# Mantle wedge oxidation from deserpentinization modulated by sediment-derived fluids

José Alberto Padrón-Navarta, Vicente López Sánchez-Vizcaíno, Manuel Menzel, María Teresa Gómez-Pugnaire, Carlos Garrido

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José Alberto Padrón-Navarta, Vicente López Sánchez-Vizcaíno, Manuel Menzel, María Teresa Gómez-Pugnaire, Carlos Garrido. Mantle wedge oxidation from deserpentinization modulated by sediment-derived fluids. Nature Geoscience, 2023, 16 (3), pp.268-275. 10.1038/s41561-023-01127-0. hal-04727041

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# Mantle wedge oxidation due to sediment-infiltrated deserpentinisation

José Alberto Padrón-navarta (✉ alberto.padron@csic.es)

CSIC https://orcid.org/0000-0003-1005-0012

Vicente López Sánchez-Vizcaíno

Universidad de Jaén

Manuel Menzel

RWTH Aachen University

María Teresa Gómez-Pugnaire

Universidad de Granada

Carlos Garrido

CSIC

## Article

### Keywords:

**Posted Date:** December 9th, 2021

DOI: https://doi.org/10.21203/rs.3.rs-1050656/v1

**License:** © 🌐 This work is licensed under a Creative Commons Attribution 4.0 International License.

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Mantle wedge oxidation due to
sediment-infiltrated deserpentinisation

Padrón-Navarta, J.A.<sup>1,2*</sup>, López Sánchez-Vizcaíno, V.<sup>3</sup>, Manuel D. Menzel<sup>4</sup>,

María Teresa Gómez-Pugnaire<sup>1,5</sup>, Garrido, C.J.<sup>1</sup>

<sup>1</sup>Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-Universidad de Granada, Av. Palmeras 4,
18100 Armilla, Granada, Spain.

<sup>2</sup>Géosciences Montpellier, Université de Montpellier & CNRS, F-34095 Montpellier cedex 5, France.

<sup>3</sup>Dpto. de Geología (Unidad Asociada al IACT, CSIC-UGR), Universidad de Jaén, Escuela Politécnica
Superior, 23700 Linares, Jaén, Spain.

<sup>4</sup>Institute of Tectonics and Geodynamics, RWTH Aachen University, Aachen, Germany.

<sup>5</sup>Departamento de Mineralogía y Petrología, Universidad de Granada, 18002 Granada, Spain

*corresponding author (e-mail: alberto.padron@csic.es)

2476 words/Summary (219 words)

The Earth's mantle is oxygen-breathing through the sink of oxidised tectonic plates at convergent margins<sup>1,2</sup>. Ocean floor serpentinisation increases the bulk oxidation state of iron relative to dry oceanic mantle and results in a variable intake of other redox-sensitive elements such as sulphur<sup>2,3</sup>. The reversibility of seafloor oxidation in subduction zones during high-pressure dehydration of serpentinite (“deserpentinisation”) at subarc depths and the capacity of the resulting fluids to oxidise the mantle source of arc basalts are highly contested<sup>4–15</sup>. Thermodynamic modelling<sup>8,10,15</sup>, experiments<sup>6,12,14</sup>, and metaperidotite study in exhumed high-pressure terrains<sup>4,15,16</sup> result in differing estimates of the redox state of deserpentinisation fluids, ranging from low to highly oxidant. Here we show that although intrinsic deserpentinisation fluids are highly oxidant, the infiltration of small fractions of external fluids equilibrated with metasedimentary rocks strongly modulates their redox state and oxidation-reduction capacity explaining the observed discrepancies in their redox state. Infiltration of fluids equilibrated with graphite-bearing sediments reduces the oxidant, intrinsic deserpentinisation fluids to oxygen fugacities similar to those observed in most graphite-furnace experiments and natural metaperidotites. However, infiltration of CO<sub>2</sub>-bearing fluids equilibrated with modern GLOSS generates sulphate-rich, highly oxidising deserpentinisation fluids. We show that such GLOSS-infiltrated deserpentinisation fluids can effectively oxidise the mantle wedge of cold to hot subduction zones potentially accounting for the presumed oxidised nature of the source of arc basalts.
37 Main (701 words)

38 Interaction of seawater with the oceanic lithosphere results in alteration of the oceanic crust and
39 hydration of mantle peridotite to serpentinite. Serpentinisation substantially increases the amount of
40 water and the ferric iron over total iron ratio (Fe³⁺/ΣFe)¹,²,¹³,¹⁷,¹⁸ (Fig.1a; 0.4–1.0 with a median and an
41 average value of 0.60 and 0.58, n = 601) relative to that of the Depleted MORB Mantle (DMM, 0.015–
42 0.04)¹⁹ (Fig. 1a), placing serpentinite among the most oxidised rocks of the altered oceanic lithosphere.
43 Seafloor serpentinisation furthermore increases the content of redox-sensitive, multivalent elements,
44 particularly sulphur, whose abundance varies by up to three orders of magnitude relative to DMM (Fig.
45 1b), reflecting a large variability of oxygen fugacity (fO₂) and S intake processes.

46 Subduction of altered oceanic lithosphere triggers the high-pressure (high-P) dehydration of
47 serpentinite at intermediate slab depths of subduction zones, a process usually referred to as
48 deserpentinisation. Deserpentinisation is considered the main volatile source of arc magmatism and
49 likely plays an important role in the recycling of redox-sensitive elements⁸,¹⁰,¹¹,¹³ and the genesis of ore
50 deposits²⁰. More controversial is the potential role of deserpentinisation aqueous fluids as oxidation
51 agents of the mantle wedge source of arc magmatism, accounting for the more oxidised nature of arc
52 basalts relative to MORBs⁵⁻⁸,¹²,¹⁴. While there is a consensus that water is mostly recycled back to the
53 crust and atmosphere by deserpentinisation, the oxidation of seafloor serpentinites (Fig. 1a) is generally
54 thought to be irreversible. While pure molecular species in aqueous fluids have a limited oxidising
55 capacity²¹, the redox capacity of deserpentinisation electrolytic fluids highly depends on their content
56 in oxidised species bearing multivalent elements, particularly sulphur⁸,²²,²³. Sulphur species with a high
57 oxidising capacity —such as sulphates— are thermodynamically predicted during deserpentinisation at
58 fO₂ near or above the hematite–magnetite oxygen buffer⁸. Most deserpentinisation experiments,
59 however, yield significantly lower fO₂¹²,¹⁴. Rare metaperidotite formed by high-P dehydration of
60 subducted serpentinite, now exhumed in metamorphic paleo-subduction terranes —Cerro del Almirez
61 (CdA) and Cima di Gagnone (CdG)— also records lower oxidising conditions than those predicted by
62 thermodynamic models⁴,¹⁵,²⁴. High-P serpentinite (metaserpentinite) and metaperidotite from CdA have
63 a rather narrow range of Fe³+/ΣFe ratio (0.60–0.64, first and third quartile, n=27 and 0.24–0.33, n = 21
64 respectively, Fig. 1a) and magnetite content (3–4 wt.%, n=19 vs. 0.3–1.1 wt.%, n=11, Supplementary
65 Fig. 1), and the transition from metaserpentinite to metaperidotite results in a 40 % decrease of its redox
66 budget (from 0.7 to 0.4 mol/kg, Methods and Supplementary Fig. 4 and Table 1). The trend of
67 decreasing Fe³+/ΣFe content from metaserpentinite to metaperidotite in CdA has been ascribed to
68 deserpentinisation¹⁶ suggesting the reversibility of seafloor serpentinisation oxidation during
69 subduction (Fig. 1a), yet the ultimate cause for such reversal has not been provided. Alternatively, this
70 trend is ascribed to different extents of oceanic serpentinisation of their protoliths⁴,¹⁵,²⁴, implying the
71 irreversibility of seafloor oxidation serpentinisation during subduction (Fig. 1a). This interpretation is
72 however at odds with numerous textural and geochemical evidence for the provenance of CdA and CdG
<figure><img src="image_2.png" /></figure>

**FIGURE 1a.** The cycle of hydration-oxidation of mantle peridotite (serpentinites, grey arrow) at the Earth's ocean floor relative to the nominally anhydrous mantle (red star, Depleted MORB Mantle⁹) and two possible deserpentinisation paths either reversing or preserving the oxidation set at the Earth's surface. The directions of the arrows for the thermodynamically modelled intrinsic and graphite metapelite-infiltrated deserpentinisation evolutions indicate the direction of H₂O loss during subduction (depicted also in Fig. 2a). The only known rock samples from high-pressure terrains recording the deserpentinisation (metaperidotites from the Cerro del Almirez, CdA, and Cima di Gagnone, CdG, localities) match the reversible path requiring an external influx of reduced fluids during dehydration. Worldwide serpentinites also include partially serpentinized orogenic peridotites (< 9.0 wt.% H₂O, see Methods). **b.** The observed increase in total S content in the CdA and CdG metaperidotites compared to serpentinites is also reproduced by the graphite metapelite-infiltrated deserpentinisation model (blue arrow), in contrast to the intrinsic deserpentinisation model (red arrow).
metaperidotites from highly serpentinised peridotite protoliths metasomatised by crustal fluids²⁵⁻²⁷.

CdA metaperidotite is also characterised by a moderate increase in S relative to its putative Ca-poor
serpentinite protolith —with a low S content (< 300 µg/g) similar to DMM; an opposite trend to that
predicted from thermodynamic modelling (Fig. 1b). Furthermore, subducted serpentinite has lower and
narrower S contents than average oceanic serpentinite (Fig. 1b), thus limiting the capacity of
deserpentinisation fluids to oxidise the mantle wedge. Thermodynamic modelling, experiments and
natural samples thus provide seemingly contradictory views on the role of deserpentinisation fluids as
potential agents for the oxidation of the mantle source of arc basalts.

The pervasiveness of subducted-sediment geochemical signatures is the hallmark of arc magmatism²⁸, an attribute increasingly ascribed to the interaction of slab crustal lithologies with slab fluids sourced from deserpentinisation. There is also overwhelming geochemical evidence on the role of infiltration of fluids equilibrated with metasedimentary rocks in the genesis of natural serpentinite-derived metaperidotite²⁵⁻²⁷,²⁹⁻³³. However, it has not yet been investigated how the interaction of sediment-derived fluids modify the fO₂ and the cargo of multivalent elements in the deserpentinisation fluids, compared to those sourced from the intrinsic (i.e. without external fluid infiltration) deserpentinisation.

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Intrinsic deserpentinisation (396 words) 91

Figure 2a shows the thermodynamic modelling of the intrinsic prograde evolution a serpentinite
subducted along the geothermal gradient of high-P metamorphic terrains. At constant bulk O₂ (red
arrows in Fig. 1a and 2a, Supplementary Fig. 2), this evolution implies a dramatic increase of the
Δlog₁₀fO₂[FMQ]⁷,¹⁰,³⁴ from +2.2 (450 °C, 0.9 GPa), where olivine is first produced, to +4.6 (660 °C,
1.7 GPa), where antigorite serpentinite dehydrates to metaperidotite. The rise in fO₂ is due to the
formation of hematite after magnetite to maintain the bulk Fe³+/ΣFe constant (Fig. 1a; red line) since
the X_Mg [Mg/(Fe²⁺+Mg)] of olivine and orthopyroxene reaction products is lower than that of reactant
antigorite (Supplementary Fig. 3 and 4). In agreement with previous models for a similar
Δlog₁₀fO₂[FMQ]⁸, our model shows that S and C of serpentinite are fully dissolved in the
deserpentinisation fluid, with HSO₄⁻(aq) and, to a lesser extent, SO₄²⁻(aq) as the dominant sulphur species
(Supplementary Table 2).

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<figure><img src="image_3.png" /></figure>

**FIGURE 2a.** Two potential evolutions of the oxygen fugacity $fO_2$ for the subducting serpentinite slab (expressed as relative to the FMQ buffer; see Methods) for a geothermal gradient crossing the antigorite (660 °C/1.7 GPa) and chlorite dehydration (820 °C/2.4 GPa) as observed at CdA and CdG computed for a representative fully serpentinized peridotite containing 5 wt.% magnetite, 11.6 wt.% $H_2O$, 170 ppm S, and 200 ppm C (Figure 1 and Supplementary Table 1). Stability fields highlighting some key mineral assemblages are superimposed as thick lines (see Supplementary Figure 2). Path-I (“Intrinsic deserpentinisation”) corresponds to the conventional, closed system, prograde metamorphism where the rock imposes the $fO_2$ and shows a dramatic increase in the $\Delta\log_{10}fO_2$[FMQ] of up to +4.5. Here we show that, alternatively, externally-derived fluid infiltration can induce complete antigorite dehydration at isobaric-isothermal conditions (horizontal Path-IIa, graphite-metapelite infiltrated deserpentinisation) with a decrease in the relative $fO_2$ and in the redox budget if the infiltrating fluid is derived from a sediment (pyrite and graphite-bearing metapelite) with a high reducing capacity⁴⁶. **b-e.** Isothermal and isobaric evolution (650 °C/1.7 GPa) along the computed, dehydration-driven infiltration path. The vertical dashed line marks the complete deserpentinization (for the Path-I intrinsic evolution see Supplementary Figure 4): **b.** Bulk-rock ferric to total iron content ($\text{Fe}^{3+}/\Sigma\text{Fe}$). **c.** $X_{Mg}$ in olivine. **d.** $\Delta\log_{10}fO_2$[FMQ]. **e.** Total sulphur content retained in the reacting rock during the external infiltration. The blue-shaded range shows the extent of fluid-rock reaction necessary to induce full dehydration of serpentinite and reproduce the observed $\text{Fe}^{3+}/\Sigma\text{Fe}$ and $X_{Mg}$ in the CdA exhumed high-pressure terrain (metaperidotite).
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The intrinsic deserpentinisation thermodynamic model fails to explain many features of natural
serpentinite and metaperidotite, such as the decrease of the bulk Fe³⁺/ΣFe⁴,¹⁶,³⁵ and the relative increase
in S content in metaperidotite³⁶ (Fig. 1); its magnetite content and lack of hematite⁴,¹⁶,²⁴,³⁵
(Supplementary Fig. 1); and the lower olivine Xₘg⁴,³⁵ (Supplementary Fig. 4). Nor does it explain the
substantially lower Δlog₁₀fO₂[FMQ] (+2.1 ± 0.3; at 800 °C, 1GPa) found in most high-P serpentinite
dehydration experiments¹²,¹⁴,³⁷⁻³⁹ that obtain olivine —coexisting with magnetite⁶,¹⁴,⁴⁰— with an Xₘg
that perfectly matches that of natural metaperidotite⁴,¹⁵,³⁵,⁴¹. Although these lines of evidence might
point to the non-validity of the thermodynamic model of intrinsic deserpentinisation, it is well known
that the fO₂ is externally buffered and biased towards lower fO₂ in high-T, long-run, graphite-furnace
experiments¹⁴. Interestingly, deserpentinisation experiments conceived to maintain high fO₂ using
LaCrO₃ instead of graphite furnace⁶ obtain high Xₘg olivine (0.94–0.97) coexisting with hematite and
magnetite, and Δlog₁₀fO₂[FMQ] ranging from +3.1 to +4.1, in excellent agreement with our
thermodynamic model predictions (Fig. 2 and Supplementary Fig. 4). As serendipitously attained in
graphite-furnace experiments¹²,¹⁴,³⁷⁻³⁹, many natural metaperidotite features would be matched if the
bulk O₂ of the system was open, externally imposed and shifted towards lower bulk O₂ values (blue
lines for CdA and CdG, respectively in Fig. 2a). This hypothesis would reconcile the seemingly
contradictory fO₂ stemming from thermodynamic modelling, some experiments, and natural samples.

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Sediment-fluid infiltration driven by deserpentinisation (553 words)

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The infiltration of fluids equilibrated with metasedimentary rocks —well documented in the genesis of
natural metaperidotite²⁵,³³,⁴²,⁴³—is a potential mechanism for modifying the fO₂ and multivalent element
cargo of deserpentinisation fluids. Lowering the fO₂ relative to the oxidising conditions of intrinsic
deserpentinisation (Fig. 2a) requires the infiltration of fluids in equilibrium with metasedimentary rocks
with a highly reducing capacity, such as graphite-bearing sediments⁴⁴,⁴⁵. Graphite-bearing sediments
and calc-silicates subducted to eclogite facies conditions during the Alpine orogeny are widely
associated with the few known natural occurrences of metaperidotite derived from high-P
deserpentinisation³³,⁴⁶⁻⁴⁸. Figures 2b–e show the results of thermodynamic modelling of the infiltration
of fluids equilibrated with graphite-bearing metapelite at a temperature 10 °C colder than the intrinsic
deserpentinisation conditions in the CdA (650 °C, 1.7 GPa). At these conditions, serpentinite releases 2
wt.% of H₂O and generates sufficient permeability for external fluid infiltration. The metapelite-derived
infiltrating aqueous fluid is rich in CH₄ and H₂S with minor H₂ (Supplementary Table 2). At the onset
of its infiltration into dehydrating serpentinite, CH₄ is oxidised to CO₂, and the H₂S and H₂
concentrations decrease at the expense of more oxidised HSO₄⁻(aq) and SO₄²⁻(aq) species that increase
compared to those in the intrinsic deserpentinisation fluid (Supplementary Table 2 and Fig. 5a).
Sediment-fluid infiltration gradually decreases the bulk Fe³⁺/ΣFe (Fig. 2b), thus decreasing its bulk rock
and olivine Xₘg (Fig. 2c) due to a lower fO₂ (Fig. 2d), and increases the bulk S content of metaperidotite

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and fluid (Fig. 2e and Supplementary Fig. 5a and Table 2). Complete dehydration occurs at isobaric and isothermal conditions with an infiltration as low as 7.4 mol/kg of a fluid equilibrated with a graphite-bearing metapelite (vertical dashed line in Fig. 2b-e).

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For an infiltration extent of 9–12 mol/kg (ca. 0.2 fluid/rock mass ratio, Fig. 2b–e), sediment-equilibrated fluid infiltration driven by deserpentinisation fully succeeds in explaining many features of natural CdA serpentinite and metaperidotite, such as the trend of decreasing bulk Fe³⁺/ΣFe (Fig. 1a; 0.17–0.10 in Fig. 2b), the relative increase in metaperidotite bulk S content (ca. 400–500 µg/g S, Fig. 1b and 2e) relative to its metaserpentinite protolith (c. 170 µg/g), and the modal magnetite content (Supplementary Fig. 1) and lack of hematite in metaperidotite. It also predicts an olivine $X_{Mg}$ (0.91–0.89, Fig. 2c) and $\Delta\log_{10}fO_2$[FMQ] (+2.7 to +2.3, Fig. 2d), in excellent agreement with the olivine $X_{Mg}$ of CdA metaperidotite⁴,¹⁵,³⁵ and the $fO_2$ and olivine $X_{Mg}$ obtained in graphite-furnace serpentinite dehydration experiments¹². Compared to the intrinsic deserpentinisation, the infiltration of 9–12 mol/kg of sediment-equilibrated fluids does not significantly modify the bulk major element contents, silicate mineral assemblage or modal proportions of metaperidotite. This petrological similarity likely explains why sediment-fluid infiltration has only been considered responsible for the cryptic geochemical signature of natural metaperidotite but largely passed unnoticed as a mechanism to modulate the redox state and cargo of redox-sensitive elements during deserpentinisation. For an external fluid infiltration of > 11 mol/kg, the rise of $H_2S$ and $HS^{−}_{(aq)}$ in the fluid results in pyrite precipitation (Supplementary Fig. 5b) —occasionally found in CdA metaperidotites— while maintaining an elevated concentration of $HSO_4^{−}_{(aq)}$, accounting for the observed increase of the $SO_4/\Sigma S$ ratio allied to a decrease in total $\delta^{34}S$ in metaperidotite relative to its serpentinite protolith reported in CdA³⁶.

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## Implications for the redox state of the subarc mantle (607 words)

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The interaction of slab fluids with the subarc mantle wedge source is increasingly recognised⁴⁰,⁴¹ as the cause of the more oxidised nature of arc basalts relative to MORBs. Deserpentinisation is the main source of slab fluids at subarc depths and thus a possible mantle wedge oxidation agent. While the extent of serpentinisation of fast-spreading mid-ocean lithosphere is largely unconstrained, serpentinite is widespread in subducted slow-spreading, mid-ocean ridge lithosphere and at the slab interface of subduction zones. At the slab interface of hot to cold subduction zones, modelling shows that serpentinite intrinsically dehydrates between 2.4–3.4 GPa and 650–660 °C under relatively oxidising conditions (+2.9 to +3.7 $\Delta\log_{10}fO_2$[FMQ]), particularly in hot subduction zones (see ID in Fig. 3a) due to the negative pressure dependence of $\Delta\log_{10}fO_2$[FMQ] for metaperidotites (Supplementary Fig. 6). The dominant fluid sulfur species in all thermal regimes is $HSO^{−}_{(aq)}$, which is slightly more abundant in deserpentinisation fluids from —relatively more oxidant— hot subduction zones (Fig. 3a). At the slab interface, serpentinite coexists with fluid-saturated metasedimentary rocks in

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<figure><img src="image_4.png" /></figure>

**FIGURE 3a.** Modification of the Δlog₁₀/O₂[FMQ] and the concentration of the main oxidising fluid species (HSO₄⁻) relative to the intrinsic dehydration (ID) serpentinite fluid when serpentinites are infiltrated by sediment-derived fluids with low reducing capacity (GLOSS) for a worldwide compilation of subduction zones (color-coded for the pressure at which the serpentinite dehydrates at the slab surface, Methods). **b.** The capacity of these modified, serpentinite-derived fluids (empty dots in a) to oxidise the mantle wedge on top of the slab at near wet-solidus conditions is computed for the hottest (Central Cascadia) and coldest (Tonga) subduction zones. A minimum value range of Δlog₁₀/O₂[FMQ] inferred for oxidised IAB source and recorded by high-pressure metasomatized mantle atop of the slab⁵²,⁵³ is given as a horizontal blue-shaded range. Sediment (GLOSS)-serpentinite derived fluids are more than two-fold more efficient (3 mol/kg) to oxidise the mantle wedge than the intrinsically-released serpentinite dehydration fluids (7 mol/kg). The metasomatized mantle wedge has an initial depleted composition. Squares and stars on the red and blue lines indicate the condition range limits at which pyrrhotite (Po), pyrite (py) or anhydrite (anh) are the stable minerals hosting S in the rocks. For an ultradepleted MORB mantle see Supplementary, Fig. 7. For an interaction with sediments with high reducing capacity (blackshale-like sediments) see Supplementary Fig. 8.
melange zones⁴⁹ and sediment-fluid infiltration driven by deserpentinisation is favoured. Modelling shows that for an extent of infiltration of 12 mol/kg —similar to that inferred from natural CdA (Fig. 2b-e)— infiltration of fluids equilibrated with modern Global Subducting Sediments (GLOSS)²¹,⁴² during deserpentinisation does not greatly modify their fO₂ relative to intrinsic deserpentinisation whatever the subduction thermal regime (Fig. 3a). However, the abundance of dominant sulphate fluid species HSO₄⁻(aq) increases notably in warm and hot subduction zones (Fig. 3a), boosting the redox capacity of deserpentinisation slab fluids relative to those produced by intrinsic dehydration.

Figure 3b shows the modelling of the fO₂ evolution of a DMM mantle wedge —at the near-vapour-saturated solidus temperature⁵⁰— infiltrated by deserpentinisation fluids for a hot (Central Cascadia) and cold (Tonga) subduction zone. Infiltration of at least 7 mol/kg of slab fluids produced by intrinsic deserpentinisation is required to oxidise the mantle wedge within the minimum range of Δlog₁₀fO₂[FMQ] inferred for the mantle source of arc magmas (FMQ +1.0⁵¹,⁵²). In contrast, GLOSS-infiltrated deserpentinisation fluids have a greater oxidising capacity; the interaction of about 3 mol/kg of these slab fluids drastically increases the Δlog₁₀fO₂[FMQ] of the mantle wedge in hot and cold subduction zones (Fig. 3b). Therefore the effectiveness of rock deserpentinisation required to oxidise the mantle wedge is increased by a factor of more than two compared to intrinsic deserpentinisation. In both thermal regimes, slab fluids can attain the inferred fO₂ of the mantle source of arc magmas by precipitating anhydrite, thought to be the main sulfur host at these fO₂ mantle wedge conditions⁵¹. Our results demonstrate that metasomatism of the mantle wedge with GLOSS-infiltrated deserpentinisation fluids is a viable mechanism to account for the oxidised nature of the mantle source of arc magmas in hot and cold subduction zones. Similar conclusions are obtained when considering a more depleted and reduced —due to dependence of the fO₂ on the peridotite bulk composition⁵³— mantle wedge (Supplementary Fig. 7), indicating that the extent of mantle wedge depletion has a subsidiary role compared to the redox capacity of the incoming slab fluids.

As attested by natural metaperidotite, the oxidising capacity of sediment-infiltrated deserpentinisation fluids depends on the redox capacity of subducted sediments. This capacity has changed throughout Earth's geological history linked to the oxidation state of the atmosphere-ocean system and varies from oxidant GLOSS²⁸,⁵⁴ to highly reducing black shales⁴⁴,⁴⁵. Modelling of deserpentinisation infiltrated by fluids equilibrated with graphite-bearing metapelite shows that in hot subduction zones the oxidant capacity of fluids is as high as for intrinsic deserpentinisation but is significantly lower for cold subduction zones (Supplementary Fig. 7 and 8).

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337

338
339 **Methods**

340 **Abundance of redox-sensitive elements.** We extended a previous compilation¹⁷ of ferric and ferrous
341 iron contents in serpentinite from different geologic settings (n=758) (Fig.1a) to include alpine
342 serpentinite from Malenco (N. Italy, n=42)⁵⁵,⁵⁶ and 33 new analyses from CdA analysed by
343 potentiometric titration. Bulk Fe³⁺/ΣFe ratios are in excellent agreement with mineral mode estimates
344 based on the measured amount of magnetite by saturation magnetization corrected for the paramagnetic
345 fraction¹⁶ and using the Fe³⁺/ΣFe content of antigorite measured by XANES spectroscopy in selected
346 samples⁵⁷. Magnetite contents (Supplementary Fig. 2) comprise serpentinites from mid-ocean ridges
347 (ODP Leg 153 MARK¹⁸,⁵⁸,⁵⁹), passive margins (ODP Leg 173 Iberia⁵⁸,⁵⁹), Puerto Rico Trench
348 (NWPRT)⁵⁹, forearc (ODP Leg 125 Marianas⁵⁸,⁵⁹), alpine serpentinites¹⁶,³⁵,⁵⁷, and prograde
349 metaperidotite from CdA⁴,³⁵,⁵⁷ and CdG⁴¹. Late-stage weathering of prograde metaperidotite may cause
350 re-oxidation during hydration, therefore we only consider samples with < 4.5 wt.% H₂O for comparison
351 with the thermodynamic predictions (Fig.1). Bulk carbon and sulphur contents are from this study and
352 ref²,¹¹,³⁶,⁴¹,⁶⁰.

353

354 **Computation of redox budget.** The redox budget⁶¹ is an extensive variable quantifying the ability of
355 a rock-fluid system to potentially induce oxidation or reduction relative to an arbitrary reference redox
356 state of interest. The specific redox budget (RB) is normalized to units of mol/kg and it is defined as,

$$
357 \quad \boxed{\text{RB} = \sum_j n_j \cdot v_j}
$$

358 where $\boxed{v_j}$ is the number of electrons required to take one mole of the element in the redox state $j$ to the
359 reference redox state, and $\boxed{n_j}$ is the number of moles per kg of the element in the redox state $j$ present in
360 the system (rock and fluid). Here (Supplementary Table 1 and Fig. 3), we consider the redox budget of
361 the solid rock as this is the only variable that can be measured after fluid extraction. For the whole
362 mantle reference redox state², the oxidation state of iron, carbon and sulphur are Fe²⁺, C⁰ and S²⁻,
363 respectively, resulting in $\boxed{v_j}$ of 1, -2, 4, 1 and 8 for Fe³⁺, Fe⁰, C⁴⁺, S⁻ and S⁶⁺. In the modelled system,
364 the RB is computed based on the number of moles per kg of rock of the phases containing Fe³⁺, Fe⁰,
365 C⁴⁺, S⁻ or S⁶⁺ (hematite, native iron, magnesite, dolomite, calcite, pyrite, pyrrhotite, pyroxenes, garnet
366 and anhydrite, Supplementary Fig. 3) multiplied by the number of redox-sensitive atoms in their
367 structural formulae. For mineral phases having ferric iron in solid solution, the RB is weighted by the
368 molar proportion of the ferric iron endmember (magnetite in spinel⁶², Fe₀.₈七₅S in pyrrhotite⁹, magnesium
369 ferri-tschermak's in orthopyroxene modified by J.A.D. Connolly from ref⁶³, and khoharite in garnet⁶⁴).
370

370

371 **Phase equilibria.** Phase equilibria and electrolyte fluid speciation were computed by Gibbs energy
372 minimization using the PerpleX algorithm⁶⁵,⁶⁶. Forward thermodynamic modelling has been performed
373 in the CFMACrSHCSO system using elements as thermodynamic system components (Ca–Fe–Mg–
374 Al–Cr–Si–H₂–C–S₂–O₂) for metaserpentinite and metaperidotite and in the NKCFMASHCSO system
(Na-K-Ca-Fe-Mg-Al-Cr-Si-H₂-C-S₂-O₂) for the graphite-bearing metapelite³³,⁴⁶ and GLOSS⁵⁴, as well as for the metasomatic infiltration models derived from them (bulk composition used are in Supplementary Table 1). Fluid saturation conditions were not imposed during the computation. Element components are preferred over oxide components when modelling electrolytic fluid speciation because C-O-H-S solvents in the fluid have a strong tendency to order for certain bulk compositions at low temperature⁶⁷. Thermodynamic data for endmembers are taken from ref⁶⁸. Considered solid solutions models were olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), chlorite (Chl), dolomite (Dol), magnesite (Mgs), epidote (Ep), staurolite (St), all of them from ref⁶⁹,⁷⁰, pyrrhotite (Po)⁶⁸, amphibole (Amph(DHP))⁷¹, garnet (Grt(WPH))⁶⁴, antigorite (Atg)³⁷, white mica (mica(CF))⁷², biotite (Bi(WPH))⁶⁴, feldspar⁷³, Cr-bearing spinel (Mag)⁶², talc (Tlc, ideal), brucite (B, ideal), anthophyllite (Anth, ideal), and pure endmembers hematite (hem), pyrite (py), anhydrite (anh), graphite (gr), quartz (q) and iron. We used models Chl(W)⁷⁴ and Cpx(HGP)⁷⁵ for graphite-bearing metapelite and GLOSS, whereas for the mantle wedge metasomatism modelling we used solid solution models specifically calibrated for the mantle⁶². The systems considered in our work allow the investigation of redox reactions among the Fe-C-S-bearing solid phases (pyrrhotite, pyrite, anhydrite, magnetite, hematite, iron, carbonates and graphite) and species (e.g. HS⁻, SO₄²⁻, CaSO₄(aq) among many others) in an electrolyte aqueous C-O-H-S fluid. Non-modelled components (Ti, Mn and Ni) typically represent less than 0.5 wt.% (expressed as oxides) of the bulk rock composition. The absence of titanium and nickel prevents the modelling of phase assemblages containing ilmenite and pentlandite which typically occur in excess in high-pressure serpentinite and metaperidotites. The initial oxidation state of the redox-sensitive elements (Fe-C-S₂) is specified by the amount of O₂ in the system and requires a knowledge of the oxidation state of iron, carbon and sulphur (Supplementary, Table 1). However, the oxidation state of iron, carbon and sulphur are not imposed but are set by the stable phases obtained by Gibbs energy minimization (Supplementary Fig. 2 and 3). The geothermal gradient used to compute Fig. 2 and Supplementary Fig. 2-4) (P[bar] = -18057 + 37.9357 * T[K]) follows the P-T path inferred for CdA⁷⁶, and also applies for peak metamorphic conditions estimated for the CdG metaperidotite⁴¹.

**Solvent and speciation in the aqueous fluid.** The neutrally charged solvent molecules considered in the fluid were H₂O–CO₂–CH₄–H₂–H₂S (thermodynamic data from ref.⁶⁸) described by a generic hybrid fluid EoS with non-linear subdivision (COH-Fluid+ from ref⁶⁷). The EoS for H₂O and CO₂ is PSEoS⁷⁷, whereas for other solvents is the MRK⁷⁸. All other species are taken as solutes following the approach of ref.⁶⁷,⁷⁹ (see Supplementary Table 2). Thermodynamic data for aqueous species are taken from the latest version of the DEW model⁸⁰. The species CO(aq) and SO₂(aq) are considered as solute species in the fluid, although they occur at very low concentrations (Supplementary Table 2). Organic species from the DEW model were excluded from the computation following ref⁶⁷. The Mg(SiO₂)(HCO₃)⁺ and H₂CO₃(aq) species were also excluded as they were found to result in unrealistically high concentrations.
411

Back- and lagged-computation methods⁶⁷ give similar results at temperatures cooler than the complete
serpentinite breakdown, but only the lagged method correctly models the complete loss of sulphur at
higher temperatures because this method imposes mass balance conservation.

412

413

414

415

**Infiltration models.** The fluids in equilibrium with the graphite-bearing metapelite were first computed by Gibbs energy minimization at the P–T conditions of interest (650 °C, 1.7 GPa, Supplementary Table 2) using MEEMUM from the PerpleX package⁶⁶ and the lagged calculation method⁶⁷. This composition (renormalized to two hydrogen moles) was then equilibrated with the model serpentinite at the same P–T conditions at steps of 0.1 mol of fluid aliquot along 150 nodes (0-d infiltration mode), without fractionation of the fluid at each node. Additional computations fractionating the fluid at each node do not significantly modify the results. An example of the reequilibrated composition after interaction of 12 mol/kg with a fluid equilibrated with graphite-bearing metapelite is presented in the Supplementary Table 2. To model global subduction zones, the same procedure was repeated for all s 56 subduction zone segments at the P and T of the intersection of the antigorite breakdown with the slab surface based on the numerical model D80 from ref.⁸¹ The intersection was found by solving the polynomial fitting of the antigorite dehydration curve from this study and the slab trajectories at the surface⁸¹ as documented in ref.⁸². MEEMUM was then computed for the 56 P–T conditions for the two different fluid sources (graphite-bearing metapelite and GLOSS sediment) resulting in 112 different fluid compositions. Once these fluids were retrieved, 112 infiltration models were run with VERTEX at the specified P–T conditions. These computations generate multigrid outputs for all available properties accessible with WERAMI, including, mineral and fluid modes, dependent chemical potentials (used to compute Δlog₁₀f/O₂[FMQ]) and the amount of solvents in the fluid (as mole fraction) and species concentration (as molalities, mol/kg). We provide a global dataset (source data) including Δlog₁₀f/O₂[FMQ], bulk fluid composition and solvent-solute fluid composition for the intrinsic and the two sediment-infiltrated deserpentinisation models (using high and low reducing capacity fluids) for the 56 subduction transects and an extent of interaction of 12 mol/kg. A Python notebook is provided under request to generate the dataset for any desired extent of interaction.

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**Mantle wedge redox conditions.** Two end-member bulk compositions were considered to compute the f/O₂ of the mantle wedge before fluid infiltration (Fig. 3 and Supplementary Fig. 6): depleted MORB mantle source (DMM)⁸³ with a Fe³+/ΣFe equal to 0.035, taken from the Primitive Upper Mantle, and adding S content of 119 µg/g; and a highly depleted mantle (sample PHN5239 from ref.⁸⁴) with a Fe³+/ΣFe equal to 0.021 and a 50 µg/g of S, taken as the lowest possible bound based on the Cu–S covariance in MORBs⁸⁵. We chose fluids from the hottest and coldest subduction zones after 12 mol/kg interaction of the two fluids (graphite-bearing metapelite and GLOSS) with the reference serpentinite.

440

441

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443

444

445
446

The aqueous fluid was fractionated at each node for mantle wedge infiltration models to reflect the low
instantaneous fluid–rock ratio expected for pervasive fluid flow at mantle conditions.

447

448

449

Acknowledgements

450

This work is part of the project DESTINE (PID2019-105192GB-I00) funded by
MICIN/AEI/10.13039/501100011033 and the FEDER program “una manera de hacer Europa”.

451

452

J.A.P.N. acknowledges a Ramón y Cajal contract (RYC2018-024363-I) funded by
MICIN/AEI/10.13039/501100011033 and the FSE program “FSE invierte en tu futuro”. This research
is part of the Junta de Andalucia research group RNM-131.

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456

Author contributions

457

J.A.P.N. conceived the project, processed the data, acquired funding and wrote the original manuscript.

458

V.L.S-V. contributed to the conceptualization, performed the computations, organized the raw data and
contributed to the writing of the manuscript. M.D.M. computed the global deserpentinisation conditions
and assisted computations. M.T.G-P. contributed to the writing of the manuscript. C.J.G contributed to
the conceptualization, acquired funding, t and writing of the manuscript.

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**Supplementary figures and tables**

**Figure 1.** Global compilation of magnetite content in serpentinite and metaperidotite (Chl-harzburgite) against the water content (measured for CdA, this work and ref$^{36}$) or loss of ignition (L.O.I.) as a proxy for water content for samples from the literature (see Methods). The observed decrease in magnetite content relative to common magnetite-bearing serpentinite is reproduced by deserpentinisation infiltrated with highly reducing fluids equilibrated with graphite-bearing metapelite. The decrease in magnetite for the intrinsic deserpentinisation model is coeval with the precipitation of hematite (dashed red line) which is not observed in natural samples.

<figure><img src="image_5.png" /></figure>
**Figure 2.** X(O₂)-P/T (along a thermal gradient, see Methods) pseudosection¹⁰ for a representative Ca-poor serpentinite from CdA (sample Al98-05a²⁵) with sulphur and carbon content from ref³⁶ (these values were confirmed by new, duplicate analyses) and ferric iron from this work (see Supplementary Table 1). The vertical line represents the intrinsic deserpentinisation for a fixed O₂ content of the system (Path I), corresponding to the bulk O₂ for sample Al98-05a (15.602 mol/kg is used instead of the measured 15.672 mol/kg for better agreement with the observed sequence of mineral assemblages at CdA; it likely reflects the amount of ferric iron in antigorite, not accounted for in the available solid solution models⁸⁶). The horizontal path (IIa) shows schematically the evolution if the system is externally infiltrated by fluids equilibrated with metasedimentary rocks with a high reducing capacity (graphite-bearing metapelite). The quantitative evolution along path IIa is shown in Figure 2b in the main text (see also Supplementary Figure 5 for the evolution of the speciation in the fluid). The path IIb corresponds to the prograde evolution after the graphite metapelite-infiltrated deserpentinisation potentially followed by CdG.

<figure><img src="image_6.png" /></figure>

Numbered fields

1 Atg Ol Chl Cpx Mag Po arag F

2 Atg Ol Chl Cpx Mag Po Dol F

3 Atg Ol Chl Tr Mag Po F

4 Atg Ol Chl Tr Mag Po Gr F

5 Atg Ol Opx Chl Tr Mag Po Gr F

6 Atg Ol Chl Tr Mag Po F

7 Atg Ol Opx Chl Tr Mag Po Py F

8 Atg Ol Opx Chl Tr Mag Py Anh F

9 Ol Opx Chl Tr Mag Po Py F

10 Ol Opx Chl Tr Mag Py Anh F

11 Ol Opx Chl Cpx Mag Po Py F

12 Ol Opx Chl Cpx Mag Po Gr F

13 Ol Opx Chl Cpx Mag Gr F

14 Ol Opx Chl Cpx Mag Py F

15 Atg Ol Tr Chl Mag Py F

16 Atg Ol Cpx Mag Py F

17 Atg Ol Tr Mag Py F

18 Atg Ol Opx Tr Mag Py F

19 Atg Ol Opx Tr Mag Anh F

20 Atg Ol Opx Tr Mag Mgs Anh F

21 Atg Ol Tr Mag Mgs Anh F

22 Atg Ol Cpx Mag Anh Dol F

23 Atg Ol Cpx Mag Py Anh Dol F

24 Atg Ol Opx Tr Chl Mag Hem Anh F

25 Atg Ol Opx Tr Mag Hem Anh F

26 Atg Ol Tr Mag Mgs Hem Anh F

27 Atg Ol Cpx Mag Dol Hem Anh F

28 Atg Opx Tr Mag Hem Anh F

29 Atg Tr Tlc Mag Mgs Hem Anh F

30 Atg Ol Opx Tr Mag Mgs Hem F

31 Atg Ol Tr Mag Mgs Hem F

32 Atg Tr Mag Dol Hem Cal Anh F

33 Atg Tr Tlc Mag Dol Hem Anh F

34 Atg Tr Mag Dol Hem Anh F

35 Atg Cpx Mag Py Dol Anh F

36 Atg Cpx Mag Po Py arag F

37 Atg Ol Cpx Mag Po arag F

38 Atg Cpx Mag Po Py Cal F

39 Atg Cpx Mag Py Dol Cal Anh F

40 Atg Cpx Mag Py Cal Anh F

41 Atg Mag Py Dol Cal Anh F

42 Atg Chl Brc Cpx Mag Po Cal F

43 Atg Chl Brc Cpx Mag Po F

44 Atg Chl Brc Mag Po Gr F

45 Atg Chl Brc Cpx Mag Po Gr F

46 Atg Chl Mag Po Cal Gr F

47 Atg Cpx Mag Dol Cal Hem Anh F

48 Atg Tr Mag Dol Cal Hem Anh F
**Figure 3.** Absolute concentration of oxygen-sensitive components in the solids (expressed as mol/kg of rock Al98-05a) for the pseudosection shown in Supplementary Figure 2 (see also Figure 2 in the main text for the contouring of oxygen fugacity relative to the buffer FMQ). All panels were computed from the absolute amounts of mineral phases containing oxygen-sensitive components and their concentration in pure and solid solution endmembers from WERAMI outputs. Computations used the back-calculated method for fluid speciation in PerpleX, except for pannel S⁶⁺ that was computed using the lagged speciation method that allows mass balance constrains in the region below the complete serpentinite dehydration. The last panel shows the redox budget referred to the whole mantle reference redox state (Methods).

<figure><img src="image_7.png" /></figure>
**Figure 4.** Evolution of key parameters along intrinsic deserpentinisation (intrinsic path I in Figure 2, red vertical line). (a) $X_{Mg}$ in antigorite and their dehydrated product olivine and orthopyroxene; (b) H$_2$O content hosted in the solid phases; (c) and (d) bulk sulphur and carbon contents retained in the solid phases; (e) evolution of the oxygen fugacity relative to the FMQ buffer. The blue region corresponds to the temperature conditions of dehydration in Cerro del Almirez (CdA). Note that none of the observables ($X_{Mg}$, S and C content, see Fig. 2b in the main text) agrees with the model predictions along with the intrinsic deserpentinisation model.

<figure><img src="image_8.png" /></figure>
**Figure 5.** a. Fluid speciation evolution during the infiltration of a partially dehydrated serpentinite with a fluid equilibrated with a graphite-bearing metapelite at 650 °C and 1.7 GPa. The solvent species H₂S and CO₂ are expressed as mole fraction, whereas the solute species are expressed as molality (mol/kg). The main oxidising species (HSO₄⁻) is represented on a linear scale whereas other less abundant species are on a logarithmic scale.

b. Modal (vol.%) pyrite abundance in the metaperidotite induced by graphite metapelite fluid infiltration.

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 6.** Intrinsic deserpentinization oxygen fugacity conditions (a) relative to the FMQ buffer, Δlog₁₀/O₂[FMQ] and in absolute values (b) for a representative metaserpentinite (sample Al98-05a, see Supplementary, Table 1) in a pressure-temperature space. Yellow dots are pressure-temperature deserpentinisation conditions at the slab surface for a worldwide compilation of subduction zones⁸¹,⁸², geographically located in (c), (d) Difference between the slab surface intrinsic deserpentinisation and the mantle wedge oxygen fugacity (expressed as Δlog₁₀/O₂[FMQ]) along the 1000°C isotherm.</figcaption></figure>
**Figure 7.** Effect of peridotite mantle wedge depletion on the Δlog₁₀/O₂[FMQ] evolution during fluxing by different types of deserpentinisation slab fluids. The left panel shows the results for (a) a depleted MORB mantle wedge source<sup>83</sup> (DMM) and (b) an ultradepleted mantle wedge source<sup>84</sup>, both for a hot (Central Cascadia) and a cold (Tonga) subduction zone. The mantle wedge Δlog₁₀/O₂[FMQ] evolution of the mantle wedge fluxed by fluids sourced from intrinsic deserpentinisation and sediment-infiltrated deserpentinisation fluids produced by the infiltration of 12 mol/kg of fluids equilibrated with GLOSS and graphite-bearing metapelite. Right panels are the contours of the initial Δlog₁₀/O₂[FMQ] of the mantle wedge for a DMM (upper panel) and ultradepleted source (lower panel) before fluxing with slab fluids; shown as yellow dots are the initial Δlog₁₀/O₂[FMQ] conditions at the 1000 °C of the mantle wedge for a worldwide compilation of dehydration conditions in hot to cold subduction zones. Central Cascadia (hot subduction) and Tonga (cold subduction) correspond, respectively, to a minimum (2.4 GPa) and maximum pressure (3.3 GPa) for the dehydration of serpentinisation at the slab surface. Note that the initial Δlog₁₀/O₂[FMQ] conditions depend on the thermal regime of the subduction zone and the depletion of the mantle wedge source, but have a subsidiary effect on the Δlog₁₀/O₂[FMQ] evolution of the mantle wedge during fluxing of different types of deserpentinisation fluids.

<figure><img src="image_12.png" /></figure>
**Figure 8.** (a) Evolution of the Δlog₁₀/O₂[FMQ] and the concentration of HSO₄⁻—relative to the intrinsic deserpentinisation fluid (ID)— during infiltration of fluids equilibrated with metasedimentary rocks with a high reducing capacity (graphite-bearing metapelite) for a worldwide compilation of subduction zones⁸¹,⁸² (colour-coded for the pressure at which the serpentinite dehydrates at the slab surface, Source Data). (b) The capacity of these modified, serpentinite-derived fluids (empty dots in a) to oxidise the mantle wedge on top of the slab at near wet-solidus conditions is computed for the hottest (Central Cascadia) and coldest (Tonga) subduction zones. A minimum value range of Δlog₁₀/O₂[FMQ] inferred for oxidised IAB source and recorded by high-pressure metasomatized mantle atop of the slab⁵²,⁸⁷⁵²,⁸⁷ is given as a horizontal blue-shaded range. Sediment (graphite-bearing)-serpentinite derived fluids have a variable capacity to oxidise the mantle wedge for hot and cold subduction zones, a variable potential that is directly related to the contrasting solubility of HSO₄⁻ for the two extreme thermal cases. The metasomatized mantle wedge has an initially depleted composition⁸³. Squares and stars on the red and blue lines indicate the condition range limits at which pyrrhotite (Po), or anhydrite (anh) are the stable minerals hosting S in the rocks. For an ultradepleted MORB mantle, see Supplementary, Fig. 7. For interaction with sediments with low reducing capacity (GLOSS), see Fig. 3 in the main text.

<figure><img src="image_13.png" /></figure>
649 Supplementary Tables

650 Supplementary Table 1. Bulk rock compositions used for thermodynamic modelling.

651 Supplementary Table 2. Fluid composition and speciation for CdA peak metamorphic
652 conditions at intrinsic dehydration and after interaction (12 mol/kg) with sediment with high
653 reducing capacity

654

655 Source data

656 Supplementary file (csv). Worldwide subduction zone database<sup>81</sup> used to compute the
657 pressure and temperature conditions for the slab surface deserpentinisation<sup>82</sup>. These pressure
658 and temperature conditions are used to compute the intrinsic fluid chemistry and the fluid
659 composition for high and low-reducing capacity sediments (graphite and GLOSS
660 respectively) that are used for infiltration at the same serpentinite dehydration pressure and
661 temperature conditions. Main species are given for the three cases: intrinsic (<code>_intr</code>) and for
662 infiltration of 12 mol/kg for the cases of graphite-bearing and GLOSS sediments derived
663 fluids (<code>_graph</code> and <code>_gloss</code>). Fluid bulk compositions are given in mol per formula unit of fluid
664 and species concentrations are given in mol/kg. This database can be generated for other
665 degrees of infiltration using a Jupyter notebook available upon request.
Supplementary Files

This is a list of supplementary files associated with this preprint. Click to download.

* ExtendedDataTable2.pdf

* ExtendedDataTable1.pdf

* ExtendedDataFigure1.pdf

* ExtendedDataFigure2.pdf

* ExtendedDataFigure3.pdf

* ExtendedDataFigure4.pdf

* ExtendedDataFigure5.pdf

* ExtendedDataFigure6.pdf

* ExtendedDataFigure7.pdf

* ExtendedDataFigure8.pdf