# Geochemical signatures of forearc serpentinites from oceanic to continental subduction

Javiera Villalobos-Orchard, Baptiste Debret, Pierre Bouilhol, Rajeev

Upadhyay, Pamela Gutiérrez, Catherine Chauvel, Frédéric Girault

► To cite this version:

Javiera Villalobos-Orchard, Baptiste Debret, Pierre Bouilhol, Rajeev Upadhyay, Pamela Gutiérrez, et al.. Geochemical signatures of forearc serpentinites from oceanic to continental subduction. Geochimica et Cosmochimica Acta, 2025, 392, pp.1-16. 10.1016/j.gca.2024.11.001. hal-04974630

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# Geochemical signatures of forearc serpentinites from oceanic to continental subduction

Javiera Villalobos-Orchard<sup>a</sup>, Baptiste Debret<sup>a, *</sup>, Pierre Bouilhol<sup>b</sup>, Rajeev Upadhyay<sup>c</sup>,
Pamela Gutiérrez<sup>a</sup>, Catherine Chauvel<sup>a</sup>, Frédéric Girault<sup>a</sup>

## ARTICLE INFO

Associate editor: Jeffrey G. Catalano

**Keywords:**

Himalaya

Serpentinite

Oceanic subduction

Continental subduction

Trace elements

Sr-Pb isotopes

## ABSTRACT

In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts.

## 1. Introduction

Elemental cycles in subduction zones are commonly approached through the analysis of (meta-)ophiolites outcropping in mountain ranges (e.g., Alt et al., 2013; Deschamps et al., 2013; Kelemen and Manning, 2015; Cannaò and Malaspina, 2018; Pagé and Hattori, 2019). In recent decades, increasing research efforts and advances on (meta-) ophiolites have particularly highlighted the potential role of serpentinites, through their unique rheological and chemical properties, in

controlling seismic activity (Guillot et al., 2015; Horn et al., 2020), water (Ulmer and Trommsdorff, 1995), fluid mobility (Deschamps et al., 2013) and redox-sensitive elemental cycles (Debret and Sverjensky, 2017) in subduction zones. These rocks are mainly formed by hydration of mantle peridotite either at mid-oceanic ridges (MOR) or in forearc (FA) environments. However, it is often difficult to unravel the origin (i. e., MOR vs. FA) of (meta-)ophiolitic serpentinites as they have undergone complex tectonic and metamorphic pathways before and during emplacement on the continent. Based on a review of serpentinites

<footnote><sup>a</sup> Université Paris Cité, Institut de physique du globe de Paris, CNRS, Paris, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, CRPG, Nancy, France</footnote>

<footnote><sup>c</sup> Department of Geology, Kumaun University, 263 002 Nainital, India</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: debret@ipgp.fr (B. Debret).</footnote>
formed in active MOR and FA environments, Peters et al. (2017) proposed a geochemical systematic to decipher the origin of serpinţinizing fluids. However, this systematic was based on only a few meta-ophiolitic systems (European Alps and Andes) and did not investigate the consequences of the change in slab composition during the transition from oceanic to continental subduction, which can lead to (meta-)ophiolite exhumation (Shervais, 2001). Thus, the implications for the geochemistry and redox evolution of FA environments during subduction evolution remain to be addressed.

Forearc serpentinites are mainly formed by hydration and metasomatism of mantle peridotite by fluids released from the subducted slab. They are generally enriched in fluid-mobile elements (FME: Li, B, As, Sb, Rb, Cs, Pb, U) compared to peridotite and act as a reservoir for fluid-mobile and volatile elements (H₂O, S and C; Hattori and Guillot, 2007; Alt et al., 2013; Deschamps et al., 2013). They are thus an important geochemical reservoir for arc magma genesis and/or volatile emissions associated with continental collision (e.g., Debret et al., 2022). Therefore, they provide information on fluid/element transfer processes during subduction and the nature of the metasomatizing agent. The nature and fraction of FME transferred in fluids are strongly dependent on both P-T conditions and source composition (Deschamps et al., 2013; Spandler and Pirard, 2013; Cannào and Malaspaña, 2018). As these two parameters change during the transition from oceanic to continental subduction, they should be reflected in the geochemical composition of FA serpentinites. In this way, we can assess how element fluxes to the forearc region evolve due to changes in slab composition and geodynamic regime, and help elucidate the role of FA serpentinites as a temporary reservoir of FME and volatile elements. Ultimately, this has important implications for understanding the global elemental cycling that accompanies the transition from oceanic to continental subduction.

To study such a transition, the NW Himalaya is a prime target, as it has preserved the geological record of a subduction system that evolved from intra-oceanic to continental subduction during the India-Eurasia convergence (Burg and Bouilhol, 2019). During the Cretaceous, a double subduction system existed in the Neo-Tethys ocean, where the Kohistan-Ladakh Arc (KLA) was part of an equatorial intra-oceanic island arc system with an Andean-type margin to Eurasia further north (Martin et al., 2020). During the Eocene, the Indian margin subducted beneath the KLA and eventually collided with the Eurasian margin (Bouilhol et al., 2013; Jagoutz et al., 2019). Relicts of these different subduction stages are preserved in the numerous ophiolitic complexes and melange zones that outcrop along the suture zone between India and the KLA in Eurasia (Guillot et al., 2000; Robertson, 2000; Mahéo et al., 2006). These relicts provide important information not only on the geodynamic evolution of the system, but also on the petrological and geochemical imprints of the different subduction stages and thus on the subduction history. In particular, FA serpentinites exhumed along the Indus Suture can record the mass transfer from the slab to the FA as the latter transitioned from oceanic to continental subduction (Hattori and Guillot, 2007). It therefore allows for a detailed study of changes in the subduction regime and their implications for elemental recycling.

In this study, we present a comprehensive geochemical characterization of a suite of FA serpentinites from Ladakh, in the NW Himalayas, as well as part of the material subducted during the oceanic and continental subduction stages. By analysing trace element ratios and radioenic isotope compositions, we trace changes in the origin of metasomatic agents added to the FA mantle from oceanic to continental subduction, providing important information on elemental recycling during the India-Eurasia convergence and pinpointing the origin of serpentinite slivers along the Indus Suture.

## 2. Samples and geological setting

The samples selected for this study are from the Indus Suture Zone in Ladakh (NW Himalaya), which welds the Kohistan-Ladakh Arc (KLA) in

the north to the Indian continent in the south (Fig. 1). The KLA formed above a northward-trending intra-oceanic subduction zone in the Neo-Tethys ocean, which was mainly active during the late Mesozoic, before colliding with the Indian continent at 50 Ma (Bouilhol et al., 2013). In Ladakh, the KLA is mainly represented by a Late Cretaceous-Early Cenozoic batholith, with little remaining of its volcanic-sedimentary cover (Fig. 1). However, part of the KLA is now included in the Indus Suture as ophiolitic remnants, volcanic flows and volcanoclastic sediments (Nidar ophiolite, Dras and Nimdam sequence; Fig. 1), which represent the forearc sequence reworked during collision (Reuber, 1989; Robertson and Degnan, 1994). As such, the Indus Suture represents a long-lived system that has recorded the history of oceanic subduction to collisional convergence, providing geological evidence for the evolution of this complex geodynamic setting. Three main localities were sampled along the Indus Suture Zone (Fig. 1): the first two, namely the Shergol-Tingdo and Kargil areas, are from ultramafic strips that mark the Indus Suture to the west (Shergol and Pushkum sections) and are associated with Cretaceous blueschists (Honegger et al., 1989; Robertson, 2000); while the third, in the eastern part of the suture, is pinched between the Tso Morari ultra-high pressure (UHP) unit and the KLA forearc along the Zildat fault (Sumdo section, Fuchs and Linner, 1996; Steck et al., 1998; de Sigoyer et al., 2004). In addition, we sampled the associated HP and UHP rocks, representing the material subducted during the oceanic (Shergol blueschists) and continental (UHP gneisses) subduction stages, as a reference for the subducted end-member components during the evolution of the Himalayas (see Supplementary Table S1).

Strips of Shergol and Kargil serpentinites outcropping in the western part of the suture are associated with the FA sequence of the KLA (Robertson, 2000). Structurally, these strips correspond to thrust zones (Fig. 1). In the west, near Shergol, the ultramafic rocks, often associated with pelagic sediments and diabase, are overthrusted by the Indian distal passive margin sediments (Lamayuru unit) and thrust over the FA sequence (volcanic-sedimentary sequence, Dras/Naktul/Nimdam units). At Pushkum, the ultramafic rocks are overthrusted by the FA sequence and thrusted over the Paleocene post-collisional sediments to the north (Indus Group sediments) (Brookfield and Andrews-Speed, 1984; Robertson, 2000). The thrust often consists of blueschist facies rocks (350 to 470°C and 0.9–1.9 GPa; Honegger et al., 1989; Groppo et al., 2016) of diverse origin, representing remnants of the Cretaceous intra-oceanic accretionary prism (Honegger et al., 1989; Mahéo et al., 2006). Between these two strips lies the deformed and faulted Nimdam/Dras/ Naktul sequence, which represents a suite of volcanoclastic and massive volcanic deposits attributed to the FA sequence of the KLA. Although the origin of the serpentinite protolith in this area remains controversial, both structural evidence and mineral chemistry point to a cold and shallow FA mantle (~500°C) that was partially metasomatized during the formation of the KLA (Bhat et al., 2017). Five variably foliated serpentinites were sampled near the villages of Tingdo and Shergol, along with two blueschist facies metasediments (Fig. 2a). The ultramafic samples show mesh and bastite textures associated with minor magnetite (Fig. 2b). Penetrative antigorite was also observed in two samples from Shergol (Supplementary Table 1).

The third group of serpentinites comes from the Tso Morari area, on the eastern part of the suture (Sumdo section). The Tso Morari HP-UHP complex is part of the Indian passive margin that was subducted during continental subduction in the Eocene (Steck et al., 1998 and references therein). It is mainly composed of Ordovician orthogneisses whose protolith intruded Precambrian to Palaeozoic sediments, all of which were metamorphosed under eclogitic conditions (2 to 4.8 GPa and 550 to 760°C; de Sigoyer et al., 1997; St-Onge et al., 2013; Wilke et al., 2015). To the north, the Tso Morari unit is bounded by the Zildat fault, which marks the contact of the HP-UHP unit with largely unmetamorphosed volcanic and sedimentary units belonging to the FA sequence of the KLA (Ribil-Karzog unit), a lateral equivalent of the Nimdam/Dras sequence. Within the Zildat fault, serpentinite boudins are found
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological map of Ladakh showing the Indus Suture within the KLA-Karakoram-India collision zone. Samples come from three main sections (inset): two from the western side of the suture (Pushkum and Shergol) and one from the eastern side (Sumdo section).</figcaption></figure>

together with strongly sheared slices of blueschist (Virdi, 1979). The serpentinite lenses are highly sheared and the style of deformation, including a top to the northeast normal shearing, is similar to that in the Tso Morari nappe, suggesting that both units were exhumed together (Guillot et al., 2001). Previous structural and geochemical studies showed that the Tso Morari serpentinites were derived from a highly refractory mantle wedge (Guillot et al., 2000, 2001; Deschamps et al., 2013). These rocks were partially serpentinized at high pressure and high temperature (HP-HT; ~650°C) during slab devolatilisation before being exhumed with continental units during the Himalayan collision (Hattori and Guillot, 2007). We studied 13 serpentinite samples collected from two different boudins near Sumdo village. These serpentinites are variably sheared. The foliation envelops blocks of massive serpentinites and meta-mafic rocks of metric size. The massive serpentinites are crosscut by large metamorphic veins 2–10 cm wide in relief of dark clinohumite and chondrodite crystals up to several centimetres wide, associated with olivine and magnesite (Fig. 2c). The serpentinite hosts consist mainly of antigorite, spinel, clinohumite, olivine, carbonate and talc (Fig. 2d-e). The amounts of clinohumite, olivine, talc and carbonate are highly variable among the samples studied, with most samples showing an antigorite-talc ±carbonate paragenesis (Supplementary Table S1). The occurrence of clinohumite, chondrodite and olivine is symptomatic of HP-HT eclogitic conditions associated with serpentinite dehydration in subduction zones (Trommsdorff and Evans, 1972; Evans and Trommsdorff, 1983; De Hoog et al., 2014; Shen et al., 2014; González-Jiménez et al., 2017). In contrast, the association of both talc and antigorite indicates lower pressure conditions (<1.5 GPa; Padrón-Navarta et al., 2010). This stage of serpentinization often partially to completely overprinted HP-HT relicts.

In this area we completed our sample set with continental material representing the subducted Indian margin. Eclogitic (ortho- and para-) gneisses of the Tso Morari nappe (Epard and Steck, 2008) were sampled along a WSW-ENE transect. As such, they have different structural positions relative to the overlying serpentinite lenses (i.e., from LD19-76 to LD19-82 and 98, the samples are closer to the serpentinites) and show a wide range of compositions (Supplementary Table 2), covering the range of rock types that make up the subducted Indian margin. The close association of slab-derived fractions with FA serpentinite is a common feature of paleo-FA environments (Guillot et al., 2001), where FA serpentinites acted as a lubricant for the exhumation of HP units.

### 3. Methods

#### 3.1. Sample preparation

Due to the intrinsic heterogeneity of the serpentinite samples and the presence of structures such as veins, hand samples were carefully examined to select the most representative sections for preparation. For example, sample LD-19-83, which contains a large carbonate vein, was split in two and the vein processed as a separate sample. Sufficient material was processed for each sample to ensure that the resulting samples were sufficiently homogenised.

The selected sections were cut into ≤1 cm thick slabs using a diamond saw and then polished with coarse sandpaper until saw marks were visibly removed to eliminate any metal contamination from the saw. The samples were then thoroughly rinsed with water and air dried. The rock slabs were crushed into chips (< 0.5 cm²) using an agate mortar and pestle cleaned with ethanol between samples, and the
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Field and thin section (micro)photographs of the Shergol (a-b) and Tso Morari (c-e) serpentinites. (a) Massive serpentinite composing the Shergol unit. (b) Mesh and bastite textures composing the massive serpentinites of Shergol. (c) Metamorphic vein of humite, olivine and carbonate crosscutting massive serpentinites in the Tso Morari unit. (d) Plane-polarised light microphotograph of a metamorphic vein. The humite minerals consist of micrometric intergrowths of clinohumite and chondrodite. They form anhedral grains more than 200 µm wide, with a pale brown colour and sometimes a dusty appearance due to numerous mineral inclusions. Clear, anhedral olivines several hundred of micrometres wide are associated with humite minerals. (e) Serpentinites composed of antigorite and crosscut by a pervasive network of carbonate and talc veins. The veins are often associated with magnetite.</figcaption></figure>
resulting chips were finally processed in an agate ball mill until fine powders were obtained (between 30 and 40 g of powder per sample).

## 3.2. Analytical methods

### 3.2.1. Major and trace elements

Major element analyses were carried out at SARM-CRPG (Service d'Analyses des Roches et des Minéraux - Nancy, France) by alkali rock fusion using LiBO₂ and concentrations measured by inductively coupled plasma optical emission spectrometry (Thermo Scientific iCap6500 ICP-OES) following the procedure of Carignan et al. (2001). Both total sulfur (SO₄) and carbon (CO₂) were quantified by infrared analysis of the loss on ignition components using a HORIBA EMIA 320V2 analyser. The samples were heated by induction with Fe, W and Sn gases. The gases produced were passed through a halogen trap and a desiccant filter prior to measurement. Calibration was performed with reference chemicals and measurements are checked with reference materials by the SARM. The water (H₂O) content was quantified by volumetric dosing using a Karl Ficher liquor. The sample was calcined at 1000°C in a tube furnace under a nitrogen stream. The released vapour was drawn into the titration vessel, initiating the reaction and titration, which is fully automatic. Results for the reference materials BHVO-2 (basalt, USGS) and UB-N (serpentinite, CRPG), analysed with the samples, were generally within 2% of the reported values and within the reported uncertainties for all major and volatile elements (Supplementary Table S2).

Additional FeO analyses were performed by automated titration at the equivalent point with potassium dichromate after dissolution of the sample in a HF/H₂SO₄ mixture in the presence of H₃BO₃ and H₃PO₄. The Fe³⁺/ΣFe ratios were then calculated from the measured Fe₂O₃ and FeO values. The error of the FeO analyses was less than 0.2 wt% (at two standard deviations) based on repeated analyses of the CRPG UB-N serpentinite standard. The reference material values for UB-N (FeO = 3.03 wt%) and BHVO-2 (FeO = 9.23 wt%) were in agreement with previous studies (e.g., Amonette and Scott, 1991; Xue et al., 2020; FeO(UB-N) = 3.01 wt%; FeO(BHVO-2) = 9.08 wt%).

For trace element analyses, 50 mg of whole rock powder was digested with concentrated HF and HCl acids in 7 mL PTFE Teflon square digestion vessels with wrench top closures in an oven at 160°C for 3 days. After evaporation of the HF and HCl, the samples were treated with a 1:3 mixture of concentrated HCl and HNO₃ and refluxed in an oven at 140°C for 3 days. Finally, the samples were brought into solution in 6 M HNO₃ prior to trace element measurements. Trace element contents were measured using an Agilent 7900 ICP-QMS at IPGP (France), except for B and F, in three analytical sessions. Li, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, As, Mo, Ba and REE (rare earth elements) were analysed in ultramafic rocks together with BCR-2 USGS basalt, UB-N CRPG serpentinite (UB-N (1)) and PCC-1 USGS peridotite. Solutions were diluted 10,000 times in HNO₃ acid. An additional run of analyses for Rb, Sr, Zr, Nb, Cs, Hf and Th was carried out using solutions diluted 10,000 times in HNO₃ and HF acids together with UB-N CRPG serpentinite (UB-N (1)). Metasediments were analysed in solutions diluted 10,000 times in HNO₃ acid together with UB-N CRPG serpentinite (UB-N (2)) and BHVO-2 USGS basalt. Elements with masses between those of titanium (47) and arsenic (75) as well as silver (107), cadmium (111) and gadolinium (157) were measured using a collision reaction cell with helium gas (5 mL min⁻¹) to remove polyatomic interferences. All other elements were measured without collision gas. Scandium, indium and rhenium internal standards were injected after inline mixing with the samples to correct for signal drift and matrix effects.

UB-N (1) and BHVO-2 standards were analysed repeatedly along the analytical sessions for ultramafic and metasediments, respectively. The reproducibility of UB-N (1) analyses was better than 10% for Li, V, Co, Zn, As, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Tb, Tm, Hf, Pb, Th, U, between 10–15% for Ti, Cu, Gd, Dy, Ho, Er, Yb, Lu and of 25% for Sb (only one value was compiled from the literature) when compared with

published values (see Appendix). The reproducibility of BHVO-2 was better than 10% for most trace elements, only Li, Y, Sb, Cs and Th have a reproducibility between 10 and 25%. It should be noted that the As reproducibility on BHVO-2 is too high compared with the standard values, although the latter are associated with a large error, comparable to that of the analytical session. In contrast, the As reproducibility of UB-N (2) is between 10 and 15% (Supplementary Table S2). Boron concentrations were measured spectrophotometrically using an Agilent CARY60 and F was determined by an ion-selective electrode at SARM. For boron, the long-term reproducibility was between 10% and 20% at 100 ppm and 10 ppm, respectively (Och and Shields-Zhou, 2012). For fluorine, the long-term reproducibility was between 3% and 20% at 1000 ppm and 20 ppm, respectively (Vernet et al., 1987). The reference material values for UB-N (B = 140 ppm; F = 96 ppm) and BHVO-2 (B = 2.5 ppm; F = 400 ppm) were consistent with previous studies (e.g., Govindaraju, 1995; Jochum et al., 2016; B(UB-N) = 140 ppm; B(BHVO-2) = 2.95 ppm; F(UB-N) = 95 ppm; F(BHVO-2) = 396 ppm).

### 3.2.2. Radiogenic isotopes

Strontium isotope analyses were performed at SARM using a ThermoFisher Triton Plus thermal ionisation mass spectrometer (TIMS) following the procedure of Bouilhol et al. (2022). Approximately 100 mg of sample powder was digested in Teflon beakers with a mixture of concentrated ultrapure acid (HNO₃ + HF) on a hotplate for two days. The solutions were then heated on a hotplate in ultrapure HCl for one day. Strontium purification was carried out using a Sr-spec resin according to Pin and Bassin (1992). Instrumental mass bias was corrected by internal normalisation using ⁸⁶Sr/⁸⁸Sr = 0.1194. The NBS987 was used as a reference standard with a reference value of ⁸⁷Sr/⁸⁶Sr = 0.710259 ± 0.000019.

Lead isotope analyses were carried out at IPGP as follows. Depending on the Pb concentration of each sample, 0.1–2.5 g of powdered samples were weighed and dissolved in a mixture of high-purity HNO₃-HF at 110 °C for at least 48h. The samples were dried and refluxed in high-purity HCl until complete dissolution. A few drops of concentrated HBr were added to the beakers before drying. Anionic chromatography was performed according to a modified version of the protocol of Manhes et al. (1984). Samples were dissolved in 0.7M HBr and loaded onto retractable Teflon columns filled with AG1-X8 resin. Large samples were divided into several aliquots and loaded onto several columns, each containing a maximum of 200 mg of dissolved sample. The matrix was eluted with 0.7M HBr and 2M HCl, and Pb was collected by elution with 6M HCl. The chromatography was repeated to obtain a pure Pb fraction.

The isotopic composition of Pb was measured at IPGP on a Thermo Scientific Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using the thallium addition technique (White et al., 2000). The NIST 981 standard was measured every two samples for normalisation, with values published by Jochum et al. (2011). Solutions were prepared to a concentration of 100 ng mL⁻¹, resulting in an applied voltage of approximately 20 V to ²⁰⁸Pb. Reproducibility and accuracy were evaluated by measuring a procedural blank, replicate samples and reference material UB-N (²⁰⁸Pb/²⁰⁴Pb = 38.324 ± 0.002 ‰; ²⁰⁷Pb/²⁰⁴Pb = 15.623 ± 0.0005 ‰; ²⁰⁸Pb/²⁰⁴Pb = 18.311 ± 0.0005 ‰) of known composition (Chauvel et al., 2011) (Supplementary Table S2). The blank content was negligible (<32 pg Pb).

## 4. Results

### 4.1. Major elements

The major element compositions of the Ladakh FA serpentinites are given in Supplementary Table S2. The Al₂O₃/SiO₂ and MgO/SiO₂ ratios of the Ladakh serpentinites follow a similar trend to that of the silicate Earth differentiation (Fig. 3), with low Al₂O₃/SiO₂ (≤ 0.05) and high MgO/SiO₂ (0.84 to 1.26), indicating that their protoliths represent a
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Major element compositions of bulk rocks, shown as MgO/SiO₂ versus Al₂O₃/SiO₂. The pink star represents the depleted MORB mantle (DMM) (Salters and Stracke, 2004), the black line marks the silicate Earth differentiation trend and fields represent natural dunite and harzburgite end-members (Godard et al., 2008). Cb: carbonate.</figcaption></figure>

refractory mantle source. There is no distinction between samples from Shergol-Tingdo, Kargil and Tso Morari, with compositions overlapping between the three groups.

The SiO₂ content of the Ladakh serpentinites varies from 29.6 to 41.3 wt% (mean 37.5 ± 1.3 wt%, 2sd/√(n), n = 21) and is negatively correlated with total CO₂ content (Supplementary Fig. S1a). This is to be expected, as a higher proportion of carbon-bearing minerals relative to silicates in the rock will increase the total carbon content. On the other hand, total CO₂ correlates positively with CaO content only in serpentinites from Shergol-Tingdo and Kargil, and samples LD19-83A, -85, -87, -91, -93, -103 and -105 from Tso Morari (Supplementary Fig. S1b), suggesting that carbon in these samples is mainly present as calcite, whereas the remaining Tso Morari serpentinites have higher carbon contents relative to CaO, suggesting the presence of additional carbon-bearing phases (e.g., magnesite).

The loss on ignition (LOI) of the Shergol-Tingdo, Kargil and most of the Tso Morari serpentinites correlates with H₂O content (Supplementary Figure S2), indicating that water in serpentine is the major volatile present in these samples. For the remaining samples (LD19-46, -84, -86, -91, -111), a broad relationship between total CO₂ content and LOI can be identified (Supplementary Figure S2).

The Fe³⁺/∑Fe of the FA serpentinites varies over a wide range, between 0.15 and 0.82 (Supplementary Table S2). While there is considerable overlap between all three groups of samples, the high temperature serpentinites from Tso Morari extend to lower values than the low temperature samples from Kargil and Shergol. The Fe³⁺/∑Fe of the Ladakh serpentinites does not covary with LOI or CO₂ content. Surprisingly, metamorphic olivine-bearing samples have significantly higher Fe³⁺/∑Fe than the rest of the Tso Morari samples, regardless of their LOI or carbon content (Fe³⁺/∑Fe = 0.48 – 0.67 versus 0.22 – 0.44; Supplementary Table S2).

Metasedimentary rocks are very heterogeneous with respect to major elements. In particular, they show important differences in SiO₂ (48.8–88.35 wt%), Al₂O₃ (2.7–18.5 wt%) and Fe₂O₃ (0.4–8.2 wt%), reflecting the important lithological heterogeneity accreted in FA environments. The Tso Morari orthogneiss has a composition close to that of the Polokongka La granite.

## 4.2. Trace elements

The analyses of the Tso Morari serpentinites are in good agreement with Deschamps et al. (2010). They are generally less REE-depleted than those from Shergol-Tingdo and Kargil (Fig. 4a). The studied samples have higher REE concentrations than other more typical FA serpentinites sampled in active environments, such as those from the Marianas (Savov et al., 2005, 2007; Debret et al., 2019). The Shergol-Tingdo and Kargil samples show LREE (light REE) and MREE (medium REE) depletion with respect to HREE (heavy REE; e.g., (La/Yb)ₙ = 0.03 – 0.68, N: chondrite normalised), whereas the Tso Morari serpentinites show overall flatter REE patterns. In all sample groups there is more variation in LREE and MREE than in HREE. In particular, the MREE patterns are quite contrasting, especially for Eu, where a negative anomaly of variable magnitude is observed for the majority of samples (Eu/Eu* = 0.18 – 1.05; median = 0.76; Eu* = (Sm + Gd)ₙ / 2).

All Ladakh samples are generally enriched in B, Sr and Cs and depleted in Rb, Zr and Hf with respect to neighbouring elements in a multi-element plot (Fig. 5a). However, in terms of fluid-mobile element enrichments, the Tso Morari samples are notably enriched in As, Sb, Pb and especially U, whereas the Shergol-Tingdo and Kargil serpentinites show relative depletion in these elements, especially As and Sb, and a notable positive B anomaly (Fig. 5b). Typically, FA serpentinites show high enrichments in Cs and Rb relative to neighbouring elements (Peters et al., 2017), which are not observed here, especially Rb. Serpentinites are also characteristically enriched in U and not significantly enriched in Th (Deschamps et al., 2013), which is reflected in the Tso Morari samples by strong positive U anomalies and high U/Th (3.0 – 1940, except sample LD19-110 with a low value of 0.9), while the Shergol-Tingdo and Kargil serpentinites have low U/Th (0.1 – 3.0, except sample LD19-26 with a high value of 53.5) and no or even slightly negative U anomalies. In addition, the Tso Morari serpentinites show strong negative Li anomalies, whereas the Shergol-Tingdo and Kargil samples mostly show relative Li enrichment.

## 4.3. Radiogenic isotopes: Sr and Pb

Initial isotopic ratios were calculated using the concentrations obtained from the ICP-MS measurements. Considering the trace element concentrations of the samples, as well as previous works (Robertson, 2000; Hattori and Guillot, 2007), we hypothesised that the samples were serpinminized in two specific environments, one most likely during an oceanic subduction stage (Shergol-Tingdo and Kargil) and one during Indian subduction (Tso Morari). We calculated the initial ratio at 100 Ma (age of blueschist in Shergol; Honegger et al., 1989) and 50 Ma (age of continental subduction; Bouilhol et al., 2013), respectively. Strontium isotope ratios are significantly different between the three serpentinite groups. The Shergol-Tingdo serpentinites have relatively unradiogenic ⁸⁷Sr/⁸⁶Sr(100) of 0.7079 for sample LD19-17 and 0.7071 for sample LD19-20, which overlap with those of the Shergol blueschist samples (⁸⁷Sr/⁸⁶Sr(100) = 0.7062 – 0.7075; Supplementary Tables S2). The two Kargil samples are less homogeneous, with sample LD19-46 having the least radiogenic Sr isotope ratio (⁸⁷Sr/⁸⁶Sr(100) = 0.7046), while sample LD19-47 has a significantly more radiogenic ⁸⁷Sr/⁸⁶Sr(100) of 0.7169. In contrast, the Tso Morari serpentinites have the most radiogenic ⁸⁷Sr/⁸⁶Sr(50) values of the Ladakh serpentinites, ranging from 0.7199 for sample LD19-85 to 0.7311 for sample LD19-91.

Meanwhile, the gneisses representing the continental material show a wide range of Sr isotope ratios. In particular, eclogitic metasediments have ⁸⁷Sr/⁸⁶Sr(50) of 0.7294 and 0.7753, and sample LD19-82, composed mainly of phengite, has a more radiogenic ⁸⁷Sr/⁸⁶Sr(50) of 0.8104, similar to that of Polokongka La granite (sample LD19-79), protolith of the Tso Morari orthogneisses (⁸⁷Sr/⁸⁶Sr(50) = 0.8143). Some paragneisses (samples LD19-76 and LD19-78) show extremely radiogenic values (⁸⁷Sr/⁸⁶Sr(50) = 0.9825 – 1.0550).

There is a negative correlation between Sr isotope ratios in the
<figure><img src="image_6.png" /><figcaption>Fig. 4. Patterns of rare earth elements normalised to chondrite composition.</figcaption></figure>

Ladakh serpentinites and B/U, a fluid-mobile element ratio that compares elements mobilised at low and high temperatures, respectively (Fig. 6a). In addition, positive correlations are observed between L-MREE/HREE and $^{87}$Sr/$^{86}$Sr(i) (Fig. 6b), suggesting that the metasomatizing agent incorporating L-MREE into the FA mantle is from a radiogenic Sr source.

In terms of Pb isotope composition, only samples from Kargil and Tso Morari could be analysed due to the overall extremely low Pb content of the Ladakh serpentinites. Sample LD19-46 has relatively unradiogenic Pb isotope ratios ($^{206}$Pb/$^{204}$Pb$_{(100)}$ = 18.092; $^{207}$Pb/$^{204}$Pb$_{(100)}$ = 15.554; $^{208}$Pb/$^{204}$Pb$_{(100)}$ = 38.101) which, as observed for Sr isotopes, are within the range of Shergol blueschists (see Supplementary Table S2). The Tso Morari serpentinites have more radiogenic Pb isotope ratios, which are quite homogeneous for three of the analysed samples, with $^{206}$Pb/$^{204}$Pb$_{(50)}$ ranging from 18.970 to 19.320, $^{207}$Pb/$^{204}$Pb$_{(50)}$ from 15.780 to 15.792, and $^{208}$Pb/$^{204}$Pb$_{(50)}$ from 39.181 to 39.274. However, sample LD19-85, which belongs to the group of Tso Morari secondary olivine-bearing serpentinites, has less radiogenic Pb isotope ratios ($^{206}$Pb/$^{204}$Pb$_{(50)}$ = 17.166; $^{207}$Pb/$^{204}$Pb$_{(50)}$ = 15.574; $^{208}$Pb/$^{204}$Pb$_{(50)}$ = 37.967). As for the Tso Morari gneiss samples, the Pb isotope ratios mostly overlap with the Tso Morari serpentinites and vary from 18.467 to 21.882 for $^{206}$Pb/$^{204}$Pb$_{(50)}$, from 15.741 to 15.945 for $^{207}$Pb/$^{204}$Pb$_{(50)}$, and from 38.815 to 39.127 for $^{208}$Pb/$^{204}$Pb$_{(50)}$. The exception is the eclogitic metasediment LD19-82, which is almost entirely phengite and probably derived from a U-rich pelite, with extremely radiogenic Pb isotope ratios ($^{206}$Pb/$^{204}$Pb$_{(50)}$ = 31.769; $^{207}$Pb/$^{204}$Pb$_{(50)}$ = 16.489; $^{208}$Pb/$^{204}$Pb$_{(50)}$ = 41.600).

## 5. Discussion

Forearc serpentinites are generally enriched in FME such as As, Sb, Cs, Pb, Th and U (Peters et al., 2017), halogens (Pagé et al., 2018), and radiogenic Sr and Pb (Deschamps et al., 2010, 2013) compared to MOR serpentinites. These geochemical features trace the influence of slab-derived fluids due to the release of sediment-derived pore fluids and the breakdown of hydrous phases in the slab, such as amphibole (Cannaò and Malaspina, 2018), during the serpentinization of FA peridotites. Previous field studies attributed the studied serpentinites to have formed in FA environments (Guillot et al., 2000, 2008; Robertson, 2000). However, the contrasting patterns of trace element enrichment suggest important differences in the conditions and/or origin of material

transferred to the FA mantle during serpentinization of the Ladakh serpentinites. In particular, the Shergol-Tingdo and Kargil serpentinites exhibit FA-like geochemical signatures as defined by Peters et al. (2017), whereas the Tso Morari serpentinites, although enriched in fluid-mobile elements, halogens, and radiogenic Sr and Pb, exhibit anomalous geochemical signatures compared to those of active FA serpentinites. In the following subsections, we examine in detail the contrasting trace element enrichment patterns of the Ladakh serpentinites and attempt to elucidate the origin of these differences in the context of Himalayan evolution.

### 5.1. Fluid-mobile element enrichments in Ladakh serpentinites

The Shergol-Tingdo and Kargil serpentinites have high B/U and Li/U ratios, low As/Th and Sb/Th ratios, and Cs/U and Rb/U ratios similar to or slightly higher than the average primitive and depleted mantle. A similar distribution of trace element enrichments can be observed for the Mariana FA serpentinites (Debret et al., 2019). These trace element ratios have been reported to be characteristic of FA serpentinites formed in the context of active oceanic subduction (Peters et al., 2017). In contrast, the Tso Morari samples tend to have low B/U, Li/U, Cs/U and Rb/U ratios and high As/Th and Sb/Th ratios as a result of their pronounced enrichment in U, As and Sb and relative depletion in alkalis.

Before assessing the role of the serpentinizing fluid on the composition of FA serpentinites, we first consider the role of the protolith. Although the L-MREE patterns are relatively flat, a slight enrichment of LREE over MREE can be observed for some samples, mainly from Tso Morari (e.g., (La/Sm)$_{N}$ = 0.63 – 2.33; median: 1.47). LREE can be influenced by both pre-serpentinization (e.g., melting and melt-rock interactions) and serpentinization processes. The LREE are more easily transported than Th (or HFSE) in aqueous solutions, whereas melt-rock interactions result in the addition of LREE and Th to the rock in approximately equal proportions (Niu, 2004; Paulick et al., 2006). The studied samples have comparable Th concentrations, depleted in Th relative to the primitive mantle (Fig. 7a), which is compatible with a refractory protolith (Deschamps et al., 2010). In contrast, the LREE/HREE ratios are about 3 orders of magnitude more variable. This rules out melt-rock interactions as the dominant factor controlling the compositional variability of LREE in the studied samples. Similar reasoning applies for U, alkali and FME elements (Fig. 7b-c), the latter showing even larger order of magnitude variations compared with fluid-
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Chondrite-normalised multi-element diagrams. (a) Full trace element diagram and (b) selected trace elements (including fluid mobile).</figcaption></figure>

immobile elements, with Tso Morari serpentinites being particularly enriched in As, Sb and U compared to Shergol and Kargil serpentinites (Fig. 7c-d). Taken together, this indicates that protolith composition does not exert a major control on LREE and FME enrichments in Himalayan FA serpentinites.

Elements such as B, As, Sb and Cs can be incorporated into supra-
subduction zone serpentinites even at low temperatures (<200-300°C;
Cannaò and Malaspina, 2018; Deschamps et al., 2013, 2010; Seyfried
and Dibble, 1980) and shallow depths (<1 GPa; Cannàò and Malaspina,
2018), as sediment compaction triggers the release of pore fluids (Moore
and Vrolijk, 1992). However, in the Mariana FA, significant As and Sb
enrichment in serpentinites was only observed in clasts recording tem-
peratures above 250°C, while B contents showed no variation with
temperature (Debret et al., 2019). This difference in serpentinite clast
composition was interpreted in terms of a change in slab fluid compo-
sition with depth, rather than temperature control (Debret et al., 2019).

Although metamorphic fluids are likely derived from oceanic sediment
compaction at shallow depths (<15 km), metamorphic devolatilisation
takes over at greater depths (Bebout, 2013; Kelemen and Manning,
2015), resulting in a change in elemental fluxes to the FA. For the
Ladakh FA serpentinites, the As and Sb contents in Shergol-Tingdo and
Kargil are in the very low range of those observed in the Marianas,
despite a comparable temperature record, whereas the Tso Morari ser-
pentinites have As and Sb concentrations at least one to two orders of
magnitude higher than those in Shergol-Tingdo, Kargil or the Marianas.
Such a wide range of As and Sb contents in the Himalayan serpentinites
can therefore most likely be explained by processes similar to those
inferred for the Marianas, i.e., a change in the origin and/or composition
of the metasomatizing fluids. Fluids originating from the oceanic slab
would be low in As and Sb but moderately enriched in Li, Cs, Sr, Rb, Ba
and U, a composition inherited from seawater and its interaction with
oceanic rocks at the seafloor (Li and Lee, 2006; Deschamps et al., 2010).
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (a) B/U ratio and (b) chondrite-normalised Gd/Lu ratio as a function of initial Sr isotope ratios of Ladakh serpentinites.</figcaption></figure>

This scenario is also consistent with the relatively unradiogenic Sr values of the Shergol-Tingdo and Kargil serpentinites (Supplementary Table 2).

The oceanic slab may also have retained As and Sb due to the stability of phases such as garnet, amphibole, phengite or accessory phases such as Ni arsenides (Cannaò and Malaspina, 2018; Pokrovski et al., 2022), further depleting the released fluids of some of these fluid-mobile elements. Indeed, recent studies on Ladakh serpentinites have shown that As is not fluid mobile at temperatures below 500°C due to reducing conditions in the fluid and the presence of Ni arsenides (Pokrovski et al., 2022). Furthermore, fluid FME concentrations increase with progressive serpentinization as the fluid rises through the FA mantle and water is consumed, leading to enrichment of FME elements in serpentinites formed from these “mature” slab-derived fluids (Hattori and Guillot, 2007). If the serpentinites were formed after mantle rocks near the trench at shallow depths, the fluid may not have evolved long enough to concentrate FME (e.g., Sb, As, U) to be incorporated into the FA mantle during serpentinization, or even to reach a high enough temperature and oxidation state to mobilise As and Sb (Pokrovski et al., 2022). On the other hand, the Tso Morari serpentinites appear to have been metasomatized by an As-Sb rich fluid (Fig. 7b). Sedimentary materials typically have radiogenic Sr, as found in the Tso Morari serpentinites, and

are particularly enriched in some fluid-mobile elements such as As and Sb (Li, 1991; Plank and Ludden, 1992). Note that the As/Th and Sb/Th of the metasediment samples analysed here are low (Fig. 7b), but this is due to very high Th contents (Th = 3.0 – 18.4 ppm) rather than low As and Sb contents (As = 0.8 – 40.8 ppm; Sb = 0.05 – 1.35 ppm).

The origin of the high As and Sb contents of the Tso Morari serpentinites has already been attributed to the subduction of high As-Sb metasediments derived from old granitic rocks of the northern Indian continental margin in an accretionary wedge, on the basis of trace element and Pb isotope data (Guillot et al., 2001; Hattori and Guillot, 2007; Deschamps et al., 2010). Here we show that this imprint is not systematic in FA serpentinites from the Indus Suture Zone and is notably absent in the low temperature Shergol-Tingdo and Kargil serpentinites. Such observations support the fact that the western serpentinites were metasomatized by fluids derived from an oceanic slab. This supports the idea that the variability in As and Sb contents in the Indus Suture Zone serpentinites is related to a change in slab composition from an oceanic Mariana-like subduction system (Shergol-Tingdo and Kargil) to a sedimentary continental-like system (Tso Morari). The absence of an early, low-temperature geochemical signature in the Tso Morari serpentinites can be attributed to either sampling bias and/or the complex metamorphic history of the massif, which was dehydrated to olivine-humite-antigorite (Fig. 2c-d) prior to a second serpentinization step to talc-antigorite (Fig. 2e). However, no geochemical differences were observed between the HP-HT and LP-LT serpentinites that make up Tso Morari. It is therefore likely that the samples retain the geochemical signatures of the HP-HT imprints. Similar observations were made by Peters et al. (2017), who showed that the chemical signatures of serpentinites from the Western Alps and Andean (meta-)ophiolites retain the HP-HT imprints.

The high alkali/U and B/U ratios in the Shergol-Tingdo and Kargil serpentinites relative to the primitive mantle (Fig. 8) are typical of FA serpentinites from oceanic supra-subduction zones (Peters et al., 2017). Boron is enriched in marine sediments (230 ppm; Li and Schoonmaker, 2014; Pagé et al., 2018) and is released from the subducted slab at low temperatures (<350°C, Bebout et al., 2007, 1999). At the same time, alkalis (Li, Rb, Cs) and U are decoupled during serpentinization in these environments due to their different mobility in subduction zones and their concentrations in the oceanic crust from which the metasomatizing fluids originate (Peters et al., 2017). Thus, alkalis and B are typically enriched in oceanic supra-subduction zone serpentinites, while U enrichment is modest. Conversely, the Tso Morari serpentinites are relatively depleted in alkalis and highly enriched in U, resulting in low alkali/U and B/U ratios (Fig. 8). Peters et al. (2017) attribute such low ratios to serpentinites generated in mid-ocean ridge contexts, where high amounts of U are transferred from circulating seawater to peridotites during serpentinization, resulting in pronounced U enrichments in MOR serpentinites of up to 10–100 CI-chondrite (Deschamps et al., 2013). Given that the compilation carried out by Peters et al. (2017) considers active FA and MOR serpentinites worldwide, the Tso Morari serpentinites really stand out as anomalous for a supra-subduction zone context. The lack of alkali enrichment in Himalayan serpentinites has previously been attributed to alkali retention in the slab (Hattori and Guillot, 2007). This is true for all three groups of serpentinites, but in the case of Shergol-Tingdo and Kargil, the alkali/U ratios are higher due to the low U content. On the other hand, the high U enrichment observed in the Tso Morari serpentinites is probably the result of metasomatism by a U-rich agent, which could be a melt or an aqueous fluid. Varying degrees of enrichment in fluid-immobile elements such as Nb and HREE are observed in the Ladakh serpentinites and were also noted by Deschamps et al. (2010) in samples from Tso Morari, where they were attributed to metasomatism of the mantle wedge by percolating melts during an intra-oceanic arc stage. Detrital sedimentary components can also supply REE and high field strength elements to the FA mantle via melts (Plank and Langmuir, 1988; Plank, 2014; Cannaò and Malaspina, 2018). Melt-rock interactions have been proposed to cause strong U enrichment as well as
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Th versus (a) La/YbN, (b) U and (c) Cs/U and (d) As/Th versus Sb/Th of Ladakh serpentinites. The Mariana FA serpentinites (Debret et al., 2019) are included in (d) for comparison. Values for the primitive (green star; Palme and O'Neill, 2007) and depleted mantle (pink star; Salters and Stracke, 2004) are also shown.</figcaption></figure>

enrichments of fluid immobile elements and REE in FA serpentinites (Deschamps et al., 2013), as seen in the Tso Morari serpentinites. The Ladakh serpentinites show additional signs of refertilisation, such as slightly elevated Ti and Yb concentrations compared to typical mantle wedge serpentinites (Deschamps et al., 2013). However, the enrichment of fluid-mobile elements such as As and Sb (see above) suggests that the U enrichment in the Tso Morari serpentinites is associated with fluid-rock interaction. The uranium content in the Tso Morari serpentinites shows a correlation with U/Yb (Fig. 8d), parallel to a constant Yb trend. Such an observation suggests that U is fractionated with respect to melt-mobile elements, as geochemical variations caused by melt depletion or melt-rock interactions prior to serpentinization will necessarily be oblique to a constant Yb trend (Peters et al., 2017). Consistent with these results, the Tso Morari samples are characterised by the absence of Th enrichment, limited Th variations and high U/Th. Given that Th is typically mobile in melts but not in slab-derived fluids (Elliott et al., 1997; Kessel et al., 2005; Freymuth et al., 2016), these observations

suggest a preferential mobility of U relative to Th, and therefore mass transfer of U to the FA mantle driven by fluids rather than melts. Thus, the high-U metasomatizing agent derived from the slab must be an aqueous fluid. Consistent with the As and Sb content observations, the transition to a high-U metasomatizing fluid therefore requires a major change in the composition of the subducting slab.

Uranium solubility is significantly higher in oxidised fluids, so surface water circulating at the MOR produces U-rich serpentinites, whereas multiple re-equilibration stages of fluids within the FA lead to reduced conditions, producing U-poor FA serpentinites (Peters et al., 2017). However, in the case of the Himalayan serpentinites, these trace element relationships do not fit the same discrimination system defined by Peters et al. (2017), suggesting different processes. The presence of continental sediments at the source of metasomatizing fluids has been inferred at Tso Morari from various geochemical proxies (e.g., Hattori and Guillot, 2007; Deschamps et al., 2013). Experimental and fluid inclusions data from eclogitic continental terrains support strong mobility
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** a, b) Alkalis versus U ratios of Ladakh serpentinites. c) U/Th as a function of Li/Th of Ladakh serpentinites. d) U/Yb as a function of U content of Ladakh serpentinites, including blueschist and eclogitic metasediments. Values for the primitive (green star; Palme and O'Neill, 2007) and depleted mantle (pink star; Salters and Stracke, 2004), as well as those for Mariana FA serpentinites (Debret et al., 2019) are shown for comparison. Light blue and beige fields represent those of Peters et al. (2007) for FA mantle and mid-ocean ridge serpentinites, respectively.</figcaption></figure>

of both LILE (Large-Ion Lithophile Element) and U during continental subduction (Cannaò and Malaspina, 2018 and references therein), whereas there is no clear evidence for U mobility in fluids derived from sediments during oceanic subduction (Moore and Vrolijk, 1992; Bebout et al., 1999; Lafay et al., 2013). Furthermore, subducted continental lithologies are characterised by U concentrations up to several parts per million (Plank, 2014) higher than in the oceanic crust (AOC = 0.07 ppm; UCC = 2.7 ppm; Wedepohl, 1995; Rudnick and Gao, 2014). These are often associated with U-rich accessory phases (e.g., monazite) that can dissolve in fluids even at moderate temperatures (e.g., Oelkers and Poitrasson, 2002), thus providing a large supply of U in continental slab fluids (Hermann et al., 2013). High U mobility during continental subduction is also consistent with the redox record of the Tso Morari serpentinites. The latter exhibit high AsV/AsIII ratios (~0.1–10), suggesting unusually high oxygen fugacity conditions (Pokrovski et al., 2022) and consistent with the high As concentrations in the Tso Morari

serpentinites relative to other FA settings (Fig. 7 and discussion above). Taken together, these observations support a major change in U mobility and slab fluxes from oceanic to continental subduction, resulting in the different geochemical signatures observed in the Indus Suture Zone serpentinites. The subducting continental slab may also produce fluids enriched in Be, B, LILE as well as As, Sb and halogens due to glauco-phane breakdown at ~500°C, followed by release of Be, B, LILE, U, Th and Ce ± As, Sb from ~500-600°C after lawsonite breakdown (Bebout et al., 2007). Such a process is consistent with the serpentinization temperature recorded in the Tso Morari serpentinites and will also explain their FME profile in the context of a transition from oceanic to continental subduction.
## 5.2. Isotopic constraints on the transition from an oceanic to a continental subduction

We further test the hypothesis of a change in elemental fluxes in FA serpentinites accompanying the transition from oceanic to continental subduction using Pb and Sr isotopes. The Pb and Sr isotopic compositions of the Kargil serpentinites are slightly more radiogenic than the DMM, suggesting that they have been metasomatized by a radiogenic component. However, this radiogenic enrichment is rather low, indicating that the metasomatic agent is unlikely to have come from an enriched source such as old continental material, as suggested by the trace element concentrations. Interestingly, the values found in this serpentinite are similar to those found in the melt-forming plutons of the KLA in the Cretaceous (Jagoutz et al., 2009), suggesting that the metasomatic agent was ultimately involved in the source of the KLA magmas. Thus, we can hypothesise that metasomatism occurred during the oceanic subduction stage, with the slab-derived metasomatic agent having a limited radiogenic component, consistent with an oceanic slab-derived fluid with little to no continental sediments. In contrast, both

HP-HT and LP-LT serpentinites of the Tso Morari massif tend to have significantly more enriched Pb and Sr isotopic ratios, strongly supporting the idea that fluids of continental origin were transferred to the Tso Morari mantle wedge. The isotopic ratios are very similar to those of the post-collisional plutons of the KLA, suggesting that the fluids responsible for metasomatism of the serpentinites may have played a role in the production of post-collisional magmas. High $^{87}$Sr/$^{86}$Sr (~0.72; Hattori and Guillot, 2007) and low $\epsilon_{Nd}$ (down to ~15; Guillot et al., 2001) have already been reported for these Himalayan serpentinites and have been attributed to contributions from ancient crustal material. The radiogenic component was therefore most likely introduced into the FA mantle by dehydration of the Indian continental margin, i.e. the Tso Morari Gneiss Complex. Indeed, the analysed metasediments and orthogneiss protolith (Polokongka La granite) show a highly variable radiogenic Sr and Pb isotopic composition that can explain the observed compositions.

We tested the above hypothesis by modelling serpentinization of the FA mantle as mixing between a depleted Indian MORB mantle (Rehka et al., 1997) and slab-derived fluids in the context of intra-oceanic and continental subduction. The fluids were modelled as being derived from

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Mixing models for the Ladakh FA serpentinites based on initial Pb and Sr isotope ratios. Panels (a) and (b) show the model for the intra-oceanic subduction stage (~100 Ma) by mixing the depleted mantle with a fluid from the average of the blueschist facies oceanic metasediments. Panels (c) and (d) show the model for the continental subduction stage (~50 Ma), where the depleted mantle is mixed with fluids from two end-member compositions of eclogitic continental metasediments (see text for details).</figcaption></figure>
the dehydration of subducted oceanic and continental gneisses using the partition coefficients of Spandler et al. (2007) at 600°C and 650°C, respectively. Details of the parameters used for these models are given in Supplementary Table S3. For the intra-oceanic subduction stage, the slab end-member is represented by a fluid from blueschist facies oceanic metasediments sampled in the Shergol area (samples LD19-30 and LD19-32). Our calculations show that the Pb and Sr isotopic signatures of sample LD19-46 from Kargil, thought to correspond to this oceanic subduction stage, can be approximated by a contribution of ~50-60% from subducted oceanic metasediments to the FA mantle (Fig. 9a,b).

The continental subduction stage requires the involvement of a slab-derived component with a different isotopic composition. Overall, the crustal slab represented by the Tso Morari gneisses shows a very large variability in composition, especially in $^{87}$Sr/$^{86}$Sr(i). Paragneiss sample LD19-82 has an anomalously high Pb isotopic composition. It is therefore difficult to identify a single end-member for the isotopic mixing model. Nevertheless, if we choose to average the two samples structurally closest to the serpentinites (LD19-81 and 98), the $^{87}$Sr/$^{86}$Sr(50) values of the Tso Morari serpentinites can be reproduced by mixing between the depleted mantle and about 60% of a fluid derived from dehydration of eclogitic metasediments (Fig. 9c,d). However, it should be noted that these sediments are not sufficiently rich in radiogenic Pb to reproduce the observations. Therefore, some amount of the highly radiogenic LD19-82 sample would be required. Considering the high variability of the isotopic composition within the Tso Morari gneiss unit (Indian passive margin), it is thus possible to explain the signatures observed in the Tso Morari serpentinites by the addition of a fluid derived from subducted continental material at high P-T to the FA mantle during continental subduction at about 50 Ma.

The model presented above is able to explain the radiogenic isotopic signatures of the majority of FA serpentinites from Ladakh. The exception is sample LD19-85 from Tso Morari, which has unusually unradiogenic Sr and Pb isotope ratios. Therefore, some variation in the composition of mantle end-members or continental material not sampled in this study may be required to explain its composition. This possible variation does not contradict our general interpretation, based on the overall pattern observed for the Ladakh FA serpentinites.

### 5.3. Tracking oceanic and continental subduction—Towards a new discrimination system for (meta-)ophiolite serpentinites?

Peters et al. (2017), who compiled a large set of active FA and MOR serpentinites, used the contrasting differences between alkalis and U to

develop a discrimination system based on their geochemical characteristics. As noted in the previous sections, in the case of the Himalayan serpentinites, these trace element relationships do not fit the same discrimination system defined by Peters et al. (2017). These discrepancies can be reconciled by a change in the composition of the subducted slab during the transition from oceanic to continental subduction. In addition to trace element and radiogenic isotope systematics, changes in subduction dynamics also affect halogen concentrations in FA serpentinites (Fig. 10). Fluorine concentrations in the Tso Morari serpentinites (up to 1820 ppm) are one to two orders of magnitude higher than those of the Mariana FA (<100 ppm; Wei et al., 2005) and those of Shergol-Tingdo and Kargil, which are often near or even below the detection limit (20 ppm; see Supplementary Table S2), at levels similar to those of the primitive and depleted mantle (~15 ppm; Kendrick et al., 2017) and MOR serpentinites (~6-30 ppm; Pagé and Hattori, 2017). Higher F contents in the Ladakh serpentinites are loosely related to higher U/Th and lower B/U ratios (Fig. 10), suggesting a common metasomatic agent for both F and U enrichment. Halogens are transferred to the FA mantle via fluids derived from subducted Indian continental sediments (Pagé and Hattori, 2017). Breakdown of glauophane in metasediments and continental crust material releases halogens from the slab at ~500°C (Debret et al., 2016). Fluorine is enriched in the continental crust (553 ppm; Rudnick and Gao, 2014) and is released from the slab at depths greater than 80 km (Debret et al., 2016; Pagé and Hattori, 2017; Pagé et al., 2018). Thus, the high F concentrations observed in the Tso Morari serpentinites also appear to be favoured by continental rather than oceanic slab subduction, by the same processes that produced U enrichments as discussed in the previous sections. The combination of both halogen and alkali/U systematics could therefore provide a powerful tool for distinguishing not only between MOR or FA environments, but also between oceanic and continental FA environments (Fig. 8), and therefore warrants further investigation.

The distinction between oceanic and continental FA serpentinites also complements previous field studies aimed at characterising the nature of the material composing the KLA basement. Indeed, previous structural, geochemical and geochronological reconstructions of the history of Himalayan subduction and collision at the regional scale have already established the existence of an intra-oceanic subduction stage (~150–55 Ma), followed by the Indian continental subduction (< 55–50 Ma; Mahéo et al., 2004; Guillot et al., 2008; Bouilhol et al., 2013; Jagoutz et al., 2019). The trace element ratios presented in this study could thus be useful as a first-order tool for distinguishing between

<figure><img src="image_12.png" /><figcaption>Fig. 10. Fluorine content of Ladakh serpentinites as a function of (a) U/Th and (b) B/U.</figcaption></figure>
serpentinite samples from the continental and oceanic subduction stages of Himalayan evolution, without the need for dating or obtaining radiogenic isotope data. Indeed, on the western side, the serpentinites are intimately associated with pelagic sediments and MORB-like conformably overlying the ultramafic rocks, and in many places over-lain by volcanoclastics and lava flows associated with the Nimdam and Dras or Naktul arc sequence (Robertson, 2000). As such, the western strips of ultramafic rocks would correspond to the mantle basement on which the arc developed, without any continental affinity, providing confirmation to previous studies that established the intra-oceanic nature of the KLA (Reuber, 1989; Robertson and Degnan, 1994; Bouilhol et al., 2013). Our data show that the different geochemical signatures of the (meta-)ophiolites composing the Indus Suture Zone can be reconciled in a subduction zone system transitioning from oceanic to continental subduction. Thus, the multiple strips forming the western suture correspond to the sliced basement repeated by the multiple top-to-north backthrusts and do not necessarily correspond to different suture zones, confirming previous field studies (see discussion in Robertson, 2000). Ultimately, the availability of such a simple geochemical tool can greatly contribute to improving our understanding of the evolution of the Himalayan subduction stages on a broader scale by facilitating the study of large sample sets in a time- and cost-effective manner.

## 6. Conclusions

The geodynamic evolution of the Himalayan orogen, from intra-oceanic to continental subduction, is recorded in the geochemistry of the FA mantle through serpentinization. Trace element ratios, such as alkali/U ratios and radiogenic (Sr-Pb) isotope compositions in FA serpentinites from Ladakh, allow identification of changes in fluid sources and fluxes at different stages of Himalayan evolution. Serpentinites from Shergol-Tingdo and Kargil, formed under low temperature and pressure conditions, show B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios, consistent with input from fluids derived from subducted oceanic crust. On the other hand, the Tso Morari serpentinites show enrichment in fluid-mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotope compositions, indicating that the metasomatizing agent corresponds to a fluid derived from subducted continental material. Consistent with this interpretation, the Sr-Pb isotopic composition of the Kargil serpentinites can be reproduced by mixing between an Indian MORB depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. The results obtained from the study of the Ladakh FA serpentinites allow us to propose a new discriminative tool for Himalayan serpentinites, which could be tested in other orogenic contexts to improve our understanding of serpentinite evolution from oceanic to continental subduction.

### CRediT authorship contribution statement

**Javiera Villalobos-Orchard:** Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. **Baptiste Debret:** Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Investigation, Funding acquisition, Data curation, Conceptualization. **Pierre Bouilhol:** Writing – review & editing, Writing – original draft, Supervision, Resources, Investigation, Funding acquisition, Conceptualization. **Rajeev Upadhyay:** Investigation, Conceptualization. **Pamela Gutiérrez:** Writing – original draft, Methodology, Formal analysis, Data curation. **Catherine Chauvel:** Writing – original draft, Supervision, Methodology, Formal analysis, Data curation. **Frédéric Girault:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Funding acquisition, Conceptualization.

## Data availability

Data are available through the Institut de Physique du Globe de Paris Research Collection, https://dataverse.ipgp.fr/dataset.xhtml?persistentId=doi:10.18715/IPGP.2024.m0b9ncwg.

## Declaration of competing interest

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

## Acknowledgments

We would like to thank Christian Nicollet, Pietro Sternai and Sébastien Castellort for their help and support during the fieldwork in Ladakh. We are also grateful to Pierre Burckel, Dimitri Rigoussen and Tu-Han Luu for their invaluable help with the geochemical analyses. We thank J. Deng, C. Heviou, J. Huang and two anonymous reviewers for critical comments on an earlier version of this paper and Weidong Sun for careful editing. This work was supported by the LabEx UnivEarthS (ANR-10-LABX-0023 and ANR-18-IDEX-0001) and is an integral part of the MYSTIC project led by F.G. Parts of this work were supported by the IPGP multidisciplinary programme PARI and by the Ile-de-France Region SESAME grants no. 12015908 and EX047016, the IdEx Université de Paris grant, ANR-18-IDEX-0001, and was also supported by OTELO (Nancy) and the CNRS INSU Syster programme.

## Appendix A. Supplementary material

Supplementary material to this article can be found online at [http://doi.org/10.1016/j.gca.2024.11.001](http://doi.org/10.1016/j.gca.2024.11.001).

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