Baptiste Debret
email: ba.debret@gmail.com

Carlos J Garrido

Marie-Laure Pons

Pierre Bouilhol

Edward Inglis

Vicente López Sánchez-Vizcaíno

Helen Williams

Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zonesa

Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ 56 Fe and δ 66 Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6-1.9 GPa and 680-710 °C). The mean δ 56 Fe of all the Cerro del Almirez metaserpentinites (+0.05 ± 0.01 ‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03 ‰). A positive correlation between δ 56 Fe and indices of peridotite protolith fertility (e.g., Al 2 O 3 /SiO 2 ) suggests that the δ 56 Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34-67 ppm) and isotope signatures (δ 66 Zn = +0.18 -+0.55 ‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ 66 Zn = +0.16 ± 0.06 ‰). The Atgserpentinites ([Zn] = 34-46 ppm; δ 66 Zn = +0.23 ± 0.06 ‰) display similar [Zn] and δ 66 Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ 66 Zn increase in transitional lithologies ([Zn] = 45-67 ppm; δ 66 Zn = +0.30 ± 0.06 ‰) and Chl-harzburgites with granofels ([Zn] = 38-Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29-45 ppm) and a positive correlation exists between [Zn] and δ 66 Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ 66 Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ 66 Zn record the interaction of the transitional lithologies and the Chlharzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO 3 (H 2 O) 5+ ] or [ZnCO 3 (H 2 O) 3 ]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ 66 Zn signature and hence explain the δ 66 Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ 66 Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.

Introduction

In subduction zones, tracing and quantifying the fluxes of redox-sensitive elements, such as those of iron, carbon or sulfur, between the slab, the mantle wedge and the deep mantle is crucial for understanding Earth chemical evolution and habitability (e.g., [START_REF] Duncan | Rise of Earth's atmospheric oxygen controlled by efficient subduction of organic carbon[END_REF]. Among subducted lithologies, serpentinites are significant reservoirs of water and redox-sensitive elements [START_REF] Alt | The role of serpentinites in cycling of carbon and sulfur : Seafloor serpentinization and subduction metamorphism[END_REF]. Their dehydration at depth could, therefore, trigger large scale metasomatism of the mantle wedge area and influence redox sensitive and volatile element cycles between the Earth's interior and surface [START_REF] Debret | Highly oxidising fluids generated during serpentinite breakdown in subduction zones[END_REF]Cannao and Malaspina, 2018;[START_REF] Iacovino | Aqueous fluids are effective oxidizing agents of the mantle in subduction zones[END_REF]. However, tracing the cycling of these elements in subduction zones remains an active research frontier and is subject to considerable uncertainty (e.g., [START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up[END_REF][START_REF] Plank | Subducting carbon[END_REF][START_REF] Menzel | Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones[END_REF].

One of the main challenges is determining the mobility and speciation of the redox sensitive and volatile elements in fluids released during prograde metamorphism in subduction zones. Although several studies have investigated this problem using traditional stable isotopes (e.g., carbon and sulfur, C and S), the significant overlap between the C and S isotope compositions of seafloor-altered protoliths and highpressure rocks suggests that these isotope systems are insensitive tracers of geochemical exchanges between the slab and the mantle wedge in subduction zones (e.g., [START_REF] Alt | The role of serpentinites in cycling of carbon and sulfur : Seafloor serpentinization and subduction metamorphism[END_REF]. Non-traditional stable isotopes, such as Fe (δ 56 Fe) and Zn (δ 66 Zn), can, however, complement traditional stable isotope systems by providing key constraints on elemental mobility and mass balance as Fe and Zn equilibrium fractionation between different phases (serpentine minerals, Fe-oxides, sulfides, carbonate and fluids) is driven by contrasts in element bonding environment, such as co-ordination chemistry and oxidation state [START_REF] Polyakov | The use of Mossbauer spectroscopy in stable isotope geochemistry[END_REF][START_REF] Schauble | Fractionation theory to new systems[END_REF][START_REF] Fujii | Density functional theory estimation of isotope fractionation of Fe, Ni, Cu, and Zn among species relevant to geochemical and biological environments[END_REF]. Furthermore, the effects of seafloor alteration (e.g., serpentinization) on Fe and to a lesser extent on Zn isotope systematics of mantle peridotites are comparatively minor [START_REF] Craddock | Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth[END_REF]Debret et al., 2018a), such that subduction-related fractionation processes should be relatively straightforward to identify. Recent studies have successfully applied Fe and Zn stable isotopes in subduction settings to identify processes associated with high-pressure metamorphism in metabasites, meta-serpentinites and metasedimentary rocks (e.g., [START_REF] Debret | Isotopic evidence for iron mobility during subduction[END_REF][START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF][START_REF] Inglis | The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites[END_REF][START_REF] Korh | Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)[END_REF]Debret et al., 2018b;[START_REF] Turner | Sub-arc xenolith Fe-Li-Pb isotopes and textures tell tales of their journey through the mantle wedge and crust[END_REF][START_REF] Gerrits | Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet[END_REF][START_REF] Huang | Iron and magnesium isotopic compositions of subductionzone fluids and implications for arc volcanism[END_REF][START_REF] Chen | Tracking Fe mobility and Fe speciation in subduction zone fluids at the slab-mantle interface in a subduction channel: A tale of whiteschist from the Western Alps[END_REF][START_REF] Debret | Redox transfer at subduction zones: insights from Fe isotopes in the Mariana forearc[END_REF]. In these studies, Fe and Zn stable isotopic variations were attributed to redox reactions and associated metal mobility in sulfur, carbon and chlorine-bearing fluids during metamorphic processes. These results highlight the potential of these isotope systems as powerful tools that can be used to trace redox-sensitive element mobility and quantify redox budget variations associated with prograde metamorphism in subduction zones.

Here we present a study of Fe and Zn isotopes in samples from the eclogite-facies ultramafic massif of Cerro del Almirez (Nevado-Filábride Complex, Betic Cordillera, S. Spain). This massif preserves a unique reaction front between antigorite-bearing serpentinite (Atg-serpentinite) and their high-pressure dehydration product, chlorite-bearing harzburgite (Chl-harzburgite) [START_REF] Trommsdorff | High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain[END_REF][START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. Our new data on well-characterized Chl-harzburgite and their precursor Atg-serpentinite allow us to investigate the role of seafloor serpentinization versus subduction dehydration on the distribution and mobility of metals. We show that the change in δ 66 Zn observed across the contact between Atg-serpentinites and Chl-harzburgites can be explained by the open-system percolation of external fluids. In this scenario, the δ 66 Zn of the Chl-harzburgites are most consistent with metasomatism by carbonate-bearing fluids released during metasedimentary rock devolatilization at high pressures and temperatures.

Geological setting

The Cerro del Almirez ultramafic massif lies in the Nevado-Filábride Complex, a subduction complex composed of metapelites, marbles, orthogneisses, meta-serpentinites, and metabasites, exposed in the core of an E-W elongated, antiformal dome structure in the internal zones of the Betics orogenic belt (SE Spain) (Fig. 1a) (Gómez-Pugnaire et al., 2019, and references therein). The Nevado-Filábride Complex results from the subduction of the Mesozoic extended SE Iberian margin beneath the Miocene extended Alborán domain during the westward rollback of the Alpine Tethys slab in the mid-Miocene (15-18 Ma) (López [START_REF] Sánchez-Vizcaíno | Middle Miocene high-pressure metamorphism and fast exhumation of the Nevado-Filabride Complex, SE Spain[END_REF][START_REF] Behr | Kinematic and thermal evolution during two-stage exhumation of a Mediterranean subduction complex[END_REF][START_REF] Booth-Rea | Continental subduction, intracrustal shortening, and coeval upper-crustal extension: PT evolution of subducted south Iberian paleomargin metapelites (Betics, SE Spain)[END_REF][START_REF] Gómez-Pugnaire | Alpine Metamorphism in the Betic Internal Zones[END_REF]. The complex was then uplifted along the subduction channel and exhumed in the extensional Sierra Nevada antiformal dome (Fig. 1a) [START_REF] Martínez-Martínez | Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain)[END_REF][START_REF] Behr | Kinematic and thermal evolution during two-stage exhumation of a Mediterranean subduction complex[END_REF][START_REF] Booth-Rea | Continental subduction, intracrustal shortening, and coeval upper-crustal extension: PT evolution of subducted south Iberian paleomargin metapelites (Betics, SE Spain)[END_REF].The Cerro del Almirez massif is the largest (2.3 km 2 ) of several ultramafic massifs that occur along shear zones in the uppermost units of the Nevado-Filábride Complex (Fig. 1b) [START_REF] Trommsdorff | High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain[END_REF][START_REF] Martínez-Martínez | Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain)[END_REF][START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. The massif is composed of strongly foliated antigoritebearing serpentinites (Atg-serpentinites) overlying a 70-100 m sequence of unfoliated, coarse-grained chlorite-bearing harzburgites (Chl-harzburgites). The Atg-serpentinites are made up of antigorite (Atg) + olivine (Ol) + chlorite (Chl) + magnetite (Mag) + ilmenite (Ilm) ± diopside (Di) ± tremolite (Tr) ± titanian clinohumite (Ti-Chu). The Chl-harzburgites were formed by dehydration of Atg-serpentinites during subduction [START_REF] Trommsdorff | High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain[END_REF]. They are composed of Ol + orthopyroxene (Opx) + Chl + Mag + Ilm ± titano-hematite ± Tr ± Ti-Chu. Two main textures, namely granofels and spinifex, were identified in the Chl-harzburgites [START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. Granofels Chl-harzburgites are made up of coarse anhedral olivine, chlorite flakes, and prismatic orthopyroxene, with an interlocked texture, whereas spinifextextured Chl-harzburgites are characterized by a criss-cross arrangement of olivine single crystals (up to 12 cm in length), and by centimetre-scale radial aggregates of orthopyroxene with variable grain size and orientation. Granofels and spinifex Chl-harzburgites occur in interspersed lenses that are thought to record, respectively, sequences of slow and fast fluid draining events during serpentinite dehydration [START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF][START_REF] Dilissen | Textural evolution during high-pressure dehydration of serpentinite to peridotite and its relation to stress orientations and kinematics of subducting slabs: Insights from the Almirez ultramafic massif[END_REF]. Both textural types of Chl-harzburgite are locally recrystallized along grain size reduction zones (Padron-Navarta et al., 2010a). This grain size reduction is also accompanied by a decrease of orthopyroxene modal amount and a change of olivine color, from brown to colorless. These features have been interpreted in terms of reaction pathways for fluid flow during high pressure metamorphism (Padrón-Navarta et al., 2010a). The minimum temperature of crystallization of Chlharzburgites is constrained by the limit of antigorite stability field, between 660 and 680 °C at 1.6 and 2.5

GPa (e.g., [START_REF] Ulmer | Serpentine Stability to Mantle Depths and Subduction-Related Magmatism[END_REF][START_REF] Wunder | Antigorite : High-pressure stability in the system MgO-SiO2-H2O (MSH)[END_REF]Padrón-Navarta et al., 2010b), In addition, the occurrence of tremolite in the Chl-harzburgites limits the maximum temperature from 680 to 710 °C for 1.9 and 1.6 GPa, respectively (Padrón-Navarta et al., 2010b). These P-T conditions compare well with independent estimates for metarodingite (1.6-1.9 GPa and 660-684 °C; [START_REF] Laborda-López | High-P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones[END_REF] and ophicarbonate lenses hosted within the Chl-harzburgites (18 kbar, 650-670 °C; [START_REF] Menzel | Subduction metamorphism of serpentinite-hosted carbonates beyond antigorite-serpentinite dehydration (Nevado-Filábride Complex, Spain)[END_REF].

These studies suggest that the peak metamorphic conditions experienced by the Cerro del Almirez massif were 680-710 °C and 1.6-1.9 GPa, notably high in temperature for a given pressure as compared to other alpine meta-ophiolites (Fig. 2).

In the massif, the transition from Atg-serpentinites to Chl-harzburgites is marked by a sharp increase in the abundance of olivine, orthopyroxene and chlorite concomitant with a steady disappearance of antigorite [START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. In the field, the transition from Atg-serpentinites to Chl-harzburgites takes place over few meters (< 2 m) where serpentinites are composed of Atg + Chl + Ol ± Opx and correspond to partly dehydrated serpentinites that have been termed "transitional" lithologies in previous studies [START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. The contact between Atg-serpentinites and transitional lithologies developed obliquely to the foliation of the Atg-serpentinite and shows no signs of tectonic activity or offset. In addition to the Atg-serpentinite, Chl-harzburgite and transitional lithologies, the massif also contains minor metarodingite boudins and ophicarbonates lenses that, together with the Atg-serpentinites, preserve a textural and geochemical record of seafloor serpentinization and carbonation processes [START_REF] Alt | Recycling of water, carbon, and sulfur during subduction of serpentinites: A stable isotope study of Cerro del Almirez, Spain[END_REF][START_REF] Laborda-López | High-P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones[END_REF][START_REF] Menzel | Subduction metamorphism of serpentinite-hosted carbonates beyond antigorite-serpentinite dehydration (Nevado-Filábride Complex, Spain)[END_REF], which likely took place during the opening of the western branch of the Alpine Tethys Ocean in the Mesozoic [START_REF] Puga | Petrology, geochemistry and U-Pb geochronology of the Betic Ophiolites: Inferences for Pangaea break-up and birth of the westernmost Tethys Ocean[END_REF].

In this study, we selected samples from the three distinctive lithologies from the Cerro del Almirez massif: Atg-serpentinites, transitional lithologies, and Chl-harzburgites with granofels, spinifex and recrystallized textures (Table 1). In order to evaluate the potential contribution of sediment-derived external fluids, we also analyzed two metasedimentary mica-schists surrounding the Cerro del Almirez massif.

Methodology

Whole rock powders were dissolved using concentrated HF and HNO 3 acids in 7 mL PTFE Teflon square digestion vessels with wrench top closures in an oven at 160 °C for 3 days. Following evaporation of the HF and HNO 3 the samples were subsequently treated with a 1:1 mix of concentrated HCl and HNO 3 and refluxed at 140 °C for 3 days in an oven. Prior studies comparing this method to Parr-bomb dissolution techniques [START_REF] Debret | Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes[END_REF] have shown that it can digest all refractory phases that may be present in serpentinites, including spinel (see Results section for further discussion). Finally, samples were brought into solution in 6 M HCl and 1.5 M HBr before Fe and Zn column chemistry, respectively. Quantitative purification of Fe was achieved by chromatographic exchange, using Biorad AG1-X4 anion exchange resin in an HCl medium following the procedure of [START_REF] Williams | Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts[END_REF]. The quantitative separation of Zn from matrix elements was achieved using Teflon shrink-fit columns filled with 0.5 mL of Biorad AG1-X4 anion exchange resin following the procedure adapted from [START_REF] Moynier | Isotopic composition of zinc, copper, and iron in lunar samples[END_REF]. All reagents used in the chemistry and mass spectrometry procedures were distilled in sub-boiling Teflon two-bottle stills at Durham University (UK), except for the HBr acid, which was ultra-pure grade ROMIL grade. The total amount of Fe and Zn processed through the columns was typically around 650 µg and 1250 ng, respectively.

The total procedural blank contribution was <50 ng of Fe and <10 ng of Zn, which is negligible compared to the amount of Fe and Zn in the samples.

Iron and Zn isotope analyses were performed by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS; Thermo Scientific Neptune Plus) at Durham University (UK). Instrumental mass fractionation was corrected using the sample-standard bracketing technique for Fe, and combined sample-standard bracketing and external empirical normalisation with a Cu standard for Zn (e.g., [START_REF] Maréchal | Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry[END_REF]. In the case of Fe analyses, solutions consisted of 2 ppm natural Fe in 0.1 M HNO 3, which were introduced into the mass spectrometer using a quartz SIS (stable introduction system; ThermoFisher) and PFA 50 µl/min nebuliser. The international Fe isotope standard IRMM014 was used as both the bracketing and delta notation reporting standard. To resolve the polyatomic interferences 40 Ar 16 O, 40 Ar 14 N, and 40 Ar 16 OH + that occur on masses 56 Fe, 54 Fe, and 57 Fe, respectively, the instrument was run in medium resolution mode, which gave a mass resolving power (mass/Δmass) of ~8000. The standard Fe beam intensities typically varied between 25 and 35 V for 56 Fe using a 10 10 Ω resistor. Data were collected in 25 cycles of 4.194 second integrations. Mass dependence, long-term reproducibility, and accuracy were evaluated by analyzing an in-house FeCl 2 -salt standard. The values obtained for repeated measurements of this standard solution yielded average δ 56 Fe values of -0.70 ±0.05 ‰ and δ 57 Fe values of -1.03 ±0.09 ‰ 2sd, n =24. These average values are in excellent agreement with previous measurements [START_REF] Williams | Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts[END_REF]. The international rock reference material USGS BIR-1a (Icelandic basalt) was analyzed in the same analytical sessions as the samples, yielding δ 56 Fe values of 0.06 ± 0.02 ‰ and δ 57 Fe = 0.08 ± 0.01 ‰ (2sd, n = 2; see appendix A).

Zinc isotope compositions were measured on the purified sample solutions. These solutions were prepared at 750 ng/g of natural Zn and doped with 250 ng/g of pure Cu solution. This gave a Zn intensity of ~ 4V for 64 Zn. The solutions were introduced to the instrument using a PFA 50 µl/min nebuliser coupled to a quartz cyclonic spray chamber. Owing to a lack of polyatomic interferences across the mass range for Zn the machine was operated in low mass resolution mode. Mass of 62 Ni was monitored to correct for the potential interference of 64 Ni on 64 Zn. Data were collected in 40 cycles of 4.194 second integrations. Because the widely used Zn isotope standard (JMC-Lyon) is exhausted, an in-house pure Zn solution obtained from Alfa-Asear was used as the bracketing standard. This standard solution was calibrated relative to the JMC-Lyon solution, and an offset for δ 66 Zn of +0.28‰ was observed. This allowed for the measured δ 66 Zn Alfa- Asear to be corrected to δ 66 Zn JMC-Lyon by applying a correction value of +0.28‰ to all values. The mass dependence, long-term reproducibility and accuracy of this method were evaluated by the analysis of the external reference material USGS BHVO-2, which yielded a δ 66 Zn value of 0.35 ± 0.05 ‰ (2sd, n = 2). This value is in excellent agreement with previously published values (see appendix A).

Results

Iron and zinc isotopic ratios were measured in the Cerro del Almirez samples and given in Table 1. Major and trace element data, including iron and zinc concentrations, are from [START_REF] Marchesi | Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain)[END_REF]. The δ 56 Fe values of Cerro del Almirez rocks range from -0.06 ± 0.04 ‰ to +0.12 ± 0.04 ‰ with a mean δ 56 Fe value of +0.05 ± 0.02 ‰ (2sd/√n; n = 24), comparable to that of the primitive mantle (+0.03 ± 0.03 ‰; [START_REF] Craddock | Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth[END_REF][START_REF] Poitrasson | On the iron isotope heterogeneity of lithospheric mantle xenoliths : implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth[END_REF][START_REF] Sossi | Iron isotope systematics in planetary reservoirs[END_REF]Fig. 3a). Among Cerro del Almirez samples, Chlharzburgites display a larger range of δ 56 Fe (from -0.06 ± 0.04 to +0.09 ± 0.03 ‰) relative to Atgserpentinites (+0.01 ± 0.01 to +0.10 ± 0.03 ‰) and transitional lithologies (+0.00 ± 0.01 to +0.08 ± 0.01 ‰). The mean value of Chl-harzburgites decreases from granofels (+0.07 ± 0.02 ‰, 2sd/√n; n = 4) to spinifex (+0.04 ± 0.03 ‰, n = 5) and recrystallized textures (-0.01 ± 0.05 ‰, n = 3; Fig. 3a). The FeO contents of these samples range from 6.7 to 8.7 wt% and are indistinguishable from those observed in abyssal and orogenic serpentinites (e.g., [START_REF] Deschamps | Geochemistry of subduction zone serpentinites: A review[END_REF].

Zinc concentrations [Zn] and isotope ratios (δ 66 Zn) range broadly from 34 to 74 ppm and from +0.09 to +0.55 ‰, respectively. The [Zn] and δ 66 Zn values of Cerro del Almirez samples are generally higher than those assumed for the primitive mantle ([Zn] = 55 ppm; mean δ 66 Zn = +0.16 ± 0.06 ‰; [START_REF] Sossi | Zinc isotope composition of the Earth and its behaviour during planetary accretion[END_REF].

Given that spinels commonly display high [Zn] and δ 66 Zn values relative to mantle silicates and sulfides [START_REF] Wang | Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle[END_REF][START_REF] Ducher | Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations[END_REF], the incomplete dissolution of spinel in our samples cannot explain these variations as this should lead to apparently lighter δ 66 Zn and low [Zn] relative to mantle values. The δ 66 Zn values observed in our samples increase from Atg-serpentinites ([Zn] = 34-46 ppm; δ 66 Zn = +0.23 ± 0.06‰), to transitional lithologies (Zn = 45-67 ppm; δ 66 Zn = +0.30 ± 0.06‰), granofels ([Zn] = 38-59 ppm; δ 66 Zn = +0.33 ± 0.04‰) and spinifex ([Zn] = 48-66 ppm; δ 66 Zn = +0.43 ± 0.09‰) Chl-harzburgites (Fig. 3b). The recrystallized Chl-harzburgites display scattered δ 66 Zn values (from +0.21 ± 0.04 ‰ to +0.49 ± 0.03 ‰) with a mean of +0.33 ± 0.12‰. Two samples of the mica-schists surrounding the Cerro del Almirez massif were also analyzed. They are characterized by similar FeO (6.44 and 8.26 wt%) and δ 56 Fe (+0.05 ± 0.02 and +0.11 ± 0.02 ‰) and high [Zn] (113 and 125 ppm) and δ 66 Zn (+0.33 ± 0.04 and +0.46 ± 0.03 ‰) compared with meta-serpentinites (i.e., the Atg-serpentinites, transitional lithologies, granofels, spinifex and recrystallized -textured Chl-harzburgites).

Discussion

Serpentinites are significant reservoirs of water and redox-sensitive elements (e.g., [START_REF] Alt | The role of serpentinites in cycling of carbon and sulfur : Seafloor serpentinization and subduction metamorphism[END_REF]. Their high-pressure dehydration to meta-peridotite at depth in subductions zones is the main source of water for arc magmatism, triggers large scale metasomatism of the mantle wedge area, and influences redox sensitive element cycles between the Earth's interior and surface [START_REF] Debret | Highly oxidising fluids generated during serpentinite breakdown in subduction zones[END_REF]Cannao and Malaspina, 2018;[START_REF] Iacovino | Aqueous fluids are effective oxidizing agents of the mantle in subduction zones[END_REF][START_REF] Menzel | Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones[END_REF]. The subduction-related Cerro del Almirez complex provides an unmatched opportunity to investigates these processes because this is the only exhumed massif worldwide that preserves a reaction front between Atg-serpentinites and their high-pressure dehydration product, Chl-harzburgites [START_REF] Trommsdorff | High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain[END_REF][START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF][START_REF] Alt | Recycling of water, carbon, and sulfur during subduction of serpentinites: A stable isotope study of Cerro del Almirez, Spain[END_REF][START_REF] Marchesi | Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain)[END_REF].

Before addressing the significance of the Cerro del Almirez Fe and Zn stable isotope data in the context of high-pressure dehydration of serpentinite in subduction zones, it is essential to first investigate the effects of high temperature petrogenetic processes and seafloor hydrothermal alteration that may have modified the oceanic lithosphere prior to subduction. The major, trace element and stable isotope composition of the oceanic mantle lithosphere is influenced by high temperature melting and melt extraction processes, which ultimately define its "fertility" (i.e., the degree to which mantle peridotite has experienced prior melt extraction and its capacity to produce partial melts). To explore how these processes could have influenced Fe and Zn isotope systematics we need to evaluate potential correlations between δ 66 Zn, δ 56 Fe and chemical indicators of mantle fertility. The degree of mantle peridotite fertility is manifested in the modal abundances of pyroxenes and hence can be qualitatively estimated using Al 2 O 3 contents, with fertile lherzolites displaying high Al 2 O 3 relative to refractory harzburgites and dunites (e.g., [START_REF] Bodinier | Orogenic, Ophiolitic, and Abyssal Peridotites[END_REF]. In serpentinites Al 2 O 3 /SiO 2 is considered a better proxy of peridotite protolith composition than Al 2 O 3 concentrations because the Al 2 O 3 /SiO 2 ratio remains relatively unchanged during the serpentinization process, whereas Al 2 O 3 concentrations can be influenced by volume changes accompanying serpentinite formation (see [START_REF] Paulick | ODP Leg 209: Implications for fluid / rock interaction in slow spreading environments[END_REF][START_REF] Deschamps | Geochemistry of subduction zone serpentinites: A review[END_REF]. Although serpentinite bulk-rock MgO values can retain some magmatic signals [START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF], serpentinized peridotites mostly display MgO/SiO 2 ratios that mostly fall below the terrestrial mantle array (Figure 4a). Previous authors have attributed this observation to a loss of MgO during low temperature seafloor weathering (e.g., [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Paulick | ODP Leg 209: Implications for fluid / rock interaction in slow spreading environments[END_REF][START_REF] Deschamps | Geochemistry of subduction zone serpentinites: A review[END_REF]. In the following discussion we therefore use the Al 2 O 3 /SiO 2 ratio instead of Al 2 O 3 , MgO contents or MgO/SiO 2 to quantify fertility variations in Cerro del Almirez protolith(s).

Previous studies have shown that the δ 56 Fe values of peridotites can be highly influenced by melt extraction, with higher δ 56 Fe preserved in lherzolites relative to harzburgites and dunites [START_REF] Williams | Systematic iron isotope variations in mantle rocks and minerals: The effects of partial melting and oxygen fugacity[END_REF][START_REF] Weyer | Partial melting and melt percolation in the mantle: The message from Fe isotopes[END_REF], where the latter display light δ 56 Fe as a consequence of losing isotopically heavy Fe to a melt phase. The range in δ 56 Fe (-0.06-0.12 ‰) and the Al 2 O 3 /SiO 2 (0.03-0.09) ratios observed in the Cerro del Almirez samples are comparable to those of mantle peridotites and abyssal serpentinites (δ 56 Fe -0.14-+0.08 ‰; Debret et al., 2018a; Al 2 O 3 /SiO 2 = 0.01-0.09; [START_REF] Godard | Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge[END_REF] and a broad correlation between Al 2 O 3 /SiO 2 and δ 56 Fe is observed (Fig. 4b). These observations suggest that the δ 56 Fe variations of Cerro del Almirez samples are mainly controlled by peridotite protolith δ 56 Fe compositions, which in turn may record prior melt extraction and/or refertilisation processes.

In contrast to Fe, there are significant variations in the Zn isotope compositions of the Cerro del Almirez samples (+0.09 ± 0.02 ‰ to +0.55 ± 0.09 ‰; Fig. 3), which are well resolved from primitive mantle values (+0.16 ± 0.06 ‰; [START_REF] Sossi | Zinc isotope composition of the Earth and its behaviour during planetary accretion[END_REF]. The absence of any correlations between δ 66 Zn and indices of fertility such as Al 2 O 3 /SiO 2 (Fig. 4c) suggests that the Zn isotope compositions of the Cerro del Almirez samples have been overprinted by subsequent metamorphic processes, taking place during either oceanic stage serpentinisation or prograde subduction related metamorphism. In a recent study, [START_REF] Bretscher | The influence of oceanic oxidation on serpentinite dehydration during subduction[END_REF] speculated that the Atg-out reaction front between the Cerro del Almirez Chl-harzburgites and Atgserpentinites may correspond to a sharp oxidation seafloor serpentinization front, with the Chl-harzburgite protolith representing the deep part of the oceanic lithosphere and the Atg-serpentinite protolith formed at shallower depths. In the scenario proposed by [START_REF] Bretscher | The influence of oceanic oxidation on serpentinite dehydration during subduction[END_REF], abyssal serpentinization processes control the contrasting mineralogical and chemical variations observed across the contact between Atgserpentinites and Chl-harzburgites in the Cerro del Almirez, rather than prograde subduction-related metamorphism and devolatilisation processes [START_REF] Padrón-Navarta | Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro del Almirez, Nevado-Filábride complex, Southern Spain)[END_REF]. These scenarios can be distinguished using Zn stable isotopes. Abyssal peridotites and serpentinites display a large range of δ 66 Zn (from 0.13‰ to 0.47‰), with highly serpentinized peridotites displaying low [Zn] and high δ 66 Zn relative to fresh mantle peridotites (Fig. 5a; Debret et al., 2018a). Debret et al., (2018a) attributed these variations to the preferential mobility and loss of isotopically light Zn in fluids during abyssal serpentinization, with Zn stable isotope fractionation being controlled by the dissolution of both mantle sulfides and/or spinels and Zn complexation with chlorine in fluids. In the Cerro del Almirez massif, transitional lithologies (δ 66 Zn = +0.30 ± 0.06‰) granofels (δ 66 Zn = +0.33 ± 0.04‰) and spinifex (δ 66 Zn = +0.43 ± 0.09‰) Chl-harzburgites display higher δ 66 Zn values relative to Atg-serpentinites (δ 66 Zn = +0.23 ± 0.06‰), as showed by a broad negative correlation between δ 66 Zn and Loss On Ignition (LOI, a broad measure of the presence of hydrous phases such as serpentine) values (Fig. 5b). Furthermore, the studied Chl-harzburgites display high [Zn] (38-74 ppm) relative to abyssal peridotites and serpentinites (29-45 ppm, Debret et al., 2018a;Fig. 5) and a broad positive correlation is observed between δ 66 Zn and [Zn] (Fig. 5a).

These results contrast with previous observations in abyssal settings, where negative correlation between δ 66 Zn and [Zn] was interpreted as reflecting spinel dissolution during peridotite serpentinization and Zn leaching in hydrothermal fluids (Fig. 5; see also Debret et al., 2018a). This comparison demonstrates that the observed variations in the Cerro del Almirez meta-serpentinites cannot be explained by differences in the degree of seafloor serpentinization (c.f. [START_REF] Bretscher | The influence of oceanic oxidation on serpentinite dehydration during subduction[END_REF]. Instead, the Zn elemental and isotopic variations observed in the Cerro del Almirez massif are most likely to reflect the mobility of Zn in metamorphic fluids at high pressures and temperatures during the progressive dehydration of Atgserpentinites into transitional lithologies and Chl-harzburgites. The observed Zn isotope fractionation can therefore be used to place constraints on the composition of slab derived fluids during subduction, as we discuss in detail below.

Zinc isotope fractionation is highly influenced by the (re-)crystallization of Fe-oxides (e.g., spinels), sulfides and carbonates, which are the main carriers of Zn in ultramafic rocks (e.g., [START_REF] Wang | Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle[END_REF]Debret et al., 2018a). The subduction of the oceanic lithosphere initiates a continuum of metamorphic redox reactions and dehydration/rehydration events between the downgoing slab and the slab/mantle wedge interface and/or the mantle wedge that influence Fe-oxides, sulfides and carbonates stability resulting in stable isotope fractionation [START_REF] Debret | Isotopic evidence for iron mobility during subduction[END_REF]. The Zn concentration and isotope composition of the Cerro del Almirez Atg-serpentinites are consistent with those observed in other high-P slab serpentinites, such as those from the Western Alps meta-ophiolites [START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF] (Fig. 5a). However, in contrast to the Western Alps, in the Cerro del Almirez massif the dehydration of Atg-serpentinites to prograde Chl-harzburgites is accompanied by an increase in both bulk rock δ 66 Zn values and [Zn] (Fig. 5). Furthermore, the metamorphic recrystallization of Atg-serpentinites into Chl-harzburgites is accompanied by a decrease of magnetite modal abundance in bulk rock [START_REF] Debret | Redox state of iron during high-pressure serpentinite dehydration[END_REF], suggesting a change in serpentinite redox budget during subduction. One possible explanation for these observations is the dissolution of Zn-bearing spinels and the subsequent leaching of isotopically heavy Zn in metamorphic fluids during serpentinites dehydration.

However, Fe-oxides tend to display high δ 66 Zn and [Zn] concentrations relative to silicates [START_REF] Ducher | Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations[END_REF][START_REF] Wang | Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle[END_REF], suggesting this scenario is unlikely.

Another possibility is that Zn stable isotope variations in the Cerro del Almirez meta-serpentinites were produced by the influx of external fluids (i.e., open-system processes), rather than by the in situ (closedsystem) dehydration of serpentinites during subduction. Recent studies show that Zn can be highly mobile in slab derived fluids forming carbonate (e.g., Zn-CO 3 ) and/or sulfate (e.g., Zn-SO 4 ) aqueous complexes, and that these preferentially complex isotopically heavy Zn relative to silicates, Fe-oxides, sulfides and/or carbonates (Fuji et al., 2014;[START_REF] Liu | Zinc isotope evidence for a large-scale carbonated mantle beneath eastern China[END_REF][START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF][START_REF] Ducher | Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations[END_REF]Debret et al., 2018b).

The elevated δ 66 Zn values and Zn concentrations of the Cerro del Almirez Chl-harzburgites relative to those of Atg-serpentinites (Figs 3 and5) may therefore be interpreted in terms of the dehydration of Atgserpentinite in conjunction with the influx of external fluids with heavy Zn isotope signatures, such as those released during slab devolatilization [START_REF] Liu | Zinc isotope evidence for a large-scale carbonated mantle beneath eastern China[END_REF][START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF]Debret et al., 2018b).

Previous geochemical studies have shown that the Cerro del Almirez Chl-harzburgites are abnormally enriched in Th, U, Nb, Ta, Pb, large ion lithophile elements (LILE, Cs, Rb, Ba, Sr) and light δ 11 B relative to Atg-serpentinites [START_REF] Garrido | Enrichment of HFSE in chlorite-harzburgite produced by high-pressure dehydration of antigorite-serpentinite: Implications for subduction magmatism[END_REF][START_REF] Marchesi | Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain)[END_REF][START_REF] Harvey | 11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle[END_REF]. These enrichments cannot be explained by closed-system dehydration of serpentinite [START_REF] Marchesi | Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain)[END_REF]. Furthermore, these geochemical characteristics (i.e., LILE, Sr and B enrichments) are also shared with fluid inclusions trapped in metamorphic olivine and orthopyroxene from Chl-harzburgites (Scambelluri et al., 2004a, b), providing strong evidence for the equilibration of the Chl-harzburgites with a fluid phase enriched in these elements. Taken together, these observations show that dehydration must have occurred in an open system involving external fluids that equilibrated with other lithologies such as metasedimentary rocks [START_REF] Jabaloy-Sánchez | Subduction-and exhumation-related structures preserved in metaserpentinites and associated metasediments from the Nevado-Filábride Complex (Betic Cordillera, SE Spain)[END_REF]), meta-rodingites (Laborda-López et al., 2018), meta-ophicarbonates [START_REF] Menzel | Subduction metamorphism of serpentinite-hosted carbonates beyond antigorite-serpentinite dehydration (Nevado-Filábride Complex, Spain)[END_REF], and/or an exotic slab-derived component [START_REF] Marchesi | Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain)[END_REF][START_REF] Harvey | 11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle[END_REF]. Interestingly, the Cerro del Almirez samples display broad positive arrays between δ 66 Zn and Sr/Y, Ba/Ce or Rb/Ce ratios with the Chlharzburgites displaying high δ 66 Zn, Sr/Y, Ba/Ce and Rb/Ce ratios relative to the other lithologies as well as higher and more variable U/Yb ratios (Fig. 6). Similarly, the metasedimentary rocks also display high δ 66 Zn, Sr/Y, U/Yb, Ba/Ce and Rb/Ce and plot in the same area as the Chl-harzburgites in Fig. 6.

The mean δ 66 Zn of clastic sediments is +0.28 ± 0.13 ‰ (n = 105; [START_REF] Moynier | The isotope geochemistry of zinc and copper[END_REF] and is highly influenced by the presence of carbon, sulfur and iron-bearing phases, which typically display isotopically heavy signatures relative to silicate-bearing minerals [START_REF] Fujii | Density functional theory estimation of isotope fractionation of Fe, Ni, Cu, and Zn among species relevant to geochemical and biological environments[END_REF][START_REF] Ducher | Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations[END_REF]. The Zn isotope signatures of micaschists (+0.33 ± 0.04 and +0.46 ± 0.03 ‰) surrounding the Cerro del Almirez ultramafic massif are extremely similar to the clastic sediment mean value, such that it is difficult to estimate the magnitude and direction of any Zn stable isotope fractionation that may take place during the high-P dehydration of these sedimentary rocks. However, given the similarity in micaschist and Chl-harzburgite δ 66 Zn values and the observation that Cerro del Almirez metasedimentary rocks also display high [Zn] (113-125 ppm), it is plausible to consider that fluids in equilibrium with the micaschists could, at the very least, have inherited their heavy δ 66 Zn signatures. Infiltration of the the Chl-harzburgites with such heavy-δ 66 Zn sediment-derived fluids could thus have modified their Zn isotope signatures, explaining the heavy δ 66 Zn values observed. Furthermore, the devolatilization of metasedimentary rocks at high pressures is likely to release carbon-bearing fluids due to carbonate devolatilization (e.g., Debret et al., 2018b). Given that these aqueous complexes preferentially incorporate heavy Zn isotopes (Fuji et al.,2014) relative to Zn-bearing Feoxides, sulfides or carbonates [START_REF] Ducher | Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations[END_REF], the fluids derived from metasedimentary rocks could be even more fractionated to even heavier δ 66 Zn values than the sedimentary (micaschist) residues from which they were derived.

The recent study of [START_REF] Ferrando | Gradual and selective trace-element enrichment in slabreleased fluids at sub-arc depths[END_REF] on high-pressure fluid inclusions in kyanite reveals that the progressive dissolution of phengite and carbonate in metasedimentary rocks results in the preferential release of LILE, U, Th and Sr to a fluid phase. The fluid inclusions in the Chl-harzburgites display similar anomalies in LILE and Sr (Scambelluri et al., 2004a, b) to those described by [START_REF] Ferrando | Gradual and selective trace-element enrichment in slabreleased fluids at sub-arc depths[END_REF], reinforcing the idea of a sediment-derived fluid input to the Cerro del Almirez meta-serpentinites. Similarly, the high F contents and 20 Ne/ 36 Ar ratios of the Cerro del Almirez Chl-harzburgites have been ascribed to the open-system influx of F -, 40 Ar, 4 He-and 20 Ne-rich fluids derived from metasedimentary rocks [START_REF] Kendrick | Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon[END_REF]. Hence, we suggest that the heavy Zn isotope composition of Chl-harzburgites, as well as their trace element compositions, is consistent with metasomatism by carbonate-bearing fluids derived from the metasedimentary rocks. This hypothesis is also in agreement with the observation of Mg-carbonates associated with high-pressure minerals in Cerro del Almirez Chl-harzburgites [START_REF] Menzel | Subduction metamorphism of serpentinite-hosted carbonates beyond antigorite-serpentinite dehydration (Nevado-Filábride Complex, Spain)[END_REF], those being commonly interpreted as subduction related (e.g., [START_REF] Scambelluri | Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling[END_REF].

In order to further test this hypothesis, we first attempted to model the Zn isotope composition of carbon bearing fluids released during sediment dehydration at HP. Carbon can exist in deep fluids as either carbonate and/or organic molecules, which can be stable, at low oxygen fugacity and/or pH, up to 6 GPa and more than 600 °C in subduction zones [START_REF] Sverjensky | Important role for organic carbon in subduction-zone fluids in the deep carbon cycle[END_REF]. We therefore considered both carbonate [ZnCO 3 (H 2 O) 3 , ZnHCO 3 (H 2 O) 5+ ] and organic carbon [Zn(cit) 2 4-] as potential complexes for Zn in metasediment derived fluids. These different carbon-bearing species were chosen based on their different partition functions, or β-factors, and contrasting oxidation state of carbon. The influence of organic ligands on Zn isotope partitioning was approximated by using citrate as a representative ligand in our calculations; moreover, no other ab initio calculation results are currently available for other organic complexes, such as acetate (see review in [START_REF] Moynier | The isotope geochemistry of zinc and copper[END_REF], which are potentially more representative of subduction-zone fluids [START_REF] Sverjensky | Important role for organic carbon in subduction-zone fluids in the deep carbon cycle[END_REF]. Using these, the compositions of the Chl-harzburgites was modelled in the form of a binary mixture between metasediment-derived fluids and Atg-serpentinites. The elemental and isotopic behaviour of Zn in carbonate-bearing metasedimentary rocks (i.e., the β-factor) was approximated to that of smithsonite (ZnCO 3 ). We used the ab initio calculations of [START_REF] Fujii | Ab initio calculation of the Zn isotope effect in phosphates, citrates, and malates and applications to plants and soil[END_REF] δ 66 Zn Final = (1000+ δ 66 Zn Initial ) × (F (α-1) -1) + δ 66 Zn Initial where α is the fractionation factor between smithsonite and a fluid phase complexing Zn as

[ZnHCO 3 (H 2 O) 5+ ], [ZnCO 3 (H 2 O) 3 ] or [Zn(cit) 2 4-]
at temperatures ranging from 500 to 700 °C. F is the fraction of Zn remaining in the rock, ranging from 1 (unreacted) to 0 (all of Zn lost to the fluid phase). As we expect the mobility of Zn to be low in metasediment-derived fluids (see discussion above), we set F to be 0.9 in our models. The δ 66 Zn Initial was set to 0.4 ‰, which is the mean value of countryside metasedimentary rocks surrounding the Cerro del Almirez massif. The composition of the Cerro del Almirez samples was then modelled as a binary mixture between Atg-serpentinites and metasediment-derived fluids according to the following equation: Our models can also be used to explore the effect of reduced carbon species in fluids, such organic carbon (illustrated on Figure 7 as citrate), on Zn isotope systematics and to determine whether percolation by such fluids can explain the δ 66 Zn variation in the Chl-harzburgites. However, reduced carbon species do not fractionate Zn isotopes to the extent that they can create fluids with sufficiently heavy δ 66 Zn. Models using available data for Zn isotope partitioning into such fluids were not able to reproduce the observed δ 66 Zn variations in the Chl-harzburgites (Fig. 7). These observations are consistent with presence of titanohematite in Chl-harzburgites, which is a phase expected to crystallize at relatively high oxygen fugacities (between +1 and +4 relative to quartz-fayalite-magnetite buffer), under which carbon is stable under oxidized form [START_REF] Debret | Redox state of iron during high-pressure serpentinite dehydration[END_REF][START_REF] Debret | Highly oxidising fluids generated during serpentinite breakdown in subduction zones[END_REF]. Furthermore, recent studies and thermodynamic modelling of Cerro del Almirez meta-ophicalcites indicate that both CO 2 and HCO 3 -species are likely to dominate fluids circulating in the massif during antigorite breakdown [START_REF] Menzel | Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones[END_REF], in agreement with our hypothesis.

The geochemical imprint of metasediment-derived fluids also occurs in other subduction-related Chlharzburgites. This is seen in Western Alps meta-ophiolites (Fig. 2), such as the Cima di Gagnone (Lepontine Alps) and Erro Tobbio [START_REF] Scambelluri | The water and fluid-mobile element cycles during serpentinite subduction. A review[END_REF]Cannao et al., 2020). These observations point to a widespread association between open-system dehydration processes and oxidized fluids conditions (i.e., associated carbonate-or sulfate-bearing fluids). Such processes may therefore be ubiquitous in subduction zones, particularly near the slab-wedge interface, where serpentinites are continuously percolated by fluids with sediment-like signatures.

Long-range and large-scale fluid flows, such as the migration of slab derived fluids to the mantle wedge, are known to occur in subduction zones. Although the sources and compositions of fluids in subduction zones are expected to be highly variable, recent studies have proposed that fluids circulating in subduction zones are likely to display fractionated Zn and/or Fe isotopes signatures [START_REF] Debret | Isotopic evidence for iron mobility during subduction[END_REF][START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF]Debret et al., 2018b;[START_REF] Gerrits | Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet[END_REF][START_REF] Chen | Tracking Fe mobility and Fe speciation in subduction zone fluids at the slab-mantle interface in a subduction channel: A tale of whiteschist from the Western Alps[END_REF][START_REF] Liu | Tracing the Deep Carbon Cycle Using Metal Stable Isotopes : Opportunities and Challenges[END_REF]. In particular, the observation of high δ 66 Zn in Chl-harzburgites is consistent with the results of previous studies showing that isotopically heavy Zn is likely to be mobile during carbonate dissolution at high pressure in subduction zones [START_REF] Liu | Zinc isotope evidence for a large-scale carbonated mantle beneath eastern China[END_REF]Debret et al., 2018b;[START_REF] Liu | Tracing the Deep Carbon Cycle Using Metal Stable Isotopes : Opportunities and Challenges[END_REF]. However, the lack of significant Fe isotope variation in the Cerro del Almirez samples is surprising as isotopically light Fe is also expected to be mobile in carbonate-bearing fluids [START_REF] Fujii | Density functional theory estimation of isotope fractionation of Fe, Ni, Cu, and Zn among species relevant to geochemical and biological environments[END_REF]. Indeed, previous studies working in forearc mantle wedge or shallow slab/mantle interface areas have shown that large Fe isotope variations in ultramafic rocks (down to -0.26 ‰) may be attributed to carbonate-bearing metasomatism [START_REF] Inglis | The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites[END_REF]Debret et al., 2018b[START_REF] Debret | Redox transfer at subduction zones: insights from Fe isotopes in the Mariana forearc[END_REF] and the dissolution of Ca-(i.e., calcites, aragonite) and/or Fe-(i.e., siderite and ankerite) bearing carbonates in metamorphic fluids at low temperature (i.e., below 400 °C; [START_REF] Facq | In situ Raman study and thermodynamic model of aqueous carbonate speciation in equilibrium with aragonite under subduction zone conditions[END_REF][START_REF] Milesi | Formation of CO2, H2 and condensed carbon from siderite dissolution in the 200-300°C range and at 50MPa[END_REF]. Both Ca-(down to -0.92 ‰; Craddock and Dauphas, 2011) and Fe-(down to -3.9 ‰; [START_REF] Belshaw | High precision measurement of iron isotopes by plasma source mass spectrometry[END_REF][START_REF] Johnson | Iron isotopes constrain biologic and abiologic processes in Banded Iron Formation genesis[END_REF] bearing seafloor carbonates are known to concentrate isotopically light Fe relative to silicate and oxide phases. However, in contrast to Ca-/Fe-carbonates, Mg-rich carbonates (e.g., magnesite) are comparatively insoluble at low pressures and become progressively soluble in water with increasing pressure and temperature (e.g., [START_REF] Pan | Dielectric properties of water under extreme conditions and transport of carbonates in the deep Earth[END_REF]. It is therefore possible that the sequential dissolution of Ca-/Fe-bearing carbonates to Mg-bearing carbonates during slab burial could generate carbonate-bearing fluids with a spectrum of Zn and Fe isotope signatures, with shallow fluids resulting from Ca-and Fe-bearing carbonate dissolution displaying isotopically light Fe signatures, and deep fluids resulting from Mg-carbonate dissolution displaying isotopically heavy Zn isotope compositions. This hypothesis is also consistent with the low Fe content of magnesite, such that the dissolution of these carbonates and the low-Fe fluids released will not significantly impact the Fe isotope compositions of carbonated mantle peridotites. Interestingly, the recent study of [START_REF] Shen | Subducted Mg-rich carbonates into the deep mantle wedge[END_REF] reported light δ 26 Mg values for deep mantle wedge peridotites from the Dabie orogen (5.3-6.3 GPa; 800 °C) that were interpreted as evidence for the dissolution and transfer of sedimentary Mg-rich carbonate by supercritical fluids at subarc depths (∼160km), a scenario consistent with our observations from the Cerro del Almirez massif.

Conclusion

Our study results demonstrate the decoupled nature of the Fe and Zn isotope variations in Cerro del Almirez meta-serpentinites. The δ 56 Fe variations of ultramafic rocks are compatible with a mantle signature, as shown by covariations between δ 56 Fe and indices of melt extraction (i.e., Al 2 O 3 /SiO 2 ). The δ 66 Zn and Zn concentrations of the Atg-serpentinites are similar to those of slab derived serpentinites from high-pressure meta-ophiolites. However, the Chl-harzburgites are abnormally enriched in isotopically heavy Zn that is accompanied with an increase of Zn concentrations and Sr/Y, U/Yb, Ba/Ce and Rb/Ce ratios. These observations are concordant with the percolation of external carbon-bearing fluids enriched in isotopically heavy Zn. Geochemical models show that Zn isotope fractionation could be driven by [ZnHCO 3 (H 2 O) 5+ ] and [ZnCO 3 (H 2 O) 3 ] complexes in metamorphic fluids at temperatures ranging from 500 °C to 700 °C. Our models show that reduced carbon species cannot reproduce the observed δ 66 Zn variations in the massif. We conclude that the decoupling behavior of Fe and Zn during Chl-harzburgite metasomatism is consistent with the dissolution and mobility of Mg-(Zn)-rich and Fe-poor carbonates in oxidizing fluids at high pressures during subduction. 1 for more details on sample lithology). [START_REF] Schwartz | Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites[END_REF]3: Agard et al., 2001); Erro Tobbio (4: [START_REF] Hermann | The importance of serpentinite mylonites for subduction and exhumation of oceanic crust[END_REF]; Monviso (5: Schwartz et al., 2001 ;Angiboust et al., 2011);Lanzo (6: Pelletier and Muntener, 2006); Zermatt Saas ophiolite complex (7a: [START_REF] Martin | An eclogitized oceanic palaeo-hydrothermal field from the St Marcel valley (Italian Western Alps)[END_REF]7b:Li et al., 2004;7c: Reinecke et al., 1998;7d: Groppo et al., 2008;7e: Luoni et al., 2018); Central Alps including Cima di Gagnone (8a), Monte Duria (8b) and Alpe Arami (8c; [START_REF] Nimis | Revised thermobarometry of Alpe Arami and other garnet peridotites from the Central Alps[END_REF]; AL: Cerro del Almirez [START_REF] Sánchez-Vizcaíno | Petrology of titanian clinohumite and olivine at the high-pressure breakdown of antigorite serpentinite to chlorite harzburgite (Almirez Massif, S. Spain)[END_REF][START_REF] Laborda-López | High-P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones[END_REF]. and Liu et al. (2019). Orogenic peridotites values are from [START_REF] Weyer | Partial melting and melt percolation in the mantle: The message from Fe isotopes[END_REF], [START_REF] Debret | Isotopic evidence for iron mobility during subduction[END_REF], [START_REF] Pons | Zinc isotope evidence for sulfaterich fluid transfer across subduction zones[END_REF], [START_REF] Wang | Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle[END_REF], [START_REF] Sossi | Zinc isotope composition of the Earth and its behaviour during planetary accretion[END_REF] and Liu et al.(2019).

  and Moynier et al. (2017) for [ZnHCO 3 (H 2 O) 5+ , Zn(cit) 2 4-] and [ZnCO 3 (H 2 O) 3 ] complexes in fluids and those of Ducher et al. (2016) for smithsonite. The δ 66 Zn composition of the metasediment derived fluids was modelled using a simple Rayleigh distillation law, according to the following equations: 10 3 ln(α fluid-mineral ) = 10 3 ln(β fluid ) -10 3 ln(β mineral )

δ

  66 Zn mixture = (N a δ 66 Zn a +N b δ 66 Zn b ) / (N a +N b ) where N a and N b are the amounts of Zn transported by fluids and the initial abundance of Zn in Atgserpentinites, respectively. The results of these models are plotted in Figure 7 and serves to demonstrate that the high [Zn] and δ 66 Zn of Cerro del Almirez Chl-harzburgites is consistent with the percolation of [ZnHCO 3 (H 2 O) 5+ ] and [ZnCO 3 (H 2 O) 3 ] fluids at temperatures ranging from 500 °C to 700 °C.

Figure 1 :

 1 Figure 1: (a) Geological map of the Nevado-Filábride Complex (NFC) in the central part of the Betic

Figure 2 :

 2 Figure 2: Compilation of P-T data for alpine meta-ophiolites. Cold (4°C/km) and hot (8.5 °C/km) slab

Figure 3 :

 3 Figure 3: Fe and Zn isotopic ranges of each major lithological division of the Cerro del Almirez massif. The

Figure 4 :

 4 Figure 4: Plots of Al 2 O 3 /SiO 2 versus (a) MgO/SiO 2 , (b) δ 56 Fe and (c) δ 66 Zn in Cerro del Almirez ultramafic rocks. The thick grey line on Fig. 4a represent the bulk silicate earth ("terrestrial array" after Jagoutz et al.,

Figure 5 :

 5 Figure 5: Plots of δ 66 Zn versus (a) Zn concentrations and (b) Loss On Ignition (LOI) in Cerro del Almirez

Figure 6 :

 6 Figure 6: Plots of δ 66 Zn versus (a) Sr/Y, (b) U/Yb, (c) Rb/Ce and (d) Ba/Ce ratios in Cerro del Almirez

Figure 7 :

 7 Figure 7: Plot of δ 66 Zn (‰) versus Zn concentrations in Cerro del Almirez samples and presenting the results of the mixing models: additions of Zn by ZnCO 3 (H 2 O) 3 , ZnHCO 3 (H 2 O) 5 + and Zn(cit) 2 4-fluids at temperatures ranging from 500°C to 700°C.

AKNOWLEDGMENT We thank G. Nowell (Durham University, UK) for technical support and K. Burton (Durham University, UK) for fruitful discussions. This work was supported by a Natural Environment Research Council (NERC) Deep Volatiles Consortium Grant (NE/M000303/1), a European Research Council (ERC) Starting Grant (HabitablePlanet; 306655) to H. Williams, and by the TelluS Program of CNRS/INSU. B Debret acknowledges financial support by LabEx UnivEarthS (ANR-10-LABX-0023 and ANR-18-IDEX-0001). M. T. Gómez-Pugnaire, J. A. Padrón-Navarta, and C. Marchesi are thanked for early work and characterization of the samples investigated in the present study. We are grateful to the Sierra Nevada National Park authorities for providing the permit for fieldwork and sampling at the Cerro del Almirez massif. E.I. is supported as a postdoc on an ERC Horizon 2020 advanced grant (SHRED; 833632) awarded to Dr. C. Chauvel (IPG, Paris). C.J.G. and V.L.S.-V. acknowledge funding from the "Spanish Ministry of Science and Innovation" and "Agencia Estatal de Investigación (AEI)" grants no. CGL2016-75224-R and CGL2016-81085-R, and from the "Junta de Andalucía" research group grants RNM-131 and RNM-374. We thank S.-A. Liu, R. Halama and two anonymous reviewers for critical comments on earlier version of this article and careful editorial handling by D.A. Ionov.