Sulfur-enriched sub-arc fluids drive deep sulfur cycling in subduction zones

Domain: neuroscience
Published: 2026-04-07
Editorial outcome: accept

Abstract
Arc magmas are enriched in sulfur relative to mid-ocean ridge basalts, commonly attributed to slab-derived sulfur inputs during subduction. However, the contribution of slab fluids remains debated because sulfur concentrations in sub-arc fluids have not been directly measured. Here we quantify sulfur in slab-derived fluids preserved as multiphase fluid inclusions composed of H O, calcite, and chalcopyrite in omphacite from ultrahigh-pressure eclogites in the Sumdo orogenic belt. Three-dimensional Raman spectroscopy reveals high sulfur concentrations averaging ~6 wt.%. Mass-balance calculations indicate that such fluids can efficiently enrich the mantle wedge and supply up to ~70% of the sulfur emitted by arc volcanism. We further suggest that chalcopyrite formed through post-entrapment reduction of oxidized sulfur species by host omphacite, followed by precipitation with co-entrapped copper and iron. Our findings identify sub-arc depths as a critical window for slab sulfur release and provide key constraints on deep sulfur cycling and copper mobilization in arc systems.

Introduction
Sulfur is a key volatile element that profoundly influences a wide range of Earth system processes, including volcanic outgassing, ore genesis, climate regulation, and biological evolution1-3. Subduction zones represent the primary pathway for sulfur exchange between Earth's surface and interior, transferring sulfur from the subducting slab into the mantle4-5. However, the ultimate fate of subducted sulfur remains uncertain, particularly the extent to which it is transferred into the mantle wedge to sustain arc-related sulfur outputs versus retained in the slab and recycled into the deep mantle6-10.

Arc volcanic rocks are characteristically enriched in sulfur, a feature widely attributed to sulfur input from slab-derived fluids11-21. However, mass-balance calculations indicate that such fluids contribute a highly variable fraction of arc-related sulfur emissions, revealing a mismatch between the observed arc outputs and the estimated slab-derived inputs18-22. For example, sulfur isotopic compositions of ultrahigh-pressure (UHP) metamorphic rocks, combined with thermodynamic modeling, suggest that slab-derived fluids may account for only ~20% of the sulfur budget of arc magmas22,23. In contrast, sulfur enrichments recorded in melt inclusions from natural arc basalts imply substantially larger contributions of ~40–70%20. This discrepancy may partly reflect the compositional diversity of slab-derived fluids among different subduction systems. However, it may also arise from the lack of direct constraints on sulfur concentrations and fluxes in natural slab-derived fluids themselves. Resolving this uncertainty requires direct quantification of sulfur in slab-derived fluids at sub-arc depths, where fluid-mediated sulfur transfer exerts the strongest influence on the overlying mantle wedge.

To date, estimates of sulfur concentrations in subduction zone fluids have relied predominantly on indirect approaches, including analyses of sulfur concentrations and isotopic compositions in exhumed metamorphic rocks, melt inclusions in arc basalts and tephra, high-temperature and high-pressure (HT–HP) experiments, and thermodynamic modeling18-22,24-29. While these approaches have provided valuable insights, they are subject to substantial uncertainties related to pressure–temperature–redox conditions, phase relations, and fluid–rock equilibria. As a result, reported sulfur concentrations in slab-derived fluids span a wide range, from <1 wt.% to several weight percent22,26-29. For example, thermodynamic calculations based on the Deep Earth Water (DEW) model22 predict sulfur concentrations of ~0.5–1.0 wt.%, which may be underestimated owing to extrapolation beyond the model’s calibration range. Experimental studies24,25 have reported sulfur contents of 1.58–14.83 wt.% in sulfide-bearing basaltic systems, but these experiments were conducted at temperatures higher than those characteristic of globally dominant cold subduction zones and employed simplified starting compositions that may not fully capture the complexity of natural fluid–rock interactions. Crucially, direct measurements of sulfur concentrations in natural sub-arc fluids have not yet been reported, remaining a major barrier to quantifying slab-to-mantle sulfur transfer.

A more direct approach to constraining slab-derived fluid compositions involves the study of fluid inclusions preserved in HP–UHP metamorphic rocks30,31. Among the major sulfur reservoirs entering subduction zones—altered oceanic crust, marine sediments, and serpentinized peridotite—the altered oceanic crust typically dominates sulfur input in oceanic subduction systems18,21. During prograde metamorphism and dehydration, fluids released from this source may be trapped in co-crystallizing metamorphic minerals, thereby preserving snapshots of fluid compositions at the time of entrapment, commonly with limited post-entrapment modification32. In particular, fluid inclusions hosted in omphacite formed under eclogite-facies conditions are especially valuable, as they provide rare access to the chemistry of slab-derived fluids at sub-arc depths. Despite this potential, however, direct quantitative constraints on sulfur concentrations in such fluid inclusions remain scarce.

Here, we present in situ sulfur measurements from chalcopyrite-bearing multiphase fluid inclusions hosted in omphacite from UHP eclogites in the Sumdo orogenic belt, southern Tibet. These inclusions preserve evidence of fluids released during the dehydration of altered oceanic crust at sub-arc depths. High-resolution three-dimensional (3D) Raman spectroscopy reveals average sulfur concentrations of ~6 wt.%, providing the direct quantification of sulfur in natural subduction zone fluids. Our results establish sub-arc depths as a critical window for sulfur mobilization and provide key insights into sulfur cycling between the subducting slab, mantle wedge, and arc outputs. Furthermore, we highlight the role of sulfur-rich oxidized sub-arc fluids in facilitating efficient copper mobilization, with important implications for the formation of porphyry copper deposits in arc settings.

Results
Geological setting and multiphase fluid inclusions
The Sumdo eclogite belt, located in the east-central Lhasa terrane of the Tibetan Plateau, represents a well-preserved segment of a fossil subduction zone formed during the northward subduction of the Paleo-Tethys Ocean beneath the North Lhasa terrane33 (Fig. S1). The belt extends over 100 km in an east-west direction and is 5–10 km wide. It comprises a diverse lithological assemblage including eclogite, blueschist, greenschist, marble, mica schist, quartzite, and volcanic rocks33-35 (Fig. S1). The coexistence of eclogite- and blueschist-facies rocks points to a cold subduction regime characterized by a low geothermal gradient. Peak metamorphic conditions recorded by eclogites reach ~2.6–2.7 GPa and 630–780 °C33, corresponding to sub-arc depths in a typical oceanic subduction setting36. The occurrence of coesite pseudomorphs in garnet and omphacite further attests to UHP metamorphism37. The investigated eclogites are composed primarily of garnet and omphacite, with minor rutile, phengite, and amphibole, and sparse sulfide minerals—mainly pyrite35(Fig. S2). Garnet grains display well-developed compositional zoning, recording crystal growth from prograde metamorphism through peak conditions and into the early retrograde stage35,37. In contrast, omphacite is compositionally unzoned and occurs both as porphyroblasts and as inclusions within garnet, consistent with crystallization during a single metamorphic stage—most likely at peak conditions37.

Fluid inclusions occur in both garnet and omphacite, but their abundance, distribution, and preservation differ markedly between the two host minerals. Inclusions in garnet are generally sparse and commonly show evidence of modification during retrograde metamorphism. In contrast, omphacite hosts abundant, well-preserved inclusions that are spatially restricted to crystal cores. These omphacite-hosted inclusions display several petrographic features, as outlined below. First, the fluid inclusions are confined to the cores of omphacite grains and do not form trails that crosscut the host crystals (Fig. 1), as would be expected for secondary inclusions trapped along healed fractures. Detailed petrographic observations further reveal no microfractures or healed cracks in the host omphacite that could have acted as pathways for late-stage fluid infiltration (Fig. S2). Second, the fluid inclusions commonly occur as isolated or as arrays parallel to the crystallographic c-axis of omphacite (Fig. 1). Individual fluid inclusions are typically ~5–60 μm in length and ~2–8 μm in width and display a consistent columnar morphology that mimics the elongated crystallographic habit of the host mineral. This negative-crystal–like geometry is consistent with primary entrapment and provides supporting, though not diagnostic, evidence for a primary origin38. Third, the fluid inclusions show uniform filling degrees of ~80–90% at room temperature and contain consistent proportions of liquid, solid, and vapor phases (Fig. 1), a feature inconsistent with heterogeneous secondary trapping. Combined with the fact that omphacite crystallized exclusively at the peak stage of UHP conditions37, corresponding to sub-arc depths36, these observations suggest that omphacite-hosted fluid inclusions provide the most reliable recorders of slab-derived fluid compositions at sub-arc conditions.

Raman spectroscopic characterization and multiphase fluid composition reconstruction
High-resolution Raman spectroscopy analysis of all well-preserved primary multiphase fluid inclusions reveals a consistent three-phase assemblage comprising chalcopyrite, calcite, and aqueous fluid (Figs. 2, S3, S4; see Methods). The volumetric proportions of solid and liquid phases are remarkably uniform across inclusions (Supplementary Data 1, Fig. S5), indicating entrapment of a compositionally homogeneous fluid. To quantitatively reconstruct the bulk composition of the trapped fluids, we applied three-dimensional (3D) Raman mapping to quantify the relative volumes of each phase, followed by mass-balance calculations using published density and compositional data for chalcopyrite, calcite, and brine (Supplementary Data 1; see Methods). The reconstructed fluid compositions show a high degree of consistency among inclusions (Fig. S5), yielding an average concentration of 6.33 wt.% sulfur, 6.33 wt.% copper, 5.54 wt.% iron, 10.16 wt.% CaO, 15.23 wt.% carbon (as CO 2-), 3.22 wt.% sodium, 5.25 wt.% chlorine, and 47.96 wt.% H O (Supplementary Data 2).

Methods
3D imaging and spectral acquisition
Three-dimensional (3D) Raman mapping of multiphase fluid inclusions hosted in omphacite was conducted at the State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. Analyses were performed using a WITec alpha300R confocal Raman microscope equipped with a 488 nm cobalt laser. The laser power at the sample surface was maintained at 25 nW, which is sufficiently low to minimize laser-induced heating or damage to the inclusions while still ensuring robust detection of Raman signals from both the daughter minerals and associated fluid phase. Spectra were collected over the range of 100–4,400 cm-¹ to fully capture the Raman features of the multiphase assemblages. Mineral phases were identified and discriminated from compositionally similar counterparts based on established Raman spectral databases (https://rruff.info/) and previous studies (e.g., Frezzotti et al.81). For sulfide phases, chalcopyrite (CuFeS ) is characterized by a strong Raman peak at 293 cm-¹, whereas other common sulfides display distinct and non-overlapping diagnostic peaks, including pyrite (FeS ) at 428 cm-¹, marcasite (FeS ) at 324 cm-¹, sphalerite (ZnS) at 349 cm-¹, and galena (PbS) at 136 cm-¹. In this study, the dominant sulfide Raman peak consistently occurs at 292–293 cm-¹ (Fig. S3), with no additional peaks indicative of other sulfide species, supporting the identification of chalcopyrite as the sole sulfide phase within the inclusions. For carbonate phases, calcite is characterized by strong Raman bands at 284 and 1086 cm-¹, whereas dolomite shows peaks at 299 and 1097 cm-¹, and magnesite at 329 and 1094 cm-¹. The carbonate daughter minerals in the Sumdo inclusions consistently exhibit a peak combination at 283–284 and 1086 cm-¹ (Fig. S3), which is diagnostic of calcite and clearly distinct from other carbonate species. The aqueous fluid phase is identified by a broad Raman band between 3300 and 3600 cm-¹ (Fig. S3), corresponding to O–H stretching vibrations, indicating the presence of H O-rich fluid. Host omphacite is identified by its characteristic Raman peaks near 670 and 1012 cm-¹; in our analyses, peak positions at 672 and 1015 cm-¹ are consistently observed (Fig. S3), confirming the host mineral identity. Furthermore, no solid NaCl daughter minerals were detected in the fluid inclusions (which would be expected to show a strong Raman peak near 358 cm-¹). We therefore infer that NaCl is present as a dissolved component in the aqueous fluid phase. Furthermore, 3D maps were acquired via point-by-point scanning with spatial resolutions of 1 μm along the x-axis and 0.5 μm along the y- and z-axes. Acquisition times for individual points ranged from 0.5 to 2 s, with total scan durations of 5–20 hours per fluid inclusion. This methodology has been successfully employed in our previous investigation of multiphase fluid inclusions from a UHP metamorphic vein in the Dabie orogenic belt, China30.

Reconstruction of fluid composition
Following the analytical protocol established in a previous study30, we reconstructed the bulk compositions of the multiphase fluid inclusions investigated here. Volumetric reconstructions of multiphase fluid inclusions were carried out using WITec Project Plus and ImageJ software. Absolute volumes of individual daughter minerals were determined from the 3D maps (Supplementary Data 1) and combined with published mineral densities to calculate mass fractions for each phase. Idealized mineral chemistries were assumed for the reconstruction, including calcite (CaCO ), chalcopyrite (CuFeS ), and an aqueous brine composed of H O with 15 wt.% NaCl.37. Details of the salinity measurements of brine have been reported previously37, showing that omphacite-hosted multiphase fluid inclusions trapped at peak metamorphic conditions, corresponding to sub-arc depths, are systematically characterized by high salinities (10–22 wt.% NaCl). For the purpose of bulk composition reconstruction, we therefore adopted an average salinity of 15 wt.% NaCl. Densities of 2.71 g/cm3 for calcite, 4.10 g/cm3 for chalcopyrite, and 1.148 g/cm3 for the brine were used in the mass-balance calculations. This approach enables quantitative reconstruction of the bulk compositions of slab-derived fluids recorded by omphacite-hosted multiphase inclusions from the Sumdo eclogites. The robustness of the reconstructed fluid compositions is supported by several independent lines of evidence. In addition to the use of an established analytical framework30. First, analytical reproducibility was evaluated through independent repeat analyses of two representative fluid inclusions (Nos. 2 and No. 20). These repeat measurements yielded highly consistent daughter-mineral assemblages, phase volume proportions, and reconstructed bulk compositions (Supplementary Data 1 and 2), demonstrating good analytical reproducibility. Second, ternary compositional diagrams summarizing all 22 analyzed fluid inclusions show tight clustering of data points (Fig. S5), indicating limited compositional variability and strong internal consistency among inclusions. Finally, standard deviations are reported for all calculated major components, providing a quantitative estimate of the analytical uncertainties associated with the Raman-based reconstruction (Supplementary Data 2).

Electron probe microanalysis of host omphacite across fluid inclusions
To evaluate potential post-entrapment interaction between the trapped fluid and the host mineral, a representative multiphase fluid inclusion was selected for detailed analysis and carefully polished to near-surface exposure. The inclusion was initially ground using 10,000-grit silicon carbide sandpaper lubricated with ethanol until it approached the sample surface. Final polishing was performed on a polishing cloth using 10,000-grit carbon powder mixed with ethanol. Throughout the polishing process, the distance between the inclusion and the sample surface was continuously monitored under an optical microscope to avoid breaching the inclusion. Following sample preparation, the major element composition of the host omphacite surrounding the exposed fluid inclusion was analyzed by EPMA at the University of Science and Technology of China. Three linear analytical traverses were arranged to cross the inclusion boundary. Each traverse consisted of ten analytical points, with a spacing of 6 μm between adjacent points. The electron beam diameter was set to 3 μm. Analyses were performed at an accelerating voltage of 15 kV and a probe current of approximately 200 nA. The EPMA results are reported in Supplementary Data 3 and illustrated in Fig. S6. The data reveal minor but systematic variations in major-element compositions of the host omphacite surrounding the fluid inclusion. However, the magnitude of these variations is small, indicating that post-entrapment interaction between the host mineral and the inclusion was limited and did not significantly modify the bulk chemical composition of the trapped fluid.

Thermodynamic modeling of the sulfur speciation in sub-arc fluids and dehydration of altered oceanic crust under Sumdo conditions
Thermodynamic calculations were carried out by Gibbs free energy minimization using Perple_X (version 7.1.13)82. Phase equilibria involving both solid and fluid phases were modeled in the Mn-Na-Ca-K-Fe-Mg-Al-Si-Ti-C-O -H -S system. Thermodynamic data for condensed phase and molecular volatile species were taken from the revised Holland and Powell dataset (DS633, 2018)83, and solute species data were adopted from the DEW model of Huang and Sverjensky84(revised in 2019). Electrolytic fluid compositions were calculated using the lagged speciation algorithm, which ensures mass balance between coexisting solid and fluid phases. The thermodynamic properties of H O, CO , CLH , H S, and SO were described using the solvent reference state, whereas CO and C H were treated relative to the solute reference state owing to their negligible abundances. The equation of state of Pitzer and Sterner85 was employed to calculate the thermodynamic properties of pure H O and CO , while the modified Redlich–Kwong equation of state of Connolly and Cesare86 was applied to all other solvent species and used to compute the activities of solvent components. The solid-solution models used in the calculations, together with their sources, are listed in Supplementary Data 4. Thermodynamic simulations were conducted using the bulk-rock composition of a representative Sumdo eclogite (T18237,39). The model input composition (in molar proportions) was specified as Si = 20.3960, Al = 8.6834, Ti = 0.4259, Fe = 3.7864, Ca = 6.1017, Mg = 5.6123, K = 0.1146, Na = 1.7421, Mn = 0.0761, H = 10.9999, C = 1.2661, S = 0.0666, and O = 42.8356. The Fe³⁺/ΣFe ratio (0.51), carbon content (2 wt.%), and sulfur content (1500 ppm) were adopted from typical sulfur-rich altered oceanic crust compositions60,87,88. Following recent thermodynamic assessments89,90, the Mg(SiO )(HCO )+, Fe(HCOO)+, and H CO (aq) species were excluded to avoid potential artifacts from their unrealistically high predicted concentrations. Open-system fluid-fractionation modeling was applied along the P–T trajectory of progressive rock devolatilization26. The results of the thermodynamic modeling, including sulfur and carbon speciation and progressive dehydration behavior with increasing pressure and temperature, are summarized in Supplementary Data 4 and 5. At sub-arc depths of approximately 80–90 km, the modeled slab-derived fluids are dominated by oxidized sulfur species, primarily SO ²⁻ and HSO ⁻. In addition, the calculations indicate that approximately 60% of the water initially stored in subducted altered oceanic crust is released at these depths, consistent with efficient slab dehydration under cold subduction conditions.

Sulfur flux released from the altered oceanic crust at sub-arc depths
The peak metamorphic pressures recorded by omphacite in the Sumdo eclogite (~2.6–2.7 GPa33) correspond to sub-arc depths of approximately 80–90 km36. Our thermodynamic modelling constrained by the geological and P–T conditions of the Sumdo system indicates that ~60% of the water initially stored in altered oceanic crust is released at these depths (Fig. S6). Treating the Sumdo system as representative of altered oceanic crust dehydration in cold subduction zones, we further integrate this dehydration fraction (~60%) with previously reported estimates of (i) the average H O content in altered oceanic crust61, (ii) a global subduction zone length of ~55,000 km62 and (iii) an altered crust thickness of 0.6 km60 in order to quantify the global H O flux released from altered oceanic crust at sub-arc depths (~80–90 km). This approach yields an H O flux of ~84.5 Mt/yr. Notably, this value is in good agreement with independent global estimates of H O flux released from dehydrating altered oceanic crust (~91 Mt/yr22), supporting the robustness of our parameterization. The consistency of these values provides confidence in adopting this H O flux as a key parameter for calculating sulfur flux released from altered oceanic crust at sub-arc depths (~80–90 km).

Using the sulfur and H O concentrations reconstructed from the multiphase fluid inclusions in this study, together with the estimated H O flux (~84.5 Mt/yr), we calculate the corresponding sulfur flux released by altered oceanic crust-derived fluids at sub-arc depths as:

𝐹sulfur = 𝐹H2O×𝐶sulfur = 11.7±3.3 Mt/yr (1)
fluid 𝐶H2O fluid

Here, 𝐹sulfur and 𝐹 represent the sulfur flux and H O flux, respectively. 𝐶sulfur and 𝐶 denote the sulfur and water concentrations in slab-derived fluids. This calculation provides a direct and reasonable quantitative constraint on the sulfur flux released during dehydration of altered oceanic crust at sub-arc depths on a global scale.

Desulfurization efficiency at sub-arc depths
To evaluate the extent of sulfur loss from the altered oceanic crust at sub-arc depths on a global scale, we compare the calculated sulfur flux in fluids (𝐹sulfur), with the total sulfur input from subducted altered oceanic crust (𝐹sulfur). The global sulfur input flux from the subducted altered oceanic crust is approximately 48.09 Mt/yr4. The desulfurization efficiency (η), defined as the proportion of sulfur released into slab-derived fluids at sub-arc depths is given by:

𝜂 = 𝐹f s l u u l i f d ur =25±7% (2)
𝐹i s n u p lf u u t r

This result implies that approximately one-third of the sulfur initially stored in the altered oceanic crust is mobilized into fluids at sub-arc depths, with the remainder either released at shallower levels or retained for recycling into the deeper mantle.

Sulfur enrichment in the sub-arc mantle by infiltration of sulfur-rich fluids
To evaluate the potential of slab-derived fluids to enrich the sub-arc mantle in sulfur, we apply a mass-balance framework to estimate the fluid proportions required to elevate sulfur concentrations in metasomatized mantle domains to levels typical of arc-source regions (250–500 ppm12,63). Given that the mass of infiltrating fluids is much smaller than that of the mantle, the sulfur content of the metasomatized sub-arc mantle can be expressed as:

𝐶sulfur =𝐶sulfur + 𝐶sulfur×𝑓 (3)
metasomatized sub−arc mantle depleted mantle fluid

In this equation, the𝐶sulfur ,𝐶sulfur , and𝐶sulfurrepresentthe sulfur concentrations in metasomatized sub−arc mantle depleted mantle fluid
metasomatized sub-arc mantle, depleted mantle, and sub-arc fluids, respectively. The variable 𝑓 represents the proportion of sulfur-rich fluids added to the metasomatized sub-arc mantle.

To further evaluate sulfur enrichment in the metasomatized sub-arc mantle over geological timescales, we estimate the mass of the sub-arc mantle domain between 70 and 100 km depth. Based on the total length of the global subduction zone (approximately 55,000 km62) and the estimated width of the sub-arc mantle per arc (approximately 150 km), the total volume of the 70–100 km sub-arc mantle is calculated to be 6.6 × 1017 m3. With an average mantle density of 3300 kg/m³, the total mass of the 70–100 km sub-arc mantle is approximately 8.17 × 1020 kg. Using the sulfur flux determined in this study, the time-dependent sulfur content of this reservoir can be calculated as:

𝐶sulfur =𝐶sulfur +(𝐹sulfur×𝑡)/𝑀 (4)
metasomatized sub−arc mantle depleted mantle fluid sub−arc mantle

Here, 𝐶sulfur and 𝐶sulfur are the sulfur concentrations in the metasomatized sub-arc mantle and depleted mantle. 𝐹sulfur is the calculated sulfur flux from sub-arc fluids, 𝑡 is the duration of fluid infiltration (typically taken as 1 Myr), and 𝑀 is the mass of 70–100km sub-arc mantle.

Comparison of sulfur input via slab fluid and arc volcanic output
To evaluate the global significance of sulfur released from altered oceanic crust at sub-arc depths, we compared the calculated sulfur flux (𝐹sulfur) carried by the slab-derived fluids with the estimated global sulfur output from arc volcanic volcanism (𝐹sulfur), which is approximately 16.7 Mt/yr4. The contributed proportion (Φ) of slab-derived fluids' sulfur flux to the arc volcanic sulfur output can be calculated as follows:

Φ = 𝐹f s l u u l i f d ur =70±20% (5)
𝐹o su u l t f p u u r t

This result indicates that slab-derived fluids may account for a dominant fraction of the sulfur released by arc volcanism. The calculated range overlaps well with previous estimates of ~40–70% derived from indirect constraints based on natural arc basalts20, supporting the consistency and geological plausibility of our flux-based assessment.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Dear Authors,\nI read your paper with great interest.Your new omphacite-hosted fluid inclusion data from Sumdo eclogite belt are a useful\naddition to our understanding of fluid transfer and fluid compositions in subduction zone settings. I think this material is of\nbroad interest. Your 3D Raman fluid inclusion data does provide direct evidence of sulfur-rich fluid compositions in a past\nsubduction zone, which is great and confirms lot of model results already published in the literature.\nHowever, I do have some reservations with the manuscript as it is, so I recommend that revisions are made to the paper so\nthe results presented are more robust, also suggest making the paper a bit broader interest with the inclusion of other\nelements in parts of your model calculations.\nOne of my main criticism is the limited, and in my view, skewed, use of references in the paper. You often cite Li et al. 2020\n(Nat Comms) as the reference for many of your statements and ignore a large number of papers that deal with sulfur in\nsubduction settings and suggest (as you also do), that sulfur is strongly fluid-mobile and added to the mantle wedge from the\nslab in large quantities. I listed a few references in my detailed comments that you should probably include (some you\nalready do, but don’t always cite when you should). Your discussion should be more nuanced, as the way it is currently\nwritten, it feels like you are the first to prove efficient S transfer via slab-derived fluids, but there is already plenty of evidence\nfor this form arc volcanoes and model results. Limitations of your data are not acknowledged – surely, one sample from one\neclogite belt will be representative of one P-T-X subduction path, not global variability.\nSecondly, I’m not convinced that your inclusions are primary (they can be secondary based on the evidence you provide),\nand this needs to be addressed or acknowledged in the text. I’m also not convinced that your inclusions have not undergone\npost-entrapment modification. You kind of brush this under the carpet in the text and provide no evidence that rules out Ca\nscavenging from the host (to form calcite from a CO2-rich fluid), or FI and host electron exchange (between Fe3+ present in\nomphacite and S6+ that may have been in the fluid). In summary, I’m not sure that your inclusions were reduced, as you\nsuggest. This needs to be considered and addressed, as oxidised fluids thought to carry large quantities of S, based on most\nrecent DEW models, and may fit better with your data.\nYour arguments that Cl influences S behaviour at the end of the manuscript feel very speculative. These are two fluid mobile\nelements, and Cl is elevated in all subduction zone magmas, while S is not always enriched. So I don’t really feel there is\nany causality here, and that you provide sufficient proof that there is.\nI’m not sure who your flux models work – is this for a fictional subduction zone, or for a global S flux from the slab? It needs\nto be better explained.\nData tables are provided as a Word document, which is not particularly useful for other researchers who may want to use\nyour data – you probably have an excel/csv table for these – please provide data in that format instead.\nHere are my detailed comments, line-by-line:\nLines 47-48: While the 20-30% fluid contribution from the slab to the mantle wedge, and\nsubsequently to the arc, is indeed the value in Li et al. (2020) you cite, but this is one of only few papers that argue for low amounts of S being transferred from\nthe slab to the wedge, and there are many others that say the opposite. For example, in Central America, we estimated 40-\n70% sulfur addition to the mantle wedge from the slab in Taracsák et al. 2023. The way it is currently written implies there is\nonly this 20-30% range in the literature, which is not the case.\nLine 51: This implies that people argued for extra S in the mantle before subduction. I have not seen papers suggesting high\nS contents in the subarc mantle (relative to normal, depleted upper mantle) under arcs prior to subduction-derived S\naddition. Is there a reference for this?\nLine57: a minor issue but melt inclusion paper you cite here (Muth & Wallace 2021) used tephra samples, not lavas.\nLine70-72: you cite Evans 2012 (a redox focussed review paper) and Bekaert et al. 2021 (a volatile cycle review paper),\nrather than papers that look at subducting S budgets (de Moor et al. 2022 Chem Geol, Peccia et al. 2025 Chem Geol), that\nare much more detailed and more relevant to your work. Crucially, both of these papers suggest it is the primary igneous\ncrust that has the largest S budget followed by the AOC – and the total igneous package (AOC+ primary) are the most\nimportant sources of S. Is there any proof that your eclogites represent AOC, or can they be metamorphosed primary crust\n(unaltered)?\nLine 99: the presence of pyrite is interesting. Looking at the recent model results of Beaudry & Sverjensky 2024 (G-cubed),\nwho extended the Deep Earth Water model’s calibration range and included oxidised S species, they suggest that under\nconditions similar to what you imply for these samples (~3GPa, 600-800 Celsius), pyrite is present at FMQ-1 to FMQ+2 (at\nhigher T, it is more restricted to FMQ+0.5 to +1.5). Critically, their model results provide S contents in fluids in equilibrium\nwith mafic eclogite that reach the S contents you propose based on your Fis (and even higher values), so it is a very relevant\npaper to your results, and you currently don’t cite or integrate this into your paper. I think you need to carefully consider their\nfindings regarding the S carrying capacity of fluids at higher fO2 than those modelled in Li et al. 2020, as they predict up to\nan order of magnitude more S in the fluid.\nLine125-128: the reconstructed fluid compositions are very interesting, particularly with respect to their high Cu contents.\nThis is because, as far as I’m aware, Cu has been suggested not to mobilise during slab devolatilisation (e.g. Jenner,\nor Cl. For example, to see if your FIs have enough H2O to enrich the mantle wedge in H2O and match estimates for mantle\nwedge H2O contents that range from 0.3 to 1wt% in the Marianas (Kelley et al. 2010 J. Petrol). The 0.1% to 1% fluid addition\nin Fig 3A seems a bit too low for that. Also, how much Cu would be added to the mantle under the same circumstances,\nconsidering your Fis have 5 wt%, and the upper mantle only ~30 ppm?\nLine 136: I have some issue with your interpretation of the FIs being primary. In line 110, you mention FIs are parallel with\nthe C axis (a cleavage direction for cpx). You also mention they are confined to cores and have negative crystal\nmorphologies. But together, these observations cannot rule out that the inclusions are in fact secondary, entrapped in cracks\nwithin the omphacite after formation, followed by further crystal growth along the rims. Negative crystal morphologies alone\ncannot be used to argue for a primary origin (see Goldstein 2003 review on FI petrography). In any case, you need to provide\nsome further arguments why your inclusions are representative for peak metamorphic conditions, considering a potential\nsecondary origin relative to omphacite cores.\nLine 140: I doubt that you can rule out post entrapment re-equilibration. H+ diffuses rapidly within cpx, so you could gain or\nlose it depending on H activity around the grains. Fe2+ can also diffuse in and out. Both of these elements could influence\nthe redox within your inclusions. This leads me to think that the S may have been oxidised in the fluid originally, not reduced,\nand the chalcopyrite formed due to post-entrapment processes. Can you rule this out, and if yes, how? If not, this needs to be\nconsidered.\nLine145-146: I don’t understand this sentence. What is replaced? Feels like the subject of the sentence is not defined.\nLine157: you say that your S contents at 3-10 wt% is similar to the literature range, but in fact it is narrower by an order of\nmagnitude, and you exclude the lower end. So rather than saying it is similar, I would say it is more restricted.\nLine160-161: Well, you argue that your results are more faithful than thermodynamic models to the real fluid composition.\nBut surely, your one sample is restricted to one P-T-X path, so hardly a representation of global subduction zone variability\nthat include hot and cold, steep and shallow, and old and young slabs (see e.g. Syracuse et al. 2010). The restricted S\ncontent range restricted range may be representative of the fact that you only look at a single P-T-X.\nLines164-167: While I very much agree with this statement, you kind of put this here in a vacuum, and somewhat feels like\nyou are implying this is a new idea (and cite no other paper that suggested this). In fact, lots of papers argued for extensive S\ntransfer from slab to mantle wedge, both old and new (Métrich et al. 1999 EPSL, de Hoog et al. 2001 EPSL, Walters et al.\nTaracsák et al. 2023 EPLS, Kawaguchi et al. 2022 JPetrol, just to list a few). Yes, your results confirm this, and are valuable,\nbut they add to existing evidence that you should mention here.\nLine169: I think this subsection title misses a word, or mechanism is not the best way to put it. “Formation of …” may be\nbetter?\nLine173: reference 33 is in fact a review of a paper in Nature Geo, not the paper itself that presents the results – that would\nbe Farsang and Zajacz 2024 Nature Geo.\nLine176: again, oxidised species are not considered here, yet these can carry more S than reduced species – see earlier\ncomments.\nLines178-180: I already mentioned this earlier; but pyrite is not an indicator of low fO2 (pyrrhotite would be), and in fact a\nhigher fO2 may be needed for pyrite. Can you be sure that the cpy is not formed by post entrapment reduction of S,\nconsidering Fe and S can readily exchange electrons? Omphacite can contain aegirine and Fe3+ - in fact, Weller et al. 2016\nmeasured a Fe3+/Fe(T) of 0.33-0.45 in peak omphacite, so there is plenty of it to reduce the little S (with respect to total\nmass) present in the FIs. And a similar argument can be made for calcite – can the Ca scavenged from the omphacite (it has\n18 wt% CaO)? A similar process is described for opx-hosted, CO2-rich FIs from mantle xenoliths (Berkesi et al. 2012 EPSL),\nwhere reactions between the fluid and the host formed carbonates.\nLine228-230: if I understand correctly, you argue that sulfur is more mobile because of the high Cl content of fluids, and that\nthere is a coupling effect. Yet, there is plenty of evidence that S is fairly fluid mobile as it is, especially under oxidising\nconditions. Cl is always very fluid mobile (elevated in all arcs) – so this can simply be the case of two fluid mobile elements\nbehaving similarly, because they have similar affinity to fluids under certain conditions? The citation you provide in line 227\ndoes not add to your argument – it is a melt inclusions paper from 22 years ago, that highlights there is a lot for Cl and S in\narc melt inclusions, and they modelled a fluid composition based on mass balance – it is not an observed value for fluid S\nand Cl content.\nLine240-242: Even for one arc, these estimates vary a lot – see de Moor et al 2022 for central America, who estimate that\nbetween 10-60% of S is lost to the mantle wedge from the slab. So, most S may enter the deep mantle, but also, most may\ngo into the arc.\nLines248-271: the flux calculations are confusing because you do not actually explain what you want to show, and how you\ndo it; are these values global fluxes, or just for a specific subduction zone? Is this for a certain thickness of crust. It feels a bit\nconfusing.\nLine275: the paper you cite on sediment melt S solubility says 1800 ppm at the SCAS – which is not that high (similar to a\nbasalt at the SCSS) – combine this with the fact that sediments rich in S are usually reduced (e.g. Peccia et al. 2025 Chem\nGEol), makes me question if sediments melts can in fact carry substantial amounts of S as you state.\nLine318: I cannot find this water flux anywhere in the referenced paper. You should really explain what this “flux” is, not just\nsay that this is the water flux at 90 km depth (which means not much – every subduction zone will have a different H2O flux\ndue to different P-T-X and crustal thickness). Is this for a certain thickness of crust dehydrating?\nLine329: Same question applies for the sulfur input flux – what does this value represents?\nLine364-365: This value does not significantly exceed what was suggested for Central America (between 40-70% of S being\nslab-derived in arcs) in Taracsák et al. 2023 – so in a sense your results confirm what was already suggested based on arc\nmelt inclusion data.\nMethods section: This is very little detail considering these Raman analyses are the base of the paper - what peaks/spectral\nfeatures were used to identify phases? How sure can we be that each phase is a pure mineral phase and not something\nsimilar, yet compositionally different? (e.g. cpy vs other sulfides, calcite vs dolomite or magnesite?). While I appreciate that\nyou provide four representative spectra in the supplementary, I think you should give a more\ndetailed summary here.\nLine 535: “Approximately this sulfur is eventually” sounds strange, I suggest rewording this sentence.\nFigure 2: It would be interesting to add a ternary plot with water, cpy, and cc as the three endmembers – if the inclusions\nhave a common origin, the fractions of the three phases should be similar across all inclusions. Maybe this can go in as a\nsupplementary figure.\nTable 1: you could add standard deviation and minimum/maximum values, as you use those in your Fig. 3.\nI hope these comments are helpful and will assist improving your ms.\nBest wishes,\nZoltán Taracsák", "reviewer_2": "This manuscript presents the first in situ quantification of sulfur contents in natural slab-derived fluids from sub-arc depths,\nbased on multiphase fluid inclusions hosted in omphacite from UHP eclogites in the Sumdo belt, southern Tibet. The\nauthors convincingly demonstrate that these fluids contain exceptionally high sulfur concentrations (~6 wt.%), potentially\naccounting for up to 75% of the global arc volcanic sulfur output. This is an important contribution to understanding sulfur\ncycling agents and fluxes in subduction zones, and the study is timely, original, and methodologically sound. The dataset is\nof high quality, the interpretations are generally well supported, and the implications are broadly relevant to geochemistry,\nsubduction dynamics, and volatile transfer processes. Overall, the manuscript is suitable for publication in Nature\nCommunications after moderate revisions in order to improving methodological transparency and quantitative rigor.\n________________________________________\nMajor Comments\n1. Uncertainty and reproducibility of Raman-based quantification\nThe 3D Raman mapping and volumetric reconstruction form the technical foundation of this study. However, no explicit\nuncertainty propagation is provided for the calculated bulk compositions (Table 1). Please quantify and discuss the\nanalytical uncertainties and reproducibility of the Raman-based measurements.\n2. Variability among subduction settings\nPlease discuss potential variability of slab-derived sulfur contents among different subduction zones (e.g., Alps,\nCaledonides, Japanese arcs), and clarify the limitations of extrapolating the Sumdo data to global sulfur flux estimates.\n3. Sulfide mineralogy and sulfur speciation\nChalcopyrite is the only sulfide phase observed in the fluid inclusions, whereas pyrite dominates in the host eclogite.\nWhy is this the case? In addition, no oxidized S-bearing volatiles (e.g., SO₄²⁻) were detected in the omphacite inclusions,\nalthough experimental and thermodynamic models suggest that oxidized sulfur species should be more mobile under subduction\nconditions. Please discuss this discrepancy.\n4. Role of calcite and carbonate components\nWhat is the role of calcite in the fluid inclusions? Could CO₃²⁻ enhance sulfide solubility instead of (or in addition to) NaCl?\nThis should be addressed quantitatively or at least conceptually.\n5. Potential overestimation of global sulfur fluxes\nThe limited omphacite inclusions from UHP eclogites in the Sumdo belt may represent locally sulfur-enriched portions of the\nsubducting slab. Consequently, using these inclusions to estimate global slab-derived sulfur contributions might lead to\noverestimation. Please discuss this limitation explicitly.\n________________________________________\nMinor Comments\n• Line 118: Please include full microscopic images of the mineral assemblage in the Supplementary Materials and specify\nthe trapping stage of the fluids (during subduction or exhumation). Are the omphacites vein minerals, as reported in other\nsubduction settings?\n• Lines 144–145: Clarify why chalcopyrite appears as the only sulfide phase within the inclusions, while pyrite dominates the\nhost eclogite. Does chalcopyrite have higher solubility under subduction conditions?\n• Line 146: The pronoun “that” is ambiguous—does it refer to pyrite or chalcopyrite? Please clarify whether decomposition is\npressure- or temperature-driven, and cite any reports of pyrite in metamorphic fluid inclusions.\n• Line 176: Explain why reduced sulfur species (S²⁻) are released rather than oxidized ones (SO₃²⁻). Note that reduced\nsulfur may be re-trapped by Fe²⁺ in the host rocks.\n• Lines 221–222, 235–236: Provide details about the salinity measurements and indicate whether NaCl is present in the\nSumdo inclusions. Could CO₃²⁻ enhance sulfide solubility?\n• Line 234: The discussion of “supercritical-like” fluids is speculative without supporting evidence. Use this term cautiously or\nreplace it with “near-critical, solute-rich fluids,” and cite relevant experimental studies (e.g., Mibe et al., 2011).\n• Lines 248–250: When calculating the slab-derived sulfur flux, it would be helpful to provide the estimated water flux for the\nSumdo subduction zone, rather than relying solely on global averages.\n• Line 295: Include details of laser power and acquisition parameters in the Methods section to ensure reproducibility.\n• Lines 328–329: The global sulfur input flux from subducted altered oceanic crust (~48.09 Mt/yr) may not directly apply to\nthe Sumdo subduction zone. Please provide uncertainty ranges (e.g., 75 ± ? %) to reflect the robustness of your global\nestimate."}, "author_response": "Point-by-point response\nReviewer 1 Dr. Zoltán Taracsák’s comment\nI read your paper with great interest. Your new omphacite-hosted fluid inclusion data from\nSumdo eclogite belt are a useful addition to our understanding of fluid transfer and fluid\ncompositions in subduction zone settings. I think this material is of broad interest. Your 3D\nRaman fluid inclusion data does provide direct evidence of sulfur-rich fluid compositions in a\npast subduction zone, which is great and confirms lot of model results already published in the\nliterature. However, I do have some reservations with the manuscript as it is, so I recommend\nthat revisions are made to the paper so the results presented are more robust, also suggest\nmaking the paper a bit broader interest with the inclusion of other elements in parts of your\nmodel calculations.\nReply: We sincerely thank Dr. Zoltán Taracsák for the careful, constructive, and positive review\nof our manuscript. We greatly appreciate the reviewer’s constructive comments and thoughtful\nsuggestions and have thoroughly revised the manuscript accordingly. All changes are clearly\nindicated in the revised version (shown in blue), and detailed point-by-point responses are\nprovided below. For the reviewer’s convenience, we summarize the major revisions here.\n(1) We have substantially expanded the citation of literature related to sulfur cycling in\nsubduction zones in the revised manuscript and have incorporated these references at\nappropriate locations throughout the text.\n(2) To assess the potential effects of post-entrapment modification, we selected a representative\nfluid inclusion, polished it to the surface, and conducted systematic electron probe\nmicroanalysis (EPMA) traverses across the surrounding host omphacite. By quantifying major-\nelement variations in the host mineral as a function of distance from the inclusion, we find that\nalthough limited post-entrapment modification may have occurred, its overall magnitude is\nminor. Importantly, sulfur is absent in the host omphacite, implying that any post-entrapment\nmodification would, if anything, lead to a minor underestimation of the original sulfur contents\nof the trapped fluids. Therefore, this process does not compromise, but rather reinforces, our\nconclusion that sub-arc fluids were sulfur-enriched.\n(3) Following the reviewer’s suggestion, we have clarified the sulfur speciation of the initially\nentrapped sub-arc fluids and explicitly discussed the potential influence of fluid–host\ninteraction. (i) EPMA profiles reveal slightly lower Fe contents in omphacite adjacent to\ninclusions relative to more distal regions (0.4 wt.% FeO variations), suggesting limited Fe\nexchange between host and inclusion. Such redistribution could have locally modified sulfur\nspeciation within the inclusions, potentially shifting it from initially oxidized forms toward\nreduced species during or after entrapment. (ii) To further constrain the primary sulfur\nspeciation, we performed thermodynamic modeling using the DEW framework under the\ngeological P–T conditions of the Sumdo system. These calculations indicate that slab-derived\nfluids at sub-arc depths are dominated by oxidized sulfur species (e.g., SO ²⁻, HSO ⁻) at the\n4 4\ntime of entrapment. Importantly, although sulfur speciation may be locally modified by fluid–\nhost interaction, the total sulfur contents of the fluids remain unaffected. Accordingly, our\ncentral conclusion—that slab-derived fluids at sub-arc depths can contain exceptionally high\nsulfur concentrations—remains robust.\n(4) We have strengthened and clarified the discussion regarding extrapolation from the Sumdo\nsystem to broader subduction contexts by explicitly stating the assumptions, rationale, and\nlimitations. In summary: (i) The Sumdo eclogites record a cold subduction regime, which\nrepresents the dominant style of subduction operating globally. As such, the sulfur systematics\ndocumented here are expected to be broadly relevant to other cold subduction zones. (ii)\nThe studied inclusions capture fluid compositions at sub-arc depths, a crucial window for slab-\nderived sulfur release. We further clarify that sulfur-rich sub-arc fluids are not unique to the\nSumdo system, but have been reported in multiple subduction settings worldwide. Moreover,\nour measurements not only validate previous experimental and theoretical predictions of\nelevated sulfur contents in sub-arc fluids but also provide more restricted natural constraints on\nthe range of sulfur concentrations in slab-derived fluids. (iii) We performed new\nthermodynamic modelling constrained by the geological context and P–T conditions of the\nSumdo system to quantify slab dehydration. The results indicate that ~60% of the water initially\nstored in altered oceanic crust is released at sub-arc depths. Treating the Sumdo system as\nrepresentative of altered oceanic crust dehydration in cold subduction zones, and integrating\nthis dehydration fraction (~60%) with the average H O content of altered oceanic crust, global\n2\nsubduction zone length, and crustal thickness (Alt et al., 1999; Stern et al., 2002; Ribeiro et al.,\n2026), we estimate a global H O release flux of ~84.5\n(~91 Mt/yr), supporting the robustness of our water-flux parameterization. The independently\nconstrained H O flux thus provides a critical parameter for refining estimates of sulfur flux\n2\ntransported by slab-derived fluids at sub-arc depths. (iv) Based on the calculated sulfur fluxes,\nwe further quantify the efficiency of slab-derived sulfur release at sub-arc depths and evaluate\nits contribution to sulfur enrichment in arc systems. These estimates are in quantitative\nagreement with independent experimental, theoretical, and geochemical constraints, supporting\nboth the reasonableness of our parameterization and the reliability of extrapolating results from\nthe Sumdo system to global cold subduction zones.\n(5) In response to the reviewer’s insightful comment regarding the elevated copper\nconcentrations measured in the fluid inclusions, we have newly incorporated a concise\ndiscussion of their broader geochemical implications. Our results highlight the role of sulfur-\nrich oxidized sub-arc fluids in promoting efficient copper mobilization, with significant\nimplications for the formation of porphyry copper deposits in arc magmatic systems.\nIn addition to the major revisions outlined above, we have carefully addressed all other\nconstructive comments and suggestions in the revised manuscript. We sincerely hope that the\nrevised version meets the reviewer’s expectations. We are grateful to Dr. Zoltán Taracsák for\nthe thoughtful and insightful comments, which have substantially strengthened the clarity, rigor,\nand overall quality of this work.\nGeneral comments\n(1) One of my main criticism is the limited, and in my view, skewed, use of references in the\npaper. You often cite Li et al. 2020 (Nat Comms) as the reference for many of your statements\nand ignore a large number of papers that deal with sulfur in subduction settings and suggest (as\nyou also do), that sulfur is strongly fluid-mobile and added to the mantle wedge from the slab\nin large quantities. I listed a few references in my detailed comments that you should probably\ninclude (some you already do, but don’t always cite when you should). Your discussion should\nbe more nuanced, as the way it is currently written, it feels like you are the first to prove efficient\nS transfer via slab-derived fluids, but there is already plenty of evidence for this form arc\nvolcanoes and model results. Limitations of your data are not acknowledged – surely, one\nsample from one eclogite belt will be representative of one P-T-X subduction path, not global\nvariability.\nReply: We greatly appreciate the reviewer’s constructive and thoughtful comments. We fully\nagree that these aspects required clearer treatment, and we have revised the manuscript\naccordingly.\n(1) We acknowledge that the original version of the manuscript placed disproportionate\nemphasis on Li et al. (2020). In the revised manuscript, we have substantially expanded and\nrebalanced the citation of the literature on sulfur cycling in subduction zones. This includes the\nreferences suggested by the reviewer, as well as additional key studies published up to 2026.\n(2) We have revised the manuscript to avoid implying that our study is the first to demonstrate\nefficient sulfur transfer from the slab to the mantle wedge. Instead, we now clearly emphasize\nthat our primary contribution is the first direct, in situ quantification of sulfur concentrations in\nnatural slab-derived fluids at sub-arc depths, achieved through three-dimensional (3D) Raman\ncharacterization of multiphase fluid inclusions. Importantly, our findings provide a critical\nobservational bridge linking experimental constraints, thermodynamic models, and sulfur-\nenriched signatures observed in arc magmas, thereby strengthening and integrating existing\nframeworks rather than replacing or contradicting them.\n(3) Finally, we now explicitly acknowledge the limitations associated with extrapolating results\nfrom the Sumdo system to global subduction zones. We recognize that subduction zones\nworldwide exhibit substantial variability in thermal regimes (e.g., cold versus hot subduction),\nslab dip angles, and convergence rates, all of which may influence the behavior of sulfur during\nsubduction. These limitations are now clearly stated in the revised manuscript.\nAt the same time, we also clarify why the Sumdo system nevertheless provides meaningful\nconstraints on the deep sulfur cycle at the global scale, based on the following considerations.\n(i) The Sumdo system records a cold subduction regime, which represents a widespread and\ngeodynamically important class of subduction zones operating globally. Although results from\na single system cannot capture the full range of global variability, cold subduction zones\ncontribute substantially to global volatile recycling, allowing the extrapolating results from the\nSumdo system provide broadly applicable constraints on subduction zone sulfur budgets. (ii)\nSub-arc depths constitute a primary window for sulfur release from the subducted slab, and the\nsamples investigated here directly record this critical stage of sulfur transfer. Notably, the sulfur\nconcentrations quantified in our study fall within the range predicted by previous experimental\nand theoretical studies, supporting the robustness and reliability of our observations. (iii)\nUsing geological constraints from the Sumdo system, we performed thermodynamic calculations to\nestimate slab dehydration extents and corresponding H O fluxes at sub-arc depths (80–90 km).\n2\nfurther supporting the representativeness of the Sumdo system within the context of cold\nsubduction. (iv) By integrating the measured sulfur concentrations with the calculated H O\n2\nfluxes, we estimated sulfur fluxes carried by sub-arc fluids and evaluated the efficiency of slab\nsulfur release at sub-arc depths and its contribution to sulfur enrichment in arc systems. These\nestimates are in quantitative agreement with independent experimental, theoretical, and\ngeochemical constraints, supporting a cautious yet reasonable extrapolation of our findings\nfrom the Sumdo system to global subduction zones.\n(2) Secondly, I’m not convinced that your inclusions are primary (they can be secondary based\non the evidence you provide), and this needs to be addressed or acknowledged in the text. I’m\nalso not convinced that your inclusions have not undergone post-entrapment modification. You\nkind of brush this under the carpet in the text and provide no evidence that rules out Ca\nscavenging from the host (to form calcite from a CO -rich fluid), or FI and host electron\n2\nexchange (between Fe3+ present in omphacite and S6+ that may have been in the fluid). In\nsummary, I’m not sure that your inclusions were reduced, as you suggest. This needs to be\nconsidered and addressed, as oxidised fluids thought to carry large quantities of S, based on\nmost recent DEW models, and may fit better with your data.\nReply: We thank the reviewer for these insightful and important comments. We agree that the\norigin of the fluid inclusions, the possibility of post-entrapment modification, and the sulfur\nspeciation require careful evaluation and clearer treatment. In the revised manuscript, we have\nsubstantially revised the text to explicitly address these issues, as summarized below.\n(1) We have clarified and strengthened the criteria supporting a primary origin for the studied\nmultiphase fluid inclusions. The revised manuscript (Lines 125–142) now states: “Multiple\nindependent lines of evidence support a primary origin for the omphacite-hosted fluid\ninclusions. First, the fluid inclusions are confined to the cores of omphacite grains and do not\nform trails that crosscut the host crystals (Fig. 1), as would be expected for secondary inclusions\ntrapped along healed fractures. Detailed petrographic observations further reveal no\nmicrofractures or healed cracks in the host omphacite that could have acted as pathways for\nlate-stage fluid infiltration (Fig. S2). Second, the fluid inclusions commonly occur as isolated\nor as arrays parallel to the crystallographic c-axis of omphacite (Fig. 1). Individual fluid\ninclusions are typically ~5–60 μm in length and ~2–8 μm in width and display a consistent\ncolumnar morphology that mimics the elongated crystallographic habit of the host mineral. This\nnegative-crystal–like geometry is consistent with primary entrapment and provides supporting,\nthough not diagnostic, evidence for a primary origin38. Third, the fluid inclusions show uniform\nfilling degrees of ~80–90% at room temperature and contain consistent proportions of liquid,\nsolid, and vapor phases (Fig. 1), a feature that is inconsistent with heterogeneous secondary\ntrapping. Taken together, these textural and morphological characteristics indicate that the\nstudied inclusions represent primary fluid inclusions. Combined with the fact that omphacite\ncrystallized exclusively at the peak stage of UHP conditions37, corresponding to sub-arc\ndepths36, these observations suggest that omphacite-hosted fluid inclusions provide the most\nreliable recorders of slab-derived fluid compositions at sub-arc conditions.”\n(2) We fully agree that potential post-entrapment modification must be carefully assessed. To\ndirectly address this concern, we selected a representative multiphase fluid inclusion, polished\nit to the surface, and conducted systematic EPMA transects across the host mineral (omphacite)\nsurrounding the fluid inclusions. By quantifying major-element variations as a function of\ndistance from the inclusion, we find that some degree of post-entrapment modification did occur,\nbut its overall extent is limited (Fig. S6). Importantly, sulfur is absent in the host omphacite.\nTherefore, any fluid–host interaction during post-entrapment modification would not result in\na decrease of the sulfur concentrations preserved within the inclusions. Consequently, our\ncentral conclusion—that the trapped fluids were sulfur-rich—remains robust and unaffected by\npost-entrapment processes.\n(3) Furthermore, we now explicitly acknowledge that post-entrapment processes may have\ninfluenced sulfur speciation within the inclusions. The newly added EPMA data reveal slight\nvariations in Fe contents in omphacite adjacent to the investigated fluid inclusions, which may\nreflect a weak Fe exchange between the host mineral and the inclusions (Fig. S6). Such electron\nexchange could plausibly modify sulfur speciation after entrapment. To address this more\nrigorously, we have newly conducted a thermodynamic modeling using the DEW framework,\nconstrained by the geological and P–T conditions of the Sumdo system. These calculations\nindicate that sulfur in slab-derived fluids at sub-arc depths is dominantly present as oxidized\nspecies (e.g., SO 2-, HSO -)(Fig. S7). Accordingly, we have revised the manuscript to clarify\n4 4\nthat the primary slab-derived fluids at sub-arc depths were likely oxidized and sulfur-rich,\nwhich may be reduced in the fluid inclusions after entrapment.\nWe sincerely thank the reviewer once again for raising these important and constructive points.\nAddressing these issues has substantially improved the clarity, rigor, and overall robustness of\nthe manuscript!\nFig. S6 in the supplementary information. Major-element variations in host omphacite across\na representative multiphase fluid inclusion. (A-G) Electron probe microanalysis transects\nshowing variations in FeO, CaO, SiO , MgO, Al O , Na O, and TiO (wt.%) in host omphacite\n2 2 3 2 2\nacross a representative multiphase fluid inclusion. Three parallel analytical transects (Lines 1–\n3) were conducted across the inclusion, with the grey shaded region marking the position of the\nfluid inclusion. (H) Schematic illustration showing the relative positions of the three transects\nand the fluid inclusion polished to (near-)surface. Overall, major-element compositions of\nomphacite show only slight variations across the inclusion (e.g., ~0.4 wt.% for FeO and ~0.6\nwt.% for CaO), indicating that post-entrapment modification of the host mineral is minor.\nFig. S7 in the supplementary information. Thermodynamic modeling of sulfur speciation in\nslab-derived fluids constrained by the Sumdo system. The results show that slab-derived fluids\nat sub-arc depths are dominated by oxidized sulfur species (e.g., SO 2-, HSO -).\n4 4\n(3) Your arguments that Cl influences S behaviour at the end of the manuscript feel very\nspeculative. These are two fluid mobile elements, and Cl is elevated in all subduction zone\nmagmas, while S is not always enriched. So I don’t really feel there is any causality here, and\nthat you provide sufficient proof that there is.\nReply: We are grateful to the reviewer for raising this important point. We agree that the original\ndiscussion placed too much emphasis on a potential causal role of Cl in controlling sulfur\nbehavior. In the revised manuscript, we have substantially restructured this section.\n(1) Following the reviewer’s suggestion, and guided in particular by the recent experimental\nand thermodynamic study of Beaudry and Sverjensky (2024), we now interpret the trapped\nfluids as being dominated by oxidized sulfur species and focus instead on the role of major\ncations (Ca and Na) in dissolving sulfur in fluids at sub-arc conditions. Beaudry and Sverjensky\n(2024) demonstrate that oxidized sulfur can be efficiently mobilized in high-pressure fluids\nthrough complexing with Ca and Na (e.g., CaHSO ⁺ and Na SO 0), resulting in significantly\n4 2 4\nenhanced sulfur solubility under sub-arc conditions. Importantly, this revised interpretation is\nfully consistent with our observations of elevated CaO (10.16 wt.%) and Na (3.22 wt.%), and\nhigh sulfur contents in the multiphase fluid inclusions. We therefore believe that the revised\ndiscussion presents a more appropriate interpretation of the sulfur enrichment mechanism in\nslab-derived fluids.\n(2) As noted by the reviewer, chlorine is itself highly fluid-mobile, and the apparent covariation\nof sulfur and chlorine in arc systems may simply reflect their shared affinity for slab-derived\nfluids rather than a direct causal relationship. We therefore adopt a more cautious treatment of\nits role. Specifically, we now retain only a limited discussion of two possible secondary\ninfluences of chlorine: (i) experimental results at relatively low pressures and temperatures\n(<0.5 GPa) indicating a positive correlation between Cl concentration and CaSO solubility\n4\n(Newton, 2004); and (ii) the stronger affinity of Cl for Fe relative to sulfur, which may indirectly\naffect sulfur transport by promoting metal–chloride complexation and influencing fluid–rock\ninteraction processes. Accordingly, in the revised manuscript (Lines 288–314), we have\nsoftened statements regarding the role of chlorine in sulfur enrichment and limited the\ndiscussion to a concise and cautious interpretation. We hope that this revised and more cautious\ntreatment adequately addresses the reviewer’s concern!\n(4) I’m not sure who your flux models work – is this for a fictional subduction zone, or for a\nglobal S flux from the slab? It needs to be better explained.\nReply: We thank the reviewer for this constructive comment. We agree that the description of\nthe sulfur flux calculations in the original manuscript was not sufficiently clear. In the revised\nmanuscript, we have substantially clarified both the rationale underlying our flux estimates and\nthe limitations associated with their extrapolation. As discussed above, the justification for\nextending results from the Sumdo system to global subduction zones can be summarized as\nfollows.\n(1) Representativeness of the Sumdo system. Although our study focuses on a single natural\nsystem, the Sumdo belt records a well-constrained cold subduction regime that represents a\nwidespread and geodynamically significant class of subduction zones globally. While it does\nnot encompass the full spectrum of global subduction variability (e.g., hot subduction systems),\nthe Sumdo system provides a robust natural reference for sulfur cycling in cold subduction\nenvironments. As such, it provides a useful natural reference for the general assessment of\nsulfur cycling processes in subduction zones at the global scale, particularly within cold\nsubduction settings that play a major role in volatile recycling.\n(2) Direct constraints on sulfur concentrations in slab-derived fluids. Our study provides the\nfirst direct, in situ quantification of sulfur concentrations in natural slab-derived fluids released\nfrom the altered oceanic crust at sub-arc depths. Notably, the measured sulfur contents fall\nwithin the range predicted by previous experimental, thermodynamic, and arc geochemical\nstudies. This consistency lends confidence to the use of our measured sulfur concentrations as\na geologically realistic parameter for estimating sulfur fluxes transported by sub-arc fluids at\nthe global scale.\n(3) Constraints on H O flux as a key parameter in sulfur flux calculations. In calculating sulfur\n2\nfluxes, the H O flux represents the other key parameter. In the original manuscript, we relied\n2\ndirectly on literature estimates of H O fluxes at sub-arc depths (80–90 km). In the revised\n2\nmanuscript, to provide a more rigorous calculation, we performed thermodynamic calculations\nconstrained by the geological and P–T conditions of the Sumdo system to estimate slab\ndehydration extents at sub-arc depths. The results indicate that ~60% of the water initially\nstored in altered oceanic crust is released at depths of 80–90 km. Based on this\ndehydration fraction, and integrating average H O contents of altered oceanic crust, global\n2\nsubduction zone length, and crustal thickness (Alt et al., 1999; Stern et al., 2002; Ribeiro et al.,\n2026)(see revised Methods for details), we estimate a global H O flux of ~82.5 Mt/y released\n2\nfrom subducted oceanic crust at sub-arc depths. This estimate closely matches independent\nliterature constraints (~91 Mt /y, Li et al., 2020), supporting the robustness of our water-flux\nparameterization.\n(4) Sulfur flux calculations in sub-arc fluids supported by previous independent studies. By\ncombining the measured sulfur concentrations in slab-derived fluids with the calculated global\nH O fluxes released from the altered oceanic crust at-sub depths, we estimate the sulfur flux\n2\ntransported by sub-arc fluids and quantify the efficiency of slab-derived sulfur release at sub-\narc depths and its contribution to sulfur enrichment in arc systems. These estimates are\nquantitatively in agreement with independent experimental, theoretical, and geochemical\nconstraints (Walters et al., 2020; Li et al., 2022; Taracsák et al., 2023), lending support to a\ncautious and reasonable extrapolation from the Sumdo system to global subduction zones.\nWe consider this extrapolation to be reasonable, while fully acknowledging the associated\nlimitations. In the revised manuscript (Lines 331–360, Lines 539–550), we now provide a\ndetailed discussion of both the rationale and the constraints involved in extending results from\nthe Sumdo system to broader subduction zone settings. We sincerely hope that these revisions\nadequately address the reviewer’s concerns!\n(5) Data tables are provided as a Word document, which is not particularly useful for other\nresearchers who may want to use your data – you probably have an excel/csv table for these –\nplease\nReply: We appreciate the reviewer for this helpful suggestion. We have converted original Table\n1 and Table S1 from a Word document into Excel format and now provide them as Tables S1\nand S2 in the Supplementary Information. In addition, we have expanded these tables to include\nstandard deviations (SD) as well as minimum and maximum values.\nFurthermore, we have added several new datasets to the Supplementary Information. These\ninclude (i) EPMA major element compositions of host omphacite across the investigated\nmultiphase fluid inclusions (Table S3), and (ii) New thermodynamic modelling results\nconstrained by the geological and P–T conditions of the Sumdo system, including calculated\nsulfur speciation in slab-derived fluids and estimates of slab dehydration extents (Table S4 and\nS5).\nDetailed comment\nLines 47-48: While the 20-30% fluid contribution from the slab to the mantle wedge, and\nsubsequently to the arc, is indeed the value in Li et al. (2020) you cite, but this is one of only\nfew papers that argue for low amounts of S being transferred from the slab to the wedge, and\nthere are many others that say the opposite. For example, in Central America, we estimated 40-\n70% sulfur addition to the mantle wedge from the slab in Taracsák et al. 2023. The way it is\ncurrently written implies there is only this 20-30% range in the literature, which is not the case.\nReply: We apologize that the original wording did not adequately reflect the full range of\npublished estimates for slab-derived sulfur transfer to the mantle wedge.\nIn the revised manuscript, we now explicitly acknowledge that estimates of slab-derived sulfur\naddition to arc magma sources span from ~20–30% to as high as ~40–70%, based on both\nthermodynamic modeling and geochemical constraints from natural arc systems (e.g., Li et al.,\n2020, 2021; de Moor et al., 2002; Muth and Wallace, 2022; Taracsák et al., 2023). We further\nclarify that this variability likely reflects differences among subduction zone settings, but also—\nperhaps more fundamentally—differences in the approaches used to quantify sulfur transfer,\nincluding theoretical modeling and mass-balance calculations based on arc volcanic rocks. We\nfurther clarify that this wide range likely reflects not only variability among subduction zone\nsettings, but also fundamental differences in these indirect methodological approaches.\nAlthough these indirect approaches provide important constraints, they do not directly measure\nsulfur concentrations in slab-derived fluids and therefore may yield inconsistent conclusions\nregarding the contribution of slab-derived sulfur to arc magma budgets. In this context, we\nemphasize that our study provides direct observational constraints on sulfur concentrations in\nnatural slab-derived fluids at sub-arc depths. These data offer an independent line of evidence\nthat helps bridge the gap between model-based predictions and sulfur enrichments inferred from\narc magmas.\nAccordingly, we have revised the relevant text (Lines 54–63), which now reads: “However,\nmass-balance calculations indicate that such fluids contribute a highly variable fraction of arc-\nrelated sulfur emissions, exposing a fundamental mismatch between the observed arc outputs\nand the estimated slab-derived inputs18-22. For example, sulfur isotopic compositions of\nultrahigh-pressure (UHP) metamorphic rocks, combined with thermodynamic modeling,\nsuggest that slab-derived fluids may account for only ~20% of the sulfur budget of arc\nmagmas22,23. In contrast, sulfur enrichments recorded in melt inclusions from natural arc basalts\nimply substantially larger contributions, on the order of ~40–70%20. This discrepancy may\npartly reflect the compositional diversity of slab-derived fluids among different subduction\nsystems. More fundamentally, however, it likely arises from the lack of direct constraints on\nsulfur contents and fluxes in natural slab-derived fluids themselves. ”\nLine 51: This implies that people argued for extra S in the mantle before subduction. I have not\nseen papers suggesting high S contents in the subarc mantle (relative to normal, depleted upper\nmantle) under arcs prior to subduction-derived S addition. Is there a reference for this?\nReply: We agree that the original wording was unclear. Our intention was to convey that,\nalthough slab-derived sulfur represents a major and essential source for arc magmatic systems,\nit may not always be sufficient on its own to account for the total sulfur released in arc settings.\nIn such cases, additional contributions—such as mantle-inherited sulfur—may also play a role\nin the overall sulfur budget of arc magmas. To avoid ambiguity or potential misinterpretation,\nwe have removed this statement in the revised manuscript.\nLine57: a minor issue but melt inclusion paper you cite here (Muth & Wallace 2021) used tephra\nsamples, not lavas.\nReply: Thanks. We have corrected the text in the revised manuscript (Lines 65–69), which now\nreads: “To date, estimates of sulfur concentrations in subduction zone fluids have relied\npredominantly on indirect approaches, including analyses of sulfur concentrations and isotopic\ncompositions in exhumed metamorphic rocks, melt inclusions in arc basalts and tephra,\nhigh-temperature and high-pressure (HT–HP) experiments, and thermodynamic modeling18-22,24-29.”\nLine70-72: you cite Evans 2012 (a redox focussed review paper) and Bekaert et al. 2021 (a\nvolatile cycle review paper), rather than papers that look at subducting S budgets (de Moor et\nal. 2022 Chem Geol, Peccia et al. 2025 Chem Geol), that are much more detailed and more\nrelevant to your work. Crucially, both of these papers suggest it is the primary igneous crust\nthat has the largest S budget followed by the AOC– and the total igneous package (AOC +\nprimary) are the most important sources of S. Is there any proof that your eclogites represent\nAOC, or can they be metamorphosed primary crust (unaltered)?\nReply: We thank the reviewer for the insightful suggestion and for drawing our attention to\nthese important references. These references have now been incorporated into the revised\nmanuscript (Line 85).\nRegarding the nature of the protolith of the Sumdo eclogites, previous work has constrained\ntheir origin based on whole-rock major and trace element systematics and Sr–Nd isotopic\ncompositions (Yang et al., 2009; Liu et al., 2019a). As summarized in Fig. 1 (after Liu et al.,\n2019a), these geochemical data indicate that the Sumdo eclogites were derived predominantly\nfrom altered oceanic crust, with contributions from relatively fresh igneous components. This\nclarification has now been added in the revised manuscript (Lines 185–188).\nFig.1 (a) Chondrite-normalized rare earth element diagram for the Sumdo eclogite samples. (b)\nPlot of initial Sr-Nd isotopic compositions of the Sumdo eclogite. Figures are cited from Liu et\nal. (2019).\nLine 99: the presence of pyrite is interesting. Looking at the recent model results of Beaudry &\nSverjensky 2024 (G-cubed), who extended the Deep Earth Water model’s calibration range and\nincluded oxidised S species, they suggest that under conditions similar to what you imply for\nthese samples (~3GPa, 600-800 Celsius), pyrite is present at FMQ-1 to FMQ+2 (at higher T, it\nis more restricted to FMQ+0.5 to +1.5). Critically, their model results provide S contents in\nfluids in equilibrium with mafic eclogite that reach the S contents you propose based on your\nFis (and even higher values), so it is a very relevant paper to your results, and you currently\ndon’t cite or integrate this into your paper. I think you need to carefully consider their findings\nregarding the S carrying capacity of fluids at higher fO than those modelled in Li et al. 2020,\n2\nas they predict up to an order of magnitude more S in the fluid.\nReply: We sincerely appreciate the reviewer for sharing this valuable reference. We have\ncarefully examined the work of Beaudry and Sverjensky (2024). It is indeed a nice paper. Their\nfindings provide strong independent support for our results. Specifically, (1) the sulfur\nconcentrations predicted for slab-derived fluids at sub-arc conditions in Beaudry and\nSverjensky (2024) overlap well with the sulfur contents quantified in our study (3.2–10.3 wt.%);\n(2) their thermodynamic modeling demonstrates that fluids equilibrated with pyrite at sub-arc\ndepths can be dominated by oxidized sulfur species. This is fully consistent with our\nobservations, including the presence of residual pyrite in the Sumdo eclogites (Fig. S2) and the\ninference of oxidized sulfur species in the initially entrapped fluids based on our DEW\nmodelling (Fig. S7). We have now incorporated and discussed Beaudry and Sverjensky (2024)\nat several appropriate locations in the revised manuscript (Line 69, 73, 198, 208, 225, 233, 288–\n290). We thank the reviewer again for drawing our attention to this important and relevant\ncontribution!\nLine125-128: the reconstructed fluid compositions are very interesting, particularly with\nrespect to their high Cu contents. This is because, as far as I’m aware, Cu has been suggested\nnot to mobilise during slab devolatilisation (e.g. Jenner, 2017 Nature Geo). Your paper could\nbe more broad interest I think if you explored other elements alongside S, such as Cu, H O or\n2 2\nmatch estimates for mantle wedge H O contents that range from 0.3 to 1wt% in the Marianas\n2\n(Kelley et al. 2010 J. Petrol). The 0.1% to 1% fluid addition in Fig 3A seems a bit too low for\nthat. Also, how much Cu would be added to the mantle under the same circumstances,\nconsidering your Fis have 5 wt%, and the upper mantle only ~30 ppm?\nReply: We thank the reviewer for these excellent and constructive suggestions.\n(1) We agree that H O fluxes in subduction zones may vary among different tectonic settings\n2\nand that precise values are inherently uncertain. In the revised manuscript, we therefore\nincorporated thermodynamic modelling constrained by the geological and P–T conditions of\nthe Sumdo system to quantify slab dehydration at sub-arc depths (~80–90 km). The results\nindicate that approximately 60% of slab-bound water is released at these depths (Fig. S6).\nTreating the Sumdo system as representative of altered oceanic crust dehydration in cold\nsubduction zones, and integrating this dehydration fraction (~60%) with the average H O\n2\ncontent of altered oceanic crust, global subduction zone length, and crustal thicknes (Alt et al.,\n1999; Stern et al., 2002; Ribeiro et al., 2026), we estimate a global H O release flux of ~84.5\n(~91 Mt/yr, Li et al., 2020), supporting the robustness of our water-flux parameterization.\nTherefore, this approach provides an estimate of the average global H O flux from the altered\n2\noceanic crust at sub-arc depths, while we explicitly acknowledge that the precise magnitude of\nH O flux may vary among individual subduction systems due to differences in tectonic and\n2\nthermal conditions.\n(2) Furthermore, we agree that the reconstructed fluid compositions have implications\nextending sulfur, particularly in light of the unexpectedly high Cu concentrations measured in\nthe inclusions. Given the close geochemical relationship between sulfur and copper (Richards,\n2015; Farsang and Zajacz, 2025), we have added a concise discussion in the revised manuscript\n(Lines 391–422) to explore the implications of slab-derived sulfur-rich, oxidized fluids for Cu\nmobility in the subduction zone. This new discussion focuses on two main aspects:\n(1) We acknowledge that Cu mobility during slab devolatilization remains debated. Some\nstudies argue that Cu is largely retained in the slab and is not efficiently transferred by slab-\nderived fluids (e.g., Jenner, 2017; Zou et al., 2024; Ren et al., 2025), whereas other work\nsuggests that Cu can be mobilized under specific conditions, particularly when complexed by\nsulfur-bearing fluids (e.g., Li et al., 2013; Walters et al., 2021). Our fluid inclusion data provide\ndirect natural evidence that slab-derived fluids at sub-arc depths can contain elevated Cu\nconcentrations. In the revised manuscript, we therefore emphasize that the combination of\nsulfur-rich and oxidized fluid compositions provides a plausible mechanism for Cu\nmobilization in subduction zones.\n(2) We further address the apparent discrepancy between Cu-rich slab-derived fluids and the\nlack of systematic Cu enrichment in many arc magmas. Many geochemical signatures of many\narc basalts indicate that their mantle sources are not systematically enriched in copper, implying\nlimited net addition of slab-derived copper to the arc magma source (Muth et al., 2022; Lee et\nal., 2012; Lee and Tang, 2020; Zhao et al., 2022). In contrast, some metasomatized sub-arc\nxenoliths show elevated copper compared to the depleted mantle compositions (Mcinnes et al.,\n1999; Kepezhinskas et al., 2002; Lorand et al., 2013; Tassara et al., 2018). In the revised\nmanuscript, to reconcile this apparent mismatch, we propose three possible processes. (i) Slab-\nderived Cu may be efficiently sequestered or “filtered” during fluid–rock interaction at the slab–\nmantle interface or within the mantle wedge, for example, through sulfide precipitation.\nConsistent with this interpretation, Cu enrichment has been found at eclogite–peridotite\ninterfaces (representing slab-mantle boundary), including in CCSD samples reported in our\nrecent study (Tan et al., 2025, GRL), where Cu concentrations at the interface are significantly\nhigher than in adjacent eclogite and peridotite. We note that the previous study reported Cu\nconcentrations for the interface and adjacent lithologies only; a detailed mechanistic\ninvestigation of Cu enrichment at these interfaces is beyond the scope of the previous study\n(Tan et al., 2025) and will be addressed in future work. (ii) Copper transport may be spatially\nheterogeneous, with focused fluid flow through channels, leading to localized Cu enrichment\nwithin the mantle wedge rather than uniform distribution. During ascent and interaction with\nsurrounding mantle peridotite, Cu may be preferentially unloaded via sulfide formation,\nlimiting its transfer to arc magmas. Sulfide saturation and magmatic differentiation processes\nmay buffer Cu concentrations in arc melts, thereby reducing contrasts between MORB and arc\nlavas despite variable slab-derived Cu inputs (Walters et al., 2021).\nTaken together, these considerations indicate that Cu enrichment in arc magmas and the\nformation of copper deposits are governed by a combination of factors, including the capacity\nof slab-derived fluids to mobilize copper, the influence of sulfide sequestration during fluid\ntransport, and subsequent magmatic differentiation processes. Nevertheless, our results\ndemonstrate that sulfur-rich, oxidized sub-arc fluids are capable of effectively mobilizing Cu\nfrom the subducted slab. This provides a necessary material basis for copper transfer into the\nmantle wedge and, ultimately, for copper enrichment in arc magmas, highlighting the potential\nrole of slab-derived fluids in the genesis of porphyry copper deposits in arc settings.\nWe thank the reviewer again for encouraging us to broaden the scope of the discussion. We\nbelieve that these revisions improve the overall significance and appeal of the manuscript to a\nbroad readership!\nLine 136: I have some issue with your interpretation of the FIs being primary. In line 110, you\nmention FIs are parallel with the C axis (a cleavage direction for cpx). You also mention they\nare confined to cores and have negative crystal morphologies. But together, these observations\ncannot rule out that the inclusions are in fact secondary, entrapped in cracks within the\nomphacite after formation, followed by further crystal growth along the rims. Negative crystal\nmorphologies alone cannot be used to argue for a primary origin (see Goldstein 2003 review\non FI petrography). In any case, you need to provide some further arguments why your\ninclusions are representative for peak metamorphic conditions, considering a potential\nsecondary origin relative to omphacite cores.\nReply: We appreciate the reviewer for the comments on the origin of fluid inclusion.\n(1) In the revised manuscript (Lines 125–142), we have further clarified this issue and now\npresent several independent lines of evidence that collectively support a primary origin for the\nfluid inclusions. (i) The fluid inclusions are consistently confined to the cores of individual\nomphacite grains and do not occur as planar trails or arrays that crosscut the host crystal.\nSecondary inclusions are typically associated with healed fractures or crack-related inclusion\ntrails; however, detailed petrographic examination reveals no evidence for healed cracks or\nmicrofractures in the host omphacite that could have acted as pathways for secondary\nentrapment (Fig. S2). (ii) The inclusions exhibit remarkably uniform phase assemblages and\nsimilar filling degrees across multiple grains. In contrast, secondary inclusions commonly\ndisplay greater variability in phase proportions and compositions due to episodic trapping along\nfractures. (iii) The inclusions display elongate, negative-crystal–like morphologies that mimic\nthe crystallographic habit of the host omphacite. This negative-crystal–like geometry is\nconsistent with primary entrapment and provides an additional line of evidence for a primary\norigin, although it is not, by itself, a definitive criterion (Goldstein, 2001). Taken together, we\nconsider all the criteria and interpret these lines of evidence as most consistent with primary\nentrapment during omphacite growth.\n(2) Regarding the stage and P–T conditions of the fluid inclusions hosted in omphacite, previous\nstudies have tightly constrained the P–T conditions of omphacite growth in the Sumdo eclogites\n(Table below, Liu et al., 2019b). This study suggested that omphacite crystallization was\nrestricted to the peak metamorphic stage. Accordingly, we interpret the omphacite-hosted fluid\ninclusions to record fluids trapped during peak to near-peak UHP metamorphic conditions,\ncorresponding to sub-arc depths.\nThis table summarizes the growth stages of various metamorphic minerals, showing that\nomphacite occurs exclusively at the peak metamorphic stage. Table is from Liu et al. (2019b).\nLine 140: I doubt that you can rule out post entrapment re-equilibration. H+ diffuses rapidly\nwithin cpx, so you could gain or lose it depending on H activity around the grains. Fe2+ can also\ndiffuse in and out. Both of these elements could influence the redox within your inclusions.\nThis leads me to think that the S may have been oxidised in the fluid originally, not reduced,\nand the chalcopyrite formed due to post-entrapment processes. Can you rule this out, and if yes,\nhow? If not, this needs to be considered.\nReply: Many thanks for this constructive comment. As noted by the reviewer, H⁺ can diffuse\nrelatively rapidly within clinopyroxene, and Fe²⁺–Fe³⁺ exchange between the host omphacite\nand the inclusions is also possible. Both processes could modify the redox state within fluid\ninclusions after entrapment.\nWith respect to H diffusion, this process is difficult to directly constrain in natural samples,\nand we acknowledge that it cannot be excluded in this study (Lines 241–243). Regarding Fe\nexchange, we have added new EPMA profiles across host omphacite surrounding the\nrepresentative multiphase fluid inclusions. These data show that omphacite in the immediate\nvicinity of inclusions contains slightly lower FeO contents (0.3 wt.% variations) compared to\nregions farther from the inclusions (Fig. S6). Such Fe content variations could plausibly have\nmodified the redox state within the inclusions after entrapment.\nTo further constrain the primary sulfur speciation in the entapped fluids, we performed\nthermodynamic modeling using the DEW framework, constrained by the geological and P–T\nconditions of the Sumdo system. These calculations indicate that sulfur in slab-derived fluids\nat sub-arc depths is dominantly present as oxidized species (e.g., SO ²⁻ and HSO ⁻) (Fig. S7).\n4 4\nOn this basis, we now interpret the entrapped fluids as being most likely oxidized at the time\nof entrapment at-sub depths.\nWe therefore revise our interpretation to explicitly consider that post-entrapment processes,\nincluding Fe exchange between the host omphacite and the inclusions, may have driven\nlocalized redox reactions that partially reduced sulfur species and promoted chalcopyrite\nprecipitation within the inclusions. Alternatively, hydrogen diffusion across the inclusion–host\nboundary may also have contributed to such redox modification, although its quantitative effect\nremains difficult to assess. This clarification regarding sulfur speciation has been incorporated\nin the revised manuscript (Lines 226–253).\nAccordingly, in the revised manuscript, we clarify that the presence of reduced sulfur phases\nwithin the inclusions does not necessarily reflect the primary redox state of the slab-derived\nfluids, but may instead record post-entrapment modification of initially oxidized, sulfur-rich\nfluids. Furthermore, we also note that although post-entrapment modification may affect sulfur\nspeciation within the inclusions, it is unlikely to significantly reduce the measured sulfur\nconcentrations, given the limited sulfur content of the host omphacite. If anything, such post-\nentrapment processes would tend to elevate the preserved sulfur content relative to the original\ntrapped fluid. Therefore, while we revise our interpretation of sulfur speciation, this does not\nweaken our conclusions regarding high sulfur contents in slab-derived fluids; instead, it further\nsupports the robustness and importance of our findings.\nLine145-146: I don’t understand this sentence. What is replaced? Feels like the subject of the\nsentence is not defined.\nReply: Thanks. Our original intention was to convey that sulfides occurring within the fluid\ninclusions are chalcopyrite, whereas sulfides in the host eclogite are dominated by pyrite,\nthereby excluding mechanical transfer of sulfides from the host into the inclusions. To avoid\nany ambiguity or confusion, we have removed this sentence from the revised manuscript.\nLine157: you say that your S contents at 3-10 wt% is similar to the literature range, but in fact\nit is narrower by an order of magnitude, and you exclude the lower end. So rather than saying\nit is similar, I would say it is more restricted.\nReply: Thanks for this helpful suggestion. The revised text (Line 196–205) now reads:\nFurthermore, the sulfur concentrations measured in this study (~3.2–10.3 wt.%, Table S2) fall\nwithin the broader range (~0.5–15 wt.%) inferred for subduction zone fluids from\nthermodynamic models and high-pressure experiments22,24-29. The wide variability predicted by\nthese indirect approaches likely reflects uncertainties in pressure–temperature–redox\nconditions, fluid-rock interactions, and mineral equilibria. In contrast, our analysis of primary\nmultiphase fluid inclusions avoids many of these limitations and provides a direct quantitative\nconstraint on the sulfur concentrations in slab-derived fluids at sub-arc depths. These\nmeasurements not only validate previous experimental and theoretical predictions of elevated\nsulfur contents in sub-arc fluids but also provide more restricted natural constraints on the range\nof sulfur concentrations in slab-derived fluids. ”\nLine160-161: Well, you argue that your results are more faithful than thermodynamic models\nto the real fluid composition. But surely, your one sample is restricted to one P-T-X path, so\nhardly a representation of global subduction zone variability that include hot and cold, steep\nand shallow, and old and young slabs (see e.g. Syracuse et al. 2010). The restricted S content\nrange restricted range may be representative of the fact that you only look at a single P-T-X.\nReply: We thanks the reviewer for this comment and fully acknowledge the variability among\nsubduction zones, such as differences between hot and cold subduction regimes. Accordingly,\nsulfur contents in slab-derived fluids are expected to vary among different subduction settings.\nIn our dataset, sulfur concentrations measured in the fluid inclusions also show variability,\nranging from 3.2 to 10.3 wt.% (Table S2). We suggest that this range may, at least in part,\noverlap with sulfur contents characteristic of fluids generated in different subduction\nenvironments. To account for this variability in subsequent discussion and modelling, we\ntherefore use the mean sulfur concentration (6.3 wt.%) together with its associated standard\ndeviation (SD = 1.8 wt.%) as representative values in our calculations.\nFurthermore, we also recognize that extrapolating sulfur concentrations from the Sumdo system\nto global subduction zones involves inherent limitations. Nevertheless, the Sumdo belt\nrepresents a well-characterized cold subduction regime, which constitutes a widespread and\ngeodynamically significant class of subduction zones. On this basis, we suggest that sulfur\nconcentrations comparable to those documented here may also occur in similar cold subduction\nenvironments, although confirmation from additional natural systems will require future\ninvestigation. Importantly, when using the average sulfur content of sub-arc fluids (~6 wt.%)\nin our flux calculations, the resulting estimates of sulfur release efficiency and contributions to\narc systems are quantitatively consistent with previous indirect constraints (Walters et al., 2020;\nLi et al., 2022; Taracsák et al., 2023). This agreement suggests that the sulfur characteristics\nobserved in the Sumdo system are unlikely to be anomalous, but may instead reflect a broader\nfeature of sulfur-rich slab-derived fluids in comparable subduction settings. These points have\nbeen clarified in the revised manuscript (Lines 205–213, 331–360). We sincerely hope that this\nexplanation satisfactorily addresses the reviewer’s concern!\nLines164-167: While I very much agree with this statement, you kind of put this here in a\nvacuum, and somewhat feels like you are implying this is a new idea (and cite no other paper\nthat suggested this). In fact, lots of papers argued for extensive S transfer from slab to mantle\nwedge, both old and new (Métrich et al. 1999 EPSL, de Hoog et al. 2001 EPSL, Walters et al.\nMuth and Wallace 2022 EPSL, Taracsák et al. 2023 EPLS, Kawaguchi et al. 2022 JPetrol, just\nto list a few). Yes, your results confirm this, and are valuable, but they add to existing evidence\nthat you should mention here.\nReply: We appreciate the reviewer for this important and well-taken suggestion. We fully agree\nthat extensive sulfur transfer from the slab to the mantle wedge has been proposed and discussed\nin numerous previous studies, and our original wording did not sufficiently acknowledge this\nbody of work. In the original manuscript, our intention was not to imply that efficient slab-to-\nwedge sulfur transfer is a new concept. Rather, we aimed to emphasize that sulfur-rich fluids\nrecorded in HP–UHP metamorphic rocks and associated veins are not unique to the Sumdo\neclogites, but have been documented in multiple subduction settings worldwide (Phillippot et\nal., 1991; Svensen et al., 1999, 2001; Frezzoti and Ferrando, 2015). What distinguishes our\nstudy is that it provides the first direct, in situ quantification of sulfur concentrations in slab-\nderived fluids at sub-arc depths, thereby filling a critical observational gap between arc magma-\nbased inferences and slab dehydration processes.\nThe revised text (Lines 205–213) now reads: “Importantly, sulfur-rich slab-derived fluids are\nnot unique to the Sumdo system. Numerous studies based on observations of arc magmas and\nmelt inclusions, together with experimental constraints and thermodynamic modeling, have\ndemonstrated extensive sulfur transfer from the subducting slab to the mantle wedge,\nhighlighting the critical role of slab-derived fluids11-21,27. Moreover, sulfide- and sulfate-bearing\nfluid inclusions have been documented in HP–UHP eclogites and associated veins from other\norogenic belts worldwide, such as the Western Alps and the Caledonides32,41-43. Collectively,\nthese observations indicate that sulfur enrichment in slab-derived fluids at sub-arc depths is\nlikely a common feature of deep subduction zones rather than an isolated characteristic of the\nSumdo system.”\nLine169: I think this subsection title misses a word, or mechanism is not the best way to put it.\n“Formation of …” may be better?\nReply: Thanks, we have revised it (Line 215).\nLine173: reference 33 is in fact a review of a paper in Nature Geo, not the paper itself that\npresents the results – that would be Farsang and Zajacz 2024 Nature Geo.\nReply: We appreciate this suggestion and have now cited the original study by Farsang and\nZajacz (2024, Nature Geoscience) instead of the review article (Line 393).\nLine176: again, oxidised species are not considered here, yet these can carry more S than\nreduced species – see earlier comments.\nReply: We thanks the reviewer for this insightful comment. We have revised the manuscript to\nfully incorporate oxidized sulfur species in the discussion. The revised text (Lines 217–225)\nnow reads: “In altered oceanic crust, sulfur is initially hosted in both sulfate- and sulfide-phases,\nwhich are progressively destabilized with increasing depth during subduction43,44. Some studies\nsuggest that sulfate-bearing minerals mostly decompose and release oxidized sulfur species\n(SO₄²⁻) at relatively shallow forearc depths (<70 km), whereas sulfide phases remain stable to\ngreater depths and break down under eclogite-facies conditions (>70 km), liberating reduced\nsulfur (e.g., S²⁻, HS⁻) into coexisting fluids22,23,43,45. However, more recent experimental,\nthermodynamic, and geochemical studies indicate that oxidized sulfur species may remain\ndominant even at sub-arc depths, despite increasing pressure and temperature18-21,26,27.”\nLines178-180: I already mentioned this earlier; but pyrite is not an indicator of low fO\n2\n(pyrrhotite would be), and in fact a higher fO may be needed for pyrite. Can you be sure that\n2\nthe cpy is not formed by post entrapment reduction of S, considering Fe and S can readily\nexchange electrons? Omphacite can contain aegirine and Fe3+ - in fact, Weller et al. 2016\nmeasured a Fe3+/Fe(T) of 0.33-0.45 in peak omphacite, so there is plenty of it to reduce the little\nS (with respect to total mass) present in the FIs. And a similar argument can be made for calcite\n– can the Ca scavenged from the omphacite (it has 18 wt% CaO)? A similar process is described\nfor opx-hosted, CO -rich FIs from mantle xenoliths (Berkesi et al. 2012 EPSL), where reactions\n2\nbetween the fluid and the host formed carbonates.\nReply: We are grateful to the reviewer for this important and detailed comment. In the revised\nmanuscript, we have fully reconsidered the possibility of post-entrapment modification and re-\nevaluated the sulfur speciation of the fluids initially trapped at sub-arc depths.\nAs noted by the reviewer, pyrite is not an indicator of low oxygen fugacity, and its presence in\nthe host eclogite does not require reducing conditions. We therefore do not use pyrite as\nevidence for reduced sulfur speciation at the time of fluid entrapment. Instead, thermodynamic\nmodeling constrained by the P–T conditions of the Sumdo system indicates that slab-derived\nfluids at sub-arc depths are dominantly characterized by oxidized sulfur species (SO 2- and\nprofiling of host omphacite across representative exposed fluid inclusions. These analyses\nreveal limited but measurable compositional gradients, with maximum variations of ~0.4 wt.%\nFeO and ~0.6 wt.% CaO between regions adjacent to inclusions and those farther away (Table\nS3, Fig. S6). These results indicate that post-entrapment element exchange between the trapped\nfluids and the host omphacite did occur, albeit to a limited extent. Accordingly, we now\nexplicitly acknowledge that Fe exchange between the entrapped fluids and Fe³⁺-bearing\nomphacite could have promoted localized redox reactions after entrapment. Such reactions may\nhave partially reduced initially oxidized sulfur species and led to the precipitation of\nchalcopyrite within the fluid inclusions.\nFinally, we emphasize that although post-entrapment fluid–host interactions may have modified\nsulfur speciation, they do not affect our primary conclusion regarding sulfur concentrations.\nSulfur is absent in the hosted omphacite, and therefore, post-entrapment exchange processes\ncannot significantly modify the total sulfur concentrations recorded by the fluid inclusions.\nLine228-230: if I understand correctly, you argue that sulfur is more mobile because of the high\nCl content of fluids, and that there is a coupling effect. Yet, there is plenty of evidence that S is\nfairly fluid mobile as it is, especially under oxidising conditions. Cl is always very fluid mobile\n(elevated in all arcs) – so this can simply be the case of two fluid mobile elements behaving\nsimilarly, because they have similar affinity to fluids under certain conditions? The citation you\nprovide in line 227 does not add to your argument – it is a melt inclusions paper from 22 years\nago, that highlights there is a lot for Cl and S in arc melt inclusions, and they modelled a fluid\ncomposition based on mass balance – it is not an observed value for fluid S and Cl content.\nReply: We appreciate the reviewer’s constructive comment and agree that the original\nmanuscript overemphasized the role of chlorine in enhancing sulfur concentrations in slab-\nderived fluids. In response, we have made the two revisions as follows:\n(1) Guided by recent thermodynamic results (Beaudry & Sverjensky, 2024), we now focus on\nthe role of major cations—particularly Ca and Na—which exert a stronger and more\nquantifiable control on sulfate solubility and sulfur transport under UHP conditions. This\ninterpretation is more directly supported by our observations of elevated CaO (10.16 wt.%) and\nNa (3.22 wt.%) in association with exceptionally high sulfur contents in the Sumdo fluid\ninclusions.\n(2) Previous studies have also suggested a potential role for chlorine in promoting sulfur\nenrichment in subduction zone fluids (Newton, 2004). In addition, chlorine may also exert an\nindirect influence through its stronger affinity for iron relative to sulfur, potentially favoring\nFe–Cl complexation and thereby suppressing sulfide reprecipitation during fluid transport.\nSuch a mechanism could facilitate longer-distance sulfur migration within the mantle wedge.\nThe elevated concentrations of chlorine (3.5–6.4 wt.%), iron (2.80–8.98 wt.%), and high sulfur\nobserved in the Sumdo fluid inclusions are broadly consistent with this possibility. Nevertheless,\nwe also note that whether the coupled enrichment of these two fluid-mobile elements, chlorine\nand sulfur, reflects similar behavior under sub-arc conditions—and the extent to which chlorine\nenhances sulfur transport—remains to be further investigated. Overall, in the revised\nmanuscript (Lines 288–314), we have weakened the role of Cl in enhancing sulfur\nconcentrations in sub-arc fluids. We hope that this revised and more cautious treatment\nadequately addresses the reviewer’s concern!\nLine240-242: Even for one arc, these estimates vary a lot – see de Moor et al 2022 for central\nAmerica, who estimate that between 10-60% of S is lost to the mantle wedge from the slab. So,\nmost S may enter the deep mantle, but also, most may go into the arc.\nReply: We thank the reviewer for this important comment. We have revised the manuscript\naccordingly (Lines 319–323). The revised text now reads: “Previous sulfur mass-balance\nestimates for subduction zones indicate that the fate of subducted sulfur is highly variable, with\nsulfur either being transferred into the mantle wedge or retained within the slab and transported\ninto the deeper mantle6,8,10,18-20,26. For example, studies of Central American arc basalts suggest\nthat ~10 to 60% of slab-derived sulfur may be incorporated into the mantle wedge18.”\nLines248-271: the flux calculations are confusing because you do not actually explain what you\nwant to show, and how you do it; are these values global fluxes, or just for a specific subduction\nzone? Is this for a certain thickness of crust. It feels a bit confusing.\nReply: We thank the reviewer for pointing out the lack of clarity in this section. We agree that\nthe original presentation of the flux calculations was insufficiently explained. In the revised\nmanuscript, we have therefore restructured this section to clearly define what is being calculated,\nat what scale, and how the calculations are performed. The aim of the flux calculations is to\nprovide a cautious extrapolation from the Sumdo system to broader subduction zone contexts.\nThis extrapolation is based on several reasonable considerations.\n(1) The Sumdo eclogites record a cold subduction regime, representing a widespread and\ngeodynamically significant class of subduction zones globally. Accordingly, the sulfur\nsystematics documented here are expected to be broadly applicable to other cold subduction\nsettings, while recognizing that they do not encompass the full diversity of global subduction\nenvironments.\n(2) The investigated samples record fluid compositions at sub-arc depths, representing a critical\nwindow of slab-derived sulfur release. As discussed above, sulfur-rich sub-arc fluids are not\nunique to the Sumdo system but have been documented in multiple subduction settings\nworldwide. Furthermore, our study not only validates previous experimental and theoretical\npredictions of elevated sulfur contents in sub-arc fluids but also provides more restricted natural\nconstraints on the range of sulfur concentrations in slab-derived fluids. As such, we suggest that\nour measured sulfur concentrations can serve as an important and reasonable parameter for\nsulfur flux calculations.\n(3) To further strengthen the calculation, we re-evaluated the second key parameter controlling\nsulfur fluxes, namely the global H O flux released from the altered oceanic crust. We performed\n2\nthermodynamic calculations constrained by the geological and P–T conditions of the Sumdo\nsystem to estimate dehydration. The results indicate that approximately 60% of the water\ninitially stored in altered oceanic crust is released at depths of 80–90 km. Based on this\ndehydration fraction, and integrating average H O contents of altered oceanic crust, global\n2\nsubduction zone length, and crustal thickness (Alt et al., 1999; Stern et al., 2020; Ribeiro et al.,\n2026), we estimate a global H O flux of ~82.5 Mt/y released from subducted oceanic crust at\n2\nsub-arc depths. This estimate closely matches independent literature constraints (~91 Mt /y,\nLi et al., 2020), providing confidence in the adopted parameterization.\n(4) Finally, based on these parameters, we calculate sulfur fluxes carried by sub-arc fluids and\nfurther estimate the efficiency of sulfur release from the slab at sub-arc depths, as well as its\ncontribution to sulfur enrichment in arc systems. Importantly, these estimates are quantitatively\nconsistent with independent constraints derived from previous experimental, theoretical, and\ngeochemical constraints (Walters et al., 2020; Li et al., 2022; Taracsák et al., 2023), providing\nadditional support for the reasonableness and reliability of extrapolating our results from the\nSumdo system to global subduction zones.\nWe explicitly acknowledge in the revised manuscript that sulfur fluxes may vary among\ndifferent subduction zones and even within a single subduction system. Accordingly, our\ncalculations are not intended to represent precise values for all subduction zones, but rather to\nprovide a general estimate of sulfur cycling at sub-arc depths. We believe that our study can\nprovide a good case for future comparative studies of sulfur cycling across diverse subduction\nenvironments. The above clarifications have been clearly indicated in the manuscript (Lines\n331–390).\nLine275: the paper you cite on sediment melt S solubility says 1800 ppm at the SCAS – which\nis not that high (similar to a basalt at the SCSS) – combine this with the fact that sediments rich\nin S are usually reduced (e.g. Peccia et al. 2025 Chem GEol), makes me question if sediments\nmelts can in fact carry substantial amounts of S as you state.\nReply: Thanks. In the revised manuscript (Lines 332–335), we have softened this statement.\nNow it reads: “In unusually hot subduction systems, slab-derived melts have been proposed as\nimportant sulfur carriers, with sulfur concentrations reaching up to several thousand ppm55-59.\nHowever, these concentrations remain substantially lower than those measured in slab-derived\nfluids in this study. ”\nLine318: I cannot find this water flux anywhere in the referenced paper. You should really\nexplain what this “flux” is, not just say that this is the water flux at 90 km depth (which means\nnot much – every subduction zone will have a different H O flux due to different P-T-X and\n2\ncrustal thickness). Is this for a certain thickness of crust dehydrating?\nReply: We thank the reviewer for this insightful comment. We now explicitly clarify the\ncalculation of global H O flux release from altered oceanic crust at sub-arc depths of 80–90 km.\ngeological and P–T conditions of the Sumdo system to quantify slab dehydration extents. These\ncalculations indicate that ~60% of the water initially stored in altered oceanic crust is released\nat sub-arc depths. Treating the Sumdo system as representative of altered oceanic crust\ndehydration in cold subduction zones, we combined this dehydration fraction (~60%) with\naverage H O contents of altered oceanic crust, global subduction zone length, and altered\n2\ncrustal thickness (Alt et al., 1999; Stern et al., 2020; Ribeiro et al., 2026). On this basis, we\nestimate the global H O release flux of ~84.5 Mt/yr from altered oceanic crust at sub-arc depths.\nreported\nin the caption of Fig. 6 in Li et al., 2020), supporting the robustness of our water-flux\nparameterization. These clarifications have been incorporated into the revised manuscript\n(Lines 331–360, 538–550).\nLine329: Same question applies for the sulfur input flux – what does this value represents?\nReply: We thank the reviewer for this clarification request. In the revised manuscript, we now\nexplicitly clarify what the sulfur input flux represents and how it is used in our study.\nOur goal is to place the Sumdo-derived observations within a broader framework for assessing\nsulfur cycling in subduction zones. To this end, we calculate the global sulfur flux transported\nby sub-arc fluids derived from altered oceanic crust based on three key considerations: (i) the\nSumdo eclogites record a cold subduction regime, which represents a widespread and\ngeodynamically important class of subduction zones globally; (ii) sulfur-rich sub-arc fluids\nhave been documented in multiple subduction settings and are not unique to the Sumdo system;\nand (iii) the global H O fluxes at sub-arc depths (~80–90 km) from altered oceanic crust\n2\nadopted in our calculations are constrained by thermodynamic modeling and are consistent with\nindependent estimates reported in the literature.\nTo evaluate the significance of the calculated sulfur flux at sub-arc fluids, we compare it with\nthe initial sulfur input flux carried by subducted altered oceanic crust, estimated to be ~48.09\nMt/yr (Evans, 2012). On this basis, we estimate a sulfur release efficiency of 25 ± 7% at sub-\narc depths. This value is broadly consistent with previous theoretical and modeling-based\nestimates (~45%, Walters et al., 2020). This agreement supports the internal consistency of our\nsulfur flux calculations as well as the reasonableness of the adopted sulfur input flux from\naltered oceanic crust. These clarifications have now been incorporated into the revised\nmanuscript (Lines 331–360).\nLine364-365: This value does not significantly exceed what was suggested for Central America\n(between 40-70% of S being slab-derived in arcs) in Taracsák et al. 2023 – so in a sense your\nresults confirm what was already suggested based on arc melt inclusion data.\nReply: We thank the reviewer for this insightful comment. By explicitly incorporating the\nanalytical uncertainty (SD = 1.8 wt. %) of concentrations into our calculations, we now estimate\nthat slab-derived fluids contribute 70 ± 20% of the sulfur released in arc systems. We have\nrevised the manuscript accordingly (Lines 600–602). The sentence now reads: “The calculated\nrange overlaps well with previous estimates of ~40–70% derived from indirect constraints\nbased on natural arc basalts20, supporting the consistency and geological plausibility of our flux-\nbased assessment.”\nMethods section: This is very little detail considering these Raman analyses are the base of the\npaper - what peaks/spectral features were used to identify phases? How sure can we be that\neach phase is a pure mineral phase and not something similar, yet compositionally different?\n(e.g. cpy vs other sulfides, calcite vs dolomite or magnesite?). While I appreciate that you\nprovide four representative spectra in the supplementary, I think you should give a more\ndetailed summary here.\nReply: We thank the reviewer for this insightful suggestion. In the revised manuscript, we have\nadded a detailed description of the criteria used to identify daughter minerals and fluid phases\nbased on their diagnostic Raman spectral features, drawing on established Raman databases\n(https://rruff.info/) and previous work (e.g., Frezzotti et al., 2012).\n(1) For sulfide phases, chalcopyrite (CuFeS ) is characterized by a strong Raman peak at ~293\n2\ncm⁻¹, whereas other common sulfides display distinct and non-overlapping diagnostic peaks,\nincluding pyrite (FeS ) at ~428 cm⁻¹, marcasite (FeS ) at ~324 cm⁻¹, sphalerite (ZnS) at ~349\n2 2\ncm⁻¹, and galena (PbS) at ~136 cm⁻¹. In this study, the dominant sulfide Raman peak\nconsistently occurs at ~292–293 cm⁻¹ (Fig. S3), with no additional peaks indicative of other\nsulfide species, supporting the identification of chalcopyrite as the sole sulfide phase within the\ninclusions.\n(2) For carbonate phases, calcite is characterized by strong Raman bands at ~284 and ~1086\ncm⁻¹, whereas dolomite shows peaks at ~299 and ~1097 cm⁻¹, and magnesite at ~329 and ~1094\ncm⁻¹. The carbonate daughter minerals in the Sumdo inclusions consistently exhibit a peak\ncombination at ~283–284 and ~1086 cm⁻¹ (Fig. S3), which is diagnostic of calcite and clearly\ndistinct from other carbonate species.\n(3) The aqueous fluid phase is identified by a broad Raman band between ~3300 and 3600 cm⁻¹\n(Fig. S3), corresponding to O–H stretching vibrations, indicating the presence of H O-rich fluid.\nour analyses, peak positions at ~672 and ~1015 cm⁻¹ are consistently observed (Fig. S3),\nconfirming the host mineral identity. Furthermore, no solid NaCl daughter minerals were\ndetected in the fluid inclusions (which would be expected to show a strong Raman peak near\n~358 cm⁻¹). We therefore infer that NaCl is present as a dissolved component in the aqueous\nfluid phase. Consistent with our previous work (Liu et al., 2019b), microthermometric\nconstraints indicate salinities of ~10–22 wt.% NaCl equivalent, with an average value of ~15\nwt.% adopted for bulk fluid composition calculations. These clarifications have been\nincorporated in the Method section of the revised manuscript (Lines 434–453).\nFig. S3 in the supplementary information. Representative Raman spectra of daughter\nminerals and fluid phases in multiphase fluid inclusions. Raman spectra of chalcopyrite, calcite,\nand aqueous fluid phases within multiphase inclusions hosted in omphacite from the Sumdo\neclogite.\nLine 535: “Approximately this sulfur is eventually” sounds strange, I suggest rewording this\nsentence.\nReply: Thanks. We have rephrased the sentence (Lines 879–881) accordingly. It now reads:\nThis sulfur is ultimately returned to the surface via arc volcanism, accounting for ~70 % of the\nglobal sulfur output flux of ~16.8 Mt/y.”\nFigure 2: It would be interesting to add a ternary plot with water, cpy, and cc as the three\nendmembers – if the inclusions have a common origin, the fractions of the three phases should\nbe similar across all inclusions. Maybe this can go in as a supplementary figure.\nReply: We thank the reviewer for this helpful suggestion. In response, we have added two\nternary diagrams to the supplementary materials (Fig. S5). The first diagram (A) illustrates the\nrelative phase proportions of the inclusions, with the three endmembers defined as brine (H O\n2\n+ dissolved NaCl), calcite, and chalcopyrite. The second diagram (B) shows the reconstructed\nbulk compositions of the inclusions, with endmembers defined as H₂O + NaCl, CuFeS , and\n3\ncompositions of the fluid inclusions cluster tightly, indicating a high degree of compositional\nuniformity among the inclusions. This consistency supports a common origin for the trapped\nfluids.\nFig. S5 in the supplementary information. Phase proportions and reconstructed bulk\ncompositions of multiphase fluid inclusions.\nTable 1: you could add standard deviation and minimum/maximum values, as you use those in\nyour Fig. 3.\nReply: Thanks. We have revised Table 1 to include the standard deviations (SD) as well as the\nminimum and maximum values for each quantified component. For sulfur, concentrations range\nfrom 3.2 to 10.3 wt.%, with an average value of 6.3 wt.% and a standard deviation of 1.8 wt.%.\nIn addition, following the reviewer’s earlier suggestion, Table 1 and Table S1 in the original\nmanuscript have been converted into an Excel-format dataset, and the complete data are now\nprovided in Tables S1 and S2.\nI hope these comments are helpful and will assist improving your ms.\nReply: We once again thank Dr. Zoltán Taracsák for the detailed and constructive\ncomments and thoughtful suggestions, which have significantly improved the clarity, rigor,\nand overall quality of the manuscript!\nReviewer 2’s comment\nThis manuscript presents the first in situ quantification of sulfur contents in natural slab-derived\nfluids from sub-arc depths, based on multiphase fluid inclusions hosted in omphacite from UHP\neclogites in the Sumdo belt, southern Tibet. The authors convincingly demonstrate that these\nfluids contain exceptionally high sulfur concentrations (~6 wt.%), potentially accounting for up\nto 75% of the global arc volcanic sulfur output. This is an important contribution to\nunderstanding sulfur cycling agents and fluxes in subduction zones, and the study is timely,\noriginal, and methodologically sound. The dataset is of high quality, the interpretations are\ngenerally well supported, and the implications are broadly relevant to geochemistry,\nsubduction dynamics, and volatile transfer processes. Overall, the manuscript is suitable for\npublication in Nature Communications after moderate revisions in order to improving\nmethodological transparency and quantitative rigor.\nReply: We sincerely thank the reviewer for the positive evaluation of our manuscript. We\ngreatly appreciate the constructive comments and insightful suggestions, which have helped to\nfurther improve the clarity and quality of our study. In response, we have carefully addressed\nall points raised by the reviewer. All revisions are clearly indicated in the tracked‐changes\nversion of the manuscript (shown in blue), and a detailed, point‐by‐point response is provided\nbelow.\nMajor Comments\n1. Uncertainty and reproducibility of Raman-based quantification\nThe 3D Raman mapping and volumetric reconstruction form the technical foundation of this\nstudy. However, no explicit uncertainty propagation is provided for the calculated bulk\ncompositions (Table 1). Please quantify and discuss the analytical uncertainties and\nreproducibility of the Raman-based measurements.\nReply: We thank the reviewer for this insightful comment. In the revised manuscript, we have\ntaken several steps to further strengthen the robustness of our Raman analysis results.\n(1) The three-dimensional (3D) Raman mapping, phase identification, and volumetric\nreconstruction approach used in this study follows the same analytical protocol established in\nour previous work (Jin et al., 2023), where the methodology was validated and shown to provide\nreliable quantitative results for multiphase fluid inclusions. We have now clarified this point\nexplicitly in the revised manuscript (Lines 456–462).\n(2) To directly assess analytical reproducibility, we performed independent repeat analyses on\ntwo representative fluid inclusions (No. 2 and No. 20). The results, reported in Tables S1 and\nS2, show highly consistent daughter mineral assemblages, phase volume proportions, and\nreconstructed bulk fluid compositions, demonstrating good reproducibility. This clarification\nhas now been incorporated into the revised manuscript (Lines 476–480).\n(3) We now report the standard deviations (SD) for the calculated contents of all major\ncomponents (Table S2), providing a quantitative assessment of the analytical uncertainty\nassociated with the Raman-based reconstruction.\n(4) We have additionally included a ternary compositional diagram (Fig. S5) summarizing the\nreconstructed phase volume proportions and bulk fluid compositions of all 22 analyzed fluid\ninclusions. The tight clustering of data points in this diagram indicates limited compositional\nvariability among inclusions and further supports the internal consistency and reproducibility\nof the Raman reconstructed results.\nTaken together, these multiple lines of evidence demonstrate that the Raman-based\nmeasurements are robust and reproducible, and that the reconstructed bulk compositions\nfaithfully represent the original slab-derived fluids preserved within the inclusions.\nFig. S5 of the supplementary information. Phase proportions and reconstructed bulk\ncompositions of multiphase fluid inclusions. (A) Ternary diagram showing relative phase\nproportions of the inclusions, with endmembers defined as brine (H O + dissolved NaCl),\n2\ncalcite, and chalcopyrite. (B) Ternary diagram illustrating reconstructed bulk compositions of\nthe inclusions, with endmembers defined as H O + NaCl, CuFeS , and CaCO . In both diagrams,\n2 2 3\nthe data cluster tightly, indicating a high degree of compositional uniformity among the\ninclusions and supporting a common origin for the trapped fluids.\n2. Variability among subduction settings\nPlease discuss potential variability of slab-derived sulfur contents among different subduction\nzones (e.g., Alps, Caledonides, Japanese arcs), and clarify the limitations of extrapolating the\nSumdo data to global sulfur flux estimates.\nReply: We thank the reviewer for this important comment.\n(1) We fully acknowledge that the precise sulfur contents of slab-derived fluids at sub-arc\ndepths may vary among different subduction zones, such as the Alps, Caledonides, and\nJapanese arcs, reflecting differences in slab composition, thermal structure, redox state, and\nfluid–rock interaction histories. This potential variability is now explicitly stated in the revised\nmanuscript. Nevertheless, sulfur enrichment in sub-arc-depth fluids is not unique to the Sumdo\nsystem. Sulfur-rich daughter minerals and sulfur-bearing fluid inclusions have been widely\nreported from HP–UHP metamorphic rocks and associated veins in multiple subduction settings\nworldwide (Phillippot et al., 1991; Svensen et al., 1999, 2001; Frezzoti and Ferrando, 2015). In\naddition, geochemical studies of natural arc basalts consistently indicate sulfur enrichment in\nsub-arc mantle sources (de Hoog et al., 2001; Muth and Wallace, 2022; Taracsák et al., 2023),\nsupporting an important role for slab-derived fluids in sulfur transfer. Importantly, the sulfur\nconcentrations measured in this study (~3.2–10.3 wt.%) fall within the broader range predicted\nfor subduction zone fluids from thermodynamic models and high-pressure experiments (Li et\nal., 2020; Jégo and Dasgupta, 2013, 2024; Beaudry and Sverjensky, 2024; He et al., 2024;\nMaffeis et al., 2024). Our measurements not only validate previous experimental and theoretical\npredictions of elevated sulfur contents in sub-arc fluids but also provide more restricted natural\nconstraints on the range of sulfur concentrations in slab-derived fluids. Collectively, these\nobservations indicate that sulfur enrichment in slab-derived fluids at sub-arc depths is likely a\ncommon feature of deep subduction zones rather than an isolated characteristic of the Sumdo\nsystem. These clarifications are now explicitly stated in the revised manuscript (Lines 196–\n213).\n(2) Furthermore, we have also clarified the assumptions, rationale, and limitations involved in\nextrapolating the Sumdo data to global sulfur budgets. (i) The Sumdo system records a cold\nsubduction regime, which represents a widespread and geodynamically important class of\nsubduction zones operating globally. Although results from a single system cannot capture the\nfull range of global variability, cold subduction zones contribute substantially to global volatile\nrecycling, allowing the extrapolating results from the Sumdo systeto provide broadly applicable\nconstraints on subduction zone sulfur budgets. (ii) The investigated samples record fluid\ncompositions at sub-arc depths, representing a critical window of slab-derived sulfur release.\nAs discussed above, sulfur-rich sub-arc fluids are not unique to the Sumdo system but have\nbeen documented in multiple subduction settings worldwide. Furthermore, our study not only\nvalidates previous experimental and theoretical predictions of elevated sulfur contents in sub-\narc fluids but also provides more restricted natural constraints on the range of sulfur\nconcentrations in slab-derived fluids. As such, we suggest that our measured sulfur\nconcentrations can serve as an important and reasonable parameter for sulfur flux calculations.\n(iii) To further strengthen the calculation, we re-evaluated the second key parameter controlling\nsulfur fluxes, namely the global H O flux released from the altered oceanic crust. We performed\n2\nthermodynamic calculations constrained by the geological and P–T conditions of the Sumdo\nsystem to estimate dehydration. The results indicate that approximately 60% of the water\ninitially stored in altered oceanic crust is released at depths of 80–90 km. Based on this\ndehydration fraction, and integrating average H O contents of altered oceanic crust, global\n2\nsubduction zone length, and crustal thickness (Alt et al., 1999; Stern et al., 2020; Ribeiro et al.,\n2026), we estimate a global H O flux of ~82.5 Mt/y released from subducted oceanic crust at\n2\nsub-arc depths. This estimate closely matches independent literature constraints (~91 Mt /y, Li\net al., 2020), providing confidence in the adopted parameterization. (iv) Finally, based on these\nparameters, we calculate sulfur fluxes carried by sub-arc fluids and further estimate the\nefficiency of sulfur release from the slab at sub-arc depths, as well as its contribution to sulfur\nenrichment in arc systems. Importantly, these estimates are quantitatively consistent with\nindependent constraints derived from previous experimental, theoretical, and geochemical\nstudies (Walters et al., 2020; Li et al., 2022; Taracsák et al., 2023), providing additional support\nfor the reasonableness and reliability of extrapolating our results from the Sumdo system to\nglobal subduction zones.\nOverall, we consider the extrapolation from the Sumdo system to broader assessments of sulfur\ncycling in subduction zones to be reasonable. We acknowledge that sulfur transfer fluxes may\nslightly vary among different subduction zones; however, our calculations provide an efficient\nand general assessment for understanding sulfur cycling in subduction zones at the global scale.\nThese clarifications are now explicitly stated in the revised manuscript (Lines 331–360, 538–\n550).\n3. Sulfide mineralogy and sulfur speciation\nChalcopyrite is the only sulfide phase observed in the fluid inclusions, whereas pyrite\ndominates in the host eclogite. Why is this the case? In addition, no oxidized S-bearing volatiles\n(e.g., SO₄²⁻) were detected in the omphacite inclusions, although experimental and\nthermodynamic models suggest that oxidized sulfur species should be more mobile under\nsubduction conditions. Please discuss this discrepancy.\nReply: We thank the reviewer for this constructive comment. In the revised manuscript, we\nhave carefully re-evaluated sulfur speciation in the trapped fluids under UHP conditions and\nclarified the origin of the chalcopyrite within the fluid inclusion.\nBased on the geological setting and P–T conditions of the Sumdo belt, we performed\nthermodynamic calculations using the Deep Earth Water (DEW) model. These calculations\nindicate that sulfur in slab-derived fluids at sub-arc depths is dominantly present as oxidized\nspecies, primarily SO 2- and HSO - (Fig. S7). This result suggests that sulfur was initially\n4 4\nentrapped in the fluid inclusions predominantly in an oxidized form, consistent with previous\nexperimental and thermodynamic studies predicting high mobility of oxidized sulfur species\nunder deep subduction conditions (Walters et al., 2020; Beaudry and Sverjensky, 2024; Maffeis\net al., 2024).\nWe further propose that the sulfur speciation recorded in the fluid inclusions may have been\nmodified after entrapment. Electron probe microanalysis (EPMA) of host omphacite across\nfluid inclusions exposed at the surface reveals slightly but systematically lower Fe contents in\nomphacite adjacent to inclusions compared with regions farther away (Fig. S6). Although the\nmagnitude of this variation is small, it suggests that post-entrapment Fe exchange between the\ntrapped fluids and the host omphacite may have promoted localized redox reactions, leading to\npartial reduction of sulfur species and subsequent precipitation of chalcopyrite within the\ninclusions. Alternatively, hydrogen diffusion across the inclusion–host boundary may also have\ncontributed to post-entrapment redox modification, although this process is difficult to quantify\ndirectly.\nTaken together, we now clarify in the revised manuscript (Lines 226–253) that sulfur in slab-\nderived fluids at sub-arc depths was initially present predominantly in oxidized forms, whereas\npost-entrapment interactions between the trapped fluids and the host omphacite likely resulted\nin sulfur reduction and the precipitation of chalcopyrite as the dominant sulfide phase in the\ninclusions.\nFig. S6 of the supplementary information. Major-element variations in host omphacite across\na representative multiphase fluid inclusion. As shown in (A), the host omphacite exhibits slight\nbut systematic variations (0.4 wt.%) in Fe content, with lower Fe concentrations observed in\nregions adjacent to the fluid inclusions compared to areas farther away. This Fe variation is\nconsistent with limited Fe exchange between the trapped fluids and the host omphacite, which\nmay have facilitated post-entrapment redox modification of sulfur species within the fluid\ninclusions.\nFig. S7 of the supplementary information. Thermodynamic modeling of sulfur speciation in\nsubduction zone fluids constrained by the Sumdo system. The results show that slab-derived\nfluids at sub-arc depths are dominated by oxidized sulfur species (e.g., SO 2- and HSO -).\n4 4\n4. Role of calcite and carbonate components\nWhat is the role of calcite in the fluid inclusions? Could CO₃²⁻ enhance sulfide solubility instead\nof (or in addition to) NaCl?\nThis should be addressed quantitatively or at least conceptually.\nReply: We thank the reviewer for this insightful comment. The presence of calcite within the\ninclusions indicates that the trapped fluids were carbon-enriched at the time of entrapment,\nlikely with CO , CO ⁻ , or HCO - as an important dissolved carbon component (Fig. S7).\n2 3 3\nWith respect to whether CO 2⁻ directly enhances sulfide solubility, we note that, to our\n3\nknowledge, no experimental studies have quantitatively evaluated the effect of CO 2⁻ on sulfur\n3\nsolubility in deep subduction zone fluids. However, observations from natural samples\nconsistently show that slab-derived supercritical fluids recorded in fluid inclusions for UHP\nrocks are commonly enriched in both sulfur and carbon (e.g., Zhang et al., 2008; Jin et al.,\n2023), suggesting a close association between carbon-rich fluids and sulfur enrichment.\nRecent studies suggest that the presence of carbon can lower the P–T conditions of the second\ncritical endpoint in supercritical fluids (Li et al., 2020; Ni et al., 2025). Supercritical fluids\ngenerated under such conditions are characterized by enhanced solubilities for volatiles (e.g.,\nsulfur) and metal elements. We therefore suggest that carbon plays an indirect role in promoting\nsulfur enrichment in slab-derived fluids. The estimated trapping conditions of the Sumdo\ninclusions (~2.7 GPa, Yang et al., 2009) approach the second critical endpoint of supercritical\nfluids reported for mafic systems at ~3.0 GPa (Mibe et al., 2011), and the reconstructed fluid\ncompositions exhibit characteristics consistent with near-supercritical or supercritical behavior.\nWe therefore suggest that the Sumdo inclusions most likely record supercritical fluids, in which\nsulfur enrichment is facilitated by the physicochemical properties of the supercritical state. In\naddition, following your insightful suggestion, we now describe the entrapped fluids in the\nmultiphase inclusions as near-supercritical, solute-rich fluids.\nMoreover, guided by recent thermodynamic results (Beaudry and Sverjensky, 2024), we\nhighlight that Ca and Na cations in fluids can substantially enhance sulfate solubility under\nsubduction zone conditions. Beaudry and Sverjensky (2024) demonstrate that oxidized sulfur\ncan be efficiently mobilized in high-pressure fluids through complexing with Ca and Na (e.g.,\nCaHSO ⁺ and Na SO 0), resulting in significantly enhanced sulfur solubility under sub-arc\n4 2 4\nconditions. This interpretation is fully consistent with our observations: the Sumdo fluid\ninclusions are enriched in both CaO (10.16 wt.%) and Na (3.22 wt.%), and the measured sulfur\nconcentrations (~3.2–10.3 wt.%) agree well with experimentally and thermodynamically\npredicted solubilities at comparable P–Tconditions.\nCollectively, we therefore interpret Ca and Na as the primary controls on sulfur solubility in\nthese fluids, while carbonate components exert an indirect influence by modifying fluid\nphysicochemical properties. These interpretations have now been clarified in the revised\nmanuscript (Lines 263–272, 288–293, 308–314).\n5. Potential overestimation of global sulfur fluxes\nThe limited omphacite inclusions from UHP eclogites in the Sumdo belt may represent locally\nsulfur-enriched portions of the subducting slab. Consequently, using these inclusions to\nestimate global slab-derived sulfur contributions might lead to overestimation. Please discuss\nthis limitation explicitly.\nReply: We thank the reviewer for raising this constructive comment. In the revised manuscript,\nwe now explicitly discuss and clarify the limitations, assumptions, and rationale underlying our\nsulfur flux estimates.\n(1) Representativeness and limitations of the Sumdo system. The Sumdo belt represents a well-\ncharacterized cold subduction zone system, which constitutes a dominant end-member of global\nsubduction styles, despite the presence of less common hot subduction zones. We therefore\nargue that the Sumdo system provides a reasonable analogue for global cold subduction\nenvironments.\n(2) Consistency with sulfur-rich fluids at sub-arc depths. The sulfur concentrations measured\ndirectly in the Sumdo fluid inclusions are consistent with recent experimental and\nthermodynamic predictions of sulfur-rich slab-derived fluids at sub-arc depths (e.g., Li et al.,\n2020; Jégo and Dasgupta, 2013, 2024; Beaudry and Sverjensky, 2024; He et al., 2024; Maffeis\net al., 2024). These independent constraints suggest that sulfur enrichment in slab-derived fluids\nat sub-arc depths is likely a recurrent feature of deep subduction systems, rather than an isolated\ncharacteristic unique to Sumdo.\n(3) Water flux constraints. Because H O flux is a key parameter in sulfur flux calculations, we\n2\nnow include a dehydration model constrained by the geological and P–T conditions of the\nSumdo system (2.6–2.7 GPa and 630–780 °C, Yang et al., 2009). These calculations indicate\nthat ~60% of the water initially stored in altered oceanic crust is released at sub-arc depths.\nScaling this result to the global subduction system using published average H O contents of\n2\naltered oceanic crust (Alt et al., 1999), a global subduction zone length of ~55,000 km (Stern\net al., 2002), and an altered crustal thickness of ~0.6 km (Ribeiro et al., 2026) yields an\nestimated H O release flux of ~84.5 Mt/yr. This value closely matches independent global\n2\nestimates of ~91 Mt/yr at ~90 km depth (Li et al., 2020), supporting the robustness of our water-\nflux parameterization.\n(4) Sulfur flux estimates and uncertainty. Using directly measured sulfur concentrations in slab-\nderived fluids together with reasonably calculated H O fluxes, we calculate sulfur fluxes at sub-\n2\narc depths and compare them with both the sulfur input from subducted altered oceanic crust\nand sulfur outputs from arc systems. We now explicitly incorporate uncertainty by accounting\nfor the observed variability (standard deviation) in sulfur concentrations, yielding an estimated\ncontribution of 70 ± 20% of arc sulfur output from slab-derived fluids. This range overlaps with\nprevious indirect estimates (e.g., Walters et al., 2020; Li et al., 2022; Taracsák et al., 2023),\nsuggesting that our results do not represent a substantial overestimation.\nCollectively, we acknowledge that sulfur transfer fluxes may slightly vary among different\nsubduction zones. However, as we discussed in detail above, we consider the extrapolation from\nthe Sumdo system to broader assessments of sulfur cycling in subduction zones to be reasonable.\nThese clarifications are now explicitly stated in the revised manuscript (Lines 331–360, 538–\n558). We sincerely hope this clarification can meet the reviewer’s approval!\nMinor Comments\nLine 118: Please include full microscopic images of the mineral assemblage in the\nSupplementary Materials and specify the trapping stage of the fluids (during subduction or\nexhumation). Are the omphacites vein minerals, as reported in other subduction settings?\nReply: We thank the reviewer for this helpful comment. Representative microscopic images\ndocumenting the mineral assemblages of the Sumdo eclogites have now been added to the\nSupplementary Information (Fig. S2). The investigated eclogites are composed predominantly\nof garnet and omphacite, with minor amphibole, rutile, and pyrite (Fig. S2).\nFig. S2 in the Supplementary information. Representative photomicrographs illustrating the\nmineral assemblages of the Sumdo eclogites. The samples are composed predominantly of\nomphacite (Omp) and garnet (Grt), with minor rutile (Rt), amphibole (Amp), and pyrite (Py).\nFurthermore, previous petrological and geochemical evidence indicates that omphacite in the\nSumdo eclogites formed during a single metamorphic stage, most likely corresponding to peak\nUHP conditions (Liu et al., 2019b). The omphacite compositions are generally homogeneous,\nwith jadeite contents ranging from ~20 to 39 mol.% (Liu et al., 2019b). Accordingly, the fluid\ninclusions hosted in omphacite are interpreted to have been entrapped at or near peak\nmetamorphic conditions, thereby providing reliable records of slab-derived fluids at sub-arc\ndepths under UHP conditions.\nGenerally, omphacite is a characteristic eclogite-facies mineral and occurs as elongated\nprismatic crystals in the Sumdo eclogites. Following the principle of surface-energy\nminimization during inclusion entrapment, primary fluid inclusions commonly adopt\nmorphologies that reflect the crystallographic habit of their host minerals. The prismatic\nmorphology of the inclusions hosted in omphacite is therefore interpreted as a key textural\nfeature indicative of primary fluid entrapment, rather than secondary trapping during\nexhumation. Similar prismatic fluid inclusions hosted in omphacite have been widely\ndocumented in HP–UHP metamorphic rocks and associated veins from subduction zones\nworldwide (e.g., Zhang et al., 2008; Frezzotti and Ferrando, 2015; Jin et al., 2023). These\nclarifications have now been incorporated into the revised manuscript (Lines 130–135, 139–\n142, 209–211).\nLines 144–145: Clarify why chalcopyrite appears as the only sulfide phase within the inclusions,\nwhile pyrite dominates the host eclogite. Does chalcopyrite have higher solubility under\nsubduction conditions?\nReply: We thank the reviewer for this insightful comment. As discussed above and now clarified\nin the revised manuscript (Lines 226–253), we provide a detailed interpretation for the origin\nof chalcopyrite within the fluid inclusion. We infer that sulfur in the fluids was dominantly\npresent as oxidized species at the time of entrapment. We infer that sulfur in the fluids was\ndominantly present as oxidized species at the time of entrapment. Following entrapment, post-\nentrapment redox modification—most likely driven by limited Fe exchange between the host\nomphacite and the trapped fluids—may have led to partial reduction of sulfur within the\ninclusions. Under these conditions, reduced sulfur species could have reacted with dissolved\nCu and Fe in the co-entrapped fluids, resulting in chalcopyrite precipitation. We therefore\ninterpret the occurrence of chalcopyrite within the inclusions not as evidence for intrinsically\nhigher chalcopyrite solubility under subduction zone conditions, but rather as the product of\npost-entrapment redox reactions and metal availability within a closed inclusion system.\nLine 146: The pronoun “that” is ambiguous—does it refer to pyrite or chalcopyrite? Please\nclarify whether decomposition is pressure- or temperature-driven, and cite any reports of pyrite\nin metamorphic fluid inclusions.\nReply: We apologize for the ambiguity caused by our previous wording. In the original\nmanuscript, our intention was to distinguish between sulfide phases occurring in the host rock\nand those present within the fluid inclusions: sulfides in the host eclogite are dominantly pyrite,\nwhereas chalcopyrite is the only sulfide observed in the fluid inclusions. This distinction\nindicates that the sulfides in the inclusions are unlikely to result from mechanical entrainment\nor physical transfer from the host minerals.\nIn the revised manuscript, we clarify that sulfur was most likely initially trapped in the fluid\ninclusions as oxidized species. In addition, recent experimental and thermodynamic studies\nfurther demonstrate that pyrite can act as a source of oxidized sulfur in slab-derived fluids under\nsubduction zone conditions (2–3 GPa, 400–800 °C) across a broad range of oxygen fugacities\n(e.g., Beaudry & Sverjensky, 2024). This mechanism is consistent with observations from the\nSumdo eclogites, including the presence of residual pyrite in the host rocks (Fig. S2) and the\nclose correspondence between sulfur-rich fluid entrapment conditions (~2.6–2.7 GPa, 630–\n780 °C; Yang et al., 2009) and those predicted for oxidized sulfur mobilization. We have\nrephrased and reorganized the relevant discussion accordingly in the revised manuscript (Lines\n226–253).\nLine 176: Explain why reduced sulfur species (S²⁻) are released rather than oxidized ones\n(SO₃²⁻). Note that reduced sulfur may be re-trapped by Fe²⁺ in the host rocks.\nReply: We thank the reviewer for this insightful comment. In the revised manuscript, we clarify\nthat sulfur in the fluids initially trapped by the inclusions was dominantly present as oxidized\nspecies rather than reduced S²⁻. This interpretation is supported by thermodynamic modelling\nunder the P–T conditions of the Sumdo system (Fig. S7). We further propose that post-\nentrapment fluid–rock interaction, particularly Fe²⁺ exchange between the trapped fluids and\nhost omphacite, may have facilitated partial redox re-equilibration. Such localized redox\nmodification could have promoted the reduction of initially oxidized sulfur species within the\ninclusions, leading to the generation of reduced sulfur species and the precipitation of sulfide\nphases. This revised interpretation has now been clarified in the revised manuscript (Lines 226–\n253).\nLines 221–222, 235–236: Provide details about the salinity measurements and indicate whether\nNaCl is present in the Sumdo inclusions. Could CO ²⁻ enhance sulfide solubility?\nmeasurements were previously reported by Liu et al. (2019b), who documented fluid inclusions\nspanning a wide range of salinities. The inclusions investigated in the present study specifically\ncorrespond to fluids trapped at peak metamorphic conditions at sub-arc depths, and are\ncharacterized by high salinities of approximately 10–22 wt.%. For the purpose of bulk\ncomposition reconstruction, we adopted an average salinity of 15 wt.%. In addition, no solid\nNaCl daughter minerals were observed in the Sumdo fluid inclusions, suggesting that NaCl was\npredominantly present as a dissolved component in the aqueous phase. These points have now\nbeen explicitly clarified in the Methods section (Lines 451–453, 467–471).\nWith respect to the potential role of CO 2- in enhancing sulfide solubility, we agree that this is\n3\nan important yet currently unresolved issue. To our knowledge, no experimental studies have\ndirectly quantified the influence of carbon on sulfur solubility in UHP fluids, and we therefore\navoid speculative interpretations. Nevertheless, as discussed above, we have added a concise\ndiscussion in the revised manuscript proposing a possible indirect role of carbon in sulfur\nenrichment in subduction zone fluids. Specifically, carbon may lower the pressure conditions\nrequired for the generation of supercritical fluids (Li et al., 2020; Ni et al., 2025), thereby\nmodifying fluid properties and potentially enhancing sulfur solubility indirectly. These\nclarifications and additions have now been incorporated into the revised manuscript (Lines\n263–272).\nLine 234: The discussion of “supercritical-like” fluids is speculative without supporting\nevidence. Use this term cautiously or replace it with “near-critical, solute-rich fluids,” and cite\nrelevant experimental studies (e.g., Mibe et al., 2011).\nReply: We thank the reviewer for this helpful suggestion and agree that the term “supercritical-\nlike” should be used with caution. In our study, the reconstructed compositions of the\nmultiphase fluid inclusions are consistent with those expected for supercritical fluids, and the\nestimated pressure conditions of fluid entrapment (~2.7Gpa, Yang et al., 2009) approach the\nsecond critical endpoint reported for mafic systems in experimental studies (3.0 Gpa, Mibe et\nal., 2011). This combination of chemical characteristics and near-critical pressure conditions\nmotivated our original use of the term. However, to avoid over-interpretation and to adopt more\nrigorous terminology, we have revised the manuscript to replace “supercritical-like fluids” with\n“near-supercritical, solute-rich fluids” throughout the text (Lines 256, 269, 272, 311). We have\nalso added appropriate experimental references to support this revised interpretation.\nLines 248–250: When calculating the slab-derived sulfur flux, it would be helpful to provide\nthe estimated water flux for the Sumdo subduction zone, rather than relying solely on global\naverages.\nReply: We thank the reviewer for this constructive comment. In the revised manuscript, we now\nprovide an explicit estimate of the slab-derived water flux. We performed thermodynamic\nmodelling constrained by the geological and P–T conditions of the Sumdo system to quantify\ndehydration of altered oceanic crust. The results indicate that approximately 60% of the water\ninitially stored in the altered oceanic crust is released at sub-arc depths (Fig. S7). Given the\nsimilarity between the Sumdo system and other cold subduction regimes worldwide, we treat\nSumdo as representative of altered oceanic crust dehydration in cold subduction settings. We\nthen scale this dehydration estimate to the global subduction system using previously reported\naverage H O contents of altered oceanic crust (Alt et al., 1999), a global subduction zone length\n2\nof ~55,000 km (Stern et al., 2002), and an altered crustal thickness of ~0.6 km (Ribeiro et al.,\n2026). This approach yields an estimated H O release flux of ~84.5 Mt/yr at 80–90 km of sub-\n2\narc depths. This value is consistent with independent global assessments, which suggest a slab-\nderived water flux of ~91 Mt/yr at ~90 km depth (Li et al., 2020). The close agreement between\nthese estimates supports the robustness of our water-flux parameterization. We therefore\nconsider the revised H O flux (~84.5 Mt/yr) adopted in our subsequent sulfur flux calculations\n2\nto be reasonable and well constrained. These clarifications and calculations have now been\nincorporated into the revised manuscript (Lines 505–534, 538–550).\nLine 295: Include details of laser power and acquisition parameters in the Methods section to\nensure reproducibility.\nReply: We thank the reviewer for this helpful suggestion. In the revised manuscript (Lines 430–\n434), we have now explicitly clarified the key Raman analytical parameters to ensure\nreproducibility.\nFor the reviewer’s convenience, the newly added text reads: “The laser power at the sample\nsurface was maintained at ~25 nW, which is sufficiently low to minimize laser-induced heating\nor damage to the inclusions while still ensuring robust detection of Raman signals from both\nthe daughter minerals and associated fluid phase. Spectra were collected over the range of 100–\n4,400 cm⁻¹ to fully capture the Raman features of the multiphase assemblages. ”\nLines 328–329: The global sulfur input flux from subducted altered oceanic crust (~48.09 Mt/yr)\nmay not directly apply to the Sumdo subduction zone. Please provide uncertainty ranges (e.g.,\n75 ± ? %) to reflect the robustness of your global estimate.\nReply: We thank the reviewer for this insightful comment. In the revised manuscript, we clarify\nthat the global sulfur input flux from subducted altered oceanic crust (~48.09 Mt/yr, Evans,\n2012) is used as a reference framework, rather than being directly applied to the Sumdo\nsubduction zone. Because the Sumdo system represents a cold subduction end-member—\ncorresponding to a widespread and geodynamically significant class of global subduction\nregimes—it provides a reasonable basis for cautious extrapolation to broader subduction zone\ncontexts.\nTo strengthen this extrapolation, we incorporate thermodynamic dehydration modeling based\non the P–T conditions of the Sumdo system and apply it to estimate the global H O release flux\n2\nfrom the altered oceanic crust at sub-arc depths (~80–90 km). The resulting H O flux (~84.5\nin the original manuscript, providing confidence in the water-flux parameter used. Moreover,\nthe sulfur concentrations measured in this study (~3.2–10.3 wt.%) fall within the broader range\ninferred for subduction zone fluids from thermodynamic models and high-pressure experiments\n(~0.5–15 wt.%)(Li et al., 2020, Jégo and Dasgupta, 2013, 2014, Beaudry and Sverjensky, 2024;\nHe et a., 2024; Maffeis et al., 2024), but define a more restricted range. Together, on this basis,\nwe calculate sulfur fluxes carried by sub-arc fluids derived from the subducted altered oceanic\ncrust and compare them with the total sulfur input from subducted altered oceanic crust to\nprovide a general assessment of sulfur release efficiency at sub-arc depths and its potential\ncontribution to arc sulfur output. Importantly, we now explicitly include uncertainty associated\nwith sulfur concentrations by incorporating the measured standard deviation (SD) of sulfur\ncontents in the fluid inclusions. Using an average sulfur concentration of 6.3 ± 1.8 wt.%, our\nrevised calculations indicate that slab-derived fluids may contribute 70 ± 20% of the sulfur\nreleased to arc magmatic systems. This uncertainty range overlaps well with independent\nestimates derived from natural arc rocks and melt inclusions (40–70%, Taracsák et al., 2023),\nsuggesting that our results do not represent a substantial overestimation.\nThese clarifications have been incorporated into the revised manuscript (Lines 331–361, 381–\n385). We hope that this revised explanation satisfactorily addresses the reviewer’s concern.\nWe once again sincerely thank the reviewer for recognizing the novelty of our work and\nfor the constructive and insightful suggestions. These comments have significantly\nimproved the clarity, rigor, and overall quality of the manuscript!\nReferences\nAlt, J. C. & Teagle, D. A. H. The uptake of carbon during alteration of ocean crust. Geochim.\nCosmochim. Acta 63, 1527–1535 (1999).\nBeaudry, P. & Sverjensky, D. A. Oxidized sulfur species in slab fluids as a source of enriched\nsulfur isotope signatures in arcs. Geochem. Geophys. Geosyst. 25, e2024GC011542\n(2024).\nde Hoog, J. C. M., Taylor, B. E. & van Bergen, M. J. Sulfur isotope systematics of basaltic\nlavas from Indonesia: implications for the sulfur cycle in subduction zones. Earth Planet.\nSci. Lett. 189, 237–252 (2001).\nde Moor, J. M., Fischer, T. P. & Plank, T. Constraints on the sulfur subduction cycle in Central\nAmerica from sulfur isotope compositions of volcanic gases. Chem. Geol. 588, 120627\n(2022).\nEvans, K. A. The redox budget of subduction zones. Earth-Sci. Rev. 113, 11–32 (2012).\nFarsang, S. & Zajacz, Z. Sulfur species and gold transport in arc magmatic fluids. Nat. Geosci.\n18, 98–104 (2025).\nFrezzotti, M. L. & Ferrando, S. The chemical behavior of fluids released during deep\nsubduction based on fluid inclusions. Am. Mineral. 100, 352–377 (2015)\nGoldstein, R. H. Fluid inclusions in sedimentary and diagenetic systems. Lithos 55, 159–193\n(2001).\nHe, D. Y., Qiu, K. F., Simon, A. C., Pokrovski, G. S., Yu, H. C., Connolly, J. A. D., Li, S. S.,\nTurner, S., Wang, Q. F., Yang, M. F. & Deng, J. Mantle oxidation by sulfur drives the\nformation of giant gold deposits in subduction zones. Proc. Natl Acad. Sci. U.S.A. 121,\ne2404731121 (2024).\nHogg, O. R., Edmonds, M., Wieser, P. E., Gleeson, M., Jenner, F. E. & Blundy, J. Copper-rich\nfluids arising from sulfide resorption by hydrous arc melts. Sci. Rep. 16, 29115 (2026).\nJenner, F. E. Cumulate causes for the low contents of sulfide-loving elements in the continental\ncrust. Nat. Geosci. 10, 524–528 (2017).\nJégo, S. & Dasgupta, R. Fluid-present melting of sulfide-bearing ocean-crust: Experimental\nconstraints on the transport of sulfur from subducting slab to mantle wedge. Geochim.\nCosmochim. Acta 110, 106–134 (2013).\nJégo, S. & Dasgupta, R. The fate of sulfur during fluid-present melting of subducting basaltic\ncrust at variable oxygen fugacity. J. Petrol. 55, 1019–1050 (2014).\nJin, D., Xiao, Y., Tan, D. B., Wang, Y. Y., Wang, X., Li, W., Su, W. & Li, X. Supercritical\nfluid in deep subduction zones as revealed by multiphase fluid inclusions in an ultrahigh-\npressure metamorphic vein. Proc. Natl Acad. Sci. U.S.A. 120, e2219083120 (2023).\nKepezhinskas, P., Defant, M. J. & Widom, E. Abundance and distribution of PGE and Au in\nthe island-arc mantle: implications for sub-arc metasomatism. Lithos 60, 113–128 (2002).\nLee, C. T., Luffi, P., Chin, E. J., Bouchet, R., Dasgupta, R., Morton, D. M., Le Roux, V., Yin,\nQ. Z. & Jin, D. Copper systematics in arc magmas and implications for crust–mantle\ndifferentiation. Science 336, 64–68 (2012).\nLee, C.-T. A. & Tang, M. How to make porphyry copper deposits. Earth Planet. Sci. Lett. 529,\n115868 (2020).\nLi, J.-L., Gao, J., John, T., Klemd, R. & Su, W. Fluid-mediated metal transport in subduction\nzones and its link to arc-related giant ore deposits: constraints from a sulfide-bearing HP\nvein in lawsonite eclogite (Tianshan, China). Geochim. Cosmochim. Acta 120, 326–362\n(2013).\nLi, J. L., Schwarzenbach, E. M., John, T., Ague, J. J., Huang, F., Gao, J., Klemd, R., Whitehouse,\nM. J. & Wang, X. S. Uncovering and quantifying the subduction zone sulfur cycle from\nthe slab perspective. Nat. Commun. 11, 514 (2020).\nLi, J.-L., Schwarzenbach, E. M., John, T., Ague, J. J., Tassara, S., Gao, J. & Konecke, B. A.\nSubduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction.\nGeochim. Cosmochim. Acta 297, 40–64 (2021).\nLi, W. & Ni, H. Dehydration at subduction zones and the geochemistry of slab fluids. Sci. China\nEarth Sci. 63, 1925–1937 (2020).\nLi, Y.-B., Chen, Y., Su, B., Zhang, Q.-H. & Shi, K.-H. Redox species and oxygen fugacity of\nslab-derived fluids: Implications for mantle oxidation and deep carbon–sulfur cycling.\nFront. Earth Sci. 10, (2022).\nLiu, H., Sun, H., Xiao, Y., Wang, Y., Zeng, L., Li, W., Guo, H., Yu, H. & Pack, A. Lithium\nisotope systematics of the Sumdo eclogite, Tibet: tracing fluid–rock interaction of\nsubducted low-Taltered oceanic crust. Geochim. Cosmochim. Acta 246, 385–405 (2019a).\nLiu, H., Xiao, Y., van den Kerkhof, A., Wang, Y., Zeng, L. & Guo, H. Metamorphism and fluid\nevolution of the Sumdo eclogite, Tibet: constraints from mineral chemistry, fluid\ninclusions and oxygen isotopes. J. Asian Earth Sci. 172, 292–307 (2019b).\nLorand, J.-P., Luguet, A. & Alard, O. Platinum-group element systematics and petrogenetic\nprocessing of the continental upper mantle: a review. Lithos 164–167, 2–21 (2013).\nMaffeis, A., Frezzotti, M. L., Connolly, J. A. D., Castelli, D. & Ferrando, S. Sulfur\ndisproportionation in deep COHS slab fluids drives mantle wedge oxidation. Sci. Adv. 10,\nadj2770 (2024).\nMcInnes, B. I. A., McBride, J. S., Evans, N. J., Lambert, D. D. & Andrew, A. S. Osmium\nisotope constraints on ore metal recycling in subduction zones. Science 286, 512–516\n(1999).\nMétrich, N., Schiano, P., Clocchiatti, R. & Maury, R. C. Transfer of sulfur in subduction\nsettings: an example from Batan Island (Luzon volcanic arc, Philippines). Earth Planet.\nSci. Lett. 167, 1–14 (1999).\nMibe, K., Kawamoto, T., Matsukage, K. N., Fei, Y. & Ono, S. Slab melting versus slab\ndehydration in subduction-zone magmatism. Proc. Natl Acad. Sci. U.S.A. 108, 8177–8182\n(2011).\nMuth, M. J. & Wallace, P. J. Sulfur recycling in subduction zones and the oxygen fugacity of\nmafic arc magmas. Earth Planet. Sci. Lett. 599, 117833 (2022).\nNewton, R. C. Solubility of anhydrite, CaSO₄, in NaCl–H₂O solutions at high pressures and\ntemperatures: Applications to fluid–rock interaction. J. Petrol. 46, 701–716 (2004).\nNi, H., Xiao, Y., Xiong, X., Liu, X., Gao, C., Chen, Y.-X., Li, Y., Li, W.-C., Guo, X., Wang,\nY.-Y., Tan, D.-B. & Zhang, L. Formation and evolution of supercritical geofluid. Sci.\nChina Earth Sci. 68, 39–51 (2025).\nPhilippot, P. & Selverstone, J. Trace-element-rich brines in eclogitic veins: Implications for\nfluid compositions and transport during subduction. Contrib. Mineral. Petrol. 106, 417–\n430 (1991).\nRen, A., Wang, Z., Aulbach, S., Zong, K., Wang, X., Zou, Z., Shen, Y., Cheng, H., Hu, Z. &\nZhu, Z. Subduction-related transfer of sulfur and chalcophile elements recorded in\ncontinental mantle wedge peridotites. Geochim. Cosmochim. Acta 398, 11–28 (2025).\nRibeiro, J. M., Lin, J. & Ryan, J. The serpentinized oceanic mantle: a potentially substantial\nvolatile sink. Earth Planet. Sci. Lett. 674, 119730 (2026).\nRichards, J. P. The oxidation state, and sulfur and Cu contents of arc magmas: implications for\nmetallogeny. Lithos 233, 27–45 (2015).\nStern, R. J. Subduction zones. Rev. Geophys. 40, 1012 (2002).\nSvensen, H., Jamtveit, B., Yardley, B., Engvik, A. K., Austrheim, H. & Broman, C. Lead and\nbromine enrichment in eclogite-facies fluids: extreme fractionation during lower-crustal\nhydration. Geology 27, 467–470 (1999).\nSvensen, H., Jamtveit, B. & Banks, D. A. Halogen contents of eclogite facies fluid inclusions\nand minerals: Caledonides, western Norway. J. Metamorph. Geol. 19, 165–178 (2001).\nTan, D. B., Lei, J., Xiao, Y., Gu, H. O., Sun, H., Ye, X., & Guo, Z. L. Carbonic Fluids Drive\nContinental Carbon Cycling as Revealed by the Geochemistry of the Eclogite‐Garnet\nPeridotite Interface. Geophysical Research Letters, 52(24). (2025).\nTaracsák, Z., Mather, T. A., Ding, S., Plank, T., Brounce, M., Pyle, D. M. & Aiuppa, A. Sulfur\nfrom the subducted slab dominates the sulfur budget of the mantle wedge under volcanic\narcs. Earth Planet. Sci. Lett. 602, 117948 (2023).\nTaracsák, Z., Hartley, M. E., Burgess, R., Edmonds, M., Longpré, M. A., Monteleone, B. D.,\nTartèse, R. & Turchyn, A. V. The origin of sulfur in Canary Island magmas and its\nimplications for Earth’s deep sulfur cycle. Proc. Natl Acad. Sci. U.S.A. 122, e2416070122\n(2025).\nWalters, J. B., Cruz-Uribe, A. M. & Marschall, H. R. Sulfur loss from subducted altered oceanic\ncrust and implications for mantle oxidation. Geochem. Perspect. Lett. 13, 36–41 (2020).\nWalters, J. B., Cruz-Uribe, A. M., Marschall, H. R. & Boucher, B. The role of sulfides in the\nchalcophile and siderophile element budget of the subducted oceanic crust. Geochim.\nCosmochim. Acta 304, 191–215 (2021).\nWeller, O. M., St-Onge, M. R., Rayner, N., Waters, D. J., Searle, M. P. & Palin, R. M. U–Pb\nzircon geochronology and phase equilibria modelling of a mafic eclogite from the Sumdo\ncomplex of south-east Tibet: insights into prograde zircon growth and the assembly of the\nTibetan Plateau. Lithos 262, 729–741 (2016).\nYang, J., Xu, Z., Li, Z., Xu, X., Li, T., Ren, Y., Li, H., Chen, S. & Robinson, P. T. Discovery\nof an eclogite belt in the Lhasa block, Tibet: a new border for Paleo-Tethys? J. Asian Earth\nSci. 34, 76–89 (2009).\nZhang, Z.-M., Shen, K., Sun, W.-D., Liu, Y.-S., Liou, J. G., Shi, C. & Wang, J.-L. Fluids in\ndeeply subducted continental crust: petrology, mineral chemistry and fluid inclusion of\nUHP metamorphic veins from the Sulu orogen, eastern China. Geochimica et\nCosmochimica Acta 72, 3200–3228 (2008).\nZhao, S.-Y., Yang, A. Y., Langmuir, C. H. & Zhao, T.-P. Oxidized primary arc magmas:\nconstraints from Cu/Zr systematics in global arc volcanics. Sci. Adv. 8, eabk0718 (2022).\nZhu, J., Zhang, L., Li, H., Jiang, R., Zhang, L. & Tao, R. The effect of iron sulfidation on the\nstability of the trisulfur radical ion S₃•⁻ in subduction zone fluids. Geochim. Cosmochim.\nActa 415, 40–55 (2026).\nPoint-by-point response\nReviewer 1 Dr. Zoltán Taracsák\nI read the revised version of the ms. You sufficiently addressed the major comments, and I think\nthe manuscript in its present form is much more convincing and suitable for publication in\nNature Communications. I appreciate the extra modelling work and EPMA analyses you carried\nout. I have some minor comments and suggestions to the text where I think you should be more\nnuanced as I indicate below (these should be simple to do by adding a sentence or two)– once\nthese are sorted, I recommend publication.\nReply: We sincerely thank Dr. Zoltán Taracsák for the positive evaluation of our manuscript\nand for the insightful suggestions that have helped further improve its clarity and robustness.\nWe have carefully considered all the comments raised in this round, and the corresponding\nrevisions have been incorporated into the manuscript with tracked changes (shown in blue).\nDetailed point-by-point responses are provided below.\nLine62: Maybe reword the start of the sentence starting with “more fundamentally” – I suggest\n\"It is also possible such discrepancy arises\"...\nReply: Ok, the sentence has been revised accordingly (Lines 54-56) and now reads: “However,\nit may also arise from the lack of direct constraints on sulfur concentrations and fluxes in natural\nslab-derived fluids themselves.”\nLine125-142: It is good that you added this segment - but I still have an issue around the use of\n\"primary origin\"-which strictly means that the FIs were entrapped during growth of the\nomphacite cores. Crosscutting inclusions with homogeneous fill may still be secondary relative\nto the core, placing FI formation between growth of omphacite cores and rims. I accept the\narguments here that these FIs represent near-peak P-T conditions (particularly as these are in\nthe core only), and representative of fluids lost from slabs-but you can say that without\nconfirming primary origin (as you do in lines 139-142), or by mentioning that even if the FIs\nformed after the omphacite cores crystallised, they predate the rims, which is firm evidence of\neclogite facies origin.\nReply: We thank the reviewer for this important suggestion. We have removed the expression\n“primary origin” from the manuscript (Lines 119-134). Instead, we now describe the inclusions\nmore cautiously by emphasizing that they occur exclusively within the cores of omphacite.\nBecause omphacite is a high-pressure mineral stable under eclogite-facies conditions, the\noccurrence of fluid inclusions within its cores indicates that these inclusions record fluids\npresent at or near peak metamorphic conditions. As such, they provide critical constraints on\nthe composition of slab-derived fluids at sub-arc depths. These revisions improve the precision\nof the description while retaining the main conclusions of the study.\nLine164: again, use of primary fluid inclusion. I would leave primary out, and just state that\nthese fluids were present concurrent to omphacite crystallisation at high P-T.\nReply: Ok, the sentence has been revised accordingly (Lines 155-156) and now reads: “First,\nas discussed above, the investigated inclusions were trapped during omphacite crystallization\nat peak metamorphic conditions.”\nLine181-184: I agree that post-entrapment addition/loss of sulfur and Cu can be ruled out - but\nfor elements like Ca and Fe, the contribution from the host may still be non-negligible. I\nappreciate that you carried out the EPMA work, but an issue that remains is that the silicate-\nhost may have underwent diffusive re-equilibration too (partially or fully). I recommend\nchanging the text here slightly to add nuance (i.e. add that modification was negligible for\nelement other than Ca and Fe).\nReply: We thank the reviewer for pointing this out. For clarity and rigor, we have revised the\nrelevant sentence in the manuscript (Lines 172-174). The revised text now reads: “This pattern\nsuggests localized post-entrapment element exchange. However, the overall extent of\nmodification appears negligible for elements other than Ca and Fe and is unlikely to have\nsubstantially altered the bulk composition of the trapped fluids.”\nLine198-202 I don't think these two sentences add to your argument, considering that the\nquoted experiment/model based S range is, while broader, it still fully overlaps with your FIs.\nI would delete these two sentences and only leave in the revised text.\nReply: Thanks. As recommended, we have removed these two sentences from the manuscript.\nLine276: delete squarely\nReply: Ok, we have deleted it.\nLine278: mention where Dabie Shan is for broader context, and what subduction system it\nrepresent.\nReply: We have added a brief description of the Dabie Shan in the revised manuscript (Lines\n264-265), noting that it (Dabie orogenic belt) represents one of the well-known continental\nsubduction zones in China.\nLine290: Maybe this is one of my last real concern with the ms - I think it would be important\nto highlight here that Ca contents may be affected by exchange with the host here, and also the\nfact that you did not measure Na in the brine, but assumed a 15% salinity based on a published\ndataset that had a range of 12-22%. In line 291 you state these concentrations are observed, but\nreally Na is just estimated or implied. You should point out these limitations at the end of\nthe paragraph.\nReply: We thank the reviewer for this important comment. We agree that the Ca concentrations\nin the fluid inclusions may potentially be influenced by exchange with the host mineral. Indeed,\nour EPMA elemental mapping shows a variation of up to ~0.6 wt.% in Ca, suggesting that\nlimited modification may occur. However, such modification is unlikely to significantly alter\nthe overall Ca-rich nature of the fluids. Ca-rich fluids are commonly observed in subduction\nenvironments, and in this case, the high carbon contents observed in the inclusions likely\nindicate a carbonate-derived component, which would naturally contribute Ca to the fluid phase\nduring carbonate breakdown. Regarding Na, the NaCl content and salinity of the fluid were not\ndirectly measured in this study but were estimated using a salinity value of 15 wt.% NaCl. This\nvalue lies within the range (12–22 wt.% NaCl) determined in our previous fluid inclusion study\nfrom the same rock (Sumdo eclogite, Liu et al., 2019). The use of the average value allows\nconsistent comparison among different inclusions analyzed in this study. Therefore, the\ninference that the fluids were Na-rich remains robust. Following the reviewer’s suggestion, we\nhave clarified in the revised manuscript that the absolute concentrations of Ca and Na may\ncontain minor uncertainties due to potential host–fluid interaction and the estimation of salinity.\nHowever, these uncertainties do not affect the overall interpretation that the fluids are solute-\nrich and capable of transporting high sulfur concentrations.\nRelevant clarifications have now been added to the revised manuscript (Lines 279-282) and\nread as follows: “In addition, we acknowledge that the absolute concentrations of calcium and\nsodium may be slightly influenced by minor exchange with the host omphacite and by\nuncertainties associated with estimating fluid salinity using an average value. However, such\neffects do not alter the overall characterization of the fluids as being enriched in calcium and\nsodium.”\nLine311-312: delete \"capable of transporting high solute loads (solute-rich already implies this)\nReply: Ok, we have deleted it.\nLine371-373: How does the difference between isotope-based models of fluid fraction (5%)\nreinforces the robustness of your estimate of 0.2-0.6%? To me this looks like a 10x discrepancy.\nMaybe delete “thereby reinforcing the robustness of our mass-balance constraints”\nReply: Ok, we have deleted it.\nLine545: You state that you calculated “ the global H O flux from sub-arc depth”, but to me it\n2\nseems you just quantify H O from the AOC, based on your previous sentence – isn’t this a\n2\nglobal flux estimate for the AOC only, not including sediments or serpentinites?\nReply: We thank the reviewer for pointing this out. We have clarified it in the revised\nmanuscript (Lines 532-533), and the text now reads: “in order to quantify the global H2O flux\nreleased from altered oceanic crust at sub-arc depths (~80–90 km).”\nWe sincerely thank Dr. Zoltán Taracsák again for the insightful and detailed comments\nand suggestions, which have helped improve the clarity and quality of the manuscript.\nReviewer 2 Prof. Lifei Zhang\nI just have checked this revised manuscript carefully. All my questions and suggestions have\nbeen accepted and changed in the revised version. I suggest to accepted for the publication in\nNC.\nReply: We sincerely thank Prof. Lifei Zhang for the positive evaluation of our work and for the\nconstructive comments provided during the previous review round, which have significantly\nimproved the quality of the manuscript.\nReferences\nLiu, H., Xiao, Y., van den Kerkhof, A., Wang, Y., Zeng, L. & Guo, H. Metamorphism and fluid\nevolution of the Sumdo eclogite, Tibet: constraints from mineral chemistry, fluid inclusions and\noxygen isotopes. J. Asian Earth Sci. 172, 292–307 (2019)."}]

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