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# Massive Mg-rich fluid release across the brucite + serpentine reaction in subduction zones

Emma Legros, Benjamin Malvoisin, Fabrice Brunet, Zaccaria El Yousfi, Valentina Batanova, Alexander Sobolev, Anne-Line Auzende

► To cite this version:

Emma Legros, Benjamin Malvoisin, Fabrice Brunet, Zaccaria El Yousfi, Valentina Batanova, et al. Massive Mg-rich fluid release across the brucite + serpentine reaction in subduction zones. Earth and Planetary Science Letters, 2025, 671, pp.119602. 10.1016/j.epsl.2025.119602. hal-05403277

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# Massive Mg-rich fluid release across the brucite + serpentine reaction in subduction zones

Legros Emma<sup>ID*</sup>, Malvoisin Benjamin<sup>ID</sup>, Brunet Fabrice<sup>ID</sup>, El Yousfi Zaccaria<sup>ID</sup>, Batanova Valentina<sup>ID</sup>, Sobolev Alexander<sup>ID</sup>, Auzende Anne-Line<sup>ID</sup>

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, Grenoble, 38000, France

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## ARTICLE INFO

Editor: F.-Z. Teng

**Keywords:**

Brucite

Metamorphic olivine

Serpentinite dehydration

Thermodynamic modeling

LFE

Zermatt-Sass ophiolite

## ABSTRACT

In subduction zones, serpentinized oceanic mantle is expected to dehydrate above 600°C due to antigorite breakdown (Atg-out reaction). Analysis of compilatory bulk composition of serpentinites shows that brucite (Brc) should also be an important hydrous component, capable of carrying water to depth. The Atg + Brc = Ol + H₂O reaction (R1) which occurs at lower temperature than Atg-out, is thus highly relevant for fluid release in subduction zones. Depending on the initial composition of the serpentinized mantle, R1 can produce more water than the Atg-out reaction. The consideration of most recent thermochemical data for brucite and serpentine solid-solutions shows that the R1 reaction proceeds in a narrow temperature range (< 10 °C), implying relatively high dehydration rates. Thermochemical modeling also shows that the fluid released during R1 is highly magnesian ([Mg]/[Si] > 100), i.e., likely to promote Mg metasomatism. In parallel, metamorphic olivine veins formed according to R1 in the Zermatt-Saas meta-ophiolite were examined. They are interpreted as magnesium-rich segregations resulting from the interaction between an Mg-rich fluid and the host serpentinite for around a hundred years. Furthermore, they are crosscut by brucite veinlets formed at R1 conditions or higher, which confirms, in agreement with thermodynamical modeling, that R1 is not a brucite-out reaction. Finally, P-T conditions of R1 were found to coincide with the location of Low Frequency Earthquakes recorded in the Mexican, Nankai and Cascadian subductions; this indirectly supports the role played by R1 as a significant source of fluid in subduction zones.

## 1. Introduction

Subduction is the only present-day process that can transport water into the deep Earth. Among hydrated slab components, serpentinized oceanic mantle has been identified as the major water carrier into subduction zones (Schmidt and Poli, 1998; Rüpke et al., 2004). Within the incoming plate, serpentinite dehydration reactions liberate aqueous fluids that are partially transferred to the mantle wedge where they are believed to trigger arc magmatism (Hattori and Guillot, 2003; Scambelluri et al., 2019). Serpentinite dehydration may also play a role in the generation of intermediate-depth earthquakes (Peacock, 2001; Jung et al., 2004). Fluids released by dehydration reactions could also be the source of Low Frequency Earthquakes (e.g. Shapiro et al., 2018). However, the level of hydration of subducting oceanic mantle remains uncertain (Grevemeyer et al., 2007; Cai et al., 2018) and the mineralogy of the (partially) hydrated incoming mantle is itself a matter of

debate. Phase relationships calculated over a range of peridotite compositions predict that lizardite (or antigorite) should co-exist with brucite, Mg(OH)₂, for temperatures up to 400 - 500 °C (Plümper et al., 2017; Kempf et al., 2020) in the subducted slab. Whereas serpentine minerals are recognized as the dominant hydrous mineral phases in subducted serpentinites, brucite is barely described. The presence and fate of brucite in the incoming serpentinized peridotite slab is an important issue given the large amount of water hosted in this mineral (around 30 wt.% H₂O).

There is growing evidence that brucite and more specifically, Fe-bearing brucite, (Mg,Fe)(OH)₂, is abundant in obducted oceanic mantle as in the Ligurides (Schwarzenbach et al., 2016), Oman (Malvoisin et al., 2021; Templeton et al., 2021) and New Caledonia (Frost et al., 2013). The lack of brucite in dredged abyssal peridotites has supported the notion that serpentinized oceanic lithosphere is poor in brucite (brucite-I) (Mével, 2003). However, a recent study by Klein et al. (2020) shows that

<footnote>* Corresponding author.
E-mail address: emma.legros@univ-grenoble-alpes.fr (E. Legros).</footnote>
in samples from deep oceanic and ophiolitic drilling projects, brucite is a major component in serpentinites and should therefore be present in subducting slabs. Furthermore, due to its wide stability field in the MgO-H₂O system, brucite can potentially host water in the deep mantle, i.e., down to the topmost lower mantle under very cold geotherms (Sun et al., 2025). Brucite-I, produced in the course of oceanic serpentinization, typically occurs as a submicrometer phase often intergrown with lizardite, the dominant serpentine mineral in the hydrated oceanic mantle (Beard et al., 2009; Frost et al., 2013; Templeton and Ellison, 2020; Malvoisin et al., 2021). Brucite-I is therefore highly reactive and can thus be easily altered when exposed to subsurface or seafloor conditions (Jöns et al., 2017; Klein et al., 2020), which explains why it can be absent from surficial rock samples.

The same type of brucite paradox applies to serpentinized oceanic mantle exhumed in subduction-collision zones where “oceanic brucite” (brucite-I) has been reported in lizardite serpentinites (Iyer et al., 2008; Steinthorsdottir et al., 2022; Ressico et al., 2024) whereas it is generally absent from antigorite serpentinites, except in the specific contexts of high-pressure serpentinization within the mantle wedge (Kawahara et al., 2016; Debret et al., 2019; Nagaya et al., 2022) and thrust faulting in forearc setting (Menzel et al., 2018). This is all the more surprising given that the Mg/Si ratio in antigorite is lower than in lizardite, so the lizardite-antigorite transition should produce brucite (brucite-II) rather than consume it. A notable exception to these observations, however, consists in high-grade serpentinites in which brucite (brucite-III) is found in association with metamorphic olivine and is interpreted as a product of (metamorphic) olivine hydration under retrograde conditions (Kempf et al., 2020).

The apparent contradiction here is that in serpentinites containing antigorite + metamorphic olivine, olivine is inferred to have formed according to the reaction:

$$ \text{antigorite} + \text{brucite} \rightarrow \text{olivine} + \text{H}_2\text{O} \quad (\text{R1}) $$

although the prograde brucite (Brc) + antigorite (Atg) assemblage is not observed in subducted mantle units exhumed in collision zones. The only notable exception is Brc + Atg within Brc + Atg + Ol paragenesis interpreted as evidence for R1 (Peretti et al., 1992; Caurant et al., 2023). A possible explanation for the apparent absence of the (olivine-free) Atg + Brc I/II assemblage in meta-ophiolites is that, as for dredged abyssal peridotite samples, serpentine samples recovered in ophiolites from subduction-collision zones have lost their brucite-I (and possibly brucite-II, if any) either before subduction or upon exhumation in the subduction channel.

Metamorphic olivine formed according to R1 is often described as occurring as sub-monomineralic veins with varying degrees of deformation or within shear zones, e.g., in the Erro-Tobio meta-ophiolite (Voltri Massif, Italy; Scambelluri et al., 1995; Hermann et al., 2000; Plümper et al., 2017), in the Zermatt-Saas meta-ophiolite (Switzerland; Li et al., 2004; Kempf et al., 2020; Muñoz-Montecinos et al., 2024) or at Cerro del Almirez, (SE Spain; Jabaloy-Sanchez et al., 2022). Such olivine veins formed as a result of serpentine dehydration have recently attracted significant attention to understand how fluid and chemical elements are transported in subduction zones (e.g. Plümper et al., 2017; Peters et al., 2020; Kempf et al., 2020; Muñoz-Montecinos et al., 2024; Ulrich et al., 2024). Two end-member models of fluid release have been proposed. A first one considers that dehydration is controlled by inherited chemical heterogeneities leading to porosity generation and fluid channelization. The fluid release is progressive, extending over a hundred of °C (Plümper et al., 2017; Kempf et al., 2020), and olivine vein formation is controlled by inherited chemical heterogeneities, element diffusion, and fluid pressure variations induced by the R1 reaction (Plümper et al., 2017; Huber et al., 2022). Based on microstructural observations, a second type of models considers high pore-fluid pressure induced by dehydration leading to dilational fracturing and brittle-ductile-shearing, with possible consequences for intermediate-depth seismicity (Muñoz-Montecinos et al., 2024).

The monomineralic character of some of the olivine veins formed according to R1 is interpreted as a consequence of fluid enrichment in dissolved magnesium (Ulrich et al., 2024; Jabaloy-Sanchez et al., 2022). Consequently, R1 is also prone to modify the bulk-rock MgO/SiO₂ ratio which has been reported to decrease in some samples from Alpine Corsica (Ressico et al., 2024). Other data acquired on samples from Erro Tobbio suggest that the fluid-rock interaction proceeded isochemically during the subduction (Peters et al., 2020). A compilation of bulk-rock compositions from samples collected in subducted serpentinites revealed a more widespread distribution of MgO/SiO₂ and Al₂O₃/SiO₂ ratios than in samples collected on the seafloor (Deschamps et al., 2013). The reason for this difference remains unclear.

In the present work, in order to evaluate the role played by brucite in terms of water (and magnesium) transfer in subduction zones, we first assessed the expected modal amount of brucite in a fully serpentinized oceanic mantle. Then, phase relationships and dehydration fluid composition were modeled using recent thermochemical data for the iron-component of brucite, Fe(OH)₂, amakinite (Carlin et al., 2024), and a new solid-solution model for antigorite including ferric iron (Eberhard et al., 2023). Finally, a new type of brucite occurrence is described in partially dehydrated serpentinites collected near the Unterer Theodulglacier, Zermatt (Zermatt-Saas meta-ophiolite) and interpreted in light of the modeling results. Implications for the onset of serpentine dehydration and for possible dynamic events in present-day subduction zones are also discussed.

## 2. Methods

### 2.1. Meta-serpentinite composition

The bulk composition of serpentinites from various geological settings was taken from literature compilations (Supp. Mat. 1). Serpentinite compositions were divided into three groups: abyssal serpentinites (AS), low-grade subduction/collision meta-serpentinites (LGS) and high-grade subduction/collision meta-serpentinites (HGS). The latter two groups were distinguished based on the presence of lizardite (LGS) and antigorite (HGS) as the dominant serpentine mineral, respectively. In order to assess the effect of subduction metamorphism on serpentine bulk-chemistry, the compositions of LGS and HGS sampled in the same locality were specifically compared. At each locality, the initial composition of the oceanic mantle was assumed to be the same and bulk-chemistry differences were interpreted in the frame of metamorphism in an open system, in terms of Mg budget in particular.

Simplified normative mineralogy was calculated on the basis of a mineral assemblage composed of antigorite ± brucite ± chlorite ± diopside, i.e., for P-T conditions beyond the lizardite stability but below R1. The calculation thus aims at quantifying the amount of brucite present in serpentinized peridotites at the onset of R1. Mg/(Mg+Fe) ratio in the latter minerals was set constant by taking typical values measured in HGS for antigorite (0.95), chlorite (0.95), diopside (0.98), and brucite (0.99). Normative diopside was directly obtained from the bulk Ca-content. Chlorite, (Mg₀.95Fe₀.05)₅.₂Al₀.₈Si₃.₂Al₀.₈O₁₀(OH)₈, was only considered for bulk-rock compositions with Al/Si > 0.045, i.e., maximum Al/Si ratio in antigorite above which normative chlorite is required for hosting Al. The remaining Si and Al were used to calculate the antigorite content, assuming a Tschermak's substitution, that is Si + Al/2 = 34. At the end, remaining Mg, if any, was incorporated into brucite. The calculation of normative mineralogy performed here thus relies on three assumptions: the Mg/(Mg+Fe) ratio in each mineral, the incorporation of all the Ca into diopside, and the maximum Al content in antigorite. Modal compositions were also calculated using Mg/(Mg+Fe) ratios in silicates of 0.91 and 1, by considering carbonate instead of diopside as Ca host, and by dividing the maximum amount of Al in antigorite by three (Fig. S1). The so-derived brucite and antigorite modes proved marginally different due to the fact that brucite modal fraction is controlled at first order by the bulk Mg/Si ratio rather than by the content
in other elements with lower concentrations such as Fe, Ca and Al. The
results of mode calculation are provided in Table S1.

Phase relationships were calculated for three simplified antigorite
serpentinite compositions corresponding to a fully hydrated dunite,
harzburgite and lherzolite (Table S2). These compositions were re-
trieved from the compilation of abyssal serpentinite compositions (AS).
MgO, Al₂O₃ and SiO₂ contents for each type of ultramafic rocks were
calculated by dividing the MgO/SiO₂ vs. Al₂O₃/SiO₂ space into three
domains. The average dunite composition was calculated with samples
having an MgO/SiO₂ ratio above 1.15. The average harzburgite com-
position was calculated from samples with MgO/SiO₂ ratio below 1.15
and Al₂O₃/SiO₂ ratio below 0.03. The samples with MgO/SiO₂ ratio
below 1.15 and Al₂O₃/SiO₂ ratio above 0.03 were used to determine
the lherzolite composition. The FeO content was determined by assum-
ing a molar Mgr/Fe+Mg of 0.91, typical for ultramafic rocks (Kelemen et
al., 2021). The Fe³⁺/Fe²⁺+Fe³⁺ ratio was fixed to 0.4, consistently with the
data compilation of Mayhew and Ellison (2020). The amount of water in
each simulation was taken as the maximum water amount which can be
bound to the solid in the investigated P-T range. This amount was cal-
culated for each simulation by running an exploratory calculation with
excess water.

## 2.2. Thermodynamic modeling

Phase relationships in meta-serpentinites were modeled with
Perple_X (version 7.1.13) using the thermodynamic database of Hol-
land and Powell (2011) including properties of aqueous species from
the Deep Earth Water model (DEW19HP633ver_elements.dat; Huang and
Sverjensky, 2019). A generic hybrid molecular fluid EoS solution model
was used for an H-O fluid (Connolly, 1995). Aqueous species were
only considered in a single simulation; in this case, the lagged forward-
calculation of Connolly and Galvez (2018) was used.

A first series of thermodynamic calculations, hereafter referred to
as “P-type”, was devoted to reproduce previously published calcula-
tions. For antigorite, the solid-solution model of Padrón-Navarta et
al. (2013) was used, which considers Tschermak’s substitution for Al
and no ferric iron. For brucite, the ideal solid-solution implemented in
Perple_X was used, in which the thermodynamic properties of the Fe-
brucite end-member, Fe(OH)₂, are computed as a linear combination of
the properties of brucite, Mg(OH)₂, talc, Mg₃Si₄O₁₀(OH)₂ and Fe-talc,
Fe₃Si₄O₁₀(OH)₂.

A second series of simulations, referred to as “N-type”, was carried
out using the latest solid-solution models for serpentine and brucite. Ser-
pentine solid solution is that of Eberhard et al. (2023), which takes into
account both Tschermak’s and ferri-Tschermak’s solid-solutions (i.e.,
Fe³⁺ is considered) for lizardite and antigorite. Eberhard et al. (2023)
fitted the Gibbs free energy of the Fe³⁺. and Al-endmembers to ex-
perimental data with the Darkens Quadratic Formalism (DQF). Brucite
solid-solution is taken as ideal, as for P-type, but the thermochemi-
cal properties of the Fe(OH)₂ endmember are taken from Carlin et al.
(2024).

## 2.3. Sample location and analytical methods

The studied brucite-bearing samples were collected in the HGS of
the Zermatt-Saas ophiolitic Unit near the Unterer Theodulglacier (UTG,
Zermatt, Switzerland) whose recent retreat reveals fresh serpentinites
(Fig. 4A). The Zermatt-Saas meta-ophiolite is part of the Mesozoic
Piedmont-Ligurian oceanic lithosphere. It was subducted during the
alpine orogeny and reached eclogitic peak conditions at 43 - 50 Ma
(Skora et al., 2015). Peak metamorphic conditions are estimated to 530 -
600 °C and 2.0 - 2.5 GPa (Angiboust et al., 2009). These peak conditions
fall between those of the Atg + Brc → Ol + H₂O reaction (R1) and those
of the Atg-out reaction (Shen et al., 2020, and ref. therein) as evidenced
by the occurrence of metamorphic olivine + Ti-clinohumite veins within

an antigorite-rich matrix (Ulrich et al., 2024; Muñoz-Montecinos et al., 2024; Kempf et al., 2020; Clarke et al., 2020; Li et al., 2004).

The collected samples were cut and polished, and then further char-
acterized using optical microscopy, field-emission gun scanning electron
microscopy (FEG-SEM), electron microprobe (EPMA), transmission elec-
tron microscopy (TEM), and Raman micro-spectrometry. Detailed infor-
mation on sampled lithologies is presented in Table S3. EPMA data were
collected with a JEOL FEG JXA-iHP200F located at the Institut des Sci-
ences de la Terre (ISTerre), University of Grenoble Alpes (UGA, France).
Analytical conditions and standardization details are given in Suppl.
Mat. 3.1. FEG-SEM images were collected on gold-coated polished sec-
tions at the Consortium des Moyens Technologiques Communs (CMTC,
UGA, France). Acquisition conditions were 3 kV acceleration voltage,
20 µm beam aperture size and 1.5 mm working distance. Nanoscale ob-
servations were performed on Focused Iron Beam (FIB) thin sections.
FIB thin sections were extracted and thinned down to electron trans-
parency (< 100 nm) with Zeiss Crossbeal 5550L (IEM, Lille, France)
using a gallium source. TEM was performed using a JEOL 2010 micro-
scope (CINAM, Marseille, France) equipped with a LaB₆ filament and
operating at 200 kV acceleration voltage. With a point-to-point resolu-
tion of 0.23 nm, this TEM is well suited for sensitive mineral charac-
terization thanks to the high-sensitivity low-light-level camera (GATAN
Ultrascan 1000XP (2k×2k)). Raman spectra were collected with a Ra-
man Horiba Labram Soleil at ISTerre (UGA, France) using a 532 nm
laser at 5.7 mW and laser power at 10% during 2 seconds, and con-
focal hole at 100 µm and ×100 magnification. Bulk-rock compositions
were obtained on serpentinite powders (Suppl. Mat. 3.2) at Service d’-
Analyses des Roches et des Minéraux (SARM) at Centre de Recherches
Pétrologiques et Géochimiques (CRPG, Nancy, France).

## 2.4. Pressure and temperature range of low-frequency earthquakes (LFE)

To look at the pressure and temperature conditions of LFEs in
subduction zones, we gathered catalogs for the Mexico, Cascadia and
Nankai subduction zones, from Frank et al. (2014); Sweet et al. (2019);
Kato and Nakagawa (2020), respectively. To prevent the high uncer-
tainty on the vertical location of these events, we vertically projected
the location of LFE families on the slab interface models of the Mexico
and Cascadia subduction from Hayes et al. (2018) and for the Nankai
subduction from Hirose et al. (2008). We then defined the depth range
of LFEs in each region using the projected LFE families, which we used
to define the pressure and temperature from the regionally defined tem-
perature models from Gao and Wang (2017).

# 3. Results

## 3.1. Serpentinite composition and normative mineralogy

When plotted on an MgO/SiO₂ vs. Al₂O₃/SiO₂ diagram (Fig. 1A),
the compositions of antigorite serpentinites (HGS) can be distinguished
from those of abyssal (AS) and low-grade serpentinites (LGS). AS and
LGS compositions mainly cluster around values of MgO/SiO₂ and
Al₂O₃/SiO₂ ratios of ~ 1.03 and ~ 0.017, respectively. These values are
consistent with the harzburgite composition calculated as described in
the Methods section and used in thermodynamic calculations (magenta
diamond in Fig. 1A, Table S2). On the contrary, HGS compositions plot
either below or slightly above the terrestrial array and display a higher
variability in Al₂O₃/SiO₂ ratio which mostly falls within the 0.02 -
0.08 range. Only 1 sample out of the 291 HGS samples plots close to
dunitic-like composition at the top left of Fig. 1A. This sample was ac-
tually collected in the Zermatt-Saas meta-ophiolite and interpreted as a
meta-dunite (Li et al., 2004). The Unterer Theodulglacier (UTG) samples
studied here have highly heterogeneous compositions related to differ-
ent lithologies (Table S3, see also Kempf et al. (2020) and Ulrich et al.
(2024) for more detailed field studies), with a wide range of Al₂O₃/SiO₂
and MgO/SiO₂ ratios, from 0.01 to 0.12, and 0.91 to 1.28, respectively.
Plotted in Fig. 1A, their composition could either match serpentinized lherzolite, harzburgite or dunite compositions. Interestingly, three samples display an MgO/SiO₂ ratio that is higher than all the previously reported ratios in HGS; they correspond to olivine-bearing metamorphic veins.

Conversion of the above serpentinite bulk compositions into mineral modes (see Methods) shows that 86±10%, 89±8% and 67%±16 of the corresponding AS, LGS and HGS samples should contain normative brucite if totally serpentinized in the antigorite + brucite stability field. For AS and LGS, computed brucite mode ranges from 0 to 97 mol.%, but more than half of the samples displays brucite mode above 85 mol.% (Fig. 1B). This uneven distribution is due to the strong difference in Mg

content per formula unit between brucite (~1) and antigorite (~45). Using the distribution of Mg between brucite ($X_{MgB}$) and antigorite ($1 - X_{MgB}$) allows to circumvent this problem and to visually compare brucite mode between serpentinite types. $X_{MgB}$ distribution is plotted for AS, LGS and HGS samples in Fig. 1B. $X_{MgB}$ distribution for AS and HGS is unimodal with distribution maxima at 0.17 and 0.06, respectively, whereas it is bimodal for LGS with maxima at 0.07 and 0.17.

HGS samples occur to be the samples with the lowest $X_{MgB}$ which does not exceed 0.24. For comparison, maximum $X_{MgB}$ reaches 0.38 and 0.31 for AS and LGS, respectively, and olivine composition would correspond to $X_{MgB} = 0.26$. The UTG samples studied here display a wide range of $X_{MgB}$ from 0.04 to 0.36. The 3 samples collected in olivine-bearing veins have $X_{MgB}$ values equal or above olivine composition.

In order to evaluate the effect of subduction metamorphism on serpentinite normative mineralogy, $X_{MgB}$ distribution is plotted on Fig. 1C for distinct meta-ophiolites having experienced metamorphic conditions such that both LGS and HGS have encountered. In Cuba, Dominican Republic and Chenaillet-Queyras-Monviso transect, $X_{MgB}$ are generally below 0.1. They are intermediate, around 0.1, in Erro Tobbio and higher, up to 0.32, in Monte Maggiore. More importantly, average $X_{MgB}$ values are systematically lower in LGS than in HGS for all investigated meta-ophiolites. The difference between the average value of $X_{MgB}$ in LGS and in HGS varies from 0.02 for Erro Tobbio to 0.07 for Monte Maggiore.

## 3.2. Calculated phase relationships and dehydration reactions

Brucite is found as a stable phase under low-grade conditions in all simulations, even for the lherzolite composition (Figs. 2, S2 and S3) which is the least magnesian. Accordingly, serpentinized peridotites de-hydration in subduction-like conditions will proceed by the succession of two main dehydration reactions, R1 and

$$ \mathrm{Atg} = \mathrm{Ol} \pm \mathrm{Opx} \pm \mathrm{talc} (\mathrm{Tlc}) \pm \mathrm{Chl} + \mathrm{H_2O} (\mathrm{R2}) $$

R1 is located at temperatures ranging from 350 °C at 0.05 GPa to 535 °C at 3.5 GPa for the P-type models. It is shifted to higher temperatures for N-type models, i.e., from 380 °C at 0.05 GPa to 550 °C at 3.5 GPa. R2 is calculated to occur at temperatures ranging from 480 to 640 °C in P-type models and from 500 to 690 °C in N-type models. P-type models thus predict reactions at temperatures 30 - 50 °C lower than N-type ones.

The respective contribution of these two reactions to the amount of released water is calculated as a function of temperature for warm and

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Comparison of bulk-rock (wt.% ratios) and normative compositions of abyssal (AS), low grade (LGS) and high-grade (HGS) serpentinites. A: MgO/SiO₂ vs. Al₂O₃/SiO₂ diagram. The black and red dots correspond to LGS and HGS analyses, respectively. The black contours correspond to the point density in AS analyses. Contour labels correspond to the percentage of points in a 0.01 by 0.1 centered square. The black line corresponds to the terrestrial array computed from Kelemen et al. (2021). The analyses of Zermatt samples acquired here are displayed with red triangles. The blue, pink and yellow diamonds correspond to the rock composition used for thermodynamic calculations for serpentinized dunite, harzburgite and lherzolite, respectively. Olivine composition with $\frac{Mg}{Mg+Fe}$ ratio of 0.91 is displayed with a black star. The compositions of antigorite and chlorite used in normative mineralogy calculation are displayed with a pink line and a blue vector pointing towards fixed chlorite composition, respectively. B: Histogram of $X_{MgB}$ distribution. Black and gray histograms correspond to AS and LGS, respectively. The red line corresponds to histogram contour for HGS. The red arrows correspond to compositions calculated with the analyses performed here in the Zermatt meta-ophiolite. The dashed black vertical line corresponds to olivine composition. C: Average $X_{MgB}$ calculated in LGS (gray) and HGS (red). Five localities are investigated: Monte Maggiore, Cuba, Dominican Republic, Erro Tobbio and the Chenaillet-Queyras-Monviso transect in the Western Alps (see Suppl. Mat. 1 for the corresponding references). Horizontal bars correspond to standard deviation.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 2. Results of thermodynamic modeling. A: Stable mineralogical assemblages for harzburgite composition as a function of temperature and pressure. Abbreviations are after Whitney and Evans (2010). B: Logarithm of Mg solubility (mol/kg) as a function of temperature and pressure. The green lines correspond to reactions R1 and R2. The blue and pink lines correspond to P-T paths for slab tops in cold (Japan) and warm (Cascadia) subduction zones, respectively (van Keken et al., 2018).</figcaption></figure>

cold subduction paths, assuming a fully serpentinized mantle dragged into the subduction zone (Fig. 3). Both R2 temperature and the amount of water that R2 produces are the same for warm and cold subduction geotherms. R1 occurs at a temperature 70 - 80 °C higher for a cold geotherm than for a warm geotherm.

The mass proportion of water that is released by R1 decreases from ~ 80% of the total water content for serpentinized dunite to 35% for serpentinized harzburgite and down to 10% for serpentinized lherzolites (Fig. 3A, B and C). Whatever the model, N- or P-type, water release associated with R2 is sharp since it proceeds within less than 30 °C.

With the P-type model, R1 dehydration is spread over ~ 50, 60 and 70 °C for serpentinized lherzolite, harzburgite and dunite, respectively (Fig. 3D, E and F). Considering now N-type models, R1 dehydration is much sharper since dehydration spans over a very narrow temperature range, ~ 2, 4 and 8 °C for serpentinized lherzolite, harzburgite and dunite, respectively. As a consequence, the maximum rates of water release (kg H₂O/kg rock/°C) for N-type compared to P-type models are ~ 30, 6 and 2 times higher for lherzolite, harzburgite and dunite, respectively. This difference between the two types of models can also be seen on the pseudosections where the field corresponding to dehydration in which antigorite, olivine and brucite coexist, is wider in P-type models than in N-type models (Figs. S2 and S3).

On closer inspection, the rate of water release as a function of T follows two distinct peaks for R1 in the case of dunite and harzburgite compositions (Fig. 3D and E). The first peak corresponds to a change in antigorite composition upon dehydration with an increase of its aluminum content according to the following reaction:

$$Al-poor \ Atg + Brc = Al-rich \ Atg + Ol + H_2O \ (R3)$$

Antigorite Al content then reaches a maximum and chlorite starts forming. This latter event corresponds to an abrupt water release according to the reaction:

$$Al-rich \ Atg + Brc = Clc + Ol + H_2O \ (R4)$$

The reactions predicted by thermodynamic modeling are thus slightly different than the simplified reaction R1 since both Al-rich antigorite and chlorite (clinochlore) can form along with fluid and olivine. It should also be noted that the amount of clinochlore that is produced stores between 2 and 20% of the water initially present in serpentinized dunite and lherzolite, respectively.

Additional solubility calculations were carried out (see Methods) which turned out to give consistent results whatever the serpentinized peridotite composition and thermodynamic model (P-type or N-type).

The solubility of Mg, Al, Si and Fe are shown for a harzburgite composition using the N-type model (Figs. 2 and S6). Si, Al and Fe content in the equilibrium aqueous fluid is found to increase with P-T conditions. Si, Al and Fe concentrations remain below 10⁻² mol/kg for temperatures below 550 °C. Mg solubility behavior is remarkably different since it shows a maximum for temperatures comprised between 400 and 530 °C with solubilities by two orders of magnitude higher than the other chemical species (see Fig. S6 and S7). Mg solubility is at its maximum along the R1 dehydration curve where it reaches several mol/kg (Fig. 2B). For example, at the peak metamorphic conditions for the Zermatt-Saas metamorpholite, i.e., ~ 530 °C/2.2 GPa, Mg solubility is equal to 1.9 mol/kg. In all the investigated P-T domain, Mg speciation is largely dominated by the MgOaq species.

### 3.3. Brucite and metamorphic-olivine occurrences in the UTG serpentinites

In the Unterer Theodulglacier samples from the Zermatt-Saas Unit, dehydration through R1 (or R3 and R4) led to the formation of centimetric olivine veins crosscutting the serpentinite matrix (Fig. 4B and E). The matrix is mostly made of antigorite and also contains minor clinochlore and magnetite along with chromite relics. Antigorite has an Mg# of ca. 97.5 and shows relatively high Al and Fe content, around 2 wt.% of Al₂O₃ and FeO₇, with minor Cr and traces of Mn and Ni (Table S6). The matrix also contains isolated metamorphic olivine grains, between 50 and 100 µm in size.

The crosscutting metamorphic veins consist of ~90% of olivine associated with Ti-clinohumite (~5%) and subordinate magnetite, clinochlore and antigorite. Olivine grain sizes are above 500 µm (Fig. 4E and F). Olivine is chemically zoned with an increasing Mg-content from core, Mg# of 95 - 96, to rim, Mg# of 97 - 98 (Table S5, Fig. S5) and is similar in composition to those in the matrix. Antigorite within the metamorphic veins is in apparent textural equilibrium with olivine, as expected when brucite is completely consumed after R1. This antigorite has an Mg# which averages around 97 and shows relatively high Al₂O₃ and FeO₇ contents (2 wt.% for both) with minor (> 100 ppm level) Mn, Cr and Ni and traces (< 100 ppm level) of Ti, Ca, K and Cl (Table S6). Antigorite is also found as inclusion within the olivine grains where it shows sharp contact with the host olivine (Fig. 5E). The wall of the metamorphic vein is outlined by a thin magnetite rim (Fig. 4C and E) which separates the olivine vein from the antigorite matrix.

Locally, the metamorphic olivines are serpentinized, as shown by the presence of brucite (thereafter referred to as brucite-A) which is intermixed with lizardite and characterized by infra-micrometric grain sizes. This brucite-A grows in direct contact with olivine, whose surface displays numerous etch-pits (Fig. S4B and D). Raman spectra collected on the mixture display OH-stretching modes typical of brucite at 3650
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Fluid release calculated assuming thermodynamic equilibrium along subduction zone P-T paths for serpentinized dunite (A and D), harzburgite (B and E) and lherzolite (C and F). A, B and C: cumulative H₂O released along P-T paths. The black lines correspond to the highest possible amount of released water. D, E, F: H₂O release rate is defined as the derivative of H₂O release. Red and blue lines correspond to P-T paths for warm and cold subduction zones, respectively (van Keken et al., 2018). Dashed and plain lines correspond to calculation with P-type and N-type thermodynamic models, respectively.</figcaption></figure>

cm⁻¹ and of lizardite at 3685 cm⁻¹ and 3704 cm⁻¹ (Auzende et al., 2004, Fig. S4C). According to the calibration of Templeton and Ellison (2020), brucite-A composition corresponds within uncertainty to that of the Mg(OH)₂ endmember. Such brucite-A + lizardite association is also observed in former fluid inclusions trapped in metamorphic olivine. Brucite composition could not be resolved with EPMA and mixed Liz + Brc analyses (28 wt.% SiO₂, 44 wt.% MgO, Table S4) were obtained instead, yielding an Mg# of 96 - 97 for the phase mixture (Fig. S4).

Metamorphic veins are crosscut by small veinlets which contain a second type of brucite, brucite-B (Fig. 4C, D and F). Brucite-B forms single-phase aggregates (several µm) composed of euhedral grains with sizes ~ 100 nm (Fig. 5C). Raman spectra collected on brucite-B display a single peak in the OH-stretching region at 3650 cm⁻¹ (Fig. 5D), i.e., close to the Mg(OH)₂ endmember (Templeton and Ellison, 2020). This indicates that contrary to brucite-A, brucite-B is not intermixed with serpentine. Microprobe analyses yielded an Mg# of 99.4 (Table S4) that is consistent with Raman spectroscopy. Brucite-B contains minor (> 100 ppm) Si, Al, Mn, Cr and Fe as well as traces (< 100 ppm) of Ca, Na, and K (Table S4). Signal from elements such as Si, Mn, Cr and Al with concentrations above detection limits but below wt.% level (Table S4), likely originates from neighboring phases. Indeed, the beam size of the microprobe is increased to 5 µm to avoid beam damage on this sensitive phase.

At the wall of these veinlets, olivine displays highly magnesian stripes (Mg# 99; Table S5), running parallel to the veinlets direction (Figs. 4D, 5A and B, S5). This suggests that both brucite-B and Mg-rich olivine (stripes) crystallized from the same Mg-rich fluid. Associated to brucite-B, the veinlets also contain serpentine and clinochlore. Raman spectra of the serpentine show typical major peak for antigorite, i.e., 3670 cm⁻¹ and 3700 cm⁻¹, and minor peak at 3622 cm⁻¹ (Auzende et al., 2004; Schwartz et al., 2013), and no signal from brucite (Fig. 5D). This antigorite is Al- and Fe-rich (up to 2 wt.% for Al₂O₃ and up to 1.5 wt.% for FeOₜ, Table S6), with Mg# between 97 and 98 (Table S6).

Antigorite within these veinlets shows evidences of destabilization, as suggested by the development of significant porosity (Fig. 5C), irregular edges (Figs. 4D, 5A and B) and dissolution contacts with olivine and brucite-B (Fig. 5A and B and Figure S5). The Mg-stripes observed in wall olivine also propagate through the antigorite contained in the veinlets (Fig. 5B), also pointing to a growth of antigorite prior to Mg-rich stripes and, thus, brucite-B formation.

# 4. Discussion

## 4.1. Relevance of brucite in subduction zones

The vast majority of abyssal serpentinites (AS), lizardite and antigorite meta-serpentinites (LGS and HGS, respectively) have compositions that are compatible with modal brucite. While modes are based solely on rock composition and do not necessarily imply mineral stability, further thermochemical modeling on representative serpentinite compositions (Fig. 1A, S2 and S3) confirms the expected presence of brucite co-existing with serpentine minerals in subduction zones. Consequently, the Atg + Brc reaction, or R1, must be considered for its ability to liberate water in subduction zones. Petrological study of several Alpine ophiolites (Zermatt-Saas unit – Kempf et al. (2020); Monte Avic – Barale et al. (2023); Erro Tobbio unit – Hermann et al. (2000)) already proposed R1 to account for the presence of metamorphic olivine at metamorphic conditions below the Atg-out (R2) reaction although Brc I/II was not observed. To better constrain R1, an updated database is used to calculate new phase relationships (N-type calculation) for a hydrated mantle under subduction zone conditions. Calculation indicates that brucite + antigorite dehydration is more complex than R1 since it can be decomposed into two consecutive reactions with increasing T, which produce Al-rich antigorite (R3) and chlorite (R4) in addition to olivine and water. This is supported by petrological observations, showing that in the metamorphic veins of the UTG samples, (1) antigorite
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Description of the studied UTG serpentinite samples. A: Field photograph of weathered (reddish) and fresh (greenish; studied samples) serpentinites. Metamorphic olivine veins (black arrow) crosscut the antigorite matrix. B: Enlarged photograph of sample ZER_22-11A. Reddish parts are pluricentimetric metamorphic olivine/Ti-clinohumite veins (see Kempf et al., 2020, for further details on petrological description). The upper part of the picture shows the antigorite-matrix with carbonate-veins and Ti-clinohumite veins. C: Schematic representation of brucite-B veinlets distribution in metamorphic olivine veins (sample ZER_22-11A). D: BSE image of brucite-B veinlets crocutting metamorphic olivines. Olivine displays stripes enriched in magnesium (Mg# = 99) with the same orientation as the brucite-veinlets. E: Optical microscope image (plain light) of metamorphic olivine vein crosscutting antigorite matrix. F: BSE image (close-up view of Fig. 4E) showing brucite veinlets crosscutting an olivine + antigorite vein.</figcaption></figure>

is Al-rich (2 wt.% Al₂O₃, i.e., consistent with Al-content calculated for antigorite produced by R3) and (2) antigorite occurs along with chlorite, both in textural equilibrium with olivine. In addition, the stability field of the Brc + Ol + Atg + H₂O assemblage in hydrated harzburgite, dunite and lherzolite compositions spans over less than 10 °C in the calculations performed here, i.e., 4 to 15 times narrower than in previous calculations used to model fluid release during R1 (Plümper et al., 2017; Kempf et al., 2020; Huber et al., 2022). This difference is attributed to the use of updated thermochemical data for both antigorite and brucite solid-solutions (Eberhard et al., 2023; Carlin et al., 2024, respectively). A narrowing of the Brc + Ol + Atg + H₂O field can already be observed although not discussed in former simulations (Eberhard et al., 2023; Menzel et al., 2025) which were performed using the updated thermochemical data for the antigorite solid-solution. Recently, Menzel et al. (2025) showed that open system conditions leading to H₂ or O₂ exchange between the surrounding rock and external fluids can modify the temperature and the spreading in temperature of R1 dehydration reaction. For the range of $\frac{Fe^{3+}}{Fe^{2+}+Fe^{3+}}$ ratios measured in subducted natural samples (Debret et al., 2015), variations of R1 temperature and temperature spreading are both limited to less than 5 °C (Menzel et al., 2025). RedOx conditions thus have a negligible effect on R1 compared to the selected PT path and/or thermodynamic model.

Provided that equilibrium is continuously achieved upon dehydration, a direct consequence of a narrow stability field of the Brc + Ol +

Atg + H₂O assemblage is that H₂O will be released in a smaller temperature window (a few degrees instead of a few tens of degrees), i.e., at a higher rate. Assuming now that a few degrees of reaction overstepping is required to trigger R1, in the case of a hydrated lherzolite, the Atg + Brc reaction will basically proceed as an univariant one and water will be potentially released as a pulse. For higher grade conditions as encountered in Cerro de Almirez for example, the Atg-out reaction can be reached leading to a second major dehydration event. Both P- and N-type calculations predict a relatively sharp dehydration for Atg breakdown which was long considered as the main dehydration event in subducted serpentinites (Ulmer and Trommsdorff, 1995; Scambelluri et al., 2004). The temperature conditions at which these two dehydration reactions are calculated are about 40 - 50 °C higher in the N-type model compared to P-type (implying a deepening of a few km in a hot subduction zone up to ~15 km along a cold geotherm). R1 and R2 are not independent since the antigorite that is consumed by the former will no longer be available for the latter. Accordingly, in the case of the end-member dunite composition which, once hydrated, is composed of ~1 mole Brc for 1 mole Atg, no antigorite is left after R1 (Fig. 2A). The relative contribution of R1 to the total release of water depends on the initial Brc content; it is almost 100% for serpentinized dunite. It decreases to 50% for hydrous harzburgite and down to 30% for hydrous lherzolite (Fig. 3). For the vast majority of hydrous mantle compositions enter-
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Detailed characterization of the metamorphic olivine veins. A: BSE image of the contact between brucite-B + antigorite veinlets and metamorphic olivine. Olivine and antigorite show dark stripes at the contact with brucite. B: Corresponding Mg X-ray map. Mg-rich stripes are visible in both olivine and antigorite. C: FEG-SEM image of the contact between olivine and brucite-B veinlets. Mg-rich olivine (stripes) and brucite-B show a sharp contact surface with no evidence of reaction. On the contrary, antigorite is porous and seems to destabilize. D: Micro-Raman spectrum in the OH-stretching region of brucite-B (left part) and antigorite (right part) within the veinlets. E: TEM image of the sharp contact between a metamorphic olivine and one of its antigorite inclusion.</figcaption></figure>

ing in subduction zones, R1 must therefore be anticipated as a major
dehydration step.

4.2. *Strong Mg mobilization associated with the Atg + Brc reaction (R1)*

In the UTG samples, veins composed of high Mg# (~ 96) olivine ± Ti-Chu, represent local Mg-segregation. Recent studies have suggested that these metamorphic veins may have formed where brucite was initially concentrated (Plümper et al., 2017; Menzel et al., 2025). In the present work, we show that olivine veins formed first followed by the formation of brucite-B veinlets and olivine Mg-rich stripes. This observation is hard to reconcile with the thermochemical calculation performed with average bulk-rock compositions. For example, increasing the oxygen fugacity in the calculation allows to reach Mg# up to 97 in the stability field of magnetite, but without reproducing the modal composition of the veins, and, in particular, the presence of brucite-B. Local shift in composition was thus simulated by assuming a binary mixture between a harzburgite composition representing the vein host-rock composition and a pure MgO component simulating the dehydration fluid composition (Fig. S6). The equilibrium Mg# of antigorite, olivine and brucite were calculated along this binary join at 2.2 GPa and 550 °C. For the addition of less than 2 moles of MgO (fluid) per kilogram of harzburgite (rock) in the model, Ol and Atg coexist as generally observed in the

samples. The bulk-rock composition of olivine veins from the Zermatt-
Saas meta-ophiolite plots near the Ol + Atg to Ol + Brc boundary and
would correspond to the addition of 1 mole of MgO per kg of rock if the
reacting protolith had an harzburgitic composition (Fig. S8).

The olivine-veins wall rock is mainly composed of antigorite, chlorite and magnetite, and is generally depleted in Mg-rich components, i.e., olivine or brucite, compared to mode calculation (Fig. 1). This type of observation can only be reconciled with mode calculations if the vein + wall rock system is considered as a whole. This implies local Mg and/or Si transfer between wall rock and vein. Huber et al. (2022) reproduced olivine vein growth by considering Si mobility only. The calculations performed here indicate that Mg is orders of magnitude more soluble than Si at the conditions of R1. It is thus expected that Mg transfer controls olivine vein formation. Olivine precipitation in the veins led to Mg consumption, maintaining a chemical potential gradient between the reacting antigorite + brucite wall-rock and the veins and thus Mg transfer towards the veins. In such a model of vein formation triggered by diffusion in a fluid, vein size and spacing will mainly depend on the residence time of the fluid. In the outcrop of the Unterer Theodulglacier, olivine segregation is observed over a large range of scales from subcentimeter size olivine-segregation with a typical spacing of ten centimeters to several centimeter wide olivine-veins found every ten meters (Kempf et al., 2020). Using typical diffusion coefficients for aqueous species at 550 °C
~10⁻⁸m²/s; Watson and Wark, 1997), this corresponds to characteristic timescales for Mg segregation ranging from tens of days to hundreds of years. Considered diffusion coefficients here are for a free fluid. This assumption is based on the fact that dehydration in a closed system can generate tens of percents of porosity (Schmalholz et al., 2023). Nevertheless, it is not excluded that diffusion occurs at grain boundaries, at least in some parts of the rock, for which diffusivity can decrease by several orders of magnitude. The proposed timescales are thus minima. They suggest that typical fluid residence time during dehydration is short when compared to geological timescales, but long when compared to seismic events.

In addition, we report a new occurrence of brucite (brucite-B) with a highly magnesian composition in the form of veins that crosscut larger metamorphic olivine veins. This new occurrence significantly differs from the brucite (brucite-III) found in Brc + Lz associations after metamorphic olivine that is classically described in the literature. The brucite-A that we found in the UTG samples does correspond to brucite-III. Texturally, brucite-B forms individual grains which grew hydrothermally as crack filling. Detailed microtextural observations show that the fluid from which this hydrothermal brucite crystallized, interacted chemically with the olivine grains of the host vein. As a result of this interaction, metamorphic olivine with already high Mg# (~ 96) becomes even more magnesian (Mg# = 99) along stripes oriented parallel to the brucite veins. Fe-enrichment and Fe-depletion in stripes within olivine have been described in association with antigorite in samples from the Mariana forearc seamounts (Murata et al., 2009) and the Leka ophiolite complex (Plümper et al., 2012). The compositional stripes were found to occur along subgrain boundaries and interpreted as formed by fluid infiltration into olivine grains. In the UTG samples, a similar process of olivine interaction with an Mg-rich dehydration fluid may explain stripe formation. The thermodynamic simulation of MgO component addition to harzburgite composition reproduces the observed Ol + Br assemblage at 2.2 GPa and 550 °C for more than 2 moles of MgO added to 1 kg of rock (Fig. S8). Mineral compositions are reproduced when adding around 100 moles of Mg per kg of rock. The diffusion model proposed above for olivine vein formation explains phase segregation but does not allow to generate other assemblages than the one predicted at thermodynamic equilibrium for a peridotite composition (Ol + water ± Atg ± Chl). Another mechanism is thus required to reach significant Mg enrichment and subsequent brucite formation.

A notable outcome of the present thermochemical modeling is the high-magnesium concentration in the fluids that are produced according to R1. Indeed, concentration in the molal range are calculated with MgOaq as the dominant aqueous Mg species which is the dehydrated form of the Mg(OH)₂ aqueous species in the DEW database implemented in Perple_X (see Suppl. Mat. 2). MgOaq is neutral and, thus, elevated concentrations do not imply extreme pH conditions. Thermodynamic calculations are consistent with previous calculations for fluid composition at the equilibrium with mantle rocks in subduction zone conditions (Codillo et al., 2022; Okamoto and Oyanagi, 2023). Brucite-B could be formed by precipitation from such an Mg-rich fluid if a change in conditions (P, T or composition) occurs, leading to solubility decrease and precipitation. We do not observe evidence for changes in composition in the studied samples. Considering that the highest Mg solubility is calculated along reaction R1 (Fig. 2B), any changes in P and T in the stability field of olivine can favor brucite formation. In particular, fluid pressure changes associated with deformation events and dehydration fluid drainage could be involved in brucite-B crystallization from the fluid.

Thermodynamic calculations indicate that the complete reaction of the system at ~ 2 GPa and 550 °C leads to a rock at the equilibrium with a fluid containing approximately 3 mmolal of MgOaq. Considering closed system conditions, Mg in the fluid represents ~ 5 and 1% of the Mg initially contained in fully serpentinized dunite and lherzolite, respectively. The migration of such a fluid in subduction zones can thus leave an ultramafic rock depleted in several percents of Mg compared to its pre-metamorphic composition. This corresponds to a shift in the

$Mg_{brc}/(Mg_{brc} + Mg_{atg})$ ratio used here ($X_{MgB}$) of 0.07 and 0.01 for dunite and lherzolite compositions, respectively. Interestingly, these values are in the same range as the difference between the maximum value of $X_{MgB}$ in LGS and in HGS (Fig. 1B) and as the difference between mean values of $X_{MgB}$ at localities where LGS and HGS are both observed (Fig. 1C). This suggests that changes in serpentinized peridotite composition having experienced reaction R1 could be due to Mg export as an aqueous component in dehydration fluids.

Aqueous fluids transfer after reaction R1 can induce Mg-metasomatism. In their study of the δ¹⁸O composition of metamorphic-olivine bearing samples from the Zermatt-Sass meta-ophiolite, Ulrich et al. (2024) pointed out that olivine in isotopic disequilibrium with the surrounding antigorite is found in shear bands and in nearly pure Ol-veins. They concluded that these olivine grains crystallized from an external fluid originating from serpentinite dehydration, which was thus channelized. Based on this observation and our results, it can be concluded that R1 is prone to generate an Mg-rich fluid that is channelized and can thus transport magnesium over distances that still need to be determined. Numerous examples of Mg-transfer at the tens of centimeter scale in reaction zones have been reviewed by Okamoto and Oyanagi (2023) at the mantle-crust interface. They mainly consist in the development of chlorite-rich metasomatic rocks. Codillo et al. (2022) also described a reaction zone in the Voltri massif with Mg transfer between mafic and ultramafic rocks during subduction. At larger scale, Mg-metasomatism of the austroalpine continental crust (Eastern Alps) by fluid derived from the dehydration of serpentinites has been proposed, associated to the subduction of South Penninic oceanic slab (Chen et al., 2020). Evidences of Mg transfer in the alpine subduction is also provided, in the Western Alps, by the occurrence in internal crystalline massifs (Dora Maira, Gran Paradiso, Monte Rosa) of granitoïds that have encountered Mg metasomatism (Ferrando et al., 2009, and references therein). Among them, the Dora-Maira whiteschists is probably the most studied example, for which the metasomatic agent has been inferred to originate from serpentinite dehydration (Ferrando et al., 2009; Chen et al., 2016). In order to reconcile the P-T conditions of the Mg-metasomatic event with the expected Atg-out reaction conditions, Ferrando et al. (2009) needed to invoke low water-activity ($a_{H_2O}$) related to the presence of chlorine in the serpentinite source-fluid (HP brines). Actually, the consideration of R1 instead would not required such an $a_{H_2O}$ lowering. Indeed, the fluid generated by R1 contains even more Mg than that produced by the Atg-out reaction according to the present thermochemical simulations.

### 4.3. Implications for subduction zone dynamics

Based on our results and the comprehensive review of the literature (Fig. 1), we propose a series of evolutionary sketches illustrating the transition from oceanic serpentinites to partially dehydrated meta-ophiolites (Fig. 6A-E). As recently observed in drilled abyssal serpentinites (Klein et al., 2020) and supported by our compilation, oceanic serpentinites must contain a variable yet significant amount of brucite. Oceanic serpentinites are characterized by mesh and bastite textures, which result from the serpentinization of olivine and orthopyroxene, respectively (Fig. 6B). During this process, brucite (referred to as Brucite-I) forms together with lizardite (O'Hanley and Wicks, 1995). According to our thermodynamical modeling, antigorite (Atg) along with brucite (referred to as brucite-II) should begin to form at very low temperatures, below 200 °C (Fig. 6C), under subduction conditions. The joint formation of Atg and Brc II is due to the higher Si/Mg ratio of antigorite and its slightly reduced -OH content compared to lizardite. Up to this point, there is no thermochemical reason for brucite-I to disappear, which implies that antigorite and brucite (-I and -II) should coexist until the onset of R1. The metamorphic antigorite + brucite assemblage such as described on Fig. 6C is not observed in meta-ophiolites coming from subducting mantle. The rare cases of stable Atg + Brc assemblage in subduction-collision zones are interpreted as the product of HP-HT serpentinization of the mantle wedge (Debret et al., 2019; Nagaya et al.,
2022) with no earlier step of lizardite crystallization (Kawahara et al., 2016). It has been shown that lizardite can persist in low-grade serpentinites (LGS) up to approximately 350 – 400 °C (Schwartz et al., 2013) either for kinetics reasons or because the Liz-Atg transition temperature is underestimated by thermodynamics. Consequently, the effective P-T domain of Atg + Brc appearance under subduction zone conditions may be small covering a 100 °C or less. The lack of Atg + Brc association can thus result from a sampling bias, bearing in mind that brucite is both finely grained and easily altered. Alternatively, brucite associated with antigorite may not survive the conditions prevailing in the subduction channel. Brucite may dissolve or react, e.g., with silica-bearing fluids coming from nearby sediments, which may even prevent brucite formation at the Liz → Atg + Br reaction (Schwartz et al., 2013), thus explaining its absence in meta-ophiolites outcropping in subduction-collision zones.

When conditions of R1 are reached, a significant amount of water is released alongside the formation of olivine and Ti-Chu veins (Fig. 6D), up to the final development of brucite veinlets. From 1/3 to almost all the water contained in the rock is released at this stage, depending on the initial hydrated mantle composition, over a temperature range of < 12 °C. If dehydration is progressive and proceeds under equilibrium, 10 °C are covered within a 100 ky according to calculated P-T-t paths in a model slab (Peacock, 1990). Much shorter dehydration events can occur if dehydration proceeds out-of-equilibrium after significant reaction overstepping. A lower bound of the dehydration fluid residence in the dehydrating meta-serpentinite is given by the time required to form Mg segregation, i.e., olivine-rich veins, which can be estimated from Mg diffusion rate and olivine-veins geometry. In the UTG samples, this time is estimated to between several days and hundreds of years. The rare brucite veinlets and associated Mg-rich stripes in metamorphic olivine mark the latest crystallization stage from the Mg-rich fluid produced by R1. It can be tentatively interpreted as the result of Mg-fluid terminal evacuation stage (e.g., pressure drop upon connection to a larger fluid system). The absence of serpentine minerals intermixed with brucite-B at the veinlets wall coupled with olivine and brucite-B co-stability (Fig. 5C and D) suggests a prograde event and indicates that the R1 reaction is not a brucite-out reaction as sometime mentioned in the literature. The preservation of brucite in the form of veinlets (Fig. 6E) after R1 may contribute to the transport of H₂O to the deep mantle owing to the large brucite stability field (Sun et al., 2025; Hermann and Mookherjee, 2016), provided that it remains chemically isolated from antigorite.

There has been a strong drive to connect the petrological properties of the subducting slab with the rheology and dynamics of subduction zones (Reynard, 2013). Numerous studies have linked serpentine dehydration to Intermediate Depth EarthQuakes (IDEQ) (Peacock, 2001; Dobson et al., 2002; Jung et al., 2004; Gasc et al., 2017). More recently, the common observation of Slow Slip Events (hereafter referred to as SSEs) and Low Frequency Earthquakes (LFEs) that complement deformation accommodation of regular earthquakes in subduction zones worldwide (Schwartz and Rokosky, 2007; Jolivet and Frank, 2020) and their potential link to fluid release triggered by mineral dehydration (Bouchon et al., 2022) have garnered significant attention. Our microstructural data are certainly not sufficient to deduce any of the deformation processes listed above. The pressure and temperature conditions calculated here for LFEs in Mexico, Nankai and Cascadia subduction zones are displayed in Fig. 6 (scattered dots). One can however notice that there is a good correlation between the predicted conditions of reaction (R1) and the inferred P-T conditions of LFEs recorded in these subduction zones. LFEs and seismic tremors can be associated with SSE. By analogy with volcanic tremors, they are believed to be linked to fluid movement (Bouchon et al., 2022). Here, we propose that the newly calculated R1 is a reaction able to produce large amounts of fluids, at high rates, that should be able to channelize and migrate. This is in line with our finding that R1 must correspond to a major dehydration event in active subduction zone at least as important or even more important,

<figure><img src="image_10.png" /></figure>

**Fig. 6.** Model of olivine vein and brucite veinlet formation. A: Simplified phase diagram of stable mineralogical assemblages as function of pressure and temperature for harzburgite composition. Green lines underline the two main dehydration reactions (R1 is Atg + Brc = Ol + H₂O and R2 is Atg-out). Yellow path represents geotherm in actual subduction (van Keken et al., 2018). The red, blue and black dots provide the pressure and temperature conditions of LFEs in the subduction zones of Cascadia, Nankai and Mexico, respectively. The number correspond to mineral assemblages. 1: Brc + Liz + Chl + Hem. 2: Brc + Liz + Chl + Hem + Mag. 3: Atg + B + Liz + Mag. 4: Atg + Ol + Hem + Mag. 5: Atg + Ol + Hem. 6: Atg + Ol + Tlc + Hem + Mag. 7: Ol + Tlc + Chl + Hem + Mag. 8: Atg + Ol + Opx + Hem. 9: Atg + Ol + Tlc + Hem. 10: Ol + Tlc + Chl + Hem. 11: Ol + Tlc + Hem. Mineral abbreviations are after Whitney and Evans (2010). B to E: Schemes representing the fate of a serpentinized peridotites in a subduction zone deduced from the results presented here (modified after Schwartz et al., 2013). B: Peridotite completely serpentinized in the oceanic lithosphere with lizardite + brucite mixture as the main mineralogical assemblage. C: Lizardite to antigorite + brucite transition. Antigorite-associated brucite is generally not observed and thus hypothetical (question mark on the figure). D. Partial dehydration of serpentinites, with the formation of olivine + Ti-clinohumite veins due to Mg diffusion from the wall rocks. E: Formation of brucite veins during Mg-rich aqueous fluid expulsion. Fluid release probably plays a role in Mg-metasomatism and in LFE generation in subduction zones.

in terms of released fluid volume, than the generally invoked Atg-out reaction (R2).

### CRediT authorship contribution statement

**Legros Emma:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Malvoisin Benjamin:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Brunet Fabrice:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Methodology, Investigation, Conceptualization. **El Yousfi**
**Zaccaria:** Writing – original draft, Validation, Methodology, Formal analysis, Data curation. **Batanova Valentina:** Writing – review & editing, Writing – original draft, Validation, Methodology, Data curation. **Sobolev Alexander:** Writing – review & editing, Visualization, Validation, Resources, Methodology, Funding acquisition. **Auzende Anne-Line:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Acknowledgements

Prof. Yi-Xiang Chen (USTC, Hefei) and an anonymous reviewer are thanked for their constructive and detailed reviews. This project was supported by Initiatives de Recherche à Grenoble Alpes (IRGA funding program). It has also received funding from The European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Synergy Grant MEET, grant agreement no. 856555). University of Grenoble (UGA) is thanked for funding EL's PhD grant through Ecole Doctorale STEP. We are indebted to Alexandre Altié and Damien Chaudanson (CINAM, Marseille) for support with TEM, and to Rachel Martin (ISTerre, Grenoble) for support with FEG-SEM.

Appendix A. Supplementary material

Supplementary material related to this article can be found online at
https://doi.org/10.1016/j.epsl.2025.119602.

Data availability

Data will be made available on request.

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