# Isotopic evidence for iron mobility during subduction

Baptiste Debret, M.-A. Millet, M.-L. Pons, P. Bouilhol, E. Inglis, H. Williams

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Baptiste Debret, M.-A. Millet, M.-L. Pons, P. Bouilhol, E. Inglis, et al. Isotopic evidence for iron mobility during subduction. Geology, 2016, 44 (3), pp.215-218. 10.1130/G37565.1. hal-03976644

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<figure><img src="image_1.png" /></figure>
# Isotopic evidence for iron mobility during subduction

B. Debret<fnref n="1" />,<fnref n="2" />*, M.-A. Millet<fnref n="1" />, M.-L. Pons<fnref n="1" />,<fnref n="2" />, P. Bouilhol<fnref n="1" />, E. Inglis<fnref n="1" /> and H. Williams<fnref n="1" />,<fnref n="2" />

## ABSTRACT

Subduction zones are one of the most important sites of chemical interchange between the Earth's surface and interior. One means of explaining the high Fe³⁺/ΣFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SO₄), carbonate (CO₃), and/or iron (Fe³⁺) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (δ⁵⁶Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R<fnref n="2" /> = 0.72) is observed between serpentinite bulk δ⁵⁶Fe and Fe³⁺/ΣFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SO₄ or Fe(II)-Cl₂ species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids.

## INTRODUCTION

Magmas erupted at subduction zones display high ratios of oxidized to total iron (Fe³⁺/ΣFe) and are enriched in volatiles relative to other mantle-derived magmas (e.g., Kelley and Cottrell, 2009). It has therefore been suggested that the source region of arc magmas has been modified by oxidized and volatile-rich fluids released from the subducting slab. Possible oxidizing agents include sulfate- and/or carbonate-bearing fluids derived from subducted sediments or serpentinites (Kelley and Cottrell, 2009; Frezzotti et al., 2011; Evans, 2012; Debret et al., 2014a, 2015), as well as Fe³⁺ transferred directly via supercritical fluids and brines (Mungall, 2002; Kelley and Cottrell, 2009), which have been shown to mobilize other trivalent, normally fluid-immobile elements (Kessel et al., 2005). Evidence in support of direct Fe³⁺ and/or SO₄<fnref n="2" />⁻ transfer includes positive correlations between Fe³⁺/ΣFe and trace element ratios indicative of slab-derived fluids (e.g., Ba/La) in arc magma melt inclusions (Kelley and Cottrell, 2009).

The devolatilization of serpentinite within the subducting slab and the fluids released play a fundamental role in mantle wedge redox evolution (Evans, 2012) and arc magma genesis (Spandler and Pirard, 2013). At mid-oceanic ridges or oceanic-continent transition zones, serpentinites are formed by the hydration and oxidation of the oceanic lithosphere, which leads to the replacement of olivine (Ol) and orthopyroxene by Fe³⁺-rich lizardite, which contains as much as 13% water, and magnetite, increasing bulk-rock Fe³⁺/ΣFe (e.g., Andreani et al., 2013; Klein et al., 2014). During subduction, the lizardite (Liz, low-temperature form of serpentine) to antigorite (Atg, high-temperature form of

serpentine) transition is accompanied by magnetite dissolution and a decrease in serpentine mineral Fe³⁺/ΣFe (Schwartz et al., 2013; Debret et al., 2014a). At higher pressures and temperatures, antigorite breakdown to secondary olivine results in the loss of H₂O (e.g., Ulmer and Tromsdorff, 1995) and the growth of other Fe<fnref n="2" />⁺-rich minerals (Debret et al., 2015). The major consequence of serpentinite prograde metamorphism during subduction is thus a net decrease in bulk-rock Fe³⁺/ΣFe. It is, however, unknown whether this reflects (1) the direct loss of Fe³⁺ during prograde metamorphism and/or (2) the oxidation and loss of other redox-sensitive species (e.g., SO₄<fnref n="2" />⁻) to the mantle wedge.

To date there has been no direct means of tracing the loss of oxidizing components or Fe from the subducting slab and their transfer to the mantle wedge (Evans, 2012). New tools that can be used to trace and quantify the mobility of Fe during subduction-related devolatilization and metamorphism are thus required. Iron stable isotopes can serve as such a tracer, as significant isotopic variations can only be achieved by either the addition or loss of Fe-bearing components. Theory predicts that stable isotope variations will be driven by changes in oxidation state, coordination, and bonding environment (Polyakov and Mineev, 2000; Fujii et al., 2011), in particular for Fe the presence of Cl⁻ and SO₄<fnref n="2" />⁻ anions (Hill et al., 2010). Here we explore the mobility of Fe during slab devolatilization with a Fe stable isotope study of subducted serpentinites from Western Alps metaophiolites. We show a striking negative correlation between the Fe isotope compositions (δ⁵⁶Fe) of serpentinites and bulk-rock Fe³⁺/ΣFe, providing the first direct evidence for the open-system behavior of iron in serpentinites during subduction-related prograde metamorphism.

## SELECTED SAMPLES AND RESULTS

Alpine metaophiolites are interpreted as remnants of oceanic lithosphere formed and serpentinized in magma-poor settings before being metamorphosed and devolatilized at various pressure-temperature (P-T) conditions during subduction (Fig. 1A; e.g., Hattori and Guillot, 2007; Debret et al., 2013). We selected a set of 29 representative serpentinite samples encompassing the full range of prograde metamorphic grades for this study (Table DR1 and Fig. DR1 in the GSA Data Repository<fnref n="1" />). These samples have been characterized in previous petrological and geochemical studies (Schwartz et al., 2013; Debret et al., 2013; Lafay et al., 2013). They display the progressive replacement of lizardite by antigorite (Atg/Liz- to Atg-serpentinites; Fig. 1A), and the first stages of serpentinite dehydration at eclogite facies (Ol/Atg-serpentinites). The Atg-serpentinites are characterized by low amounts of magnetite relative to Atg/Liz-serpentinites (Debret et al., 2014a) and can display equilibrated hematite-magnetite assemblages (Fig. DR2). The Ol/Atg-serpentinites are generally present as metamorphic veins and shear zones composed of olivine and antigorite, and are interpreted as high-permeability reaction zones where the fluids released during serpentinite devolatilization have been localized (Debret et al., 2013). For reference we also analyzed a suite of Alpine slightly serpentinized peridotites (SSP), unmetamorphosed lizardite (Liz-) serpentinites, and abyssal serpentinites, which are considered to be representative of the presubduction lithospheric hydrated mantle. No retrograde phases (e.g., talc, chrysotile, amphibole) are observed in the studied sample suite, suggesting that the samples are poorly affected by retrograde metamorphism.

Bulk-rock Fe isotope (δ⁵⁶Fe), Fe³⁺/ΣFe, and trace element concentration data for the studied serpentinites are reported in Tables DR1 and DR2. There is a progressive increase in serpentinite δ⁵⁶Fe value with metamorphic grade (Fig. 1; Fig. DR3): the mean δ⁵⁶Fe value of the Atg-serpentinites (+0.08‰ ± 0.11‰, 2 standard deviations, sd) and Atg/Liz-serpentinites (+0.07‰ ± 0.07‰) is greater than that of the Liz-serpentinites (δ⁵⁶Fe = -0.02‰ ± 0.14‰), SSP (δ⁵⁶Fe = -0.03‰ ± 0.15‰), and abyssal serpentinites (δ⁵⁶Fe = -0.05‰ ± 0.07‰, this

<footnote n="1">¹Department of Earth Sciences, Durham University, Elvet Hill, Durham DH1 3LE, UK</footnote>

<footnote n="2">²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK</footnote>

<footnote n="1">¹GSA Data Repository item 2016064, geological settings, methods, seven supplemental figures, and iron isotope and trace element data, is available online at www.geosociety.org/pubs/ft2016.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.</footnote>

<footnote>*E-mail: ba.debret@gmail.com</footnote>
<figure><img src="image_2.png" /></figure>

**Figure 1.** Distribution of δ<sup>56</sup>Fe values in serpentinites during subduction. **A:** Schematic pressure-temperature (P-T) path of the Ligurian oceanic lithosphere showing the transitions from lizardite (Liz) to antigorite (Atg) and olivine (Ol) + antigorite assemblages. SSP—slightly serpentinized peridotite. The maximum temperatures recorded by eclogitic ophiolites are 580–620 °C (Schwartz et al., 2013), much lower than expected solidus for these rocks. Hot (~8 °C/km) and cold (~4 °C/km) geotherms are from Peacock and Wang (1999). Serp-—serpentine; Fo—forsterite; En—enstatite; V—vapor. **B:** Iron isotope compositions of bulk peridotites and serpentinites from Western Alps ophiolites (sd—standard deviation).

study; δ<sup>56</sup>Fe = +0.01‰ ± 0.08‰, Craddock et al., 2013). Furthermore, Fe isotopes are negatively correlated with serpentinite Fe<sup>3+</sup>/ΣFe (Fig. 2), suggesting that prograde metamorphism in serpentinite and the associated decrease in Fe<sup>3+</sup>/ΣFe is accompanied with the release of a low-δ<sup>56</sup>Fe fluid. In contrast, the eclogite facies Ol/Atg-serpentinites are characterized by lower δ<sup>56</sup>Fe values (δ<sup>56</sup>Fe = -0.03‰ ± 0.06‰).

DISCUSSION

Although the progressive increase in δ<sup>56</sup>Fe
during prograde metamorphism is suggestive
of open-system behavior of iron during slab

devolatilization, several processes must be first
taken into account. These are (1) the mobil-
ity of Fe and associated isotopic fractionation
during the initial ocean-floor serpentinization
of lithosphere, and (2) preexisting protolith Fe
isotope heterogeneity. The Liz-serpentinites dis-
play a broad range in δ<sup>56</sup>Fe values, which range
from -0.098‰ to +0.047‰ and are similar to
those of SSP, which range from -0.069‰ to
+0.063‰. Both sets of values overlap with the
δ<sup>56</sup>Fe values of abyssal serpentinites (-0.094‰
to +0.108‰; Craddock et al., 2013), and may
be considered representative of presubduction
oceanic lithosphere. It is important that, while

ridge-axis serpentinization processes are known
to increase Fe³⁺/ΣFe (Evans, 2008), the δ⁵⁶Fe
values of oxidized and hydrous Liz-serpentini-
nites are similar to those of the comparatively
reduced and anhydrous SSP and are uncorre-
lated with Fe³⁺/ΣFe (Fig. DR4). This suggests
that Fe behaves conservatively during ocean-
floor serpentinization and that no appreciable Fe
stable isotope fractionation takes place during
this process. Previous studies have demonstrated
that a relationship between bulk δ⁵⁶Fe values
and rock fertility exists, with pyroxene-rich and
less depleted peridotites displaying isotopically
heavier compositions (Williams et al., 2005). In
Alpine ophiolites, serpentinite protolith fertility
can be assessed using fluid-immobile elements
poorly affected by serpentinization (e.g., Al₂O₃/
SiO₂, Zr/Nb; Bodinier and Godard, 2013). As
illustrated in Figure 3, the δ⁵⁶Fe values of Alpine
Liz-serpentinites and SSP form broad positive
arrays with Al₂O₃/SiO₂ and Zr/Nb, which pro-
vide evidence for some level of source fertility
control on protolith δ⁵⁶Fe. Similar correlations
between the δ⁵⁶Fe values of Atg/Liz- or Atg-ser-
pentinites and indices of peridotite fertility are
present, but are demonstrably offset to overall
heavier δ⁵⁶Fe values (Fig. 3).

The comparatively heavy δ⁵⁶Fe values of the Atg/Liz- and Atg-serpentinites relative to the SSP and Liz-serpentinites and the correlation between bulk serpentinite δ⁵⁶Fe and Fe³⁺/ΣFe (Fig. 2) must therefore relate to the loss or addition of Fe-bearing fluids during prograde metamorphism. Sediment dehydration releases a significant amount of fluids and volatiles that can potentially interact with serpentinites and modify their isotopic signatures during subduction (Deschamps et al., 2010). However, no correlations between δ⁵⁶Fe and elemental tracers of sediment-serpentinite interaction (e.g., As, Sb, Cs) are present (Fig. DR5), suggesting that interactions between sediment-derived melts or fluids and serpentinites cannot explain the heavy δ⁵⁶Fe values of the Atg/Liz- and Atg-serpentinites. Furthermore, sediment melting requires an elevated subduction geotherm (e.g., Bouilhol et al., 2015), which is inconsistent with the inferred P-T conditions of Alpine ophiolites (Fig. 1). The most straightforward means of interpreting the δ⁵⁶Fe and Fe³⁺/ΣFe correlation is thus the release of isotopically light Fe-bearing fluid during prograde serpentinite devolatilization. In agreement with this interpretation, the Ol/Atg-serpentinites, which are interpreted as high-permeability reaction zones where fluids released during prograde metamorphism have been concentrated, display isotopically light δ⁵⁶Fe values relative to the Atg-serpentinites (Fig. 1) and high concentrations of fluid-mobile elements (Debret et al., 2013).

Iron stable isotope fractionation is usually
attributed to redox effects and experimen-
tal and theoretical studies that predict that, at

**Figure 2.** Increase in δ<sup>56</sup>Fe and Fe<sup>3+</sup>/ΣFe ratio in serpentinites during subduction. Bulk sample iron isotope composition versus Fe<sup>3+</sup>/ΣFe in subducted Alpine Liz-, Atg/Liz-, and Atg-serpentinites are compared to the one of abyssal serpentinites (small white circles). The error crosses are 2 standard deviations; the long-term reproducibility is shown for reference as an error bar in the lower left corner of this plot. The slope of the correlation is -0.92 ± 0.28 and the 95% confidence interval is represented by two lines (mean square of weighted deviates = 3.6).

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. Evolution of δ⁵⁶Fe as a function of peridotite fertility. Iron isotope compositions of Alpine slightly serpinintized peridotite (SSP, white squares), Liz-serpentinites (white circles) and subducted Atg/Liz-serpentinites (pale gray circles), and Atg-serpentinites (gray circles). A: Plotted against Al₂O₃/SiO₂. B: Plotted against Zr/Nb. Subducted serpentinites are comparatively enriched in heavy iron relative to SSP and Liz-serpentinites (black arrows). Major and trace element data are from Debret et al. (2013), Lafay et al. (2013), and this study.</figcaption></figure>

equilibrium, Fe²⁺-bearing phases should be isotopically lighter than Fe³⁺-bearing phases (Polyakov and Mineev, 2000; Hill et al., 2010). This is in apparent contradiction with the inferred loss of a low-δ⁵⁶Fe fluid and the inverse correlation between bulk serpentine δ⁵⁶Fe and Fe³⁺/ΣFe. One possible explanation is that the isotopically light composition of the fluids reflects kinetic (disequilibrium) isotope fractionation and the preferential mobility of the lighter isotope. In this case, the inverse correlation between δ⁵⁶Fe and Fe³⁺/ΣFe could reflect the direct loss of Fe³⁺ during serpentine prograde metamorphism. However, the mobility of Fe³⁺ relative to Fe²⁺ in aqueous fluids and brines is known to be low (e.g., Ding and Seyfried, 1992) and unrealistic amounts of Fe³⁺ (~3 wt% Fe₂O₃) would need to be lost to explain the observed decrease in Fe³⁺/ΣFe from 0.7 to 0.5 (Fig. DR6). An alternative explanation is that isotopically light Fe is lost in the form of Fe(II)-S or Fe(II)-Cl complexes, as theory predicts that these complexes should show a preference for isotopically light Fe (Hill et al., 2010). In agreement with

this scenario, subducted Alpine serpentinites have low S and Cl contents relative to abyssal serpentinites, providing evidence in support of the mobility of these elements in serpentinite-derived fluids (e.g., Alt et al., 2012; Debret et al., 2014b). This is confirmed by the presence of Cl- and S-rich polyphase fluid inclusions preserved within metamorphic olivine and garnet from Alpine metaophiolites (e.g., Scambelluri et al., 2015). The release of such fluids during serpentine devolatilization can have a large impact on element mobility, since many nominally immobile elements are mobile in Cl-rich fluids (e.g., Rapp et al., 2010).

Perhaps most important, the proposed loss of isotopically light Fe associated with SOₓ species during serpentine devolatilization may also have a large effect on mantle wedge redox evolution (e.g., in the case of SO₄²⁻, 1 mol of S⁶⁺ can oxidize 8 mol of Fe²⁺ as SO₄²⁻ is reduced to S²⁻ in the mantle wedge). The S budget of abyssal serpentinites is dominantly controlled by sulfides (e.g., Alt and Shanks, 2003). The decrease in serpentine S concentration with prograde metamorphism therefore suggests that serpentine dehydration is associated with sulfide breakdown and the release of sulfur (Hattori and Guillot, 2007). The presence of equilibrated hematite-magnetite assemblages (Fig. DR2) in the most depleted, low Fe³⁺/ΣFe and heavy δ⁵⁶Fe serpentinites shows that S transport in serpentine-derived fluids must have taken place at high oxygen fugacity (f₉₀), close to the hematite-magnetite buffer (Debret et al., 2015; Tumiati et al., 2015). At such P-T-f₉₀ conditions, sulfur will be mobilized as SO₄²⁻ (aq—aqueous) rather than reduced species such as HS⁻ (aq) (Fig. DR7). This is consistent with recent observations of SO₄²⁻ in

diamond-bearing fluid inclusions from Western Alps metaophiolites (Frezzotti et al., 2011). In this scenario, providing evidence in support of the mobility of these elements in serpentinite-derived fluids (e.g., Alt et al., 2012; Debret et al., 2014b). This is confirmed by the presence of Cl- and S-rich polyphase fluid inclusions preserved within metamorphic olivine and garnet from Alpine metaophiolites (e.g., Scambelluri et al., 2015). The release of such fluids during serpentine devolatilization can have a large impact on element mobility, since many nominally immobile elements are mobile in Cl-rich fluids (e.g., Rapp et al., 2010).

$$4 \text{Fe}_2\text{O}_{3(\text{serpentine-magnetite})} + \text{FeS} = 8 \text{FeO} + \text{Fe-SO}_{4(\text{aq})} \quad (1)$$

Given that typical ocean-floor serpentinites and eclogite facies Atg serpentinites have Fe³⁺/ΣFe ratios of ~0.7 and ~0.5, respectively, and both contain ~8 wt% of Fe₂O₃ (Fig. DR6), Equation 1 predicts that as much as ~800 ppm sulfur and a minor quantity of Fe (~0.1 wt%) will be released in serpentine-derived fluids. This value, although speculative, is broadly consistent with the observed decrease in sulfur content (Hattori and Guillot, 2007; Alt et al., 2012; Debret et al., 2014b) and the relatively constant Fe₂O₃ (Fig. DR6) of serpentinites during subduction. This is, however, a minimum estimate. For example, if S was originally present as S⁻ and released as SO⁻, S₂O₃²⁻, or SO₃²⁻, then the amount of S released would be significantly larger for the same amount of Fe loss. Our results therefore suggest that slab serpentine devolatilization is associated with Fe reduction and the oxidation and mobilization of sulfides, and potentially other reduced species. The migration of these fluids from the slab to the slab-mantle interface or mantle wedge and the reduction of slab fluid SOₓ to (Fe)S²⁻ and concomitant oxidation of mantle wedge Fe(II) to Fe(III) may therefore provide an oxidized mantle source region for primary arc magmas (e.g., Kelley and Cottrell, 2009; Fig. 4).

<figure><img src="image_5.png" /><figcaption>Figure 4. Schematic diagrams (not to scale) illustrating the redox and isotopic evolution of slab serpentinites and released fluids during subduction. A: Subduction zone. B: Subducting lithosphere. Prograde metamorphism enhanced the formation of antigorite (Atg) and then olivine (Ol). Liz—lizardite. These metamorphic reactions occur at high oxygen fugacity (f₉₀) and result in the release of isotopically light Fe from the serpentinites in the form of Fe(II)-SOₓ and/or Fe(II)-Cl⁻ complexes, with the Fe isotopically heavy residue returning to the deep mantle. FMQ—fayalite-magnetite-quartz. *See Evans, 2008, and references therein.</figcaption></figure>
ACKNOWLEDGMENTS

We thank G. Nowell (Durham University, UK) for tech-
nical support; L. Cottrell, S. Guillot, S. Klimm, and
N. Malaspina for constructive reviews; and J. Brendan
Murphy for his careful editorial handling. This work
was supported by a Natural Environment Research
Council (NERC) Deep Volatiles Consortium Grant
(NE/M000303/1) and a European Research Council
(ERC) Starting Grant (HabitablePlanet: 306655) to Wil-
liams. Williams also acknowledges NERC Advanced
Fellowship NE/F014295/1. Bouilhol acknowledges
support from an ERC Starting Grant (MASE; 279828).

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Manuscript received 25 November 2015

Revised manuscript received 20 January 2016

Manuscript accepted 20 January 2016

Printed in USA