# Element mobilities during serpentinization: Insights from the Ronda Massif, Spain

Bastien Audran, Marc Ulrich, Yannick Branquet, Guillaume Barré, Georges

Beaudoin, Philippe Boulvais

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Bastien Audran, Marc Ulrich, Yannick Branquet, Guillaume Barré, Georges Beaudoin, et al., Element mobilities during serpentinization: Insights from the Ronda Massif, Spain. Chemical Geology, 2026, 699, pp.123157. 10.1016/j.chemgeo.2025.123157. insu-05379904

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Element mobilities during serpentinization: Insights from the Ronda Massif, Spain

Bastien Audran, Marc Ulrich, Yannick Branquet, Guillaume Barré, Georges Beaudoin, Philippe Boulvais

PII: S0009-2541(25)00547-9

DOI: https://doi.org/10.1016/j.chemgeo.2025.123157

Reference: CHEMGE 123157

To appear in: *Chemical Geology*

Received date: 21 May 2025

Revised date: 12 November 2025

Accepted date: 18 November 2025

Please cite this article as: B. Audran, M. Ulrich, Y. Branquet, et al., Element mobilities during serpentinization: Insights from the Ronda Massif, Spain, *Chemical Geology* (2024), https://doi.org/10.1016/j.chemgeo.2025.123157

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# Element mobilities during serpentinization: insights from the Ronda Massif, Spain

Bastien Audran<sup>a</sup>,<sup>d*</sup>, Marc Ulrich<sup>b</sup>, Yannick Branquet<sup>a</sup>, Guillaume Barré<sup>c</sup>, Georges Beaudoin<sup>c</sup>, Philippe Boulvais<sup>a</sup>

* Corresponding author at Université de Lorraine, GeoRessources, F-54506, 54520, Vandœuvre-lès-Nancy, France

E-mail address: bastien.audran@univ-lorraine.fr (B. Audran).

## Abstract

Serpentinization is a widespread hydrothermal alteration of ultramafic rocks, particularly peridotites, resulting in the formation of serpentinite through hydration processes. While the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, Pb, U, Th, Cs, Sr, Ba) during serpentinization is well documented, the mobility of other elements (e.g., base metals) remains poorly constrained. We analyzed bulk rock compositions of pairs of poorly and highly serpentinized peridotite rocks from the Ronda Massif (Spain), complemented by in situ LA-ICP-MS mineral analyses. The loss on ignition (LOI) content of whole rock represents a good proxy for serpentinization intensity. Ca, Al, Cu, V, and Ti are increasingly mobilized during the initial stages of pervasive serpentinization. In a second stage corresponding to total serpentinization along cm-wide, several meters-long corridors, a systematic loss in Ca, Al, Rb, Sr, REE Hf, Ti, Cu, and V was observed. In contrast, during the third stage of serpentinization corresponding to late faulting only Ca and Cu were leached. The trace element composition of

<footnote><sup>a</sup> Géosciences Rennes, UMR 6118, University of Rennes, 35042 Rennes, France</footnote>

<footnote><sup>b</sup> IPGS-EOST, UMR 7516, University of Strasbourg, Strasbourg, France</footnote>

<footnote><sup>c</sup> Département de géologie et de génie géologique, Centre de recherche sur la géologie et le génie des ressources minérales (E4m), Université Laval, Québec, G1V 0A6, Canada</footnote>

<footnote><sup>d</sup> Université de Lorraine, CNRS, GeoRessources, F-54506 Vandoeuvre-lès-Nancy, France</footnote>
serpentine minerals, including Ti, V, and Zn, is influenced by the primary mineral assemblage, particularly olivine and pyroxenes. In contrast, the mobility of Al and Ca from millimeter to meter scale is notably linked to the bastite-pyroxene and serpentine mesh texture-olivine transformation. Co, Ni, Zn, and Cr depletion in serpentine within veins is compensated by their significant incorporation into magnetite. Copper is systematically leached from serpentinized rocks, suggesting that serpentinization likely serves as a significant source for Cu-rich hydrothermal metallogenic systems hosted in ultramafic rocks.

Keywords: Ronda Massif; serpentinization; element mobility; Cu leaching; serpentine; magnetite; fluid-rock interaction

# 1 Introduction

Serpentinization is a pervasive and ubiquitous process that involves the hydration of ultramafic rocks. This fluid-rock reaction is observed in a variety of geological settings (Deschamps et al., 2013; Guillot et Hattori, 2013), including slow-spreading mid-ocean ridges (Lissemberg et al., 2024), rifted margins (Pinto et al., 2017; Hochscheid et al., 2024), subduction zones (Faccenda et al., 2009; Deschamps et al., 2013), and even in subaerial environment after exhumation of ultramafic rocks from mantle depths (e.g., orogenic peridotites or mantle wedge) (Ulrich et al., 2020). Despite the expansion of a considerable amount of recent research (Pinto et al., 2015; Rouméjon et al., 2018; Mothersole et al., 2017; Debret et al., 2024), the element mobility during serpentinization remains a topic of interest. For instance, calcium has been identified as the most mobile element during serpentinization (Coleman and Keith, 1971; Janecky and Seyfried, 1986; King et al., 2003; Shervais, 2005; Iyer et al., 2008; Deschamps et al., 2013; Mothersole et al., 2017) and can thus participate to
carbonate precipitation and eventually CO2 storage (e.g., Coltat et al., 2021a). Also, abyssal and mantle-wedge serpentinites are enriched in fluid-mobile elements (FME), including B, Li, Cl, As, Sb, Pb, U, Cs, Sr, and Ba (Deschamps et al., 2013). These elements are efficiently incorporated in rocks during serpentinization at mid-ocean ridges thanks to interaction with seawater at varying depths (Vils et al., 2009; Kodolányi et al., 2012; Rouméjon et al., 2018). Once abyssal serpentinites are subducted, they progressively release FME during dehydration reactions. The released elements and H2O are transferred into the mantle wedge, together with some others derived from dehydration of the subducted metasediments, thereby promoting renewed serpentinization (Deschamps et al., 2013; Hughes et al., 2018; Scambelluri et al., 2019). Many of the FME (e.g., B, As, Sb, Pb, Sr) are also characteristic of front-arc magmatism (Leeman, 1996; Hattori & Guillot, 2003). Element mobility is thus critical for understanding mass transfer from the subducting lithosphere to the mantle wedge and, ultimately, arc magmatism. To date, apart from Ca and FME, there is a paucity of coherent views regarding the behavior of major (e.g. Si, Mg, Fe) and some trace elements (Cu, Co, Ni, Zn among others) during serpentinization. The issue of Al mobility also remains a subject of considerable controversy. Numerous studies indicated the mobility of Al during serpentinization (Shervais et al., 2005; Iyer et al., 2008; Jöns et al., 2010), others argued for its immobility (Deschamps et al., 2013; Malvoisin et al., 2020). This discordance can be attributed to the numerous factors that influence serpentinization processes. These include the geodynamic settings at which serpentinization occurs, volumetric variation, the physico-chemical properties of the original fluid, the heterogeneity of protoliths and the presence of adjacent magmatism.

Most studies on elemental mobilities have been developed in subduction and oceanic divergent settings (Deschamps et al., 2013; Mothersole et al., 2017; Debret et al., 2024). At mid-ocean ridges, hydrothermal processes are ubiquitous and contribute to element mobility
(Jöns et al., 2010). These authors suggest that serpentinization may enhance the mobility of elements and thereby change the chemistry of the whole rock in comparison to that of its protolith. However, distinguishing between the magmatic component and the pure hydrothermal component with respect to element mobilities remains challenging, as previously indicated by the aforementioned authors. The occurrence of modern and recent metal deposits, first documented since the late 1970s, results in a complex magmatic-hydrothermal system beneath the seafloor (e.g., Fouquet et al., 2010; Debret et al., 2018 and 2024; Coltat et al., 2023). These metal deposits bear a striking resemblance to fossilized "seafloor massive sulfide ultramafic-hosted deposits" identified in numerous ophiolites worldwide and termed "ultramafic-hosted deposits" as they formed in various tectonic settings (Peltonen et al., 2008; Fouquet et al., 2010; Patten et al., 2022). These deposits are enriched in base metals (Cu, Zn, Ni), precious metals (Au, Ag), and critical metals (Co), with Cu being systematically present. Their mineralogy is diverse but predominantly composed of sulfide minerals—pyrrhotite, pyrite, chalcopyrite, cubanite, and sphalerite. A genetic correlation between mineralization and serpentinization can be postulated, predicted on the observation that all the host rocks of these ore deposits have undergone this process (Patten et al., 2022).

The element transfers occurring during serpentinization in orogenic peridotites have received comparatively limited attention. A substantial proportion of research on orogenic peridotites has focused on magmatic mechanisms and high-temperature processes with the objective of elucidating the mantle petrogenetic history (e.g. Bodinier and Godard, 2014). However, given that hydration reactions are common and expected during mantle rocks exhumation and emplacement, the primary geochemical signature of the whole rock may be obliterated by fluid-rock reactions during exhumation (Thayer, 1966; King et al., 2003). Consequently, a comprehensive characterization of the geochemical consequences of serpentinization is required for further investigations on mantle petrology. In addition, the
potential mobility related to the serpentinization process may have a great implication on the ultramafic-hosted deposits metallogenic system in such orogenic settings. The Ronda Massif (southern Spain) offers an optimal natural laboratory for these objectives as it constitutes the largest exposure of subcontinental lithospheric mantle (SCLM) at the Earth's surface and is largely devoid of mafic magmatism (Garrido and Bodinier, 1999). The rate and typology of serpentinization recently established by Audran et al. (2024) for the Ronda Massif, facilitates and makes possible the determination of elemental budgets during serpentinization.

# 2 The Ronda Massif

## 2.1 Geological setting

The Ronda Massif is located within the internal zone of the Betic Chain, which developed in the frame of the Alpine collision (Figure 1a). The Ronda Massif is formed by ultramafic rocks of subcontinental lithosphere origin. These mantle rocks outcrop in three main Sierras (i.e., mountains), and here we focus on the Sierra Bermeja that is the largest one (Figure 1). The geodynamics leading to the emplacement of this mantle rocks over the Iberian plate has been a subject of debate for decades (Loomis, 1972; Bessière et al., 2021; Pedrera et al., 2025). Several authors have suggested that the Ronda peridotite was pre-exhumed during Late Mesozoic hyperextension of the Iberian margin (Pedrera et al., 2020; Bessière et al., 2021; Díaz-Alvarado et al., 2024), whereas others have linked its exhumation to an Oligocene–Miocene back-arc setting (Hidas et al., 2013; Frasca et al., 2017). In these models, the final emplacement of the peridotite is attributed to Early Miocene thrusting (i.e. Alpine convergence), as emphasized by inverted magmatism beneath the peridotite dated at 22.3 ± 0.7 Ma (from U–Pb datation on zircon: Esteban et al., 2010). By contrast, Mazzoli and
Martín-Algarra (2011) proposed a single Miocene transpressional-thrusting event to explain the peridotite's emplacement.

The mantle is primarily constituted of fertile lherzolites and minor harzburgites, with the occasional occurrence of dunite (Frey et al., 1985; Soustelle et al., 2009). In detail, the Ronda peridotite includes spinel tectonite, coarse granular and plagioclase tectonite domains (Figure 1; Obata, 1980; Van der Wal and Vissers, 1996). Lenoir et al. (2001) identified a plurikilometric transition zone, defined as a "recrystallization front," which propagated from the coarse granular domain into the spinel tectonite domain. The coarse granular and plagioclase tectonite domains are associated with the crustal emplacement of the Massif, which overprints the spinel tectonite domain (Lenoir et al., 2001; Soustelle et al., 2009; Van der Wal and Vissers, 1996; Hidas et al., 2013). Sulfides are commonly preserved as inclusions within the primary minerals of peridotite, including pentlandite and minor pyrrhotite, chalcopyrite, and bornite (Lorand, 1985).

Felsic magmatic dikes intruding the serpentinized peridotite are frequent (Esteban et al., 2008 and 2019; Pedrera et al., 2016; Audran et al., 2024; Bévan et al., 2025; Pedrera et al., 2025). This felsic magmatism is dated between 23 and 19 Ma (Priem et al., 1979; Sánchez-Rodríguez and Gebauer, 2000; Esteban et al., 2010; Frasca et al., 2017). At the contact between the intrusives and the host serpentinized peridotite, metasomatic alterations have been described and consist of rodingitization and chloritization (Esteban et al., 2003 and 2019, Bévan et al., 2025). Noteworthy, late serpentinization stages in the Ronda Massif occurred contemporaneously with the emplacement of the felsic dykes during the Miocene (Cuevas et al., 2006; Pedrera et al., 2016; Esteban et al., 2007 and 2019; Audran et al., 2024).
## 2.2 Serpentinization in the Ronda Massif

Only a few studies have been conducted on serpentinization in the Ronda Massif (Pedrera et al., 2016, Audran et al., 2024). The serpentinization process was reported to occur in three successive stages (Audran et al., 2024). In the initial stage (S1), a pervasive serpentinization affected the entire peridotite with an increasing serpentinization rate towards the main tectonic discontinuity (Figure 1b, red and blue line). The serpentinized peridotites have a brown granular appearance in the field (Figure 2a). The second stage (S2) forms "corridors" in serrated, centimeter-thick whitish blue planar layers (Figure 2a). The third stage (S3) is characterized by thick cataclastic shear zones and faults and the crystallization of syn-tectonic veins (Figures 2b and c). A serpentinized metasomatic halo is found at the edge of syn-tectonic veins (Figure 2b). A conceptual synthetic sketch illustrates the organization of the serpentinized objects in Figure 2d.

Figures 3a, b and c show a few examples of hand specimens from different stages of serpentinization. The fresh peridotite is composed of olivine, orthopyroxene, clinopyroxene, Cr-spinel and primary pristine sulfide. Although fresh in appearance, peridotites are often partly serpentinized and present a mesh texture (S1) with minor presence of tiny crystals of magnetite (Figure 3d). In corridors (S2), all primary minerals have been altered to serpentine, forming a recrystallization texture (Figure 3f). Some remnants of the S1 mineral assemblage (i.e. orthopyroxene, olivine and mesh texture) are still partially preserved. Orthopyroxene is turned into bastite. A network of veins and veinlets developed parallel or oblique to the direction of the corridor, with variable amounts of magnetite. In the metasomatic halo (S3bis) the same texture as in S2 can be recognized (i.e. bastite, a network of veinlets, recrystallized serpentine and magnetite) (Figure 3e). In the cataclastic shear zone (S3), the serpentine is mainly recrystallized. Remnants of bastite are partially replaced by magnetite (Figure 3f). A network of serpentine veins also developed. The serpentine veins show different textures such
as granular without magnetite or fibrous with cubic euhedral magnetite. The partial or full
transformation of Cr-spinel into ferritchromite occurs, irrespective of the stage of
serpentinization (Da Costa and Barriga, 2022; Audran et al., 2024; Figure 3f). The syn-
serpentinization paragenesis of sulfide, oxide, and alloys has been established by Lorand
(1985) and recently reworked by Lorand et al. (2021). This comprises euhedral pentlandite,
mackinawite, awaruite, native iron, native copper, and secondary sulfides such as digenite and
vallerite. The transformation of sulfides into oxides during serpentinization has been
documented both in our study and by Lorand et al. (1985).

Serpentinization in the Ronda Massif occurred during the exhumation of peridotites driven by Alpine convergence. O-H isotope compositions point to a magmatic or metamorphic fluid origin, with no seawater contribution. The process took place at temperatures lower than 300 °C (Pedrera et al., 2016; Audran et al., 2024).

# 3 Materials and methods

## 3.1 Sampling strategy and samples

Audran et al. (2024) showed that the proportion of serpentine in the partially
serpentinized peridotites of the S1 stage was comprised between 15 and 95%. In order to
characterize element mobility during S1 serpentinization, we selected samples with a variable
amount of serpentine at seven distinct outcrops in the Sierra Bermeja (Figure 1b). The modal
proportion of serpentine of each S1 sample has been estimated following the method of
Audran et al. (2024) and is between 28 and 55% (Table 1). For stages S2 (corridors) and S3
(shear zones) where samples are nearly pure serpentinites, we collected samples in the
immediate proximity of S1 samples. Doing this we ensure that the serpentinized samples S1
corresponds to the actual protolith of the more serpentinized S2 and S3 samples. It is worthy
to note that S3 samples can be grouped into two categories: S3 samples are associated with
the cataclastic shear zone, whereas S3bis samples represent either a partially or fully altered
edge of the shear zone or an altered block within the shear zone (Figures 2b, c, d). In total,
twenty-one samples were collected, representing four pairs of S1-S2, two of S1-S3 and three
of S1-S3bis.

3.2 Analytical methods

3.2.1 Composition of whole rocks

Samples have been crushed using an agate mortar. Rock powders were dehydrated for
a period of four hours at a temperature of approximately 1015°C to obtain the loss on ignition
(LOI) content. The major element concentrations were measured on pressed pellets of powder
using a micro-X-ray fluorescence Bruker M4 Tornado at the CoRTecS PeMCI platform from
the Institut Terre et Environnement de Strasbourg (ITES) in France. For each sample, 10
measurements were performed on each pellet to ensure the reproducibility at the pellet
surface. A collimated Rh source operating at 700 µA with an accelerating voltage of 50 kV
was used, resulting in a 1 mm spot-size in diameter at the sample surface. Two energy-
dispersive silicon drift detectors of 125 eV resolution, and with an active area of 60 mm² each
were used to measure fluorescence spectra (300 s counting time per spectrum). Measurements
were performed in a vacuum chamber at 2 mbar to minimize air absorption and ensure the
best signal/noise ratio.

Analyses performed on international standard are consistent with preferred reference values
for MGL-GAS serpentinite (certified standard from International Association of Geoanalysts
in 2019). Differences between reference values from the literature and our analyses of MGL-
GAS are less than 1% for all other major oxides except for Na₂O, K₂O and P₂O₅ which are present in very low concentrations.

The trace element concentrations were determined at ITES using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Thermo Scientific X series II instrument and ICAP RQ Thermo Scientific instrument. We use about 100 mg of rock powder for each sample. The analytical procedure of powder dissolution following the method of Chauvel et al. (2011), adapted by Hochscheid et al. (2024).

### 3.2.1 Composition of minerals

The major element concentrations of specific minerals were measured on 30 µm-thick polished sections using a µ-XRF Bruker M4 Tornado at the CoRTecS PeMCI platform (ITES) following the procedure developed in Hochscheid et al. (2024) and Ballay et al. (2025). Measurements were performed using a Rh anode operating at 400 µA with an accelerating voltage of 50 kV. Polycapillary lenses were used to focus the X-ray beam down to 20 µm full-width-at-half-maximum at the sample surface. The counting time per point was set to 180 s. A distinct calibration has been performed for serpentine, orthopyroxene, olivine and spinels to maximize the analytical quality of mineral composition. µ-XRF spectra were calibrated after repeated analyses of Smithsonian National Museum of Natural History mineral standards (Jarosewich, et al., 1980) of diopside (NMNH 117733) and San Carlos olivine (NMNH 111312-44), sulfide, and some in-house mineral standards of clinochlore, chrysotile and antigorite. Total Fe content was calculated on a divalent basis, as FeO.

The structural formula for serpentines was calculated on the basis of seven oxygens, resulting in the Mg₃(Si)₂O₅(OH)₄ formula. The sum of elements could be classified into two categories: (i) The tetrahedral site, equivalent to 2 apfu, is constituted by Si, Al, and Ti, and (ii) the octahedral site, equivalent to 3 apfu, is constituted by Mg and Fe elements. The data
set for this study was selected to encompass the range of 1.9 to 2.1 apfu and 2.8 to 3.2 apfu
for tetrahedral and octahedral sites, respectively. These values correspond to serpentine with
minimal or no pollution by ferritchromite, magnetite, chlorite, olivine and pyroxenes. The
excess of iron in the octahedral sheet can be attributed to the presence of tiny magnetite. The
data were converted into Fe (ppm), and the maximum represents the maximum acceptable
value for Fe content in serpentine trace elements, which is 122600 ppm. Finally, an Mg
content exceeding 265000 ppm is attributed to the presence of extremely fine-grained
serpentine-brucite intergrowths. Magnetite or ferritchromite with a maximum SiO₂ content of
5% was selected to preclude the potential contamination from serpentine and clinochlore. We
then performed a recalculation of the pristine composition of magnetite by proportionally
subtracting the serpentine composition, which was determined by analyzing the nearby
serpentine grains.

In situ trace elements measurements in different minerals (i.e. serpentine, magnetite, ferritchromite, orthopyroxene, clinopyroxene and Cr-spinel) were completed on an Applied Spectra RESOlution-SE 193 nm ArF excimer laser ablation system with a large volume S155 sample cell coupled to an Agilent 8900 ICP-QQQ-MS at Université Laval, Canada. Four thin sections per analytical session were loaded into the cell together with reference materials in epoxy mounts. A total of 57 analytes were collected. Data were acquired with 15 seconds of background measurement followed by between 30 seconds of sample ablation using laser energy between 4 to 6 J/cm², repetition rate between 8 and 10 Hz, and a spot size of 30 to 50 µm. The acquisition time per analyte was 5 ms except for ⁷Li, ¹¹B, ⁴⁷Ti, ⁵²Cr, ⁵⁵Mn, ¹⁰⁷Ag, ¹¹⁸Sn, ¹²¹Sb, ¹³⁷Ba, ¹⁸²W, ¹⁹⁷Au, ²⁰⁸Pb, ²³⁸U which were analyzed at 10 ms, and ⁵⁹Co, ⁶⁰Ni, ⁶³Cu, ⁶⁶Zn at 15 ms. Standard reference materials (SRM) were used to calibrate instrumental mass fractionation and monitor drift throughout the session. These reference materials include SRM NIST610 (used as primary standard) and NIST612 glasses (Jochum et al., 2011) and
USGS basaltic glass GSE-1 g (Guillong et al., 2005). Data were reduced through Iolite v4 (Paton et al., 2011). Internal calibration was done by micro-XRF major chemistry (i.e. Cr₂O₃ for ferritchromite and Cr-spinel, FeO for magnetite, SiO₂ for serpentine). To minimize the impact of serpentine-magnetite mixtures on the trace element composition of magnetite, we set a maximum limit of 10000 ppm of Si. This equates to 5% serpentine contamination, an acceptable limit for this type of system. The identical limit was employed for ferritchromite compositions in order to mitigate the effects of serpentine or clinochlore contamination.

# 4 Results

## 4.1 Mineral compositions

### 4.1.1 Serpentine

The chemical composition of serpentine grains is reported in supplementary material Table A and B and presented in Figure 4 for major elements and Figure 5 for trace elements. In these figures, beside the stage to which minerals belong, we have distinguished the mineral texture on which the analysis was made. Four textures were identified in thin section: mesh, vein, bastite and recrystallized serpentine.

The Mg# (i.e. Mg/(Fe(t)+Mg), in apfu) in serpentine grains varies from 0.855 to 0.995, without clear first order distinction between the serpentines from the distinct stages (see supplementary material, Table A). Figure 4a depicts a classical correlation between Mg and Fe (apfu), irrespective of the texture of the analyzed serpentines grains. In the Mg+Fe vs. Si diagram (Figure 4b), the serpentines display some variations that correspond to trends associated with Tschermak, cronstedtite, and brucite.
The trace element concentrations of serpentine grains are reported as a function of Al
(Figure 5a, b, c and d) and Cr (Figure 5e, f), which vary widely from 55.2 to 16923 ppm and
from 0.3 to 13685 ppm, respectively (see supplementary, Table B). The Al-poor serpentine
grains (i.e. less than 1000 ppm) coincide with the mesh texture and a part of recrystallized
serpentine, with only a slight enrichment in Al relative to the olivine composition. In contrast,
the Al-rich serpentine grains are predominantly bastite, with aluminium depletion about ~0.8
to 1.8 wt% compared to orthopyroxene and clinopyroxene (Figure 5a). There is a notable
correlation between Ti and Al and between V and Al (Figure 5a, b). These correlations are
enclosed by the mean composition of primary minerals. The Ca content greatly varies,
ranging from 25 to 10 000 ppm, especially in the bastite texture (Figure 5c). The mesh and
recrystallized serpentines have a Ni content close to the olivine mean centered around
2000 ppm (Figure 5d) whereas it falls down to 45 ppm for S3 Al-rich serpentine grains in
veins. As for Al, the highest Cr content is met for bastite with values identical to
orthopyroxene (Figures 5e, f). The Cr-poor serpentine grains do not correspond to any
specific texture. The minimum of a few ppm is lower than the olivine content. Co content is
of ~70ppm in average, with nevertheless large depletion of less than 30 ppm in many
serpentine grains, regardless of the texture. In contrast, the Cu content is higher than the one
of the main primary mantle minerals, with concentrations ranging from <1 ppm to >30 ppm,
with no clear relation regarding the serpentine generations (Figure 5f). The bastite exhibits
notable variation in Ca, with values ranging from 99 to 6676 ppm (see supplementary, Table
B), far below the mean values of clinopyroxene and orthopyroxene (i.e. 146306 and 8100
ppm respectively; see supplementary, Table F).
4.1.2 Oxides

The chemical compositions of oxides (Cr-spinel, magnetite and ferritchromite) are
reported in supplementary material Tables C and D and are shown in Figures 6 and 7 for
major and trace elements, respectively. In contrast to Cr-spinel, ferritchromite exhibits a
notable range in the concentration of MgO, Al₂O₃, FeO(t) and especially Cr₂O₃ (Figure 6a, b).
There is a correlation between Al₂O₃ and FeO(t) for Cr-spinel, and to some extent for
ferritchromite (Figure 6c). The concentration of Cr₂O₃ of ferritchromite evolves toward the
composition of magnetite (Figure 6b). Magnetite is primarily composed of FeO(t) more than
94.7 wt %, with a minimal concentration of Al₂O₃, and Cr₂O₃ at a level of less than 1 wt.%
(Figure 6b, c).

Cr-spinel and ferritchromite exhibit a high Zn content and notable dispersion.

Magnetite displays a distinct lower Zn content (Figure 7b). The concentration of Zn exceeds that of serpentine veins, with a mean value of 51 ppm, which is comparable to the mean Zn content of the olivine and the mesh texture. The concentration of Ni is lower in ferritchromite and Cr-spinel than in magnetite, with here again some dispersion for a given mineral species (Figure 7b). In the Ni vs. Co diagram, one can first observe a wide dispersion of the magnetite composition (Figure 7b). For Cr-spinel, there is a strong correlation between the two elements pointing toward the field of magnetite composition. Some ferritchromite compositions strictly follow this correlation, whereas some others are peculiar with a constant Ni content and variable Co contents. It is noteworthy that both magnetite and ferritchromite exhibit a low concentration of copper, falling below the detection limit or rarely reaching a value close to 1 ppm (see supplementary, Table D).
## 4.2 Whole rock

<table><thead><tr><th>samples</th><th>R9-23-1</th><th>R9-23-2</th><th>R9-23-3</th><th>R9-23-4</th><th>R10-23-1</th><th>R10-23-3</th><th>R14-23-1</th><th>R14-23-2</th><th>R14-23-3</th><th>R14-23-4</th><th>R16-23-1</th><th>R16-23-3</th><th>R16-23-4</th><th>R52-23-1</th><th>R52-23-2</th><th>R66-23-1</th><th>R66-23-2</th><th>R73-23-1</th><th>R73-23-3</th><th>R73-23-4</th></tr></thead><tbody><tr><td>serp. stages</td><td>S1</td><td>S2</td><td>S3bi s</td><td>S3</td><td>S1</td><td>S3bi s</td><td>S1</td><td>S2</td><td>S3bi s</td><td>S3</td><td>S1</td><td>S3bi s</td><td>S3</td><td>S1</td><td>S2</td><td>S1</td><td>S2</td><td>S1</td><td>S3bi s</td><td>S3</td><td>S3bi s</td></tr><tr><td>X-Y Coordinat es (WGS- 84)</td><td colspan="2">36.56061, -5.18299</td><td colspan="2"></td><td colspan="2">36.50463, - 5.17803</td><td colspan="2">36.52817, -5.20194</td><td colspan="2"></td><td colspan="2">36.62378, - 5.06493</td><td colspan="2"></td><td colspan="2">36.50256, - 5.19283</td><td colspan="2">36.53858, - 5.17504</td><td colspan="2">36.58535, - 5.06937</td><td colspan="2"></td></tr></tbody></table>

### Major elements (wt %)

<table><tbody><tr><td>SiO<sub>2</sub></td><td>40.9</td><td>38.7</td><td>38.8</td><td>38.8</td><td>38.7</td><td>38.3</td><td>39.3</td><td>38.8</td><td>38.3</td><td>38.9</td><td>37.0</td><td>38.9</td><td>39.0</td><td>41.0</td><td>38.5</td><td>39.7</td><td>39.3</td><td>38.7</td><td>39.1</td><td>37.4</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>2.81</td><td>1.40</td><td>0.46</td><td>0.51</td><td>0.75</td><td>0.96</td><td>1.43</td><td>1.12</td><td>1.41</td><td>1.20</td><td>0.55</td><td>0.91</td><td>1.21</td><td>1.29</td><td>0.34</td><td>1.34</td><td>0.32</td><td>0.51</td><td>0.64</td><td>0.82</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>9.25</td><td>7.62</td><td>8.15</td><td>7.6</td><td>8.72</td><td>6.73</td><td>8.33</td><td>8.21</td><td>7.76</td><td>7.62</td><td>8.48</td><td>8.09</td><td>7.76</td><td>8.92</td><td>7.22</td><td>8.6</td><td>7.78</td><td>8.42</td><td>8.28</td><td>8.14</td></tr><tr><td>MgO</td><td>36.7</td><td>38.3</td><td>38.8</td><td>37.3</td><td>40.4</td><td>39.7</td><td>38.8</td><td>37.5</td><td>37.0</td><td>37.1</td><td>40.9</td><td></td><td>35.9</td><td>40.4</td><td>38.6</td><td>40.4</td><td>37.5</td><td>41.9</td><td>40.9</td><td>36.8</td></tr><tr><td>CaO</td><td>3.11</td><td>0.57</td><td>0.5</td><td>0.07</td><td>0.91</td><td>0.03</td><td>1.77</td><td>1.07</td><td>0.33</td><td>0.04</td><td>1.01</td><td>1.01</td><td>0.04</td><td>0.93</td><td>0.02</td><td>1.76</td><td>0.07</td><td>0.59</td><td>0.91</td><td>0.37</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.38</td><td>0.55</td><td>0.28</td><td>0.22</td><td>0.72</td><td>0.27</td><td>0.52</td><td>0.54</td><td>0.3</td><td>0.28</td><td>0.27</td><td>0.53</td><td>0.32</td><td>0.59</td><td>0.23</td><td>0.34</td><td>0.14</td><td>0.42</td><td>0.34</td><td>0.2</td></tr><tr><td>K<sub>2</sub>O</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td></tr><tr><td>TiO<sub>2</sub></td><td>0.15</td><td>0.07</td><td>0.01</td><td>0.01</td><td>0.03</td><td>0.03</td><td>0.06</td><td>0.05</td><td>0.06</td><td>0.05</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.01</td><td>0.05</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.06</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td></tr><tr><td>LOI</td><td>5.95</td><td>12.1</td><td>12.2</td><td>14.7</td><td>9.04</td><td>13.3</td><td>9.02</td><td>12.0</td><td>14.1</td><td>14.1</td><td>11.0</td><td>10.4</td><td>15.1</td><td>5.96</td><td>14.4</td><td>7.1</td><td>14.1</td><td>4</td><td>8.56</td><td>9.08</td><td>15.5</td></tr><tr><td>Total</td><td>99.3</td><td>99.3</td><td>99.3</td><td>99.3</td><td>99.3</td><td>99.5</td><td>99.3</td><td>99.3</td><td>99.3</td><td>99.4</td><td>99.3</td><td>99.3</td><td>99.4</td><td>99.2</td><td>99.4</td><td>99.3</td><td>99.3</td><td>99.2</td><td>99.3</td><td>99.4</td></tr><tr><td>Mg#</td><td>4</td><td>9</td><td>8</td><td>6</td><td>8</td><td>5</td><td>6</td><td>7</td><td>7</td><td>2</td><td>6</td><td>8</td><td>2</td><td>2</td><td>9</td><td>7</td><td>5</td><td>9</td><td>5</td><td>2</td></tr><tr><td>modal serp %</td><td>0.88</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.92</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.91</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td></tr><tr><td></td><td>28</td><td>95</td><td>95</td><td>100</td><td>44</td><td>100</td><td>46</td><td>68</td><td>89</td><td>100</td><td>55</td><td>65</td><td>100</td><td>49</td><td>100</td><td>47</td><td>100</td><td>49</td><td>59</td><td>100</td></tr></tbody></table>

### Trace elements (ppm)

<table><tbody><tr><td>Li</td><td>8</td><td>7</td><td>3</td><td>4</td><td>9</td><td>14</td><td>8</td><td>5</td><td>5</td><td>5</td><td>5</td><td>4</td><td>4</td><td>3</td><td>4</td><td>bdl</td><td>4</td><td>3</td><td>6</td><td>11</td></tr><tr><td>V</td><td>86.7</td><td>51.8</td><td>37.2</td><td>33.5</td><td>40.1</td><td>29.0</td><td>56.3</td><td>51.2</td><td>59.7</td><td>48.0</td><td>43.3</td><td>55.4</td><td>45.9</td><td>48.2</td><td>20.8</td><td>58.1</td><td>30.6</td><td>34.8</td><td>41.1</td><td>49.0</td></tr><tr><td>Ti*</td><td>890</td><td>436.</td><td>50.5</td><td>53.4</td><td>156.</td><td>178</td><td>363.</td><td>306.</td><td>385.</td><td>288.</td><td>43.8</td><td>36.0</td><td>35.7</td><td>141.</td><td>56.2</td><td>270.</td><td>35.2</td><td>80.5</td><td>76.0</td><td>334.</td></tr><tr><td>Cr*</td><td>2624</td><td>5</td><td>5</td><td>9</td><td>7</td><td>3</td><td>4</td><td>2</td><td>6</td><td>1</td><td>2</td><td>9</td><td>1</td><td>9</td><td>2</td><td>2</td><td>2</td><td>7</td><td>2</td><td>6</td></tr><tr><td>Co</td><td>93</td><td>95</td><td>105</td><td>101</td><td>109</td><td>134</td><td>102</td><td>102</td><td>94</td><td>98</td><td>104</td><td>97</td><td>99</td><td>99</td><td>107</td><td>99</td><td>105</td><td>96</td><td>105</td><td>125</td></tr><tr><td>Ni*</td><td>1834</td><td>1</td><td>1</td><td>1</td><td>8</td><td>9</td><td>0</td><td>4</td><td>0</td><td>3</td><td>2</td><td>1</td><td>4</td><td>6</td><td>3</td><td>4</td><td>3</td><td>0</td><td>4</td><td>9</td></tr><tr><td>Mn</td><td>941.</td><td>802.</td><td>788.</td><td>811.</td><td>811.</td><td>905.</td><td>896.</td><td>820.</td><td>851.</td><td>851.</td><td>832.</td><td>883.</td><td>731.</td><td>781.</td><td>681.</td><td>856.</td><td>809.</td><td>799.</td><td>884.</td><td>922.</td></tr><tr><td>Cu</td><td>7</td><td>8.0</td><td>11.6</td><td>7.3</td><td>8.3</td><td>19.5</td><td>15.9</td><td>16.2</td><td>14.5</td><td>14.1</td><td>6.6</td><td>6.5</td><td>5.3</td><td>7.4</td><td>4.7</td><td>14.5</td><td>5.6</td><td>12.5</td><td>1.3</td><td>2.4</td></tr><tr><td>Zn</td><td>43.6</td><td>44.7</td><td>43.4</td><td>38.9</td><td>50.0</td><td>53.2</td><td>52.2</td><td>47.1</td><td>44.9</td><td>44.5</td><td>46.7</td><td>44.4</td><td>48.6</td><td>45.6</td><td>35.5</td><td>48.1</td><td>44.3</td><td>42.6</td><td>48.1</td><td>59.6</td></tr><tr><td>Ge</td><td>2.0</td><td>2.1</td><td>1.9</td><td>2.0</td><td>2.3</td><td>3.0</td><td>2.0</td><td>1.9</td><td>1.8</td><td>1.9</td><td>1.8</td><td>1.9</td><td>2.1</td><td>2.1</td><td>2.2</td><td>2.2</td><td>2.2</td><td>2.2</td><td>2.2</td><td>2.8</td></tr><tr><td>Sb</td><td>bdl</td><td>0.06</td><td>0.05</td><td>0.30</td><td>0.03</td><td>0.06</td><td>bdl</td><td>bdl</td><td>0.20</td><td>0.45</td><td>bdl</td><td>bdl</td><td>0.17</td><td>0.07</td><td>0.07</td><td>bdl</td><td>0.05</td><td>0.03</td><td>0.04</td><td>0.05</td></tr><tr><td>Cs</td><td>0.01</td><td>0.03</td><td>0.05</td><td>0.05</td><td>0.02</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.03</td><td>0.07</td><td>0.23</td><td>0.02</td><td>0.14</td><td>0.05</td><td>0.05</td><td>0.06</td><td>0.13</td></tr><tr><td>Rb</td><td>7</td><td>4</td><td>5</td><td>1</td><td>7</td><td>1</td><td>3</td><td>2</td><td>3</td><td>0</td><td>1</td><td>2</td><td>4</td><td>2</td><td>4</td><td>2</td><td>4</td><td>9</td><td>6</td><td>6</td></tr><tr><td>Ba</td><td>0.06</td><td>0.03</td><td>0.07</td><td>0.04</td><td>0.13</td><td>0.06</td><td>0.05</td><td>0.03</td><td>0.04</td><td>0.05</td><td>0.07</td><td>0.11</td><td>0.24</td><td>0.35</td><td>0.05</td><td>0.16</td><td>0.04</td><td>0.43</td><td>0.54</td><td>0.90</td></tr><tr><td>Ba</td><td>0.30</td><td>1.70</td><td>2.50</td><td>3.30</td><td>0.70</td><td>1.20</td><td>0.30</td><td>0.80</td><td>1.70</td><td>1.50</td><td>0.20</td><td>0.20</td><td>2.00</td><td>1.20</td><td>1.70</td><td>0.90</td><td>1.40</td><td>0.40</td><td>0.90</td><td>1.50</td></tr><tr><td>Sr</td><td>###</td><td>3.58</td><td>4.46</td><td>2.99</td><td>5.22</td><td>0.69</td><td>7.08</td><td>4.08</td><td>2.46</td><td>0.86</td><td>4.57</td><td>3.78</td><td>0.93</td><td>2.79</td><td>1.22</td><td>4.32</td><td>0.70</td><td>1.61</td><td>4.88</td><td>2.02</td></tr><tr><td>#</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>La</td><td>0.22</td><td>0.11</td><td>0.08</td><td>0.10</td><td>0.01</td><td>0.03</td><td>0.08</td><td>0.08</td><td>0.09</td><td>0.10</td><td>0.09</td><td>0.11</td><td>0.31</td><td>0.03</td><td>0.04</td><td>0.11</td><td>bdl</td><td>0.02</td><td>0.06</td><td>0.08</td></tr><tr><td>Ce</td><td>2</td><td>6</td><td>1</td><td>9</td><td>7</td><td>1</td><td>7</td><td>2</td><td>5</td><td>6</td><td>3</td><td>5</td><td>6</td><td>9</td><td>2</td><td>2</td><td>bdl</td><td>0.06</td><td>0.19</td><td>0.27</td></tr><tr><td>Pr</td><td>0.81</td><td>0.35</td><td>0.17</td><td>0.16</td><td>0.05</td><td>0.09</td><td>0.31</td><td>0.27</td><td>0.28</td><td>0.25</td><td>0.16</td><td>0.23</td><td>0.59</td><td>0.09</td><td>0.10</td><td>0.32</td><td>bdl</td><td>3</td><td>8</td><td>2</td></tr><tr><td>Nd</td><td>2</td><td>8</td><td>8</td><td>6</td><td>5</td><td>6</td><td>6</td><td>8</td><td>6</td><td>8</td><td>1</td><td>1</td><td>4</td><td>9</td><td>1</td><td>8</td><td>bdl</td><td>0.01</td><td>0.04</td><td>0.06</td></tr><tr><td>Sm</td><td>0.33</td><td>0.14</td><td>bdl</td><td>0.03</td><td>0.06</td><td>0.07</td><td>0.14</td><td>0.10</td><td>0.12</td><td>0.08</td><td>bdl</td><td>bdl</td><td>0.09</td><td>0.06</td><td>0.04</td><td>0.09</td><td>bdl</td><td>0.06</td><td>0.09</td><td>0.15</td></tr><tr><td>Eu</td><td>0.13</td><td>0.06</td><td>bdl</td><td>0.02</td><td>0.02</td><td>0.03</td><td>0.06</td><td>0.04</td><td>0.05</td><td>0.04</td><td>bdl</td><td>bdl</td><td>0.05</td><td>0.02</td><td>0.01</td><td>0.04</td><td>bdl</td><td>0.02</td><td>0.03</td><td>0.05</td></tr><tr><td>Gd</td><td>0.49</td><td>0.20</td><td>bdl</td><td>0.04</td><td>0.08</td><td>0.10</td><td>0.20</td><td>0.15</td><td>0.19</td><td>0.14</td><td>bdl</td><td>bdl</td><td>0.09</td><td>0.06</td><td>0.05</td><td>0.14</td><td>bdl</td><td>0.05</td><td>0.09</td><td>0.19</td></tr><tr><td>Tb</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>bdl</td><td>bdl</td><td>0</td><td>0</td><td>0</td><td>0</td><td>bdl</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Dy</td><td>0.09</td><td>0.03</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.04</td><td>0.03</td><td>0.03</td><td>0.02</td><td>bdl</td><td>bdl</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.02</td><td>bdl</td><td>0.00</td><td>0.00</td><td>0.03</td></tr><tr><td>Ho</td><td>0.15</td><td>0.05</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.04</td><td>0.03</td><td>0.03</td><td>0.02</td><td>bdl</td><td>bdl</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.02</td><td>bdl</td><td>0.01</td><td>0.01</td><td>0.04</td></tr><tr><td>Er</td><td>0.41</td><td>0.14</td><td>0.02</td><td>0.03</td><td>0.07</td><td>0.05</td><td>0.19</td><td>0.16</td><td>0.20</td><td>0.14</td><td>bdl</td><td>bdl</td><td>0.02</td><td>0.09</td><td>0.07</td><td>0.03</td><td>bdl</td><td>0.04</td><td>0.03</td><td>0.13</td></tr><tr><td>Er</td><td>6</td><td>8</td><td>2</td><td>0</td><td>1</td><td>0</td><td>4</td><td>5</td><td>5</td><td>1</td><td>7</td><td>3</td><td>0</td><td>3</td><td>7</td><td>0</td><td>bdl</td><td>0</td><td>7</td><td>3</td></tr></tbody></table>
<table><thead><tr><th></th><th>0.06</th><th>0.02</th><th>0.00</th><th>0.00</th><th>0.01</th><th>0.00</th><th>0.03</th><th>0.02</th><th>0.02</th><th>0.02</th><th>bdl</th><th>0.00</th><th>0.01</th><th>0.01</th><th>0.00</th><th>0.02</th><th>bdl</th><th>0.00</th><th>0.00</th><th>0.02</th></tr></thead><tbody><tr><td>Tm</td><td>0</td><td>5</td><td>5</td><td>7</td><td>4</td><td>9</td><td>1</td><td>3</td><td>7</td><td>0</td><td></td><td>5</td><td>6</td><td>1</td><td>6</td><td>7</td><td></td><td>9</td><td>6</td><td>2</td></tr><tr><td>Yb</td><td>0.43</td><td>0.16</td><td>0.03</td><td>0.04</td><td>0.08</td><td>0.06</td><td>0.21</td><td>0.17</td><td>0.23</td><td>0.15</td><td>0.03</td><td>0.05</td><td>0.12</td><td>0.09</td><td>0.04</td><td>0.16</td><td>0.02</td><td>0.07</td><td>0.05</td><td>0.16</td></tr><tr><td>Lu</td><td>0.06</td><td>0.02</td><td>bdl</td><td>bdl</td><td>0.01</td><td>0.00</td><td>0.03</td><td>0.02</td><td>0.02</td><td>0.02</td><td>bdl</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.02</td><td>bdl</td><td>0.01</td><td>bdl</td><td>0.02</td></tr><tr><td>Zr</td><td>7.46</td><td>3.71</td><td>0.85</td><td>0.71</td><td>0.57</td><td>0.47</td><td>3.08</td><td>2.65</td><td>2.90</td><td>2.09</td><td>0.17</td><td>0.53</td><td>0.78</td><td>0.67</td><td>0.57</td><td>2.31</td><td>0.15</td><td>0.74</td><td>2.35</td><td>2.92</td></tr><tr><td>Hf</td><td>0.23</td><td>0.12</td><td>0.07</td><td>0.05</td><td>0.04</td><td>0.03</td><td>0.09</td><td>0.07</td><td>0.07</td><td>0.06</td><td>bdl</td><td>bdl</td><td>0.03</td><td>0.05</td><td>0.02</td><td>0.08</td><td>bdl</td><td>0.04</td><td>0.07</td><td>0.12</td></tr><tr><td>Nb</td><td>0.08</td><td>0.05</td><td>0.20</td><td>0.58</td><td>0.03</td><td>0.03</td><td>0.07</td><td>0.11</td><td>0.07</td><td>0.05</td><td>0.10</td><td>0.14</td><td>0.16</td><td>0.04</td><td>0.04</td><td>0.06</td><td>bdl</td><td>0.03</td><td>0.03</td><td>0.02</td></tr><tr><td>Ta</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.02</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.13</td><td></td><td>bdl</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.01</td></tr><tr><td>Y</td><td>3.64</td><td>1.29</td><td>0.17</td><td>0.25</td><td>0.49</td><td>0.37</td><td>1.74</td><td>1.40</td><td>1.81</td><td>1.30</td><td></td><td>0.16</td><td>0.20</td><td>0.66</td><td>0.52</td><td>0.28</td><td>1.19</td><td>0.02</td><td>0.19</td><td>0.19</td><td>1.04</td></tr><tr><td>Sn</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td></td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>W</td><td>0.15</td><td>0.07</td><td>0.12</td><td>0.07</td><td>0.06</td><td>0.06</td><td>0.09</td><td>0.04</td><td>bdl</td><td>0.04</td><td></td><td>0.09</td><td>0.05</td><td>0.13</td><td>0.06</td><td>0.15</td><td>0.02</td><td>bdl</td><td>0.13</td><td>0.04</td><td>0.08</td></tr><tr><td>Th</td><td>0.05</td><td>0.04</td><td>0.17</td><td>0.62</td><td>0.06</td><td>0.02</td><td>0.05</td><td>0.12</td><td>0.03</td><td>0.05</td><td>0.03</td><td>0.09</td><td>0.12</td><td>0.08</td><td>0.05</td><td>0.16</td><td>0.37</td><td>0.03</td><td>0.05</td><td>0.08</td></tr><tr><td>U</td><td>0.00</td><td>0.00</td><td>0.03</td><td>0.02</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>bdl</td><td></td><td>0.00</td><td>0.00</td><td>0.03</td><td>0.00</td><td>bdl</td><td>0.01</td><td>bdl</td><td>0.00</td><td>0.00</td><td>0.01</td></tr><tr><td>Pb</td><td>0.16</td><td>0.15</td><td>0.06</td><td>0.09</td><td>bdl</td><td>0.07</td><td>0.50</td><td>0.30</td><td>0.13</td><td>0.11</td><td></td><td>0.32</td><td>0.46</td><td>5.67</td><td>0.19</td><td>0.50</td><td>0.40</td><td>0.25</td><td>bdl</td><td>0.12</td><td>0.11</td></tr></tbody></table>

Table 1 – Whole rock composition of the Ronda serpentinites and partially serpentinized peridotites. Mg# = Mg/(Fe(t)+Mg) is in molar proportions. The italicized value for Na₂O is indicative of the limit of quantification (LOQ). The LOQ is defined from the limit of detection (LOD), which is calculated using the following equations: LOQ = 3*LOD and LOD = 3√(BG)/S, where BG represents the background intensity and S denotes the sensitivity.

## 4.2.1 Major elements

The major elements concentrations are reported in Table 1 and presented in Figure 8 and 9 as a function of the Loss On Ignition (LOI). Our results are consistent with previously published data. Figure 8a shows a clear relationship between the LOI and the modal proportion of serpentine in the whole rock. This relationship remains the same during the different stages of serpentinization and follows a coherent continuity among these stages. In the following we thus report the content of oxides as a function of LOI as a proxy for the serpentinization rate. This will help to compare our results with those in the literature, as the amount of published LOI data far exceeds the amount of actual modal serpentine measurements in rocks. In Figure 8b we also reported a compilation of CaO content of serpentinized peridotites from the Ronda Massif, the Pyrenees Massifs and New Caledonia. The specific shape of the distribution reflects variable CaO contents at low LOI content and
very low CaO content for high LOI content. This pattern reveals a decrease in the CaO
content with increasing serpentinization of heterogeneous peridotites originally varying in
composition between low-CaO harzburgite and high-CaO lherzolite end-members.

CaO, Al₂O₃, SiO₂ and FeO and display a regular decrease in concentration with
increasing LOI, only differing in the slope of the correlation (Figure 9). For example, the
Al₂O₃ content (Figure 9b) decreases from a range of ~7-1wt % down to ~2-0.5% whereas the
FeO content (Figure 9d) slightly decreases from 9-8wt % to 8-7wt %. MgO displays a more
complex evolution with first, at low LOI, a large variation and then, at high LOI a decrease of
the content. Mg# varies from 0.887 to 0.921 (Table 1), with no correlation with LOI. Two
samples that have undergone full serpentinization exhibit a high Mg#, exceeding 0.910,
suggesting a depletion of Fe relative to Mg. In Figure 9, the data are compared with
theoretical values for partially serpentinized peridotites, where element concentrations are
assumed to decrease solely by dilution from added H₂O. Actually, the slope of the trend is
diagnostic of the governing process, dilution vs. leaching or enrichment. The data show that
CaO (Figure 9a) and Al₂O₃ (Figure 9b) plot below the theoretical dilution line with a steeper
slope, SiO₂ falls slightly below with a gentler slope (Figure 9c), while Fe₂O₃ and MgO closely
follow the theoretical hydration trend (Figure 9d, e).

4.2.2 Trace elements

The trace element concentrations of the variably serpentinized peridotites are reported
in Table 1 and presented in Figure 10 as a function of LOI, with the theoretical dilution line as
in Figure 9. Our results show good agreement with data from the literature.

A slight decrease with LOI is visible for Cr, and no clear evolution is observed for Ni,
Zn and Co. In contrast, Ti, Sr, Cu, and V decrease significantly with LOI, with data plotting
below the dilution lines. Ba is enriched by a few ppm during LOI increase (Figure 10e). Other
FME, such as Sb and U also display some enrichment, but not systematically (Table 1).

# 5 Discussion

## 5.1 Calcium leaching during serpentinization

When reporting the content of oxides of serpentinites from the literature and from our
S1 serpentinites as a function of LOI (i.e. as a marker of serpentinization intensity) some
straightforward observations can be made (Figures 8b). The specific distribution of the data
can be interpreted as the result of calcium leaching during the serpentinization of peridotites,
which varied in composition from a harzburgitic, CaO-depleted composition to a lherzolitic
composition (i.e. CaO-richer). To provide further insight, we have compared the distribution
of CaO vs. LOI to a theoretical dilution, exclusively based on the Ronda Massif samples
(Figure 9a). This theoretical distribution is based on the assumption that the content of CaO
resulted only from dilution during serpentinization. In other words, calcium adopts a state of
strict immobility when its concentration follows these dilution values. The actual content of
CaO is lower than this theoretical value and further decreases with increasing LOI, at least by
half. There is no doubt that calcium is leached out of the rock during serpentinization. This
information is not actually new, because even if such rare diagram has been built so far
(Mothersole et al., 2017), the leaching of calcium during serpentinization has been proposed
in previous studies (Coleman and Keith, 1971; Janecky and Seyfried, 1986; Coleman, 1977;
O'Hanley, 1996; Shervais, 2005; King et al., 2003; Iyer et al., 2008; Bach and Klein, 2009;
Deschamps et al., 2013).
The mineralogical composition (i.e. normative mineral composition) calculated on the
basis on whole rock chemical compositions are usually employed as modal mineralogy (e.g.
Fabriès et al., 1989; Van der Wal and Bodinier 1996; Lenoir et al., 2001; Le Roux et al., 2007;
Bodinier et al., 2008; Soustelle et al., 2009; Marchesi et al., 2013). Such reconstruction of
mineral proportions assumes that serpentinization is a pure hydration process, without
considering any loss or gain of elements. As discussed above, this hypothesis is incorrect with
respect to CaO. This incoherence is highlighted by the significant correlation between LOI
and normative proportions of olivine and clinopyroxene that clearly demonstrates that primary
mineral reconstruction is a function of the intensity of serpentinization relative to the
proportion of serpentine (Figure 11). Thayer (1966) already recognized this point, noting that
"Failure to recognize the chemical changes accompanying serpentinization has led to serious
exaggeration of the original pyroxene content of alpine peridotite." Considering this and the
results of this study we therefore propose that the harzburgitic composition of peridotite
massifs may be overestimated, while the lherzolitic composition may be underestimated. This
is exemplified by the Ronda Massif, Pyrenees Massifs (Figure 11b), and likely to some other
orogenic peridotites exposed in the Alps. To avoid this problem, if one wants to use (or re-use)
published whole rock geochemical composition it is necessary to consider the proportion of
serpentine in the peridotite, for example using methods such as XRD (Tichadou et al., 2021,
Hochscheid et al., 2024) or modal measurement by imaging (Audran et al., 2024).
## 5.2 Elemental mobility during serpentinization of the Ronda Massif

### 5.2.1 Petrographic evidence for element mobility

In order to characterize element mobilities in response to serpentinization, petrography provides robust, first order constraints. In pseudomorphic transformation, secondary minerals replacing the primary ones generally have similar compositions. Any observed elemental gain or loss reflects changes at the mineral grain scale. It is possible that the mobility scale of these elements may be confined to the limits of a whole rock sample. It is therefore necessary to conduct a detailed petrographic examination, although this is not sufficient to discuss the scale of the mobility. The use of whole rock compositions allows for the identification of gains or losses of elements on a scale larger than that of the rock sample, typically on the order of a few hundreds of cubic centimeters. In such cases, it is essential to ascertain whether the sampled rocks had the same composition prior to alteration. This necessitates an objective appraisal of the heterogeneities present in the rocks before alteration, which in our case pertains to the heterogeneities observed in peridotites prior to serpentinization.

The mesh texture represents a prominent feature of S1 serpentinites and will be considered first. The concentrations of Ti, V, Cr, Ni, and Zn in the mesh texture serpentine are nearly identical to the concentrations observed in olivine (Figures 5a, b, c, and d; see supplementary material, Table B), indicating minimal mobility. Mesh serpentine is distinguishable by a pronounced enrichment in Ca and Al compared to olivine (Figure 5c; see supplementary material, Table B), exhibiting an increase of two orders of magnitude in these elements. It can be concluded that these elements are brought by the serpentinizing fluid. In contrast, bastite, which corresponds to serpentine developed at the expense of pyroxenes, exhibits a depletion in Ca and Al relative to pyroxenes (Figure 5c; see supplementary
elements depletion, specifically Zn, Co, Ni and Cr (Figures 5 d, e). This depletion can be
compensated by the co-precipitation of nearby magnetite (Figure 3f), which serves as an ideal
host for these metals (Figure 7a, b), except for Cu (see supplementary, Table D).

The Cr-spinel is a main oxide phase that incorporates a range of metals, including Fe,
Cr, Zn, Co, and Al (Figure 7a, b). Hydrothermal alteration (i.e. oxidation) of Cr-spinel to
ferritchromite during serpentinization results in the release of Mg, Al and to a lesser extend Cr
(Figures 6a, b, c), as previously reported by Da Costa and Barriga (2022). In contrast, Fe and
Co were added to ferritchromite (Figures 6b, c and 7b), except for some Co-poor
ferritchromite. The ferritchromite does not exhibit a Ni-Co mantle-like trend in contrast to the
Cr-spinel (red line in the Figure 7b). In fact, the similar composition of magnetite indicates
that Co gain occurred during serpentinization. This assumption is consistent with the
conclusions of other authors (e.g. Lorand, 1985; Klein and Bach, 2009), who have shown that
Co are mobilized during serpentinization as the primary sulfide assemblage evolves into new
sulfides, oxides, native iron and native copper.

5.2.2 Element mobility during the distinct serpentinization stages

By collecting pairs of poorly serpentinized (i.e. S1) and highly serpentinized (i.e. S2
and S3) samples from representative, well-exposed outcrops, we developed a sampling
strategy that allowed us to characterize element mobility during stages S2 and S3, both of
which formed at the expense of stage S1. Furthermore, by using the large dataset presented in
Figure 8b, elements mobilities during pervasive serpentinization S1, characterized by mesh
textures, are also discussed.
5.2.2.1 Element mobility during S1 serpentinization

As shown by Audran et al. (2024), peridotites constituting the Ronda Massif exhibit variable degrees of serpentinization. A quantitative estimate of element mobility first requires the use of LOI as a numerical proxy of the serpentinization degree, in a similar approach to that discussed above for CaO (see section 5.1). Applying the same reasoning to other elements in the Ronda Massif, it can be deduced that Ti, Sr, V and Cu were partially leached from the rock (Figure 10e, f, g, h), while Cr, Ni and Zn (Figure 10a, b, c) likely remained immobile during S1 serpentinization. The behavior of Co remains unclear due to the dispersion around the theoretical dilution line (Figure 10d).

Particular attention was paid to Cu concentrations during S1 serpentinization. No fresh peridotite with LOI below 5 wt.% was sampled in this study for direct comparison with the S1 serpentinite. However, compiled data from the Ronda Massif indicate that fresh peridotites (LOI < 5 wt.%) contain, on average, 23.7 ± 15.7 ppm Cu (n = 27), which is higher than the 13.0 ± 6.7 ppm measured in S1 serpentinites (n = 7) (see Figure 10g). The variation in Cu mass can be quantified using an approach similar to that presented in Table 2 but based on the average composition of S1 serpentinite (with an average LOI of approximately 8.1 wt.%). This yields an estimated Cu depletion of -40.7 %. This contrast clearly points to an initial depletion of Cu during the earliest stage of serpentinization.

Aluminum exhibits a striking similarity in its specific Al₂O₃ vs. LOI distribution to that of CaO, with Al₂O₃ contents approaching zero only at very high degrees of serpentinization (Figures 9 a, b). This implies that Al is also partially leached during serpentinization, just as Ca, albeit at a lesser intensity. Such mobility could be unusual since Al is considered as one of the less fluid-mobile elements due to its very low solubility in aqueous fluids (e.g. Railsback, 2003). Our petrographic observations indicate that Al was mobile, at least at the mineral grain scale, as exemplified by the alteration of olivine to mesh texture serpentine (see Section
5.2.1). This assumption is consistent with numerical and experimental work showing that Al can be mobilized during hydrothermal alteration of Al-bearing phases (Palandri and Reed, 2004; Huang et al., 2017; 2021) and even the presence of Al(OH)₄⁻ complexes enhances the serpentinization rate by increasing olivine reactivity (Andreani et al. 2013; Pens et al., 2016). Examination of whole rock data indicates that Al may also be mobile at least at the whole rock sample scale, i.e. beyond a few tens of centimeters and even further. However, the uppermost scale limit of Al mobility during S1 is not constrained by the present analysis. These observations show that aluminum, typically considered immobile, can be mobilized during serpentinization across scales, revealing a previously underestimated geochemical behavior.

The distribution of SiO₂ vs. LOI (Figure 9c) suggests some SiO₂ leaching in S1 serpentinized rocks. Given the greater solubility of Si in aqueous fluids compared to Al, it is anticipated that Si mobility will be more extensive, which is consistent with the prevalent observation of Si deposition in ultramafic environments (e.g. listvenite, talc; Paulick et al., 2006; Bach et al., 2013). Concerning Mg, our results do not provide a definitive conclusion regarding its mobility as it falls in the field of theoretical immobility (Figure 9e), likely due to some initial heterogeneity in the protolith. Finally, the distribution of Fe is found to be well-defined, with a regular decrease with LOI, following the theoretical dilution line. At high LOI levels, a significant proportion of Fe remains in the rock, likely indicating the prevalence of magnetite formation.

### 5.2.2.2 Element mobility during S2 serpentinization

A total of four pairs of samples were collected consisting of a partially serpentinized S1 peridotite and a highly serpentinized S2 corridor. These samples correspond respectively to a protolith-product pair as defined by Grant (1986). For each pair, the measured concentration
of an element was compared to its theoretical value assuming it behaved strictly as immobile during stage S2. The result of this comparison is summarized in Table 2 and reported in Figure 12. During S2 serpentinization, elements with a negative percentage mass variation are leached from the rock. These are Ca, Rb, Sr, Al, Ti, Cu, most of the heavy REEs from Eu to Yb, Hf, V, and, to a lesser extent, the light REEs (Ce, Pr, Nd, and Sm), Zr, and Th. Besides H₂O (i.e., LOI), elements gained during S2 serpentinization include Ba, as well as Co, Ni, and Sb to some extent. Other elements exhibit variable behavior, including La, Nb, Na, Cs, Cr, Fe, Mn, Zn, Li, Ta, Pb, Sn, and W. Despite their relatively large standard deviations (i.e. 5–68%), La, Nb, Na, Cr, Fe, and Mn can nevertheless be considered as leached. In the case of Mg, however, no dominant behavior is exhibited during S2, as it was already the case for S1.

Calculations allow to quantify the mobilities of some major or trace elements, except those below the detection limit. In the following, we use Ca as an example. CaO is estimated to decrease by 78 ± 28 % during S2 serpentinization. For 1 m³ of S1 serpentinite weighing about 3000 kg, it means that ~45 kg of CaO would be lost, corresponding to 78% of the 57 kg (= 3000 x 0,019) of CaO initially present.

<table><thead><tr><th colspan="3">S1 - S2 pairs (n=4)</th><th colspan="3">S1 - S3 pairs (n=2)</th><th colspan="3">S1 - S3bis pairs (n=3)</th></tr><tr><th>Element</th><th>average of mass variation (g/100g)</th><th>standard deviation of mass variation</th><th>Element</th><th>average of mass variation (g/100g)</th><th>standard deviation of mass variation</th><th>Element</th><th>average of mass variation (g/100g)</th><th>standard deviation of mass variation</th></tr></thead><tbody><tr><td colspan="9"><strong>major element</strong></td></tr><tr><td>SiO<sub>2</sub></td><td>2</td><td>3</td><td>SiO<sub>2</sub></td><td>6</td><td>5</td><td>SiO<sub>2</sub></td><td>3</td><td>2</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>-53</td><td>26</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>100</td><td>65</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>41</td><td>20</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>-8</td><td>8</td><td>Fe<sub>2</sub>O<sub>3</sub></td><td>-1</td><td>7</td><td>Fe<sub>2</sub>O<sub>3</sub></td><td>-9</td><td>10</td></tr><tr><td>MgO</td><td>2</td><td>6</td><td>MgO</td><td>-7</td><td>3</td><td>MgO</td><td>-1</td><td>4</td></tr><tr><td>CaO</td><td>-78</td><td>28</td><td>CaO</td><td>-64</td><td>28</td><td>CaO</td><td>-14</td><td>77</td></tr><tr><td>Na<sub>2</sub>O</td><td>-14</td><td>54</td><td>Na<sub>2</sub>O</td><td>-13</td><td>43</td><td>Na<sub>2</sub>O</td><td>5</td><td>81</td></tr><tr><td>TiO<sub>2</sub></td><td>-48</td><td>26</td><td>TiO<sub>2</sub></td><td>273</td><td>378</td><td>TiO<sub>2</sub></td><td>1</td><td>3</td></tr><tr><td>LOI</td><td>104</td><td>48</td><td>LOI</td><td>68</td><td>29</td><td>LOI</td><td>18</td><td>32</td></tr><tr><td colspan="9"><strong>trace element</strong></td></tr><tr><td>Li</td><td>-2</td><td>37</td><td>Li</td><td>138</td><td>79</td><td>Li</td><td>48</td><td>62</td></tr><tr><td>V</td><td>-36</td><td>21</td><td>V</td><td>30</td><td>12</td><td>V</td><td>7</td><td>28</td></tr><tr><td>Ti</td><td>-52</td><td>30</td><td>Ti</td><td>165</td><td>271</td><td>Ti</td><td>-2</td><td>19</td></tr><tr><td>Cr</td><td>-12</td><td>32</td><td>Cr</td><td>-18</td><td>3</td><td>Cr</td><td>-20</td><td>23</td></tr><tr><td>Co</td><td>9</td><td>5</td><td>Co</td><td>19</td><td>20</td><td>Co</td><td>10</td><td>18</td></tr><tr><td>Ni</td><td>10</td><td>7</td><td>Ni</td><td>2</td><td>1</td><td>Ni</td><td>-4</td><td>5</td></tr><tr><td>Mn</td><td>-6</td><td>5</td><td>Mn</td><td>7</td><td>14</td><td>Mn</td><td>11</td><td>5</td></tr><tr><td>Cu</td><td>-39</td><td>32</td><td>Cu</td><td>-48</td><td>80</td><td>Cu</td><td>18</td><td>119</td></tr><tr><td>Zn</td><td>-5</td><td>12</td><td>Zn</td><td>29</td><td>17</td><td>Zn</td><td>6</td><td>10</td></tr><tr><td>Ge</td><td>5</td><td>6</td><td>Ge</td><td>31</td><td>7</td><td>Ge</td><td>10</td><td>23</td></tr><tr><td>Sb</td><td>3</td><td>-</td><td>Sb</td><td>78</td><td>-</td><td>Sb</td><td>71</td><td>53</td></tr><tr><td>Cs</td><td>-13</td><td>110</td><td>Cs</td><td>319</td><td>253</td><td>Cs</td><td>19</td><td>115</td></tr></tbody></table>
<table><caption>Table 2 – Average and standard deviation of major and trace element variations between pairs defining by S1-S2, S1-S3, S1-S3bis. The standard deviation marked with "-" corresponds to elements whose concentration is below the limit of detection (in particular for S1 samples, see Table 1).</caption><tr><td>Rb</td><td>-61</td><td>22</td><td>Rb</td><td>190</td><td>127</td><td>Rb</td><td>10</td><td>56</td></tr><tr><td>Ba</td><td>197</td><td>211</td><td>Ba</td><td>621</td><td>610</td><td>Ba</td><td>68</td><td>64</td></tr><tr><td>Sr</td><td>-59</td><td>21</td><td>Sr</td><td>-22</td><td>16</td><td>Sr</td><td>34</td><td>152</td></tr><tr><td>La</td><td>-30</td><td>56</td><td>La</td><td>271</td><td>151</td><td>La</td><td>91</td><td>80</td></tr><tr><td>Ce</td><td>-39</td><td>47</td><td>Ce</td><td>323</td><td>168</td><td>Ce</td><td>114</td><td>91</td></tr><tr><td>Pr</td><td>-39</td><td>48</td><td>Pr</td><td>397</td><td>279</td><td>Pr</td><td>100</td><td>73</td></tr><tr><td>Nd</td><td>-42</td><td>45</td><td>Nd</td><td>522</td><td>548</td><td>Nd</td><td>142</td><td>75</td></tr><tr><td>Sm</td><td>-53</td><td>34</td><td>Sm</td><td>168</td><td>-</td><td>Sm</td><td>36</td><td>21</td></tr><tr><td>Eu</td><td>-58</td><td>30</td><td>Eu</td><td>168</td><td>-</td><td>Eu</td><td>54</td><td>4</td></tr><tr><td>Gd</td><td>-48</td><td>39</td><td>Gd</td><td>307</td><td>-</td><td>Gd</td><td>56</td><td>36</td></tr><tr><td>Tb</td><td>-50</td><td>39</td><td>Tb</td><td>341</td><td>-</td><td>Tb</td><td>26</td><td>18</td></tr><tr><td>Dy</td><td>-55</td><td>31</td><td>Dy</td><td>673</td><td>459</td><td>Dy</td><td>33</td><td>57</td></tr><tr><td>Ho</td><td>-57</td><td>33</td><td>Ho</td><td>284</td><td>-</td><td>Ho</td><td>-12</td><td>5</td></tr><tr><td>Er</td><td>-56</td><td>36</td><td>Er</td><td>353</td><td>119</td><td>Er</td><td>0</td><td>31</td></tr><tr><td>Tm</td><td>-56</td><td>32</td><td>Tm</td><td>162</td><td>-</td><td>Tm</td><td>-33</td><td>0</td></tr><tr><td>Yb</td><td>-54</td><td>29</td><td>Yb</td><td>230</td><td>53</td><td>Yb</td><td>5</td><td>52</td></tr><tr><td>Lu</td><td>-55</td><td>36</td><td>Lu</td><td>114</td><td>-</td><td>Lu</td><td>-36</td><td>-</td></tr><tr><td>Zr</td><td>-41</td><td>38</td><td>Zr</td><td>350</td><td>244</td><td>Zr</td><td>138</td><td>132</td></tr><tr><td>Hf</td><td>-56</td><td>34</td><td>Hf</td><td>221</td><td>-</td><td>Hf</td><td>27</td><td>69</td></tr><tr><td>Nb</td><td>-17</td><td>68</td><td>Nb</td><td>19</td><td>67</td><td>Nb</td><td>15</td><td>21</td></tr><tr><td>Ta</td><td>1</td><td>76</td><td>Ta</td><td>471</td><td>503</td><td>Ta</td><td>112</td><td>247</td></tr><tr><td>Y</td><td>-56</td><td>34</td><td>Y</td><td>408</td><td>109</td><td>Y</td><td>1</td><td>23</td></tr><tr><td>Sn</td><td>-12</td><td>115</td><td>Sn</td><td>8</td><td>44</td><td>Sn</td><td>-37</td><td>37</td></tr><tr><td>W</td><td>52</td><td>84</td><td>W</td><td>212</td><td>144</td><td>W</td><td>53</td><td>115</td></tr><tr><td>Th</td><td>-57</td><td>52</td><td>Th</td><td>433</td><td>288</td><td>Th</td><td>-2</td><td>24</td></tr><tr><td>U</td><td>712</td><td>1073</td><td>U</td><td>420</td><td>-</td><td>U</td><td>-6</td><td>52</td></tr><tr><td>Pb</td><td>25</td><td>99</td><td>Pb</td><td>1743</td><td>1234</td><td>Pb</td><td>43</td><td>-</td></tr></table>

In the field, the presence of S2 corridors is not systematic, in part due to the non-continuity of the outcrops. This irregular distribution poses a significant challenge in the quantitative estimation of the prevalence of S2 corridors. However, the calculation above prompts further inquiry regarding the fate of calcium during this particular instance of serpentinization. One potential outcome of this process is the carbonation of ultramafic rocks, provided CO₂ is available in the system. The presence of calcium carbonates in such environments has been documented in previous studies, which may ultimately contribute to the formation of ophicalcites (Coltat et al., 2021a). At Ronda, these calcium carbonates are present, albeit in limited quantities, forming fine tension gashes in association with late chrysotile (Audran et al., 2024). Another potential fate of calcium is its redistribution along magmatic dikes, contributing to the rondingitization process (Esteban et al., 2003, 2007 and
2019; Bévan et al., 2025). To better understand the Ca distribution during magmatic-hydrothermal-serpentinization interaction, the focus should be on newly formed low-temperature mineral phases, such as xonotlite (Esteban et al., 2003; Bévan et al., 2025), Ca-amphibole (Esteban et al., 2019; Bévan et al., 2025), and carbonates (Audran et al., 2024).

### 5.2.2.3 Mobility during S3 serpentinization

In order to characterize element mobilities associated with S3 serpentinization (cataclasis), a reasoning approach analogous to that employed for S2 serpentinization was adopted. A variety of samples were collected to establish protolith-product pairs. The protolith, a partially serpentinized S1 peridotite, hosts the S3 cataclasite shear zones. The product is either the S3 cataclasite or its rim (S3bis) (Figure 2d). Two S1-S3 pairs and three S1-S3bis pairs were identified. The comparison is summarized in Table 2.

The chemical compositions of the serpentinized peridotite in Ronda are highly variable, due to two main factors. Firstly, the very low content of some trace elements in the protolith which are below the detection limit (e.g. Sb, U, Pb and some REE), renders such an approach hazardous (Table 1 and 2). Secondly, in stage S3, it is difficult to ensure that the rock taken as the protolith precisely corresponds to the initial rock of the altered rock taken as product. Obviously, the occurrence of cataclasites resulting from faulting is evidence of rock mass displacement, which complicates the establishment of a direct correspondence between protolith and product. In any case, it would be more than hazardous to attempt a precise quantification of element mobility with such distribution patterns and uncertainties.

It appears that Ca and Sr are systematically leached during S3 serpentinization, while Al, Ge, Si, Ba, Rb and Sb are gained. Concurrently, elements such as Fe, Mg, Na, Cr and Cu are susceptible to leaching, while V, Co, Zn, LREE, Zr and Hf appear to be gained. The behavior of Ni appears to deviate from systematic patterns, exhibiting minimal mobilization
during S3 serpentinization. A comparison between S3 and S2 reveals that many elements
display a similar trend, arguing for broadly comparable chemical conditions during the two
alterations. Al is a noticeable exception as being lost during S2 and gained during S3
serpentinization. The strong gain in Ba and possibly the little gain in Al may be attributed to
the magmatic activity that occurred during the S3 serpentinization stage (Audran et al., 2024).
Consistently, Jons et al. (2010) reported that magmatism–serpentinization interactions in an
abyssal context produce a comparable Al enrichment in serpentinites.

A progressive evolution from the first (S1) to the third (S3) stage of serpentinization
has been argued by Audran et al. (2024) based on geometric, mineralogical and isotopic
considerations. From the present analysis, it appears also that S3 serpentinization may well be
the continuation of S2 serpentinization, possibly at lower temperatures.

5.3. Implications for ultramafic-hosted metallogenic systems

Serpentinization is a significant chemical contributor to magmatic (e.g.
rodingitization) and hydrothermal systems (O'Hanley, 1996; Shervais et al., 2005). In oceanic
settings, ultramafic-hosted massive sulfides contain metals such as Cu, Ni, Co, Zn, and Au
(Patten et al., 2022). While Cu mobilization typically occurs above 350°C (Fouquet et al.,
2010; Debret et al., 2018), pervasive serpentinization at low temperature can also mobilize Cu
(Debret et al., 2018). Here, we document Cu loss from the protolith (initially at ~23.7 ppm)
during three successive stages of serpentinization in the Ronda Massif (Figures 10g and 12),
which occurs below 300°C, in the absence of seawater-derived hydrothermal fluids (Audran
et al., 2024). These results suggest that Cu mobility during low temperature serpentinization,
previously documented in oceanic settings, can also occur in orogenic contexts, highlighting a
previously unrecognized mechanism for Cu mobility in the overall Cu mass budget.
The mass variation calculation for Cu yields an estimated leaching of 39 ± 32%, for the S1–S2 sample pairs (Table 2). This estimate is more robust than the S1–S3 calculation (48 ± 80%), which relies only on two pairs (Table 2). The leaching is likely underestimated, as Cu loss during S1 further reduces the values. Assuming a S1-S2 serpentinization style and constant Cu leaching of 39 ± 32% during the serpentinization process of 1 m³ of peridotite (i.e. 3.22 tons of fresh peridotite containing 20 ppm of Cu), 25.12 ± 20.60g of Cu can be released. Besides, the massive sulfide ultramafic-hosted deposits in the Eastern Metal deposit (southern Quebec Appalachians, Canada) display a copper resource of about 1.32 million tons or 1.32 x 10⁹ kg (Patten et al., 2022). By simple calculation, depositing such a quantity of copper assuming a single provenance, would require the complete serpentinization of approximately 5.25 x 10¹⁰ m³ (e.g. cube of 3.7 km × 3.7 km × 3.7 km) of peridotites (i.e., a volume of 5.25 x 10¹⁰ m³ = (1.32 x 10⁹ kg of Cu) / (25.16 x 10⁻³ kg/m³ of Cu)). While this hypothesis remains preliminary, it is suggested that serpentinization may play a substantial role in the copper budget, potentially rivaling that of magmatic processes. Actually, such a volume of serpentinized peridotites is encountered in oceanic core complexes or hyper-extended rift margins, where the serpentinization front can reach depths of at least a few kilometers (Canales et al., 2000; Pinto et al., 2017; Lissenberg et al., 2024). Serpentinization is thus a process able for extracting and liberating sufficient amounts of Cu to feed seafloor mineralization.

Eleven reported occurrences of Cu ± Fe mineralized bodies and one occurrence of Cu-As-Zn have been documented in the peridotites of the Ronda Massif (Romero Silva, 2003). However, the majority of these occurrences are hard to access and poorly exposed. The exception is Mina El Cardenillo, where Romero Silva et al. (2013) describe a mineralized quartz breccia containing disseminated sulfides, including chalcopyrite, pyrite, and covellite. This deposit is hosted within the thrust separating serpentinized peridotite from underlying
migmatites, forming the sole of the peridotite. The sole of the Ronda peridotite has been interpreted as part of its tectono-metamorphic evolution during Alpine convergence (Esteban et al., 2008; Tubía et al., 2013) and is dated at 22.3 ± 0.7 Ma (Esteban et al., 2010). According to Audran et al. (2024), this event occurred contemporaneously with serpentinization in the Ronda Massif, a timing also supported by Pedrera et al. (2016) and Esteban et al. (2007, 2019). These observations suggest that, in this orogenic context, serpentinization could have mobilized sufficient copper to contribute to the formation of the Mina El Cardenillo deposit, in a manner analogous to its role in seafloor mineralization.

Other metals including Fe, Zn, Co, Cr, Ni and Ti may be mobilized due to the inhibition of magnetite during crystallization of serpentine minerals. The breakdown of Cr-spinel into ferritchromite can also result to the release of Cr, although this process is not commonly observed in the Ronda area compared to other regions (Da Costa and Barriga, 2022, and references therein). If it occurred under ideal physico-chemical fluid conditions (i.e. high pH and high fluid/chromite ratio), this reaction would be dominant, and Cr-spinel breakdown would result in the release of additional metals in the serpentinizing fluid (Da Costa and Barriga, 2022).

# 6 Conclusion

The present study investigates the mobility of elements at the grain scale and at the macroscopic scale using the Ronda partially serpentinized peridotites as a case study. Three stages of serpentinization are documented in the Ronda Massif and were specifically targeted to decipher element mobility.

At the mineral scale, the mesh texture, typical of the first stage of serpentinization, shows a trace element composition characteristic of olivine, particularly with respect to
concentrations of Ti, V, Cr, Ni, and Zn, while also showing notable enrichment in Al and Ca. Bastite has a composition similar to that of orthopyroxene and clinopyroxene, particularly in Cr and Ti. However, bastite shows a depletion of about half the expected amount of Al and a removal of Ca. Both mesh and bastite textures are found to be enriched in Cu. Trace elements including Al, Ti, V, Zn, Ni and Cr present in the pseudomorphic to sub-pseudomorphic serpentine textures reveal a composition evolving from olivine to pyroxenes, except for the global Cu enrichment. The serpentine veins are slightly depleted in Cu, Zn and Ni and largely depleted in Co and Cr, compared to other serpentine texture and primary minerals. The magnetite composition indicates that some elements lost during serpentine formation are retained into this mineral (i.e. Fe, Co, Ti, V, Zr, Ni, Zn, and Cr). The transformation of Cr-spinel into ferritchromite is characterized by a marked depletion of Mg and Al, accompanied by a slight reduction in Cr. In contrast to Cr-spinel, ferritchromite is enriched in Cr and Co.

At macroscopic scale, we show that Loss On Ignition (LOI) serves as an effective proxy for serpentinization. We observe complete leaching of Ca during the three stages of serpentinization. In addition, the first stage of serpentinization displays a significant decrease in Al, Cu, V, and Ti, concomitant with an increase in serpentinization intensity. In terms of mass variation, the second stage of serpentinization results in partial and systematic leaching of Rb, Sr, HREE, Sm, Hf, Al, Ti, Cu and V, while the third stage of serpentinization displays no systematic tendency. The mobility of Cu offers new insights into the metallogenic role of serpentinization as a source of metals for ultramafic-hosted massive sulfide deposits worldwide.
# 7 Acknowledgement

We thank Xavier Le Coz and Yann Lepagnot (Géosciences Rennes) for sample preparation, including thin sections and whole-rock powders. This work was supported by CNRS-Tellus-Syster grants awarded to PB and BA. This study benefited from the use of the µ-XRF analytical facilities of the CoRTecS PeMCI platform at ITES, University of Strasbourg, and from state aid managed by the "Agence Nationale de la Recherche" (ANR) as part of the FRANCE 2030 investment programme for the IMAGINE2 project under the reference "EQUIPEX ANR-21-ESRE0043". We are grateful to C. Patten and an anonymous reviewer for their careful evaluation and valuable feedback, which significantly improved the quality of this work.

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Figure captions :

Figure 1 – a) Location of the Ronda Massif in the Betic chain (simplified geological map after Alcalá et al., 2013) b) simplified geologic map of the Sierra Bermeja showing location of three mantellic domains and the sampling sites (modified after Obata, 1980; Frasca et al., 2015 and 2017).

Figure 2 – Outcrops of the serpentinization stages with the location of the studied samples (white squares): a) Massive serpentinized peridotites (S1) crosscut by serpentinite corridor (S2); b) Serpentinized peridotite (S1) crosscut by serpentinite shear vein (S3) with a metasomatic halo (S3bis), with the interface between both structures partially transformed into serpentinite (S3bis); c) Serpentinite cataclastic shear zone and thin shear veins (S3) within massive serpentinized peridotite (S1); d) Schematic representation of the three stages of serpentinization, which follows the three continuous stages of serpentinization S1, S2, and S3.

Figure 3 – a) b) c) Representative samples of the serpentinization stages S1, S2 and S3, respectively. Petrographic relationship between serpentine, magnetite, and ferritchromite in different stages of serpentinization corresponding to d) S1, in plane polarized light, e) S2, in cross polarized light, f) S3bis halos, in cross polarized light, g) S3 cataclasite and veins, in cross polarized light.

Figure 4 – Major elements compositions of serpentines from the Ronda Massif (in apfu): a) Mg vs Fe, with black lines denoting Mg# values and b) Mg+Fe vs Si. The various serpentine textures are also included. The main substitution trends are indicated.
Figure 5 – Concentrations of trace elements in serpentine grains: Al, Ti, V, Zn, Ni, Cr, Co and Cu. The mean values of olivine (ol), clinopyroxene (cpx), orthopyroxene (opx), and magnetite (mag) from this study are shown for all plots, as well as the mean values of spinel (spl) and ferritchromite (fch) for plots d), e), and f); see data in supplementary material, Tables B, D and F.

Figure 6 – Major element composition of Cr-spinel, ferritchromite and magnetite of a) MgO vs Al₂O₃ b) FeO(t) vs Al₂O₃ c) FeO(t) vs Cr₂O₃; see data in supplementary material, Table C.

Figure 7 – a) Al vs Zn and b) Ni vs Co concentrations, in Cr-spinel, ferritchromite and magnetite; see data in supplementary material, Table D.

Figure 8 – a) Correlation between loss on ignition (LOI) and the modal proportion of serpentine in our samples compared with data from the literature (Ronda from Frey et al., 1985 and Pyrenees from Tichadou et al., 2021). Pyrenean samples (Tichadou et al., 2021) plot above the Ronda samples (Frey et al., 1985) but the trends are roughly parallel. b) Evolution of CaO as a function of LOI for our samples compared with existing data from the literature: Ronda Massif (Frey et al., 1985; Gueddari et al., 1996; Van der Wal and Bodinier 1996; Lorand et al., 2000; Lenoir et al., 2001; Pereira et al., 2003; Bodinier et al., 2008; Marchesi et al., 2013; Hidas et al., 2015), North Pyrenean massifs (Fabriès and Conquéré, 1893; Bodinier et al., 1988, 1990 and 2004; Fabriès et al., 1989; McPherson et al., 1996; Le Roux et al., 2007; Tichadou et al., 2021) and the New Caledonia ophiolite (Ulrich et al., 2010; Secchiari et al., 2016 and 2020). At low LOI (below 2%), the variation in CaO between ~6 and ~0.3 in peridotites reflects the abundance of clinopyroxene, which is high in lherzolite and low in harzburgites.
Figure 9 – a) to f) Major element concentrations of variably serpentinized peridotites from the Ronda Massif as a function of LOI. Literature data are from Frey et al., 1985; Gueddari et al., 1996; Van der Wal and Bodinier 1996; Lorand et al., 2000; Lenoir et al., 2001; Pereira et al., 2003; Bodinier et al., 2008; Marchesi et al., 2013; Hidas et al., 2015. The grey lines correspond to the theoretical distribution of elements if they are solely controlled by a simple dilution process, which can be formulated as: $[element]_{initial} \times Mass_{initial peridotite} / (Mass_{initial peridotite} + LOI)$. Two dilution lines illustrate the heterogeneity of the protolith, with their origins corresponding to the estimated initial composition of the peridotite.

Figure 10 – Concentrations of Cr, Ni, Zn, Co, Ti, Sr, Cu, V and Ba trace elements as a function of LOI for the Ronda variably serpentinized peridotites. Literature data are from Frey et al. (1985); Gueddari et al. (1996); Van der Wal and Bodinier (1996); Lorand et al. (2000); Lenoir et al. (2001); Pereira et al. (2003); Bodinier et al. (2008); Marchesi et al. (2013); Hidas et al. (2015). Globally, there is good agreement between literature data and our own data. The grey lines represent the theoretical distribution of elements solely controlled by dilution during serpentinization (i.e. mass of conservation during hydration process). Two dilution lines illustrate the heterogeneity of the protolith, with their origins corresponding to the estimated initial composition of the peridotite.

Figure 11 – Distribution of normative composition of olivine (a) and clinopyroxene (b) versus LOI (Fabriès et al., 1989; Van der Wal and Bodinier 1996; Lenoir et al., 2001; Le Roux et al., 2007; Bodinier et al., 2008; Soustelle et al., 2009; Marchesi et al., 2013). The normative content of olivine and clinopyroxene is reconstructed by mass balance using the whole rock and mineral major element composition. The linear correlation between LOI evolution and calculated normative mineral content is illustrated by brown and black lines.
Figure 12 – Average and standard deviation of major and trace element variations between S1-S2 pairs (n=4). Line y = 0 represents no mass variation between pairs; elements are ordered by increasing mass variation. Note that the mass variation of Na₂O may be difficult to interpret due to its proximity to the LOD.
Caption Tables :

Table 1 – Whole rock composition of the Ronda serpentinites and partially serpentinized peridotites. Mg# = Mg/(Fe(t)+Mg) is in molar proportions. Values in italics for Na₂O is indicative of the limit of quantification (LOQ). The LOQ is defined from the limit of detection (LOD), which is calculated using the following formulae: LOQ = 3*3LOD and LOD = 3√(BG/S), where BG represents the background intensity and S denotes the sensitivity.

Table 2 – Values, average and standard deviation of major and trace element variations between pairs defining by S1-S2, S1-S3, S1-S3bis; elements are ordered by increasing mass variation. The standard deviation marked with "-" corresponds to elements whose concentration is below the limit of detection (in particular for S1 samples, see Table 1).
**Declaration of interests**

[x] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[ ] The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights:

* LOI values in peridotites are a reliable proxy for serpentinization degree.

* Serpentinization in the Ronda Massif is clearly non-isochemical.

* Ca, Al, Ti, Sr, V, and Cu are leached during serpentinization.

* Serpentinization may be a key source term in ultramafic-hosted deposits.
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