# Low-Temperature (100°C–300°C) Magnetite Production in Variably Serpentinized Lherzolites

Keanu Loiseau, Charles Aubourg, Pierre Camps, Guilhem Hoareau, Claire

Carvallo, Isabelle Moretti

► To cite this version:

Keanu Loiseau, Charles Aubourg, Pierre Camps, Guilhem Hoareau, Claire Carvallo, et al. Low-Temperature (100°C–300°C) Magnetite Production in Variably Serpentinized Lherzolites. Geochemistry, Geophysics, Geosystems, 2025, 26 (10), pp.e2024GC012127. 10.1029/2024GC012127. hal-05351922

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Submitted on 6 Nov 2025

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

## RESEARCH ARTICLE

10.1029/2024GC012127

**Key Points:**

* We test magnetite formation in a solid sample of lherzolite in the range 100–300°C using a simple experiment

* When heated for 24 hr in a humid atmosphere, magnetite is formed at over 100°C

* The greater the amount of initial magnetite, the greater the amount of magnetite formed

**Correspondence to:**

C. Aubourg,
charles.aubourg@univ-pau.fr

**Citation:**

Loiseau, K., Aubourg, C., Camps, P., Hoareau, G., Carvallo, C., & Moretti, I. (2025). Low-temperature (100°C–300°C) magnetite production in variably serpentinized lherzolites. *Geochemistry, Geophysics, Geosystems, 26*, e2024GC012127. https://doi.org/10.1029/2024GC012127

Received 4 FEB 2025
Accepted 27 SEP 2025

**Author Contributions:**

**Conceptualization:** Keanu Loiseau,
Charles Aubourg, Guilhem Hoareau,
Isabelle Moretti

**Formal analysis:** Keanu Loiseau,
Charles Aubourg, Pierre Camps

**Funding acquisition:** Isabelle Moretti

**Methodology:** Keanu Loiseau,
Charles Aubourg, Pierre Camps

**Resources:** Charles Aubourg

**Software:** Pierre Camps

**Supervision:** Isabelle Moretti

**Validation:** Keanu Loiseau,
Charles Aubourg, Pierre Camps

**Visualization:** Guilhem Hoareau

**Writing – original draft:** Keanu Loiseau,
Charles Aubourg, Guilhem Hoareau,
Isabelle Moretti

**Writing – review & editing:**
Keanu Loiseau, Charles Aubourg,
Pierre Camps

© 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.

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

## Low-Temperature (100°C–300°C) Magnetite Production in Variably Serpentinized Lherzolites

Keanu Loiseau¹,² ID, Charles Aubourg¹ ID, Pierre Camps³ ID, Guilhem Hoareau¹, Claire Carvallo⁴, and Isabelle Moretti¹ ID

¹Laboratoire des Fluides Complexes et de leurs Réservoirs (LFCR), UMR5150, IPRA, Université de Pau et des Pays de l'Adour, Pau, France, ²CVA-Engineering, Pau, France, ³Géosciences Montpellier, Université de Montpellier et CNRS, Montpellier, France, ⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Sorbonne Université, Paris, France

**Abstract** The study of the serpentinization process in ultramafic rocks has been revived in recent years in response to the problems of natural hydrogen generation. Serpentinization results in the formation of magnetite, which is often observed in the most serpentinized peridotites, at concentrations of up to a few percentage. In this study, we propose a novel experimental approach to test the formation of magnetite at temperatures of 100°C, 200°C and 300°C. The aim is to test whether magnetite can be produced at low temperatures in variably serpentinized peridotites, and how the degree of serpentinization can influence this production. This experimental approach exploits the properties of newly formed magnetites to record the magnetic field applied during the experiment. To achieve this, a study was conducted on Pyrenean peridotites, which encompassed variable degrees of serpentinization. The findings suggest that magnetite formation commences at 100°C, irrespective of the initial rock's degree of serpentinization. At 200°C, the production of magnetite remains comparable to that at 100°C. However, at 300°C, the production of magnetite is an order of magnitude higher. The most noteworthy outcome of our investigation is the demonstration that the production of magnetite is directly proportional to the initial nanomagnetite content of the peridotite. This finding suggests that magnetite may act as a catalyst, with strongly serpentinized peridotite exhibiting a greater propensity for magnetite production than weakly serpentinized peridotite under natural conditions.

**Plain Language Summary** The serpentinization process in mantle rocks produces hydrogen, a promising low-carbon energy resource. This process, which is effective at temperatures near 300°C, also produces magnetite, a magnetic mineral that can be detected using geophysical methods. In this paper, we explore the possibility of producing magnetite at low temperatures (<300°C) and in a hydrous atmosphere using a simple laboratory experiment. This method, based on the magnetic field recording property of newly formed magnetites, has been applied to Pyrenean mantle rocks that exhibit a wide range of serpentinizations. We show that magnetite is formed in rock samples as early as 100°C. The most remarkable observation of our study is that the greater the amount of initial magnetite, the greater the amount of magnetite formed during the experiment. Translated to natural processes, our results therefore suggest that at depths of a few kilometers, the potential for hydrogen production is all the greater if the rock has already undergone extensive serpentinization, with magnetite playing a catalytic role.

## 1. Introduction

The process of serpentinization, identified in mafic and ultramafic rocks, is the subject of extensive study (Klein et al., 2020; Klein & Garrido, 2011; McCollom & Bach, 2009; Moody, 1976; Seyfried et al., 2007). Recently, however, there has been a resurgence of interest in this phenomenon, largely due to its relevance in the context of contemporary discourse on natural hydrogen. This phenomenon involves the oxidation of ferrous iron (Fe²⁺) in silicates and water reduction. In peridotite, magnetite (Fe²⁺Fe³⁺₂ O₄) is a by-product of the serpentinization reaction (Klein et al., 2013; Malvoisin, Brunet, et al., 2012; Malvoisin, Carlut, et al., 2012; Seyfried et al., 2007) and may therefore be a proxy for past hydrogen production (Klein et al., 2020). Malvoisin, Carlut, et al. (2012) proposed the following Equation 1 to describe the serpentinization process of olivine:
$$ (\text{Mg, Fe}^{2+})_2\text{SiO}_4 + \text{H}_2\text{O} \rightarrow (\text{Mg, Fe}^{2+}, \text{Fe}^{3+})_3\text{Si}_2\text{O}_5(\text{OH})_4 + (\text{Mg, Fe}^{2+})(\text{OH})_2 + \text{Fe}^{3+}_2\text{Fe}^{2+}\text{O}_4 + 2\text{H}_{2,\text{aq.}} \quad (1) \\ \text{Olivine} + \text{water} \rightarrow \text{serpentine} + \text{brucite} + \text{magnetite} + \text{hydrogen} $$

Given current knowledge, magnetite can be produced via serpentinization at different temperatures, with a range of 250°C–350°C being ideal (Evans, 2010; Klein et al., 2009, 2014; McCollom & Donaldson, 2016). At temperatures close to or below 200°C, magnetite production is much lower (Klein et al., 2013), and the iron released during the serpentinization reaction can be incorporated by serpentinite or brucite (Bonnemains et al., 2016; Seyfried et al., 2007; Tutolo et al., 2020). For serpentinization processes below 200°C, it is thus possible to find highly serpentinized peridotites with a high content of magnetite (1–10 wt%) or little magnetite (<0.04 wt%) (e.g., Oufi et al., 2002). Under natural conditions of high temperature serpentinization (T > 250°C), the compilation of data shows that magnetite content increases almost exponentially with the degree of serpentinization (Oufi et al., 2002; Toft et al., 1990), indicating significant creation of magnetite in the final stages of serpentinization (serpentinization rate S > 60%) (Fujii et al., 2016).

Magnetite is a ferrimagnetic mineral, which gives it interesting properties for detection by rock magnetism or geophysical methods. Its remanent magnetization at saturation is of the order of 100 Am²/kg and its volume magnetic susceptibility (k) is of the order of ~1 SI (dimensionless in the international unit system) (O'Reilly, 1984). Its magnetic properties allow this mineral to be detected at all scales of observation. For example, Saspiturry et al. (2024) modeled the degree of serpentinization in a mantle from western Pyrenees at low depth (~10 km) from airborne magnetic anomalies. In the same area, Loiseau et al. (2024) specified the heterogeneity of serpentinization in a one-km-wide exposed lherzolite body by means of magnetic susceptibility. At the scale of a sample of a few cubic centimeters, several studies have characterized the magnetite fraction using numerous magnetic proxies (Cutts et al., 2021). Magnetic methods, including the measurement of induced magnetization or remanent magnetization (Dunlop & Özdemir, 1997) and references within) can be applied to solid samples and at low temperatures (tens of Kelvin to room temperature) to avoid mineralogical transformations. These methods can quantify grain concentrations, including nanoscale grains, with a sensitivity of merely a few parts per billion (ppb). In comparison, geochemical methods are more destructive (rock powder, chemical attacks) or less accurate. For example, X-rays or Mossbauer spectroscopy have a detection threshold of the order of 1%. Optical methods (SEM, TEM) are difficult to apply given the small size of magnetites and the low concentrations (generally <1% in rocks).

Among the hundreds of studies on serpentinization processes (Barbier et al., 2020), only one has used the magnetic properties of newly formed magnetites to study kinetics under thermal stress. Malvoisin, Brunet, et al. (2012), Malvoisin, Carlut, et al. (2012) used olivine powder ranging from 5 to 63 µm and placed them in a gold capsule with an aqueous NaCl solution (fluid/olivine ratio of 2:5). The experimental temperatures were 250°C and 350°C and a pressure of 50 Mpa. During the experiment, Malvoisin, Brunet et al. (2012), Malvoisin, Carlut, et al. (2012) measured the saturation of isothermal remanent magnetization (SIRM) without opening the gold capsule. SIRM will directly measure the contribution of initial and newly formed magnetite since olivine does not behave ferromagnetically. However, this technique is not suitable for detecting magnetite fractions smaller than ~20 nm, where the grains are in a superparamagnetic state. By using a cryogenic magnetometer with a sensitivity of 10⁻¹¹ Am², a magnetite detection of the order of 30 ppmv (parts per million by volume) can be achieved, considering the initial magnetic signal from the capsule and the olivines (Malvoisin, Brunet, et al., 2012; Malvoisin, Carlut, et al., 2012). This protocol made it possible to continuously monitor the formation of magnetite during the serpentinization process. The disadvantage of this approach is that it uses olivine powders with large exchange surfaces. Ideally, these serpentinization observations should be followed in solid rock to better approximate natural conditions.

Other magnetic proxies can also be monitored during experiments. Magnetic susceptibility, which is easy to measure in the laboratory, is not much higher than 10⁻⁸ SI in practice. In addition, magnetic susceptibility includes various magnetic contributions, mainly paramagnetism and ferromagnetism. The paramagnetic contribution can change significantly during the experiment due to the alteration of paramagnetic minerals, and this can mask the formation of the ferromagnetic mineral we wish to track. Cairanne et al. (2003) and Aubourg and Pozzi (2010) proposed the use of the remanent magnetization acquired by newly formed grains. The main idea behind this approach is to apply a magnetic field of a few milliteslas during thermal stress. Only the ferromagnetic grains beyond the blocking volume (typically a cube with 20 nm sides for magnetite) will record the magnetic
field in a stable manner over time. This acquired remanent magnetization is a combination of crystalline remanent magnetization (CRM) and thermo-viscous magnetization (TVRM). CRM is carried by ferromagnetic minerals larger than the blocking volume. TVRM, which decreases with time, is mainly carried by minerals below the blocking volume, whose state is superparamagnetic. It is possible to characterize the CRM by thermal and alternating field demagnetization. Thermal demagnetization allows us to characterize the unblocking temperature spectrum of the newly formed magnetic grains, while alternating field demagnetization provides constraints on their coercivity. Such information is crucial for identifying the newly formed mineral phases (e.g., magnetite, hematite) and constraining their grain-size range.

This technique has been successfully applied to low-temperature experiments (50°–130°C) aimed at studying the formation of magnetite in shales (Kars et al., 2023). These authors used solid argillite samples (~1 g) in a semi-confined atmosphere and under a 2 mT magnetic field for several weeks. Remanent magnetization measurements were made without removing the sample from the nylon container, and at room temperature. The protocol involved in-field heating and zero-field cooling. The CRMs and TVRMs obtained were of the order of $10^{-6}$ Am²/kg, corresponding to a quantity of nanometric magnetite of the order of tens of ppb. Monitoring showed saturation of the remanent magnetization after around 10 days (Aubourg et al., 2008; Aubourg & Pozzi, 2010; Kars et al., 2023). Thermal and alternating-field demagnetization of the CRM showed that the mineral produced is a magnetite smaller than ~30 nm. Maurel and Gattacceca (2023) successfully repeated this protocol in various sedimentary shaly rocks with solid samples from 2 to 8 g at temperatures of 350°C.

We therefore propose to apply this approach to the acquisition of remanent magnetization on solid samples of lherzolites that have undergone different degrees of serpentinization. The atmosphere will be hydrated. We will apply 3 heating temperatures: 100°C, 200°C, and 300°C. Our experiment is to test the feasibility of the magnetic approach for monitoring oxidation-reduction processes on solid samples at low temperature. This information is important because these processes are good candidates for promoting H₂ generation, in addition to serpentinization processes.

## 2. Methods

### 2.1. Magnetic Methods

Magnetic susceptibility *k* was measured with the AGICO KLY3 KAPPABRIDGE (sensitivity $3 \times 10^{-8}$ SI, frequency: 875 Hz, and a peak value for the field intensity of 380 μT). Magnetic susceptibility measures the paramagnetic, diamagnetic and ferromagnetic fractions. This device has also been used to obtain magnetic susceptibility *k* curves as a function of temperature T (range –196°C–700°C). k-T curves can be used to detect Curie temperatures (CT), magnetic transitions (Verwey (VT) for magnetite at ~–152°C), and paramagnetic behavior.

Remanence was measured with a JR5 spinner magnetometer (sensitivity $1 \times 10^{-4}$ A/m). Remanence, whether natural or artificial, is carried solely by ferromagnetic minerals. The S-ratio, which is an indicator of the coercivity of the mineral phases present, was obtained on 4 samples. The S-ratio is obtained by applying with the ASC IM-10-30 impulse magnetizer a magnetic field of 0.1 T, measuring the isothermal remanent magnetization (IRM$_{0.1T}$), then applying a magnetic field of 1 T in the opposite direction, and measuring the IRM$_{1T}$. The ratio between IRM$_{0.1T}$/IRM$_{1T}$ is between 0 and 1, with 1 indicating that all the magnetic phases have a coercivity of less than 100 mT.

First-order reversal curve (FORC) diagrams were measured on a few specific initial samples with a magnetometer (μ-VSM) from Princeton Measurements Corporation at the IPGP-IMPMC Mineral Magnetism Analytical Facility. FORC diagrams allow a qualitative characterization of the magnetic domain structure and magnetostatic interactions for materials containing a mixed grain-size assemblage (Pike et al., 1999; Roberts et al., 2000). They were calculated from the measurement of 123 FORCs with an averaging time of 100 ms, and processed with the FORCINEL software (Harrison & Feinberg, 2008) with a variable smoothing factor (Egli, 2013). Variable smoothing considerably reduces the noise levels by applying larger smoothing factors to the background, while preserving the areas along the axes with relatively small smoothing factors.

Thermal demagnetization were performed using a PYROX furnace shielded against the magnetic field. The residual field measured in its cooling chamber was lower than 5 nT. Temperature reproducibility between heating at the same step is within 1°C. This control is ensured by means of three thermocouples placed at different
positions within the heating chamber. Alternating field demagnetization were performed using the 2G600 automatic sample degaussing system. The density was calculated from the theoretical volume of the cylindrical sample (11.15 cm³) and its mass. The degree of serpentinization (S) was calculated with the modified formula of Oufi et al. (2002) (Equation 2), which considers the density (d) and the percentage of magnetite (m).

$$d_c = 3.300 - 0.785 * S \text{ with } d_c = (d - m * 5.200)/(1 - m) \quad (2)$$

The use of this formula is only approximate as it does not consider low temperature changes such as carbonation. For densities below 2.7 (serpentine density), the serpentine values are likely to be biased. For magnetic monitoring of heat stress in our samples, we followed a protocol similar to that initially outlined by Aubourg et al. (2008), but we used a paleointensity furnace as in Maurel and Gattacceca (2023). The paleointensity furnace in the Montpellier laboratory allows the application of a uniform magnetic field known with a precision of 0.1 µT and less than 1° in its intensity and on its direction, respectively. Temperature reproducibility between heatings at the same step was within 1°C. This control is ensured by means of three thermocouples placed at different positions within the heating chamber, plus three others sealed inside three dummy samples.

## 2.2. Heating Experiment

After estimating the initial magnetic properties of our samples (magnetic susceptibility, natural remanence, S-ratio, Curie temperature, magnetic transition), we demagnetized them by alternating field (200 mT) and temperature (300°C). The natural residual magnetization is then subtracted from the signal acquired during heating.

The specimens were placed in a paleointensity furnace. A series of steps are performed in a repetitive manner (Figure 1).

1. Application of a 100 µT magnetic field (twice the earth's magnetic field). Six water cups of ~10 cl are placed in the furnace to hydrate the atmosphere. The atmosphere is not controlled. Up to nine samples can be placed and spaced at a minimum distance of 5 cm to prevent possible magnetic interactions.

2. Stabilization at the set temperature under field conditions for a variable time (20–24 hr).

3. Establishment of a zero magnetic field (<5 nT). Samples are cooled in a zero field for a time shorter than that of heating (12–16 hr). In principle, this considerably reduces the thermoviscous component of the remanent magnetization (TVRM) acquired.

4. Measurement of acquired remanent magnetization (Rlab) with correction for residual natural remanent magnetization. This magnetization can only be carried by magnetic minerals ranging from single-domain to multi-domain states. In the case of magnetite, this means that only the fraction larger than 20 nm can carry this magnetization.

At the end of the heating sequences, the acquired remanent magnetization was treated by magnetic cleaning and both thermal and alternating field demagnetization on sister specimens, and then analyzed. This enabled us to characterize the unlocking temperature ranges and the coercivity of this magnetization.

# 3. Results

## 3.1. Starting Material

The northern part of the Pyrenees is known for the existence of some 40 outcrops of peridotitic bodies (Clerc et al., 2012; Lagabrielle et al., 2010). These peridotites are mainly lherzolites, with varying degrees of serpentinization. The upwelling of these lherzolites from mantle depth is associated with the rifting phase at the late Cretaceous period (~110–100 Ma), which resulted in a significant extension of the continental crust (Lagabrielle et al., 2010; Masini et al., 2014; Mouthereau et al., 2014). This, in turn, led to the exhumation of mantle bodies at the seafloor in some locations (Clerc & Lagabrielle, 2014; Lagabrielle & Bodinier, 2008; Lagabrielle et al., 2010). The ascent of these bodies to their current position is linked to diapiric activity (Labaume & Teixell, 2020; Loiseau et al., 2024) or tectonic activity (Lagabrielle & Bodinier, 2008; Lagabrielle et al., 2010). The serpentinization process is most probably synchronous with the extension involving a thickness of the serpentinized mantle of the order of 10 km (Saspiturry et al., 2024). This serpentinization and, however, is also heterogeneous at the kilometric outcrop scale (Loiseau et al., 2024).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Experimental setup. A ~10 cc solid sample was placed in a paleomagnetic oven under a 100 µT magnetic field. The hydrated atmosphere was provided by beakers filled with ~5 cc of water. At the end of the field heating experiment (20–24 hr), the magnetic field is interrupted during cooling (12–16 hr) to prevent the acquisition of thermoremanent magnetization. Newly formed grains carry a crystalline remanent magnetization, which can be characterized by thermal demagnetization and alternating field techniques.</figcaption></figure>

A total of five massifs were sampled. Proceeding from east to west, these are Lherz, Moncaup, Turon de la Técouère, Saraillé and Urdach (Figure 2). Fresh and oriented cores of 2.5 cm in diameter were taken in the field using a portable drill. In the laboratory, they were cut into standard specimens measuring 2.2 cm in height. Sampling from these diverse massifs allows for coverage across the entire spectrum of serpentinization. The data set encompasses compositions ranging from incipient serpentinization in the Lherz massif (Lagabrielle & Bodinier, 2008; Le Roux et al., 2007; Lorand et al., 2010) to full serpentinization in the Saraillé massif (B. Corre et al., 2016).

The arithmetical average magnetic susceptibility per site was calculated, together with the average density. These values are given for the lherzolites from east to west.

For the Lherz lherzolite, magnetic susceptibility is one of the lowest measured, of the order of $10^{-4}$ in the various sites studied ($k = 740 \pm 192$ µSI; $n = 34$). However, susceptibility values of the order of $10^{-2}$ SI are found in peridotite breccias set in a carbonate matrix. Density is one of the highest ($d = 3.16 \pm 0.14$ g/cm$^3$). The degree of serpentinization ranges, the following Equation 2, from 0% to 30%, except in breccias where it can reach 55%–60%. For Moncaup lherzolite, susceptibility is of the order of $10^{-2}$ to $10^{-3}$ SI ($k = 10,225 \pm 8,570$ µSI; $n = 40$) and a high average density ($d = 3.15 \pm 0.12$ g/cm$^3$), comparable to Lherz. The degree of serpentinization ranges from a few % to 50%. For the Turon lherzolite, studied by Loiseau et al. (2024), susceptibility varies from $10^{-3}$ in protomylonite to $10^{-2}$ in mylonite ($k = 12,390 \pm 10,100$ µSI; $n = 18$) and an average density of $3.09 \pm 0.13$ g/cm$^3$. This attests to highly variable serpentinization, ranging from a few % to 62%. For the Saraillé lherzolite, susceptibility is of the
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Thermomagnetic curves. Magnetic susceptibility (k) versus temperature (°C). Heating red. Cooling blue. (a) Fully serpentinized lherzolite (Saraillé). The Verwey transition (VT) and the Curie temperature (CT) indicate the presence of stoichiometric magnetite. Additional transitions at ~80°C and ~500°C suggest the occurrence of non-identified magnetic minerals. (b) Weakly serpentinized lherzolite (Lherz). The rapid decay from -160°C to room temperature points to a paramagnetic component. The non-reversible curve between heating and cooling suggests magnetite formation.</figcaption></figure>

order of $10^{-2}$ SI ($k = 61,280 \pm 11,354$ µSI; $n = 12$) and an average density of $2.54 \pm 0.10$ g/cm$^3$. This indicates very high serpentinization, ranging from 80% to 100%. For the Urdach lherzolite, susceptibility varies by two orders of magnitude from $10^{-2}$ to $10^{-4}$ SI ($k = 4,962 \pm 9,704$ µSI; $n = 26$) and an average density of $2.58 \pm 0.07$ g/cm$^3$. This attests to a very high serpentinization rate ranging from 80% to 100%.

From this very general picture, we note the high variability of susceptibility, sometimes by two orders of magnitude within the same peridotite unit (e.g., Moncaup). This reveals variability not only in the degree of serpentinization, but also in other additional factors. For example, at Urdach, which is one of our most serpentinized references, the peridotite has undergone low-temperature alteration (Clerc, 2013; Lagabrielle et al., 2019), leading to carbonation, which may explain the low susceptibility values. At Moncaup, which is one of our least serpentinized references, we find very high magnetic susceptibility values. The Turon de la Tecouère peridotite also shows high susceptibility, although the degree of serpentinization remains modest. In both examples, strong deformation of the lherzolites is observed, with the development of mylonites. It is possible that such highly deformed zones may promote intense, localized serpentinization (Loiseau et al., 2024). These examples indicate that magnetic susceptibility is not a reliable proxy for assessing the degree of serpentinization in Pyrenean lherzolites. The same observation was made in peridotites of different types (dunites, hazburgite, etc.) by Cutts et al. (2021).

We selected 23 samples for the magnetic monitoring experiments. Table 1 gives the characteristics of these samples, that is, magnetic susceptibility, S-ratio, density and natural remanent magnetization (NRM). Magnetic
<table><caption>**Table 1**
The First Nine Columns Are Dedicated to the Starting Material Characterization</caption><thead><tr><th rowspan="2">Lherzolite</th><th rowspan="2">Sample</th><th rowspan="2">Dens.</th><th colspan="3">Starting material</th><th rowspan="2">NRM (A/m)</th><th rowspan="2">Rock mag. d.</th><th rowspan="2">S-ratio</th><th colspan="5">Experiment</th><th colspan="3">Results</th></tr><tr><th>S</th><th>Bulk K m</th><th>SI %</th><th>n</th><th>Exp. T °C</th><th>n cycle</th><th>Time stab.</th><th>AF</th><th>Thermal</th><th>CRM (A/m)</th><th>MDF mT</th><th>CRM at 600°C %</th><th>m formed ppmv</th></tr></thead><tbody><tr><td>Lherz</td><td>2LH3C</td><td>3.17</td><td>17</td><td>6.13E-04</td><td>0.1</td><td>1.37E-03</td><td>2.37E-03</td><td></td><td>300</td><td>3</td><td></td><td>X</td><td>7.60E-02</td><td>15</td><td></td><td>38</td></tr><tr><td>Lherz</td><td>3LH3D</td><td>3.06</td><td>31</td><td>6.32E-04</td><td>0.1</td><td>3.09E-03</td><td>4.00E-06</td><td>Tcm. P</td><td>0.965</td><td>300</td><td>3</td><td>X</td><td>6.64E-02</td><td>42</td><td></td><td>33</td></tr><tr><td>Lherz</td><td>2LH11B</td><td>3.34</td><td>0</td><td>9.89E-04</td><td>0.1</td><td>2.30E-02</td><td>1.40E-02</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>3.30E-02</td><td></td><td>6.5%</td><td>16</td></tr><tr><td>Lherz</td><td>3LH7A</td><td>2.98</td><td>41</td><td>8.88E-04</td><td>0.1</td><td>8.66E-02</td><td>2.20E-03</td><td></td><td>200</td><td>1</td><td></td><td>X</td><td>4.90E-03</td><td>und.</td><td></td><td>2</td></tr><tr><td>Lherz</td><td>3LH4B</td><td>2.96</td><td>43</td><td>5.79E-04</td><td>0.1</td><td>7.12E-03</td><td>2.00E-05</td><td></td><td>100</td><td>1</td><td></td><td>X</td><td>3.05E-03</td><td>14</td><td></td><td>2</td></tr><tr><td>Turon</td><td>1OS12B</td><td>3.06</td><td>31</td><td>1.41E-03</td><td>0.1</td><td>6.54E-01</td><td>1.00E-04</td><td></td><td>300</td><td>3</td><td></td><td>X</td><td>2.52E-02</td><td>14</td><td></td><td>13</td></tr><tr><td>Turon</td><td>1OS18C</td><td>3.06</td><td>31</td><td>1.63E-03</td><td>0.2</td><td>3.18E+00</td><td>1.15E-03</td><td>V, Tcm. P</td><td>0.837</td><td>300</td><td>3</td><td>X</td><td>4.44E-02</td><td>13</td><td></td><td>22</td></tr><tr><td>Turon</td><td>1OS17A</td><td>3.02</td><td>37</td><td>1.71E-03</td><td>0.2</td><td>3.90E+00</td><td>2.64E-03</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>4.50E-02</td><td></td><td>1.2%</td><td>23</td></tr><tr><td>Turon</td><td>2PB8A</td><td>2.96</td><td>50</td><td>2.29E-02</td><td>2.3</td><td>8.55E-01</td><td>1.96E-03</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>2.88E-01</td><td></td><td>0.4%</td><td>144</td></tr><tr><td>Turon</td><td>1OS16B</td><td>3.24</td><td>8</td><td>1.39E-03</td><td>0.1</td><td>2.33E+00</td><td>3.70E-04</td><td></td><td>200</td><td>1</td><td></td><td>X</td><td>9.69E-03</td><td>10</td><td></td><td>5</td></tr><tr><td>Turon</td><td>2PB4B</td><td>2.88</td><td>63</td><td>3.16E-02</td><td>3.2</td><td>7.72E-01</td><td>2.80E-03</td><td></td><td>200</td><td>1</td><td></td><td>X</td><td>1.40E-01</td><td>6</td><td></td><td>70</td></tr><tr><td>Turon</td><td>1OS2A</td><td>3.20</td><td>13</td><td>1.29E-03</td><td>0.1</td><td>6.91E-01</td><td>8.50E-04</td><td></td><td>100</td><td>1</td><td></td><td>X</td><td>4.29E-03</td><td>9</td><td></td><td>2</td></tr><tr><td>Turon</td><td>2PB14B</td><td>3.12</td><td>29</td><td>2.25E-02</td><td>2.3</td><td>5.28E-01</td><td>8.80E-03</td><td></td><td>100</td><td>1</td><td></td><td>X</td><td>4.37E-02</td><td>3</td><td></td><td>22</td></tr><tr><td>Urdach</td><td>3PER15B</td><td>2.57</td><td>96</td><td>8.84E-03</td><td>0.9</td><td>1.57E-01</td><td>-3.50E-02</td><td>V, Tcm. P</td><td>0.958</td><td>300</td><td>3</td><td>X</td><td>2.17E-01</td><td>9</td><td></td><td>109</td></tr><tr><td>Urdach</td><td>3PER16B</td><td>2.61</td><td>93</td><td>1.59E-02</td><td>1.6</td><td>5.75E-01</td><td>1.80E-03</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>3.40E-01</td><td></td><td>3.2%</td><td>170</td></tr><tr><td>Urdach</td><td>3PER8A</td><td>2.56</td><td>95</td><td>1.27E-03</td><td>0.1</td><td>7.38E-02</td><td>3.80E-04</td><td></td><td>200</td><td>1</td><td></td><td>X</td><td>1.25E-02</td><td>42</td><td></td><td>6</td></tr><tr><td>Urdach</td><td>3PER19B</td><td>2.57</td><td>96</td><td>1.05E-02</td><td>1.1</td><td>1.67E-01</td><td>2.42E-03</td><td></td><td>100</td><td>1</td><td></td><td>X</td><td>5.14E-02</td><td>9</td><td></td><td>26</td></tr><tr><td>Saraillé</td><td>1SA13C</td><td>2.74</td><td>90</td><td>8.29E-02</td><td>8.3</td><td>9.80E+02</td><td>9.90E-03</td><td>V, Tcm. P</td><td>0.925</td><td>300</td><td>3</td><td>X</td><td>3.23E+00</td><td>7</td><td></td><td>1,614</td></tr><tr><td>Saraillé</td><td>1SA14B</td><td>2.74</td><td>91</td><td>6.01E-02</td><td>6.0</td><td>7.46E+02</td><td>1.13E-02</td><td></td><td>300</td><td>3</td><td></td><td>X</td><td>1.78E+00</td><td>7</td><td></td><td>889</td></tr><tr><td>Saraillé</td><td>1SA1A</td><td>2.59</td><td>100</td><td>5.78E-02</td><td>5.8</td><td>1.85E+02</td><td>-3.50E-02</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>2.92E+00</td><td></td><td>10.0%</td><td>1,457</td></tr><tr><td>Saraillé</td><td>1SA19B</td><td>2.71</td><td>99</td><td>6.87E-02</td><td>6.9</td><td>2.26E+02</td><td>6.60E-03</td><td></td><td>200</td><td>1</td><td></td><td>X</td><td>3.37E-01</td><td>5</td><td></td><td>169</td></tr><tr><td>Saraillé</td><td>1SA17A</td><td>2.86</td><td>77</td><td>5.78E-02</td><td>5.8</td><td>2.80E+02</td><td>4.40E-03</td><td></td><td>100</td><td>1</td><td></td><td>X</td><td>4.72E-01</td><td>7</td><td></td><td>236</td></tr><tr><td>Moncaup</td><td>1MA12B</td><td>3.21</td><td>12</td><td>4.10E-03</td><td>0.4</td><td>4.02E-01</td><td>-1.00E-04</td><td></td><td>300</td><td>1</td><td></td><td>X</td><td>8.39E-02</td><td></td><td>0.4%</td><td>42</td></tr></tbody></table>

Note. Dens. for the density. S for the serpentization rate derived from density and magnetic susceptibility. Bulk k: Bulk magnetic susceptibility. Dimensionless SI: m: magnetite concentration % derived from magnetic susceptibility. NRM: natural remanent magnetization before cleaning (A/m). NRM a.c.: natural remanent magnetization after cleaning, that is, after applying alternating field at 200 mT and temperature at 300°C temperature for 10 min. Rock mag: Rock magnetic properties from magnetic susceptibility versus temperature (k-T) curves. V: Verwey transition, CT Curie temperature of magnetite and P paramagnetic contribution. S-ratio: coercivity test. The five next columns are dedicated for the experiment parameters. Exp. T: temperature of experiment in °C, n cycle: number of heating experiments. Time stab: testing the decay of laboratory induced magnetization (Rlab) through time. AF: Alternative magnetic field demagnetization of CRM. Thermal demagnetization of CRM. The last four columns are the results of the experiments. CRM stands for Chemical Remanent Magnetization in A/m. MDF: median magnetic field where 50% of the CRM is canceled during AF demagnetization. CRM at 600°C: remaining CRM% after heating up to 600°C m formed: concentration of magnetite formed during experiment calculated from CRM assuming an outer ring of 10 µm where all magnetite formed.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) First Order Reversal Curve (FORC) for Saraillé lherzolite starting sample (without experimental heating). Bc: coercive field. Bu interaction field. The contours mostly closed and centered around 20 mT of coercive field indicate single domain-vortex behavior (magnetite size between ~30 nm and a few hundred nm at the most). (b) Backscattered scanning electron microscopy (SEM) of a sample from the Saraillé lherzolite without experimental heating. The submicrometric magnetite is in clusters of undefined size at this resolution.</figcaption></figure>

susceptibility varies from $5 \cdot 10^{-4}$ to $1 \cdot 10^{-1}$ SI, density from 2.5 to 3.3 g/cm$^3$ and NRM from $10^{-3}$ to $10^3$ A/m. The high magnetic susceptibility is a clear indication of the presence of magnetic minerals such as magnetite. After magnetic cleaning at 200 mT and thermal demagnetization at 300°C for 10 min, NRM dropped by several orders of magnitude (Table 1). Magnetic susceptibility versus temperature (k-T) curves were performed on selected samples (Figure 2). During heating from liquid nitrogen to over 600°C, a Verwey transition at ~150°C and a Curie temperature of 580°C were observed. These two signatures indicate the presence of stoichiometric magnetite as the main magnetic phase (Dunlop, 2002). A paramagnetic component is also visible from -150°C to room temperature. This component results from the paramagnetic susceptibility of olivines and pyroxenes, to name the main ones (Dunlop, 2002). In the highly paramagnetic Lherz sample, a Curie temperature is observed, but no Verwey transition. The magnetite probably reflects a high-temperature transformation, since the cooling curve is irreversible. In summary, all the samples, with the possible exception of those from Lherz, have stoichiometric magnetites which largely dominate the magnetic signal. The S-ratio varies from 0.84 (Turon de la Tecouère) to 0.97 (Lherz) (Table 1). This means that there is a harder magnetic phase (e.g., hematite) that we did not detect with the k-T curves, which are dominated by magnetite.

FORC diagrams were measured on some lherzolite samples. In the example shown (Figure 3a), the fully serpentinized sample from the Saraillé massif shows that most contours on the FORC diagrams are closed and centered around a coercivity around 20 mT, and some dispersion along the horizontal (interaction) axis. This pattern indicates a mixture of SD- and vortex behavior with weak interactions (Roberts et al., 2014), which points to a magnetic grain size no larger than a few hundred nm. In backscattered electron microscopy (Figure 3b), the magnetites are organized in clusters, with a close relationship between the spinel and the serpentine. At this resolution, the size of the magnetites cannot be resolved, but they appear locally submicrometric.

## 3.2. Heating Experiments

To assess the kinetics of magnetic mineral formation, we repeated the protocol on a first set of samples at the set temperature of 300°C (n = 7 samples), with a measurement every ~2 days (24 hr heating, then cooling to zero field for 8–12 hr. This experiment took place over 8 days.

This protocol makes it possible to monitor magnetization of the crystalline remanent magnetization (CRM) type, while eliminating a large part of the viscous magnetization (TVRM). With the applied field parallel to the core axis during the experiment, the acquired remanent magnetization (Rlab) is measured along the Z axis, correcting
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Magnetic monitoring of acquired remanence (Rlab) along the sample z-axis. Note the log scale of Rlab. Saraillé samples (1SA13C and 14B) > 90% serpentinization, Urdach massif (3PER15B) > 90% serpentinization, Lherz massif (2LH3C) < 20% serpentinization and Turon de la Técouère massif (1OS12B and 18C) around 30% serpentinization.</figcaption></figure>

for the NRM component. Residual NRM magnetization are less than $10^{-2}$ A/m (Table 1). More than 95% of the acquired remanence is parallel to the Z-axis of the furnace and aligned with the direction of the magnetic field applied during heating. After 24 hr, Rlab gains one to two orders of magnitude (Figure 4). Rlab then stabilizes for the remaining stages. This implies that more than 75% of Rlab is acquired in the first 24 hr of heating. It should be remembered that, during cooling, any potential grains that may form carry no magnetization, since the magnetic field is zero. The time-stability of Rlab was tested over 23 hr. A relatively linear decrease was observed, with a maximum of 6%. Rlab is therefore mainly a chemical remanent magnetization (CRM) with an insignificant presence of a TVRM component.

This first experiment allowed us to estimate that 24 hr of heating would allow signal detection for all samples at 300°C. This rapid kinetics can be explained using processes affecting the surface and the first few micrometers of the sample. We have chosen to establish this protocol at 24 hr for temperatures of 200°C (5 samples) and 100°C (5 samples). Moncaup massif was not tested in this second series of experiments. Considering the evidence presented, which indicates that Rlab is essentially a CRM carried by the new magnetic grains, it is proposed that CRM be used as a replacement for Rlab henceforth.

Figure 5 shows the CRM as a function of the initial susceptibility of the sample. Three significant results can be noted. (a) A significant signal appears (CRM > $3 \times 10^{-3}$ A/m) as early as 100°C for all samples, including the Lherz peridodites. This indicates that a ferromagnetic phase is created after 24 hr of heating. (b) CRM is an order of magnitude greater at 300°C than at 200°C and 100°C. For the latter two temperatures, the order of magnitude remains the same. (c) There is a positive correlation ($r^2 > 0.91$) using logarithmic scale between CRM and initial
<figure><img src="image_7.png" /><figcaption>**Figure 5.** CRM versus initial magnetic susceptibility (K). Note the log scale of both axes. Along the axis is also indicated the initial content of magnetite concentration inferred from the magnetic susceptibility and the content of neoformed magnetite inferred from CRM.</figcaption></figure>

susceptibility for all samples. This correlation suggests that the greater the initial magnetite content, the greater the amount of magnetite formed during the 12-hr heating period.

To characterize the carrier of the CRM, thermal demagnetization and alternating fields were carried out on a few samples for the 3 experimental heating temperature ranges (100°C, 200°C, 300°C). Thermal demagnetization (Figure 6a) shows similar unlocking temperature spectra, with 90.0%–99.5% of the signal eliminated at 600°C (Table 1). It is worth noting that the thermal demagnetization curves display a wide range of unblocking temperatures without any clear inflection around the experimental temperatures at which the magnetization were produced. In the example shown, the CRM was acquired at 300°C, and ~90% of the CRM was still preserved at this temperature during thermal demagnetization. This demonstrates that any potential thermoremanent magnetization component that might have been acquired during cooling is not detectable. Note that for the Saraillé peridotites, which show the highest concentration of magnetite, ~10% of the signal remains at 600°C, reflecting the contribution of a ferromagnetic mineral with a Curie temperature above 600°C, likely hematite. Apart from a few examples (Lherz and Urdach), the CRM is carried out by low coercivity minerals (Figure 6b). The alternating field required to destroy 50% of CRM (median destructive field MDF) is less than 20 mT (Table 1).

Taken collectively, the thermal demagnetization (unblocking temperature < 600°C) and coercivity data (MDF < 20 mT) suggest that the neoformed mineral is magnetite. The even distribution of unblocking temperatures (Figure 6) indicates a size distribution of magnetite ranging from single domain to pseudo single domains (PSD). These observations are valid irrespective of the set of the heating temperatures.

## 4. Discussion

Peridotite alteration processes have been extensively studied both on natural rocks (Li et al., 2017; McCollom & Bach, 2009; Moody, 1976; Schrenk et al., 2013; Toft et al., 1990), and in experiments on mineral or rock powders under well-constrained experimental conditions (Barbier et al., 2020; Malvoisin, Brunet, et al., 2012; Malvoisin, Carlut, et al., 2012; McCollom & Donaldson, 2016; Seyfried et al., 2007). Our study does not aim to detail precise chemical processes, but to verify that it is possible to form magnetic minerals at low temperatures and over modest laboratory times (24 hr heating). The originality of our approach also lies in the fact that we are working with a solid sample of about 10 cc. This type of experiment can be performed in conventional paleomagnetism
<figure><img src="image_8.png" /><figcaption>**Figure 6.** CRM acquired characterisation for the Saraillé massif. (a) Thermal demagnetization of a CRM acquired at 300°C (1SA1A) showing unblocking temperature around 580–600°C, note the 10% remaining signal possibly indicating other ferromagnetic mineral likely hematite. (b) Alternative field demagnetization (1SA13C, 14B, 19B and 17A) for the 3 studied temperatures, all of them showing median destructive fields <20 mT, indicating soft mineral as magnetite.</figcaption></figure>

laboratories, which allow the application of a controlled magnetic field during the experiment (Maurel & Gattacceca, 2023).

The objective of the protocol developed in this study is to monitor the acquired remanent magnetization (Rlab) during the heating laboratory experiments whereby Rlab is carried exclusively by newly formed magnetic minerals. Rlab is essentially a combination of chemical remanent magnetization (CRM), which is stable over time, and thermal viscous magnetization (TVRM), which decreases exponentially with time when the samples are stored in zero field (Aubourg & Pozzi, 2010; Kars et al., 2023). In the present study, the findings demonstrate that Rlab intensity decreases by approximately 5% after more than 23 hr in a zero magnetic field, suggesting that Rlab is predominantly influenced by CRM.

The peridotites studied are Pyrenean lherzolites, covering a wide range of serpentinization levels from a few % to 100% (Table 1). The ferromagnetic phase is mainly carried by a stoichiometric magnetite, as indicated by the observation of the Verwey transition and the Curie temperature (Figure 1). However, the presence of a more coercive ferromagnetic phase is suggested by the proxy S-ratio (Table 1), which may be locally less than 0.9. We have not determined this hard ferromagnetic phase, which may be hematite or maghemite. Magnetite occurs in clusters (Figure 3b) and is submicrometric in size. According to FORC diagrams, the magnetization carrier is in the SD-PSD state, corresponding to grain size ranging from a few tens to a few hundreds of nm (Figure 3a). This observation is in good agreement with the FORC diagrams measured by Fuji et al. (2016) in peridotites with a high (>60%) degree of serpentinization. Pyrenean lherzolites show variations in magnetic susceptibility of 1–3 orders of magnitude for the same kilometer-sized body (Turon de la Tecouère, Moncaup), reflecting the heterogeneity of processes leading to magnetite formation. Assuming that much of the magnetic susceptibility is controlled by magnetite, the maximum volume concentration can be calculated from the intrinsic magnetic susceptibility of magnetite at ~1 SI. Magnetite concentrations range from a few hundred ppmv to several percentage in the most serpentinized lherzolites (Table 1).

Magnetites may result from serpentinization processes, as we observed the absence of magnetite in the non-serpentinized reference (Lherz) and the highest magnetic susceptibility values for the most serpentinized lherzolite facies (Saraillé, Urdach). However, observations suggest that other processes are involved in the formation of magnetite. For example, in the Lherz lherzolites, breccias have magnetic susceptibility up to $10^{-2}$ SI and
density less than 3. We have no information on whether processes at classical serpentinization temperatures (~300°C) or low temperature processes (<200°C) could have led to the formation of these magnetites. The same applies to the Moncaup peridotites, where high susceptibilities (10⁻² SI) are associated with densities that do not reflect a high degree of serpentinization (mean density of 3.15 g/cm³). It is beyond the scope of this article to study these different processes, but it should be noted that in the Pyrenean lherzolites we have studied, there is no simple relationship between the degree of serpentinization inferred from the densities and the magnetic susceptibility.

The first experiment at 300°C allowed us to establish the initial kinetics over almost 100 hr of heating (Figure 2). More than 75% was produced in 24 hr of heating, followed by a very slow progression for the further steps. This rapid kinetics over 24 hr can probably be explained by the reactivity of the outer ring of the solid sample in contact with the hydrated atmosphere. We have no means of quantifying this ring. It is likely that this thickness does not exceed a few micrometers. Aubourg et al. (2008) have proposed to calculate a concentration from the CRM. To do this, half of the CRM value is used and compared with the nominal value of the mineral formed (Aubourg & Pozzi, 2010).

Alternating field and temperature demagnetization show that the CRM carrier at 300°C is a weakly coercive magnetite (MDF = 7–42 mT, Table 1) with an unblocking temperature close to 600°C (Figure 5). Given the experimental protocol with a hydrated atmosphere and oxygen, it may seem like surprising that the product formed is not a more oxidized form of the iron oxides. The fact that ~10% CRM was observed at 600°C for a sample of Saraillé lherzolites (Figure 6) suggests that hematite or maghemite may have been formed, as their unblocking temperatures can exceed 600°C.

Assuming that magnetite is the main CRM-bearing ferromagnetic mineral, we propose calculating its maximum concentration (Figure 3). The nominal remanent magnetization of magnetite is 480 kA/m (Dunlop, 2002). The calculation can be performed on the volume of the solid sample (~10 cc), but we prefer to restrict it to the volume of the ring impacted by the chemical process, assuming a ring thickness of 10 µm (volume ~0.016 cm³). By deliberately taking a low thickness of 10 µm, it is possible to calculate orders of magnitude of the maximum concentration of magnetite formed. This calculated concentration varies by two orders of magnitude, between 0.01% and 1% (Table 1, Figure 5). The evolution of this concentration is controlled to the first order by the initial magnetite concentration (Figure 5). This is also true for the Lherz samples, although little magnetite was present in the initial samples.

In single-step experiments (24 hr heating, 8–12 hr cooling), we find the same characteristics as in multi-step experiments. The CRM can be measured already at 100°C and its characterization suggests a weakly coercive phase (MDF < 15 mT) and an unblocking temperature range compatible with magnetite. The log-log proportional relationship between CRM and initial magnetic susceptibility was comparable to that found at 300°C. However, the calculated maximum magnetite concentration decreases by an order of magnitude (from a few tens of ppm to a few thousands ppm) compared to the 300°C temperature (Table 1). Magnetite production between 100°C and 200°C remains in the same order of magnitude under our experimental conditions.

This study shows that magnetite forms in a hydrous atmosphere with the presence of oxygen at temperatures as low as 100°C. This finding opens the way to experiments under more optimal conditions, in which it would be possible to control oxygen fugacity, water vapor concentration and pressure, to name but a few. However, the formation of magnetite at low temperature raises a number of questions.

As lherzolite has an initial stoichiometric magnetite fraction, it might have been possible for this magnetic phase to alter into maghemite or hematite. Characterization of the CRM shows that this is not the case, even if it remains possible for the sample from the most serpentinized massif, Saraillé, for example. It is difficult to extrapolate these results to natural conditions, but they seem to point in the direction of magnetite formation under hydrated conditions and with oxygen. This would explain the formation of magnetite in the late stage of mantle exhumation, in a temperature range below the optimum serpentinization temperature (<300°C).

No conclusive mechanism could be identified based on the experimental results. The iron may have originated from the alteration of olivines, pyroxenes, or other phases. Oxidation of Fe²⁺ to Fe³⁺ could be accompanied by hydrogen production, if oxygen does not interfere. Transposed to natural conditions, it therefore seems possible to produce magnetite and potential hydrogen under low-temperature conditions. A natural production of magnetite at 95°C has been described by Corre et al. (2023) in the ophiolites of New Caledonia, with potential H₂ generation.
The principal outcome of the study demonstrates a log-log proportional relationship between the initial magnetic susceptibility and the CRM carried by neoformed magnetite (Figure 5). A larger initial quantity of magnetite results in a bigger production of magnetite. This may suggest two main mechanisms. (a) The growth of magnetite from existing nanometric phases. Nano-crystalline magnetite shows no remanence at sizes below 20 nm (Dunlop & Özdemir, 1997). It is therefore in a superparamagnetic state. (b) a nucleation/growth mechanism stimulated by the presence of magnetite. Barbier et al. (2020) demonstrated that magnetite could act as a catalyst during the serpentinization reaction. The nucleation/growth mechanism appears to be the most favorable, as suggested by the presence of nanometric magnetite clusters (Figure 3). It better explains the relationship between the CRM produced and the amount of initial nanometric magnetite, as well as the maximum temperature spectrum for CRM (~580°C) and the low coercivity (MDF < 20 mT) (Figure 6), suggesting a magnetite population that tends toward polydomains (size greater than 1 µm).

No definitive conclusion can be drawn between the two hypotheses based on the current data. However, this observation may have implications for low-temperature magnetite production in variably serpentinized peridotites. In the case of ophiolites, for example, the rate of serpentinization is significant (Audran et al., 2024; M. Corre et al., 2023; Leong et al., 2023). Our results suggest that at low temperatures, in the range 100°C–200°C, that is, 4–8 km below the surface, it is possible to promote reactions involving oxidation-reduction phenomena. If magnetite functions as a catalyst for these reactions, it can be inferred that the greater the degree of serpentinization of the peridotite, the higher the production of magnetite. This hypothesis could explain, for example, natural hydrogen emanations from serpentinized peridotites in highly serpentinized ophiolitic massifs such as Oman (Leong et al., 2023; Vacquand et al., 2018; Zgonnik et al., 2019) or New Caleonia (Corre et al., 2023). It now remains to be determined whether secondary magnetite is detected in natural serpentinized formations.

## 5. Conclusion

The aim of this study was to demonstrate the possibility of monitoring the formation of magnetic minerals in samples of variably serpentinized peridotites using a measurement protocol that can be applied in a palaeomagnetism laboratory. The originality of our approach is to work on solid samples of a few cm³, under a weak magnetic field, so that the formed magnetic minerals can record a magnetic field. Under hydrated conditions, experimental temperatures of 100°C, 200°C and 300°C were studied. The study of 23 solid samples, distributed over 5 peridotite massif outcropping in the northern Pyrenees and homogeneously covering all degrees of serpentinization, showed the following results:

* The formation of magnetic minerals leads to the acquisition of crystalline remanent magnetization (CRM) from the first hours of heating. After 24 hr of heating at 300°C, almost 75% of the acquired remanence was reached, with a very slow progression thereafter.

* The characteristics of this CRM (unblocking temperature near 600°C, median destructive field less than 20 mT) indicate that the host mineral is mainly magnetite.

* Newly formed magnetite was detected for all heating experiments (100°C, 200°C, 300°C), regardless of the degree of serpentinization of the peridotite.

* The amount of magnetite formed is an order of magnitude greater for heating at 300°C than at 200°C and 100°C. For 100°C and 200°C, magnetite production is comparable within our experimental protocol.

* However, the amount of magnetite formed is controlled to a first order by the amount of initial magnetite in the sample. On a log-log scale, we show a linear relationship between the amount of initial magnetite and the amount of magnetite formed.

It is difficult to translate our experiment to a natural context as the amount of oxygen is not controlled, but the results suggest that it is possible to form magnetite at low temperatures, especially when the degree of serpentinization is high.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.
## Data Availability Statement

Additional data are available in the Supporting Information. The data used in this study are available on Zenodo via Loiseau, K. (2025).

## Acknowledgments

This work was funded as part of the H2NA project in the Nouvelle Aquitaine region (France). The authors would like to thank Dr. E. Mansbach and one anonymous reviewer for their constructive comments, which helped to clarify the results. The authors would also like to thank the Editor S. Tikoo. K. Loiseau would particularly like to thank C. Rigollet from CVA for his support in this work.

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