compared to air.</figcaption></figure>
<table><caption>**Table 4**
Acetate and Formate Concentrations in Sampled Fluids</caption><thead><tr><th>#Well. sample</th><th>Acetate (umol/L)</th><th>Formate (umol/L)</th></tr></thead><tbody><tr><td>DMK well#1.1</td><td>181</td><td>756</td></tr><tr><td>DMK well#1–2.4</td><td>n.d.</td><td>n.d.</td></tr><tr><td>DMK well#2</td><td>178</td><td>629</td></tr><tr><td>DMK well#3.4</td><td>n.d.</td><td>n.d.</td></tr><tr><td>DMK well#4.1</td><td>n.d.</td><td>n.d.</td></tr><tr><td>DMK well#5.4</td><td>n.d.</td><td>n.d.</td></tr><tr><td>DMK well#6.4</td><td>n.d.</td><td>n.d.</td></tr><tr><td>D.MCK well#1</td><td>191</td><td>643</td></tr><tr><td>BC well#1.4</td><td>183</td><td>635</td></tr></tbody></table>

Note. n.d. indicates compounds were below the detection limit.

well#2-2, and DMK well#3 show lower but still significant enrichment in $^4$He, with values of 2,371 (±164), 2,594 (±180), and 81 (±6) ppmv.

Helium of Tamarack well#2 samples has a crustal signature in each case with R/Ra values of 0.023 and 0.020 compared to the typical crustal value of 0.02 (Gerling et al., 1971). DMK well#1–2, DMK well#2, and DMK well#2-2 show a slightly crustal signature with R/Ra values of 0.071, 0.089, and 0.087. The R/Ra value of DMK well#3 is 0.132. A small mantle component can also be identified.

The $^4$He/$^{40}$Ar ratios of Tamarack well#2 samples and its duplicate are 4.15 and 4.48, respectively, which is coherent with the calculated ratio of the continental crust being 4.9 (Ballentine et al., 1991). The $^4$He/$^{40}$Ar ratios of DMK well#1–2, DMK well#2, DMK well#2-2, and DMK well#3 are significantly lower than the calculated ratio of the continental crust with values of 0.41, 0.31, 0.29, and 0.01, respectively.

The $^{40}$Ar/$^{36}$Ar ratios of Tamarack well#2 associated with the $^4$He enrichment do not show a crustal isotopic signature as they are similar to the atmospheric value which is coherent with the atmospheric component of the samples. The

$^{40}$Ar/$^{36}$Ar ratio of DMK well#1–2, DMK well#2, DMK well#2-2, and DMK well#3 associated with the $^4$He enrichment traduce the atmospheric component of the samples.

Noble gas analyses reveal that all the gas sampled in Tamarack well#2 has a clear crustal component ($^4$He enrichment) with a more or less important atmospheric contribution (Figure 4). DMK well#1–2, DMK well#2 (and its duplicate DMK well#2-2), and DMK well#3 have a less pronounced crustal isotopic signature overprinted by the atmospheric component of the samples as well as a potential small mantle component.

### 4.2.4. Volatile Organic Acids

HPLC analyses of the sampled fluids show that some of the samples contain hundreds of micromolars per liter of formate and acetate (Table 4). Four of the samples collected in water wells (D.MCK well#1, DMK well#1.1, DMK well#2, and BC well#1.4) show respective acetate values of 191, 181, 178, and 183 µmol/L. Formate concentration in those samples exhibits higher values of respectively 643, 756, 629, and 635 µmol/L. Those values are given with a 10% uncertainty obtained on an external standard.

## 5. Discussion

### 5.1. Helium Origin

The helium content, up to 1.3% in the Tamarack well#2 (against 5.2 ppmv for the atmospheric value), raises questions about helium origin and accumulation. $^3$He/$^4$He and $^4$He/$^{40}$Ar ratios of Tamarack well#2 are favorable for a crustal origin of helium (Figure 5). Similar helium content and $^3$He/$^4$He ratios have been measured for crustal gases from the Canadian Shield (Sherwood et al., 1988; Sherwood-Lollar et al., 1993, 1994).

In the gases from DMK well#1–2, DMK well#2, and DMK well#3, however, the isotopic signature of crustal $^4$He is overprinted by the isotopic signature of atmospheric helium, as well as a small mantellic component. R/Ra values of mantle gases are commonly higher than 5, whereas R/Ra values of gases collected during this study are lower than 1. The CO$_2$/$^3$He ratio of the MORB ranges from 1.10$^9$ to 10.10$^9$ (Marty & Jambon, 1987), while the CO$_2$/$^3$He ratios shown in Table 4 display values between 18 and 234 for the five studied wells (Table S1). Therefore, these results exclude the presence of mantellic CO$_2$ but do not fully exclude the presence of mantle helium (in small excess). Thermal neutron-induced fission of lithium (Martel et al., 1990; Solomon et al., 1992) could explain the presence of radiogenic helium. There are currently no economically viable lithium deposits in NE-Minnesota, but the MCR lies on the Quetico Subprovince, which is a granitic batholith. If deep faults are present in the area, this could potentially explain the small excess of $^3$He. The U, Th, and K contents of the rocks and the Li content of the fluids have not been characterized; thus, the proportion of such crustal and radiogenic helium in the measured gases cannot be determined. However, the gases sampled in this study have similar
<figure><img src="image_7.png" /><figcaption>**Figure 5.** $^{20}$Ne/$^{4}$He in the function of R/Ra ($^{3}$He/$^{4}$He air normalized) for each well with air, crustal, and mantellic endmembers (Additional points in gray and black are from Vacquand (2011), Guélard (2016), and Vacquand et al. (2018)). Measurements show a trend between crustal and atmospheric endmembers.</figcaption></figure>

$^{3}$He/$^{4}$He ratios to those obtained for MCR samples from Kansas, attributed to a crustal origin (Guélard et al., 2017).

The $^{4}$He/$^{40}$Ar ratios of DMK well#1-2, DMK well#2, and DMK well#3 are lower than the average mantellic ratio of 1.7 (Ballentine & Burnard, 2002) or much higher in the case of the Tamarack wells, where $^{4}$He/$^{40}$Ar corresponds to the average crustal ratio of 4.9 (Ballentine et al., 1991). The gases collected for the DMK wells were dissolved in water that was pumped. This could have induced fractionation processes. Of note, argon is more soluble than helium. This could explain the low $^{4}$He/$^{40}$Ar ratios of the DMK wells. The atmospheric component of the DMK wells could also induce an overprinting of the crustal argon signature.

Past studies have highlighted high He contents in intracratonic settings (Guélard et al., 2017; Sherwood-Lollar et al., 1994, and more recently Cheng et al., 2021, 2023). They have shown that in He-rich intracratonic sedimentary systems overlaying the Precambrian basement, cross-formational diffusive migration occurs over geological times and can explain these He accumulations. In these systems, the main flux of helium comes from the adjacent basement. Cheng et al. (2023) also highlighted that high concentrations of N$_{2}$ play a significant role in helium accumulation in these intracratonic settings. They showed that as basement sourced-N$_{2}$ is the major exsolving gas phase (because it reaches saturation in the water before He), it controls the timing and location of the He-accumulation.

In this study, the N$_{2}$ concentrations are, respectively, 32.6% and 33.1% in the high He content wells Tamarack well#2-2 and DMK well#2-2. This highlights a plausible association between N$_{2}$ and He in the MCR basement rocks. It seems plausible that NE-Minnesota rocks correspond to a primary N$_{2}$-He gas source being currently active and potentially driven using diffusive cross-formational migration and accumulation processes.

## 5.2. Origins of NE-Minnesota Methane From $^{13}$C and D Isotopic Values

### 5.2.1. Methane From the Tamarack Intrusion

The carbon and deuteric isotopic signatures of methane from NE-Minnesota Precambrian shield rocks cover a wide range of values. Figure 6 displays these values, and hypothetical gas origins based on previous studies without considering any additional information such as pH or Eh of the associated fluids or geological units encountered by the wells.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Diagram of deuterium isotopic values of methane versus carbon isotopic values of methane for the Tamarack wells, the DMK wells# 1, 2, and 3, as well as BC well#1, also plotted for comparison are usual microbial, thermogenic, and abiotic ranges (modified from Giunta et al. (2019), see references therein), as well as isotopic values of other Precambrian shield gases (Guélard et al., 2017; Sherwood-Lollar et al., 2006) and ophiolite gas (H. M. Miller et al., 2018; Vacquand et al., 2018) collected from various representative studies.</figcaption></figure>

Tamarack well#1 methane has the most ¹³C-depleted values and highly ²H-depleted values, usually interpreted as resulting from a microbial origin (Figure 6). Tamarack well#2 has the most ²H depleted values and highly ¹³C depleted values (Figure 6). These values would usually be considered thermogenic, but in a Precambrian context, other mechanisms such as microbial CH₄ production and abiotic methanation reactions should not be ruled out. Indeed, these gases bear similarities with the Witwatersrand Basin gases (Driefontein mines, South Africa). The Driefontein gases display carbon isotopic values of CH₄ between -40.2 and -55.5‰ and deuteric isotopic values of CH₄ between -218 and -403‰. The rocks of the Driefontein mines are volcano-sedimentary rocks overlying 3.0 Ga granitic rocks. The analyzed methane collected there was characterized by a main abiotic component produced by water-rock interactions (WRI) and a second microbial component produced by methanogenesis (Sherwood-Lollar et al., 2006). The Tamarack well#1 and well#2 are not associated with volcano-sedimentary rocks but with the olivine cumulate of the Tamarack intrusion, an early intrusion of the 1.1 Ga MCR that has been described as serpentinized (Goldner, 2011). The occurrence of WRI in such a geological context highlights a potential abiotic methanation production through H₂ consumption with H₂ being provided by serpentinization. However, even if very appealing, this hypothesis cannot explain the depleted ¹³C isotopic signature of this methane. Indeed, traditional interpretation suggests that abiotic gases have much heavier isotopic signatures (Etiope & Sherwood Lollar, 2013). An alternative interpretation can therefore be put forward. One could think that water is circulating through deeper rocks, such as the Paleoproterozoic metasedimentary shale layers beneath the intrusion, and then crossing the serpentinized and H₂-producing rocks of the intrusion. Methane could result from a mix of reactions: degradation of organic matter (if present) in the deepest levels, biological methanation, and localized abiotic methanation in the upper levels where H₂ is available, in fine producing an isotopically depleted gas mixture.

### 5.2.2. Methane From the Duluth Complex and Similar-Ages Intrusions

The gases from BC well#1 and DMK well#2 follow the same isotopic trend as the Tamarack well#2 gas with slightly less depleted carbon and deuteric isotopic values. Their values are outside the thermogenic typical
delimitation in Figure 6. They are more enriched in ¹³C than typical microbial or even thermogenic methane and could fit into the ¹³C-D-depleted values of CH₄ produced by abiotic mechanisms. These values are characteristic of Precambrian crystalline igneous rocks. Indeed, their ¹³C and D contents are like the ones of the Fennoscandian shield (Juuka region, Finland). The Juuka gases display carbon isotopic values of CH₄ between -22.4 and -29.3‰ and deuteric isotopic values of CH₄ between -200 and -281‰. The rocks bearing the Juuka gas accumulation are composed of serpentinized and altered ultramafic rocks, and the methane of the Juuka region has been characterized as abiotic (Sherwood-Lollar et al., 1993). BC well#1 reaches gabbroic cumulates of the Duluth Complex (depth ~ 300 feet—personal communication from the owner of the well, no depth reported by the Minnesota Department of Natural Resources). DMK well#2 reaches gabbroic rocks of the hypabyssal intrusions of the MCR and Paleoproterozoic metamorphic rocks derived from shale-rich sedimentary rocks (from 206 to 320 feet—depth and rock types are reported from the Minnesota Department of Natural Resources). The igneous intrusions of the MCR have been extensively studied over the past decades for ore exploration and bear interesting Fe-rich rocks (J. Miller et al., 2002 and references therein). Serpentinization, potentially leading to H₂ production, has even been described in peridotites of the Duluth Complex (Evans et al., 2017; Tutolo et al., 2019). The rocks reached by both wells could be the location of abiotic methane formation through the consumption of hydrogen produced by serpentinization. However, considering that some microorganisms can produce methane with high ¹³C values usually attributed to abiotic pathways (H. M. Miller et al., 2018) and that CH₄ isotopic signature can vary in function of multiple parameters (C origin, mixing processes, and isotopic fractionation), such deuteric and carbon isotopic signatures may also represent mixing processes with a main abiotic trend and a secondary biotic trend. In the case of DMK well#2, the Paleoproterozoic rocks could be prone to produce hydrocarbons through the degradation of organic matter (if this latter has been preserved through geological times), and thus a component of the gas could be thermogenic. However, organic matter conservation through such a geologic timespan is less probable.

DMK well#1 and DMK well#1-2 are two different samples of the same well taken at different times (one after a long recharge and one after a 24-hr recharge, respectively). They have the most ¹³C and D-enriched values of the study (Figure 6). Their values are out of the scope of typical microbial and thermogenic methane and are similar to the ones found in volcanic-hydrothermal systems little affected by organic matter where CH₄ is produced by abiotic pathways and gas produced in some ophiolitic contexts (Vacquand et al., 2018). DMK well#1, such as BC well#1 and DMK well#2, reaches gabbroic cumulates of the Duluth Complex from the MCR (300 feet deep for DMK well#1). Methane could originate from an abiotic reaction by consuming the H₂ produced during WRI in the igneous rocks of the MCR. However, DMK well#1 has a heavier isotopic signature than BC well#1. This heavier isotopic signature could also be explained by the presence of methanotrophic organisms consuming the CH₄. Such a process of anaerobic oxidation would leave residual methane with an enriched ¹³C signature (as ¹²C is preferentially consumed), and a low C₁/C₂⁺ ratio, as exhibited by the samples (Table 1).

### 5.3. Hypotheses on Hydrogen Production and Consumption in NE-Minnesota Intrusive Rocks

Water-rock interactions potentially leading to H₂ generation have been reported by several authors, as serpentinization has been reported in all studied areas: in some intrusions of the Duluth Complex (Evans et al., 2017; Tutolo et al., 2019), of the Beaver Bay complex (Joslin, 2004) and in part of the Tamarack intrusion (Golder, 2011; Taranovic et al., 2015). Hydrogen is present in the surface fluids collected at the Tamarack wells, BC well#1, and DMK well#1 and well#3, which reach the Duluth Complex. However, when one considers the volume of igneous rocks (5,000 km² for the Duluth Complex—J. D. Miller et al., 2010), their overall Fe-content (J. Miller et al., 2002), and the occurrence of water circulating through the rocks, it is surprising that H₂ concentrations never exceed 500 ppmv. A few hypotheses can be made regarding these low but still notable hydrogen concentrations.

The Tamarack wells that were tested in this study were former exploration wells a few hundred feet deep, whose wellheads were not fully gas-tight at the time of sampling. They are open to air, and even if a “deep” gas can accumulate, the surrounding area is air-saturated. The wells sampled in the rest of the MCR never exceed a depth of 300 feet and are also, at some point, open to air. In that sense, all samples are diluted into an air component, which could explain why concentrations never exceed 500 ppmv. Indeed, the atmospheric signature of these gases is well characterized by the helium concentration where crustal He is often overprinted by atmospheric He.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Formate and acetate concentrations in brines collected at H₂ and CH₄ producing sites. High concentrations, if associated with ¹³C-enriched values, would suggest abiotic synthesis. Data from this study (Eickenbusch et al., 2019, Kohl et al., 2016, Lang et al., 2018; Sherwood-Lollar et al., 2021).</figcaption></figure>

A recent study reported the direct observation of a hydrogen-rich pressurized reservoir within the Tisotiva ophiolite in Romania (Baciu & Etiope, 2024). The H₂-rich reservoir is hosted there within cataclastic dunites at a depth of >600 m. The gas is believed to have accumulated through geological times due to highly serpentinized layered dunites and their associated tonalite veins, which act as cap rocks. It is striking that, even if the gas accumulates at depth, the gas recorded in the subsurface never exceeds 100 ppmv (and 0.1 mg H₂/L of water), while the pressurized gas sampled at the wellhead reached ~29 vol%. This shows that H₂-rich systems can be described by a variety of gas concentrations and that surface measurements often offer only a glimpse of the real potential of deeper reservoirs because of gas being diluted into the air. The MCR bears a similar H₂ potential as the Romanian ophiolite. One could consider that H₂ is being produced at a depth and accumulates in a reservoir. The gases sampled in this current study should then be considered as diluted sub-surface manifestations of these potential deep pressurized gases.

In addition, the presence of VOA suggests that H₂ is being consumed on the surface either by abiotic synthesis or microbial activity. As demonstrated experimentally by various authors (McCollom & Seewald, 2007), Fischer-Tropsch-Type (FTT) reactions (such as the Sabatier reaction) occurring in water do not directly produce methane from H₂ and CO₂ (or CO) interactions but rather intermediate organic compounds with varying degrees of oxidation. Such species have also been described in natural sites (where H₂ is emitted) and are believed to form on the reaction pathway to CH₄ with decreasing redox conditions (Lang & Brazelton, 2020).

Amongst those intermediate species, commonly known as the “building blocks of life” (Proskurowski et al., 2008), literature studies of natural sites have mostly focused on short-chain organic acids, formate, and acetate, as they appear to be the organic compounds with the highest concentrations, and thus the easiest to detect and quantify. Whether they are formed in high pH environments (Lost City hydrothermal vent, Lang et al., 2018; Marianas mud volcanoes, Eickenbusch et al., 2019; The Cedars ophiolitic complex, Kohl et al., 2016) or near-neutral pH brines (Kidd Creek Precambrian shield, Sherwood-Lollar et al., 2021), the current consensus on their formation mechanisms lies in abiotic synthesis through interaction of H₂ and carbonate radicals. Figure 7 compares the concentrations of those species between the Lake Superior water wells (this study), the Kidd Creek mine, The Cedars, the Marianas mud volcanoes, and Lost City hydrothermal fields.

In most sites, formate is in greater concentration than acetate. This could be explained by the fact that the shortest organic compounds form first from dissolved carbonate radicals by consuming H₂ before transforming into less oxidized molecules. Kidd Creek brines, however, exhibit remarkably high levels of acetate (over 1,000 µM), higher than formate (up to 600 µM). Furthermore, the heavy isotope values of those compounds (up to -6.9‰ for acetate; Sherwood-Lollar et al., 2021) strongly suggest they could not have been produced by the acetogen microbial community or derived from the isotopically light TOC within the host rocks.

The samples collected in the MCR in water wells along Lake Superior have high concentrations of formate, just like the ones reported for Kid Creek brines. Those high values, up to 756 µmol/L, raise the possibility of abiotic synthesis reactions occurring in the subsurface. Of note, acetate formation from organic matter degradation has been identified in the Lost City fluids (Lang et al., 2010), indicating that VOA formation could also occur when TOC values are high. Furthermore, in sediment systems, acetate reaches millimolar concentrations through what may also be attributed to organic matter degradation (Heuer et al., 2020). However, along Lake Superior, within igneous rocks poor in organic matter, as these species are associated with hundreds of ppmv of H₂ and a “heavy” isotopic composition of CH₄ (-30‰ to -18‰), their high concentrations suggest an abiotic generation. The presence of VOA suggests that H₂ is being produced in those brines and consumed as it circulates through the subsurface up to the surface. Whether it is 100% abiotic or not, a partly microbial production of those volatiles through acetogenesis, for instance, would still be consuming hydrogen, suggesting its presence in the subsurface.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Synthetic schema of the CH₄-He-H₂ system hosted in NE-Minneapolis Mid-Continent Rift rocks (figure is not at scale).</figcaption></figure>

The fact that VOAs are below the detection limit in the remaining water samples may result from less favorable pH and Eh conditions for their formation through abiotic processes or H₂ biological consumption. In other words, there are many indicators for H₂ production at depth following consumption either by abiotic or biotic processes producing CH₄ and dilution of the remaining gas content with surrounding air in the surface environment.

## 6. Conclusions

This study documents the first gases and associated water data related to the northern part of the MCR in the United States. The NE-Minneapolis waters sampled in this study, associated with the Duluth Complex (or similar aged intrusions) igneous rocks, are reduced, highly alkaline, and characterized by formate, acetate, and dissolved gas exsolving when reaching the surface. This gas is made of various amounts of CH₄, H₂, He, and, to a small extent, C₂H₆.

The gases studied here seem to differentiate into three types: gas (1) from Tamarack wells, dominated by isotopically light CH₄ with up to 500 ppmv of H₂ and 1.3% of He, gas (2) from DMK well#2 and BC well#1, dominated by CH₄ with isotopic signatures supporting mixing of different origins, up to 2,350 ppmv of He, and only traces of H₂, and gas (3) from DMK well#1 and 1–2, dominated by isotopically heavy CH₄ and up to 100 ppmv of H₂ and 275 ppmv of He.

Helium appears to be of crustal origin when not diluted into an atmospheric component, highlighting a deep gas flux. In addition, the water geochemistry is typical of a system hosting serpentinization reactions in ultramafic rocks. Previous studies have already identified some WRI features in the Duluth Complex and other MCR-related intrusions. All the ingredients prone to H₂ production are thus present in this part of the MCR. The collected data suggest that H₂ has been produced by WRI in deep igneous rocks of the MCR and is now being consumed by abiotic or biotic pathways as it migrates toward the surface, leading to the formation of isotopically diverse methane gases (Figure 8).

Interestingly, lower pH and higher redox waters with small amounts (to none) of H₂-CH₄-He are associated with the North Shore Volcanic Group (NSVG) rocks of the MCR. Thus, high reactivity and H₂-CH₄-He rich gases are only seen in the intrusive rocks of the MCR in higher pH fluids.

This study has shed new light on Precambrian gas compositions in ultramafic rocks. The collected data has been used to draw a CH₄-H₂-He generation model in the NE-Minneapolis MCR, which points toward hydrogen generation and consumption in shallow subsurface fluids that are rich in organic matter and with extreme physicochemical parameters. A thorough biological analytic campaign is still needed to determine the range of microbial activity and the kinetics of H₂ consumption. Further isotopic analysis of methane doubly substituted isotopologues could also provide information on the nature and potential formation temperature of some of the gas collected. Nevertheless, this study highlights evidence of H₂ production at some very specific areas in the rift and could therefore be used as potential locations for future drilling sites and deeper sampling, as exploration for H₂ and He on the Mid Continental Rift is booming.

## Data Availability Statement

The data used to conduct this study are available on Mendeley Data (Combaudon et al., 2025).
Acknowledgments

This work was supported by E2S UPPA/
Nouvelle-Aquitaine Region and by the
"France 2030" Investment Program
managed by the French National Research
Agency (ANR-16-IDEX-0002). H.
Derluyn acknowledges the support from
the European Research Council (ERC)
under the European Union's Horizon 2020
research and innovation program (Grant
agreement No 850853). The authors wish
to acknowledge Dean Rossell for his
availability and help and the Talon Metals
company for allowing us to sample both of
their exploration wells. The authors wish to
acknowledge Amy Radakovich Block
from the Minnesota Geological Survey,
Don McKeever from McKeever Drilling,
and Bob Carlson for providing valuable
help on and off the field. The authors also
wish to acknowledge Patience Ekambas
and Elodie Muller for their help during the
sample analysis.

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