Arc magma formation through the fluid-fluxed mélange melting in subduction zones

Domain: chemistry
Published: 2026-02-21
Editorial outcome: minor_revision

Abstract
The transport of subducted slab materials to the overlying mantle plays a crucial role in arc magma formation. However, the contributions of aqueous fluids, hydrous melts, or mélange to the convective mantle remain controversial due to the lack of definitive fingerprints for these materials. Here, we report the Ba isotope composition of lavas from a typical cold-slab arc, the Izu arc, to resolve the recycled materials in the source of arc magmas. δ138/134Ba of the arc lavas show an across-arc decrease corresponding to increasing depth and positively correlates with both 87Sr/86Sr and 143Nd/144Nd. Across-arc variations of both δ138/134Ba and Ba/Th ratios support that high Ba/Th in the Izu and other arc magmas originated from slab-derived fluids rather than melts. Quantitative mixing modeling, involving contributions from mélange followed by slab-derived fluids in successive stages, provides a coherent explanation for Ba-Sr-Nd isotopic signatures observed in lavas from cold arcs. Our work reveals the combined role of slab-derived fluids and mélange diapir melting in arc magma formation.

Introduction
Volcanic arcs are key localities for element cycling and mass transfer between Earth’s surface and interior, which greatly affect crust-mantle evolution and Earth’s long-term climate change. Compared to MORB (mid-ocean ridge basalts), arc magmas are enriched in large ion lithophile elements (LILE; e.g., Ba, Cs) and depleted in high field strength elements (HFSE; e.g., Th, Nb). The enrichment of LILE is generally attributed to the incorporation of slab-derived materials into the mantle wedge1. However, the nature and transport mechanisms of these slab-derived materials remain debated. Generally, three scenarios have been proposed: (1) melting of mantle wedge metasomatized by aqueous fluids from altered oceanic crust (AOC) and/or hydrous melts from sediment2,3; (2) melting of mantle wedge metasomatized by hydrous melts from both AOC and sediment4–9; (3) melting of mélange diapir, which is formed by the mechanical mixing between slab materials and mantle peridotites in subduction channels and diapirically rise into the mantle wedge10–13. Thus, delineating the applicability of such mechanisms is crucial for understanding the genesis of arc magma.

A re-evaluation of the long-standing interpretation regarding the variation of Ba or Ba/Th in arc lavas could help resolve these controversies. Traditionally, ratios of fluid-mobile to fluid-immobile elements like Ba/Th in arc magmas were regarded as a typical indicator of the contribution from AOC-derived fluids3,14. However, significant variations of Ba/Th in arc magmas were recently ascribed to differences in melting temperature or composition of subducted sediment among different arcs6–8. For example, high-pressure/high-temperature experimental results showed that the Ba/Th ratio of sediment melt decreases with increasing temperatures (750–1050°C)15. As such, Ba/Th variation in arc magmas was ascribed to sediment melts formed at different temperatures6. However, arc lavas with high Ba/Th (e.g., Izu arc; up to 1000) do not show slab-surface temperatures beneath the arc significantly different (<100°C) from those with low Ba/Th (e.g., Lesser Antilles arc; <200)16–18. Moreover, based on significant Ba/Th variations in different types of subducted sediments, recent studies suggested that the Ba/Th variation in arc magmas reflects spatial variation of sediments with varying Ba/Th8. However, it is difficult to constrain the spatial variation in the bulk composition of subducted sediments.

Alternatively, the Sr-Nd-B-Mg isotopic variation of arc lavas was proposed to be incompatible with mixing between mantle and sediment melts and/or AOC fluids, but is consistent with the mixing of bulk sediment or serpentinite and mantle (i.e., mélange)11,12. The overlap of trace element characteristics (e.g., Ba/Th) between mélange and arc magmas furthers support that direct melting of mélange could produce such signatures of arc magmas10. However, the eclogitized oceanic crust and the underlying hydrated mantle may be too dense to ascend as diapirs into the mantle wedge. Instead, these materials might remain at subarc depths and then release fluids to hydrate the overlying mantle. Therefore, elucidating the mechanisms driving the variation of Ba or Ba/Th in arc magmas is critical for constraining the nature and transport mechanisms of slab-derived materials (fluid, melt, or mélange diapir).

Barium content and ratios to other elements are susceptible to magma differentiation, such as partial melting and fractional crystallization, while Ba isotopes exhibit minimal fractionation during these processes19,20 and are distinguishable in different geological reservoirs. The Ba isotopic compositions are expressed as δ138/134Ba (δ138/134Ba=[(138Ba/134Ba)sample/(138Ba/134Ba)SRM3104a−1]×1000‰). δ138/134Ba of low-temperature AOC (0.24±0.21‰, 2SD)21 are generally higher than those of depleted MORB mantle (DMM; 0.03 to +0.14‰)13,22,23. Fluids released from AOC dehydration are also suggested to have heavier Ba isotope compositions24,25. In contrast, seafloor sediments typically exhibit lighter Ba isotopic compositions (δ138/134Ba=0.02±0.10‰, 2SD)22,26,27. Consequently, Ba isotopes serve as a potentially excellent tool for tracing the source of Ba in subduction zones28.

The Izu-Bonin-Mariana (IBM) arc system serves as a typical example of an intra-oceanic subduction zone with a cold slab17. The Izu arc is located in the northern section of the IBM system and exhibits marked across-arc volcanic distribution29,30. Studies on chemical compositions of the across-arc volcanic chains could minimize the effects of spatial variations in subducted components, making the Izu arc an ideal location for constraining the nature and transport mechanisms of slab-derived materials.

In this study, we conducted whole-rock Ba isotope analyses on volcanic rocks from the Izu arc with varying volcano-to-slab depths, as well as the offshore sediments near the Izu trench (IODP Site 1149). The results indicate that the high Ba/Th and δ138/134Ba in global arc magmas (including those in the Izu arc) reflect a dominant contribution from AOC-derived fluid rather than sediments. By integrating Ba-Sr-Nd isotopic and trace elemental data from global subduction zones, we propose a model for the generation of arc magma in cold subduction zones, i.e., mélange ascend as bulk diapirs into the mantle wedge, which are subsequently metasomatized by AOC-derived fluids to form the source of arc magmas. This model highlights the combined role of AOC-derived fluids and the mélange melting.

Results
The Izu arc, located at the northeastern edge of the Philippine Sea Plate (PSP), is formed by the subduction of the Pacific Plate beneath the PSP and stretches over 480 km between 30.5°N and 34.5°N (Fig. 1). It is a typical intra-oceanic arc system with negligible inputs from sub-arc crust to the lavas and is also a non-accreting convergent margin with a thin sedimentary cover on the downgoing slab29–33. The across-arc chain studied here extends to the southwest from Oshima and includes three rear-arc volcanoes behind the volcanic front, Toshima, Niijima, and Kozushima (Fig. 1b)29,30,32.

This study analyzed δ138/134Ba of twenty-six samples from the frontal-arc regions and eleven samples from the rear-arc regions. The δ138/134Ba of these arc lavas range from −0.05 to +0.15‰ (Table S1). Samples from frontal-arc regions were collected from volcanic islands from Oshima to Torishima (Fig. 1), which are mainly late Quaternary tholeiitic basalts or basaltic andesites. These samples exhibit δ138/134Ba values ranging from 0.03‰ to 0.15‰, which almost overlap with mantle range (0.03‰ to +0.14‰) (Fig. 2a). No significant δ138/134Ba variations were observed along the arc front (Figure. S1). Samples from the rear-arc regions were collected from volcanic islands extending from Oshima toward the rear-arc direction (e.g., Toshima; Fig. 1b), which are late Quaternary basaltic to rhyolitic lavas. Their δ138/134Ba values are between −0.05‰ and 0.07‰, overlapping with mantle range but extending to lower values (Fig. 2a). Notably, δ138/134Ba values and Ba/Th decrease with increasing depths to the Wadati-Benioff zone (Fig. 2).

Barium isotope data of offshore sediments from IODP Site 1149 in the western Pacific Plate are presented in Table S2. The core is located at a proximal geographic position to the Izu-Bonin trench (Fig. 1), likely rendering it an ideal representation of the Ba isotope input of sediments to the Izu arc34–36. The core is approximately 440 m long, and the studied sediment samples are mainly volcanic ash, pelagic clay, radiolarian chert, and nannofossil marl (Table S2)34,36,37. Those samples show high and variable Ba concentrations (14.7–2717 µg/g, Table S2) compared to DMM (ca. 1.2 µg/g)38. δ138/134Ba of these sediments are −0.02 to 0.08‰ (except a radiolarite sample of −0.12‰; Fig. S2 and S3 and Table S2), with a weighted mean of +0.02‰ (weighted to interval mass and Ba concentrations).

All volcanic samples are fresh, with no clay minerals30. Low loss-on-ignition (LOI) values (<2 wt.%) and chemical index of alteration (CIA) values (40–52)39,40, as well as no significant correlation of these indices with δ138/134Ba (Fig. S4), indicate the negligible effect of alteration on Ba isotopes. Compared with data from subaerial arcs that do not record significant alteration13, we do not observe higher Th/Rb and Th/Cs ratios for any samples in our study (Fig. S5). Therefore, all samples investigated here are unlikely to have experienced significant alteration. Moreover, δ138/134Ba values show no systematic correlation with SiO2 or Mg# [molar ratios of Mg2+/(Mg2++Fe2+)], suggesting that magmatic differentiation has minimal impact on Ba isotope composition (Fig. S6). Therefore, the Ba isotopic characteristics of the Izu arc lavas primarily reflect the compositions of mantle source affected by slab-derived materials. Traditionally, Ba/Th ratios in arc magmas were regarded as a typical indicator of the contribution from AOC-derived fluids3,14. However, recent studies attributed Ba/Th variation in arc magmas to sediment melts formed at different temperatures6, or spatial variation in composition of sediments with varying Ba/Th8. Therefore, clarifying the source of Ba in the Izu arc lavas, especially the causes for large Ba/Th and δ138/134Ba variations, could help resolve these controversies.

The frontal arc samples exhibit remarkably high Ba/Th that decrease toward the rear-arc region (Fig. 2b). Given that slab temperature increases with greater slab depth, it may lead to the fractionation of Ba from Th (i.e., a decrease of Ba/Th) in sediment melts with increasing slab-depth6,15. Therefore, we need to evaluate whether the variation in Ba/Th results from the fractionation of Ba and Th in sediment melts at varying temperatures. First, at the slab-depth range from the frontal arc to the rear-arc regions (100–130 km; Fig. 2), the temperature variations of the slab are limited (<100°C)16–18, which would result in a minor decrease in Ba/Th (<2 times)15. Therefore, it cannot account for the wide range of Ba/Th (100–1000; Fig. 2b) observed in the Izu arc lavas, which is comparable to the global range for arc magmas. Moreover, although the increase in melting temperatures of sediment may reduce Ba/Th in sediment melts, partial melting at high temperatures is anticipated to have a limited effect on Ba isotope fractionation in sediment melts19. Therefore, the variation in melting temperatures of sediment cannot account for the remarkable decrease in both δ138/134Ba and Ba/Th with slab depths observed in the Izu arc lavas.

On the other hand, the sediments subducting into the Izu arc are characterized by terrigenous sediments in the upper layers with low Ba/Th and pelagic/carbonate sediments in the lower layer with high Ba/Th (Fig. S2 and Table S2)36,37. Their mixing may affect the Ba/Th variation in Izu arc samples, as previously proposed to explain the Mariana arc data8. However, both upper sediments and lower sediments have similar δ138/134Ba despite their largely variable Ba/Th (17–2813; Figs. S2 and S3), therefore, their mixing cannot explain the covariation of δ138/134Ba and Ba/Th in the Izu arc lavas. Moreover, the decrease in both δ138/134Ba and Ba/Th with increasing slab depth is difficult to explain by mixing between the high-Ba/Th lower sediments and the low-Ba/Th upper sediments, as this scenario would require preferential incorporation of the lower sediments in the frontal arc, followed by the incorporation of the upper sediments at greater depths in the rear arc. Similarly, the across-arc decrease in Ba/Th in the Mariana arc41 cannot be explained by the mixing of lower pelagic sediments with high Ba/Th and upper volcaniclastic sediments with low Ba/Th, given the similarity of sedimentary lithologies subducting beneath the Mariana arc to those subducting beneath the Izu arc34,36.

The Izu arc samples with high δ138/134Ba and Ba/Th (>500) exhibit notably high 143Nd/144Nd (>0.51306; Fig. 3a), indicating that their high Ba/Th cannot be attributed to sediment-dominated components. Global data show that arc lavas with low Ba/Th tend to exhibit lower 143Nd/144Nd and higher 87Sr/86Sr (Fig. 3), clearly indicating sediment contributions37. In contrast, arc lavas with high Ba/Th show relatively higher 143Nd/144Nd and lower 87Sr/86Sr (Fig. 3), consistent with AOC-derived fluid signatures34.

Thus, we conclude that AOC-derived fluids, rather than sediment melts, constitute the primary source of the high Ba/Th and δ138/134Ba characteristics in arc magmas. In most subduction zones, the oceanic crust component dehydrates to release hydrous fluids, rather than partially melting1–3.

Boron isotope ratios (i.e., δ11B) and B/Nb in the Izu and many other arc lavas show an across-arc decrease, with δ11B showing roughly linear correlations with B/Nb as well as Sr or Pb isotopes29,42–47. Therefore, the across-arc decrease in δ11B was considered to result from an across-arc decrease in relative proportion of slab-derived fluids compared to the mantle29,42,43,47, coupled with B isotope fractionation during progressive fluid loss from the slab44–46. Similar to B isotopes, Ba/Th and δ138/134Ba of the Izu arc lavas exhibit across-arc decreases, with δ138/134Ba showing positive correlations with 87Sr/86Sr and 143Nd/144Nd (Fig. 4). The high δ138/134Ba end-member has a Sr-Nd isotope composition similar to AOC, which most likely reflects the contribution of AOC fluids34. As boron and Ba are both fluid-mobile elements, the across-arc decrease in δ138/134Ba of the Izu arc lavas also suggests a reduction in the contribution of AOC-derived fluids with increasing slab depths, which is consistent with the positive correlations between δ138/134Ba and 87Sr/86Sr, Ba/Th, Cs/Nb, and Pb/Nb (Fig. 4a and S7a–c).

Previous studies have linked the decrease in δ11B to more mantle-like Sr and Pb isotope ratios, suggesting that the low δ11B end-member points to a mantle source with B isotopic compositions consistent with a DMM signature29,42,43. However, a pure DMM component cannot explain the Ba isotopic composition of the mantle end-member for the Izu arc lavas, as it has a significantly lighter Ba isotopic composition (ca. −0.10‰) than DMM (0.03–0.14‰) (Fig. 4). To address this problem, we propose an alternative scenario. The low δ138/134Ba end-member shows slightly enriched signatures in both Sr and Nd isotope compositions relative to DMM, most likely representing a mantle source hybridized by subducted metasediments. In other words, a small amount of sediment initially mixed with DMM, imparting it with a slightly enriched Sr-Nd isotope composition and sediment-like δ138/134Ba values (Fig. 4). This is likely because Ba_sediment/Ba_DMM ratio is much higher than the corresponding ratios for Nd and Sr37,38. This hypothesis is supported by the positive correlation between δ138/134Ba and 143Nd/144Nd and the negative correlation between δ138/134Ba and Th/Yb. That is, lower δ138/134Ba samples exhibit lower 143Nd/144Nd and higher Th/Yb (Fig. 4b and S7d).

The δ138/134Ba of offshore sediments near the Izu trench (Site 1149) range from −0.02 to +0.08‰ (Fig. S2 and Table S2), which is higher than the expected metasediment component (ca. −0.1‰). However, previous studies on other arc lavas and OIB-type rocks suggested that significant Ba isotope fractionation may occur during the subduction of sediments, resulting in lower δ138/134Ba (<0‰) of the metasediments at subarc depths compared to the protolith13,20,27,48,49. Such a scenario is also supported by the low δ138/134Ba (−0.16 to −0.05‰) of phengites, a primary host of Ba in metasediments, from eclogites in the Dabie ultrahigh-pressure metamorphic belt, China24,25. Therefore, metasediments beneath the Izu arc with low δ138/134Ba (ca. −0.1‰) are likely to be formed owing to the Ba isotope fractionation during slab subduction. AOC-derived fluids are subsequently added to such sediment-hybridized mantle source, forming the observed positive correlation trends. Our mixing modeling indicates that the addition of <0.8 wt.% bulk sediment into the mantle source could account for the characteristics of the low δ138/134Ba end-member (Fig. 4).

Barium isotope data from two other classic cold subduction zones (Tonga-Kermadec and Mariana arcs) also support this conclusion50,51. In δ138/134Ba vs. 87Sr/86Sr and 143Nd/144Nd plots, the mantle–sediment mixing line passes through the data array defined by most Kermadec lavas and North Mariana lavas (Fig. 5), which suggest that these lavas could be explained by mixing between mantle peridotite and sediments. In contrast, the data arrays defined by samples from the Tonga arc and the South Mariana arc require a more dominant contribution of AOC fluid with high δ138/134Ba (Fig. 5). Critically, the other end of that array extends to their intersection with the mantle-bulk sediment mixing lines (Fig. 5), implying that sediment components are incorporated into the mantle prior to the addition of AOC fluids.

The decoupling of Sr-Nd isotopes and their systematic relationships with Ba and Th in arc lavas from cold subduction zones further supports this model (Fig. S8). Backarc lavas (the lavas formed in association with a spreading center) are generally distributed along a “backarc” array (the DMM-bulk sediment mixing line), whereas arc magmas deviate from this array (Fig. S8). In contrast, the array defined by arc magmas lies between a point on the DMM-bulk sediment mixing line (backarc array) and AOC fluid. Notably, as arc lavas deviate farther from the “backarc array”, their Ba/Th increase (Fig. S8). This further indicates that an AOC fluid is added separately to a mantle source that has been hybridized by sediments. Therefore, both Ba isotope and Ba elemental evidence support the idea that, in cold subduction zones, bulk sediments are incorporated into the mantle prior to the addition of AOC fluids.

Consequently, the most likely model to explain the arc magma composition is as follows: mélange containing sediment components formed at the slab-mantle interface migrate into the mantle at depths shallower than subarc regions (i.e., forearc regions), and subsequently AOC fluids are added individually at subarc depths. Previous studies suggest that mélanges have lower densities than mantle rocks at depths <3 GPa (ca. 100 km), allowing them to migrate as diapirs into the mantle wedge10,52–54. The quantitative modeling of this process by considering the mélange formed by the hybridization of bulk sediment and DMM, which then experienced the metasomatism of AOC-derived fluids and partial melting, can explain not only the Ba-Sr-Nd isotope compositions but also the trace element compositions of the Izu arc lavas (Fig. S9), lending further support to the proposed model.

Partial melting of sediment-bearing mélange or sediment at the slab-mantle interface is another possibility that needs to be evaluated to explain these arc magma compositions. However, the slab surface temperature at depths shallower than subarc regions (<90 km) in cold subduction zones like the Izu arc (<700°C)16,17 is probably insufficient to induce the melting of mélange or sediment (>750–800°C)15,55. If AOC fluids need to be added to the mantle in a separate form, it also requires that mélange or sediment does not melt at the subarc depths and shallower depths. Moreover, mixing of sediment melts with a large range of partial melting (1% and 20%) with the mantle is inconsistent with the Sr-Nd isotope data of backarc lavas (Fig. S8). While some Sr-Nd isotope data for arc lavas maybe explained by sediment melts (Fig. S8), the differences between arc and backarc lavas cannot be accounted for by the addition of sediment melts and bulk sediments, respectively. This is because: (1) the slab surface beneath the backarc regions has higher temperatures than that beneath the arc, which favors sediment melting; and (2) the density of bulk sediments at >4 GPa depth could be too high for diapirism to occur10,52–54. Therefore, although the melting of mélange or sediment along the subduction channel may occur at forearc depths, the mélange diapir is the most feasible scenario to explain our Ba isotope data.

A model for arc magma formation. The formation of arc magma has long been attributed to the metasomatism of the subarc mantle by AOC-derived fluids and/or sediment melts. However, two competing models have been proposed: (1) melting of mantle metasomatized by AOC melts and sediment melts4–9, and (2) melting of mélange diapirs10–12. One key to resolving the debate lies in elucidating the origins of Ba or Ba/Th variation in arc magmas. This study reveals that the high δ138/134Ba and Ba/Th in Izu arc and other arc magmas originate from the contribution from AOC fluid, rather than from sediment melt. Moreover, the synchronous positive correlation between δ138/134Ba and 87Sr/86Sr and 143Nd/144Nd requires the incorporation of mélange diapirs and AOC fluid into mantle at different stages. Integrating the coupling between Ba/Th and Sr-Nd isotopes in global arc magmas, we propose a revised model. In cold subduction zones, mélanges ascend as diapirs at shallower depths (probably at the slab-mantle interface) into mantle wedge, which are subsequently metasomatized by AOC-derived fluid at subarc depths, triggering the formation of arc magmas (Fig. 6). Such mélanges might have experienced melting to some extent in the hot mantle wedge prior to the addition of AOC-derived fluids11,56. However, this process may not produce a melt that is significantly different in composition from the melt we envisioned. In hot subduction zones, AOC largely dehydrates at shallower depths, which may make mélange/sediment-derived signals predominant in Ba isotopic features of arc lavas, such as the Aleutian and Ryukyu arcs27. Previous numerical simulations predicted the occurrence of mélange at the slab-mantle interface, which can ascend to the hot mantle wedge in a diapiric form57–61. In addition, seismological studies, especially through the method of high-resolution seismic tomography, revealed the occurrence of zones with low-velocity, high-attenuation, and strong seismic anisotropy just beneath active arc volcanoes62,63, which is likely a mélange consisting of sediment and peridotite. Such a process is supported by the investigation of a deeply buried mélange “package” in Kyrgyz Tianshan, southern Altaids, which are formed at subarc depths, and subsequently propagate and dynamically mix with overlying mantle64.

This model breaks through the limitations of traditional single-stage melt/fluid processes by coupling mélange diapirs with AOC fluid contributions, thereby supporting the “multi-stage, multi-mechanism” paradigm of subduction-zone mass transfer. The mélanges in the subduction channel are enriched in volatiles and may act as a temporary sponge of water, carbon, and sulfur at the slab-mantle boundary. Therefore, the separate dehydration of these slab materials could control the pathways and budgets of the volatiles into the mantle wedge or the deep mantle. Quantitative modeling of the spatial distribution and mixing efficiency between AOC fluids and mélanges in the mantle wedge is helpful for further validation through thermodynamic simulations and geophysical evidence.

Methods
Barium isotope measurements were conducted at the Metal Stable Isotope Geochemistry Laboratory of the University of Science and Technology of China (USTC). Approximately 10–100 mg of rock powders were decomposed using a 1:3 mixture of concentrated HNO3 and HF for one to two days. Subsequently, they were dried down and fluxed with aqua regia overnight to remove fluorides. Samples were then dried down and refluxed with 2 mL 3 mol·L−1 HCl to convert nitrate compounds to chloride. Finally, the samples were dried down and dissolved in 1 mL of 3 mol·L−1 HCl for further purification. The above digestion and evaporation procedures were conducted on a hotplate with temperatures of 120 to 140°C. Barium was separated from the matrix using a Bio-Rad 200-400 mesh AG50W-X12 resin and two column procedures according to an ion-exchange chromatographic method of ref. 65,66. Matrix elements were eluted with 3 mol·L−1 HCl, and Ba was then concentrated through 3 mol·L−1 HNO3. After purification, Ba concentrates were dried and then dissolved in 2% (m/m) HNO3. The yield of this purification procedure was >99%. The total procedural blanks were <1 ng. Barium isotopes were analyzed on a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Barium isotopic ratios were measured using the double-spike (135Ba-136Ba) method in low-resolution mode under “dry” plasma conditions using an Aridus III desolvator (CETAC Technologies)65–67. The results are reported in δ138/134Ba values relative to the National Institute of Standards and Technology (NIST) Standard Reference Material 3104a [δ138/134Ba = 1000 × ((138Ba/134Ba)sample / (138Ba/134Ba)SRM-3104a − 1)]. All samples were analyzed three times, yielding 2SD uncertainties of 0.00‰ to 0.05‰. The long-term external precision is better than ±0.04‰ for δ138/134Ba (2SD) based on the measurement of two in-house standards (USTC-Ba and ICPUS-Ba)65,66. The reference materials were processed with every set of unknowns and displayed Ba isotopic compositions in agreement with previous work (Table S5)65,66,68–70.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Key results\nThis is a cross-arc study of Ba isotopes and Ba enrichment over Th in lavas from the Izu arc. Ba is fluid-mobile and the key\nisotope ratio for this study, δ138/134Ba, fractionates during slab dehydration; however, Ba isotope ratios in residual,\ndehydrated AOC may be only minimally affected. As a result, AOC at sub-arc depths may be a source of isotopically heavy\nBa. Izu lavas from the volcanic front have Ba-Sr-Nd isotope ratios pointing to AOC fluids. Among samples from the rear arc,\nthese isotope ratios trend towards a mixing line between DMM and bulk sediment. The authors conclude that their data\nsupports the sediment-dominated mélange diapir model. The diapirs ascend into the mantle wedge, where they are\nmetasomatized by AOC fluids, causing flux melting. This study adds to a growing body of geochemical literature suggesting\ndiapirs of some combination of sediment, AOC, and serpentinite are important source materials for arc lavas.\nValidity\nI do not see any major flaws in the authors’ data or general interpretations about source materials contributing to their Izu\nlavas. However, the conclusion that a sediment-dominated mélange diapir was involved seems rather speculative. It is not\nclear why this model is preferable to a mélange of similar composition along the subduction channel. The authors mention\nthe density contrast between subducted sediment and overlying mantle and cite two numerical modeling studies, but if there\nare other geophysical studies that provide stronger support for this model, I would like to see the authors refer to them.\nSignificance\nDespite my personal skepticism about the mélange diapir model (discussed elsewhere in my review), I think this is a helpful\nstudy with robust, clear data patterns, and adds important insight into our understanding of how materials are recycling\nthrough subduction systems.\nData and methodology\nThe data look reasonably sound to me. The authors used an appropriate analytical method for this kind of work. Overall\nprecision for their Ba isotope measurements is given as +- 0.05‰ (2SD). This does seem high given that the δ138/134Ba\nresults for their samples only cover a range of ~0.2‰ (Fig 2). To me, this does not call into question the validity of the data or\nthe authors’ interpretations. And, I am not familiar with the Ba isotope system, so maybe this level of precision is standard.\nAnalytical approach\nThe authors are using established methods for digesting volcanic rocks (HF-based digestion mixture).\nL 232: What are the proportions of acids in the digestion mixture? How long and at what temperature were samples\ndigested? Tabletop/hotplate digestions should be fine for these rocks but it will be good to include more detail.\nL 232-234: I am not familiar with Ba separation chemistry, but the authors are using a protocol based on prior literature and\nachieving good results (high yield, low blank).\nL 235: The Neptune is a perfectly suited instrument for this type of work.\nL 237: Can the authors describe what they mean by “dry” plasma? Did they use something like an Aridus in the inlet system?\nI think just adding a citation to a prior work that describes “dry plasma” would be fine here.\nL242-244: The authors report that their measurements of commonly used reference materials agree with literature values,\nbut do not provide those literature values. A small supplementary table summarizing the results for BCR-2 and RGM-1 for\nboth this study and prior work (averages +- 2SD) will help. I am also curious, since the authors analyzed JB-2 as an\nunknown for this study (Table S1)—this is a common reference material; are there Ba isotope measurements on it in the\nliterature?\nSuggested improvements\nThe authors have constructed what they refer to as a “qualitative model,” which is illustrated in Figures 4-5. We can see from\nthe relationships between the data and various mixing lines that the authors’ conclusions are qualitatively plausible.\nHowever, it is possible to calculate compositions that have isotope ratios that make sense but element concentrations that\nare unreasonable. I suggest the authors produce a quantitative model. Can they calculate some range of mixing proportions\nof their endmembers to produce a hypothetical melt source, and then calculate the composition of a partial melt with similar\ntrace element abundances and Ba-Sr-Nd isotope ratios similar to their Izu lavas? A reasonable quantitative model will\nstrengthen the authors’ arguments.\nClarity and context\nThis manuscript is well-written, easy to read and understand.\nWith respect to context, it may be useful for the authors to compare this study with similar prior work on B isotopes. As a fluid-\nmobile element that undergoes stable-isotope fractionation during fluid-mediated processes (L 63-64), barium seems quite\nsimilar to boron. There is an important body of work examining cross-arc trends in B abundances and isotope ratios in the\nIBM (Ishikawa and Nakamura, 1994-Nature; Ryan et al., 1995-Nature; Benton et al., 2001-EPSL; Pabst et al., 2012-Lithos),\nKurile (Ishikawa and Tera, 1997-EPSL; Li et al., 2023-Chemical Geology), and Kamchatka (Ishikawa et al., 2001-GCA)\nsystems. Of these, only Ishikawa and Nakamura (1994) is cited. The trends are generally similar: B/X enrichments and δ11B\ndecrease with increasing slab depth. Some of these studies further link decreasing δ11B to more mantle-like Sr, Nd, and Pb\nisotope ratios.\nTwo primary, and not necessarily competing, models arise from the boron literature: cross-arc changes in the relative\nproportions of sediment and AOC contributions compared to mantle, and B isotope fractionation during progressive fluid loss\nfrom the slab. I am only aware of one study that uses cross-arc B abundance/enrichment/isotope patterns to support a\nmélange-diapir model: Qiao et al. (2025), which is cited in L 50 and which shares some coauthors with the present study. In\nmy opinion, Qiao et al. (2025) also do not make a particularly convincing case that a mélange diapir is necessary to explain\ntheir data patterns.\nTo be clear, the geochemistry does appear to agree with the diapir model, but not to the exclusion of melting of a subduction\nchannel mélange. The authors do refer to numerical models indicating sediment diapirs are possible (Currie et al., 2007;\nBehn et al., 2011)—but to me, this still does not rule out melting of mélange within the subduction channel.\nI would like to see the authors either acknowledge this, or find a way to show that the diapir model is favorable beyond just\nreferring to the density contrast between a serpentinite-sediment mélange and mantle peridotite. If there is any geophysical\nevidence in the literature beyond numerical models (e.g. seismic studies?), referring to those studies would help.\nReferences\nOverall, the literature referenced here is appropriate and adequate. Please see my suggestions above (“Clarity and context”)\nfor some more useful literature. Not all of those studies need to be cited but given the behavioral similarities in B and Ba\n(fluid mobility, stable-isotope fractionation), I think the themes of the above-mentioned B work are relevant enough to warrant\na little bit more acknowledgement.\nPersonal expertise\nMy research focus is in fluid-mobile elements and stable isotope fractionation in subduction systems, so I do not think any\npart of this study is too far outside what I am able to assess. I have not studied the IBM system extensively—if my comments\nand suggestions indicate that there are details about the Izu arc that I don’t understand, it may help to clarify those details in\nthe manuscript to help other readers.\nOther specific comments\nL 114-120: First the authors cite work showing that high melting temperatures will produce melts with low Ba/Th, then other\nwork showing that sediment melts have lighter Ba isotopes than their protolith. This would connect low Ba/Th to low\nδ138/134Ba, but in L118-120, the authors then infer that low Ba/Th and low δ138/134Ba in their samples are not linked\nthrough high-temperature sediment melts. This seems contradictory, so some clarification is needed here.\nL 137: I think the statement about oceanic crust dehydrating needs to be cited—a couple of studies or a review paper would\nsuffice.\nL 142: Are the authors talking about an isotope fractionation process between AOC fluid and residue, or just a bulk loss of\nBa from an isotopically heavy source, leading to a greater contribution from lighter sources? Also, please clarify where the\n“lighter Ba isotopes” are: in the residue or in the fluid?\nL 178: Here the authors start referring to “backarc” lavas, whereas they had previously indicated some of their samples are\nfrom the Izu rear arc. To some people, “backarc” implies an association with a spreading center, so the authors should\nconsider either using “rear arc” consistently or clarifying that they are actually referring to backarc lavas.\nL 211: This sentence is unclear about what the AOC fluid is metasomatizing (I assume mantle peridotite).\nL 232: What were the proportions of different acids in the digestion mixture?\nFigures\nFigure 2: Horizontal axes have different scales on 2a and 2b, which is not necessary.\nFigure 5: This figure could be combined with Figure 4 to make it easier to compare your Izu data and modeling with the other\narc systems. The axes are also scaled differently between the Mariana and Tonga-Kermadec plots. This is probably\nbecause the endmembers plot in slightly different locations and is ultimately a minor issue, but equalizing the axes may\nallow readers to more easily compare data patterns between the plots.\nFigure 6: The Mariana data do not clearly point towards the AOC fluid endmember, and the AOC fluid endmembers for the\nTonga and Scotia systems are not isotopically distinct from some mixture of DMM and sediment melt or bulk sediment. So, I\nthink this figure may not be necessary and does not provide strong support for the authors’ conclusions, unless they\nprovide additional explanation that incorporates how these patterns relate to their Ba isotope data. Plotting ẟ138/134Ba vs Ba/Th for\nthis same mixing model seems like it would be more useful.\nAlso, some of the data on this figure are labeled as being from backarc regions…see my comment on L 178 above.\nSupplemental material\nL 83-85: How was +0.015‰ chosen for AOC at the Izu and Mariana arcs?\nTable S1: Can the authors provide longitude data in addition to latitude for their samples?", "reviewer_2": "Review of Zhang et al: Arc magma formation through the fluid-fluxed melting of mélange-metasomatized mantle\nThis manuscript presents new Ba isotope data for lavas from the Izu arc. The lavas cover the arc front as well as samples\nfrom the back arc and show a decrease in δ138/134Ba from front to back arc. These data also correlate to some extent with\nSr and Nd isotopes as well as a number of fluid sensitive trace element ratios. The authors argue that the highest\nδ138/134Ba values from the arc front relate to fluids released from altered oceanic crust (AOC), whereas the lavas from the\nback arc are dominated by a melange that contains sediments. Ba isotope data from other arcs are used to argued that\nsimilar observations can be made for most subduction zones, which lead the authors to propose a two-step model for arc\nlava generation: first fluid released from AOC with high Ba isotope values (reflecting those found in AOC sections) beneath\nthe arc front. This process is particularly evident in cold subduction zones. Second melting of a sediment-dominated\nmelange beneath the back arc.\nI found that this paper was interesting and contributes nicely to ongoing debate about processes that govern transport of\nsubducted slab material and the formation of arc lavas. The data appear of good quality, and the results are overall relatively\nclearly presented.\nTo take positives first, I think that the authors have found a very interesting offset between Ba isotopes in arc front and back-\narc in the Izu arc, which I agree seems significant and systematic. A robust interpretation of that Ba isotope evolution with\ndepth to the slab would be a very welcome addition to the literature. I also agree that AOC fluids (one way or another)\nseem reasonable to invoke for samples that display high Ba isotopes, high Ba/Th, low Nd isotopes and high Sr isotopes as shown\nin figure 6a.\nThere are, however, a number of deficiencies in both the understanding of how Ba isotopes work and to some extent a\ntendency to misrepresent the literature in order to make the presented interpretations work. I outline these below:\n1) The authors present Ba isotope values for marine sediments as if these simply cover a range with no particular pattern.\nThis eventually leads the authors to select specific values for sediments in their modeling (described in the supplement) that\nare chosen conveniently to match the model they propose. However, that approach is incorrect. As has been discussed in\ndetail in the literature, Ba isotopes in sediments correlate with the nutrient availability of the water column above the\nsediment. Oligotrophic regions (where nutrients are depleted in the surface waters) typically have the highest Ba isotope\nvalues because Ba uptake into barite is associated with a negative fractionation factor that drives the residual water to\nhigher values. The barites that eventually reach the sediment are, therefore, generally more positive (up to ~+0.1), which is\nseen in sediments from Western Pacific arcs.\nSediments underneath nutrient replete regions of the ocean (like the Southern Ocean) result in deposition of barite with\nmuch lower Ba isotope values, notably the South Sandwich arc, which has sediments with δ138/134Ba values of -0.05 to\n-0.1.\nThere is essentially no chance that sediments subducted in the Izu arc have δ138/134Ba values of -0.1 as inferred here. If\nthe authors are unwilling/unable to measure the Ba isotope composition of the subducted sediment package directly (these\nsediments should be readily available), then it would make sense to use other Western Pacific sediment packages as a\nguideline. For example, sediments found close to the Ryukyu arc exhibit δ138/134Ba ~ +0.06.\nIf the authors constructed mixing lines in their figure 4 using this value, then the majority of their samples (also several from\nthe arc front) would plot below that mixing line and, hence, outside of what can be explained with bulk mixing. This is the\nsame Ba isotope pattern that has been observed for all other arcs published to date: Ryukyu, Aleutians, South Sandwich,\nTonga. The simplest way to explain this pattern is that there is Ba isotope fractionation during release from the slab and I see\nno evidence presented by the authors here that suggests otherwise. I strongly recommend that the authors include such a\nprocess in their interpretations rather than inventing Ba isotope compositions of sediments for their own convenience.\n2) The Ba isotope composition of the upper mantle has been the topic of some debate. As a whole the range of values\nmeasured for MORB (mid ocean ridge basalts) is -0.05 to +0.15, which is clearly much larger than any analytical uncertainty.\nHence, assigning a single value for the depleted mantle is tenuous at best. In the first paper to present MORB data (Nielsen\net al Sci Adv 2018) it was argued that mixing between a sedimentary component (not necessarily a sediment melt, contrary\nto what the authors cite in their manuscript) and the uncontaminated depleted mantle could explain essentially all the\nobserved Ba isotope variation. Two subsequent studies that presented MORB data from geographically relatively restricted\nregions (Nan et al 2022, Wu et al 2023) did not find as strong a mixing relationship between the depleted and enriched\ncomponents of the upper mantle, leading them to suggest that the upper mantle is simply just reflected by a single value (the\none adopted here). However, as shown in Shu et al 2025 (Figure S6) all MORB data do in fact broadly fit the originally\nproposed mixing relationship (especially considering the fact that sediments do not have a uniform Ba isotope value) and\nthere is no evidence against the proposition that the upper mantle contains both a depleted endmember (with δ138/134Ba ~\n+0.1 to +0.15) and an enriched endmember derived from sediments (with δ138/134Ba ~ -0.05 to +0.1 as observed in modern\nmarine sediments). In other words, it is almost certainly incorrect to assign such a narrow range (0.03 to 0.05) for the upper\nmantle, when MORBs cover a much greater range. Whether the authors then prefer to simply expand their range for the\nmantle in their plots or acknowledge the only presently plausible interpretation of that range (ie the two endmembers\noutlined above) is probably less important. But it is not possible that the depleted mantle (ie uncontaminated by recycled\nmaterial) is 0.03 to 0.05 this needs to be fixed in the manuscript.\n3) As I understand the arguments, the arc front is dominated by AOC fluids that are fluxed into the mantle wedge causing\nmelting with little influence of sediments. The arguments presented in favor of a melange in the Izu arc make sense to me.\nHowever, the paper fails to explain how this melange is only present in the back-arc and not in the arc front lavas. I\nstruggle to make that work when I look at figure 6a where a positive correlation between Nd and Sr isotopes is shown. I agree that the\nhigh Ba/Th samples point towards a Sr-Nd isotope composition similar to what might be expected for AOC. However, the\nother end of that trend does not point towards a mantle wedge composition. Rather it points towards a melange composition\nthat is dominated by sediment. To me this implies that the melange formed at the top of the slab is present at all stages of\nsubduction and contributes at all stages. However, at the arc front fluids from AOC are released below the melange, which\ncauses flux melting of the melange with help from these fluids. This process would create exactly the observed mixing line\nbetween the melange and the AOC fluid. In this interpretation, the intersection of the bulk sediment-mantle mixing line and\nthe AOC fluid-melange mixing lines, indicates the amount of sediment present in the melange, which is higher than the rear\narc samples (that plot closer to the DMM along the DMM-sediment mixing line). To me this makes a lot of sense since it is\nexpected that the slab contains less sediment once material has been removed during arc front magma formation.\nWhile this change in interpretation may to some extent be a relatively minor adjustment, I think it is more intuitive because it\navoids concluding that the arc front samples have very little sediment in them. No sediment at the arc front seems physically\nimplausible if the back arc magmas do have a lot of sediment.\nI also have a few more detailed line-by-line comments:\nLine 29-31: Options 1 and 2 are essentially two different flavors of the same thing. You can similarly make several sub-\nmodels from option 3\nLine 52-53: this is a very loaded statement based on the preconceived idea that the lithospheric mantle is serpentinized.\nOften such a process is proposed to occur via fluid infiltration into bending faults, but Korenaga (EPSL, 2017) has shown\nthat such a process is highly unlikely because any fluid reaching the lithospheric mantle would quickly close up fluid\npathways because serpentinization is a volume increasing reaction. I would remove this statement, since the manuscript\ndoes not rely on lithospheric serpentinization.\nLine 63: Incorrect, the mantle range extends from -0.05 to +0.15\nLine 91: Incorrect, the mantle range extends from -0.05 to +0.15\nLine 99-101: previous work (Shu et al 2025) has used Th/Rb vs Th/Cs plots to identify weathering because alkalis (and\npossibly Ba) can get leached out of volcanic rocks.\nLine 116-118: neither of these papers concluded that sediment melts exist. So it is not a correct context to cite here\nLine 121-129: a useful way to test these ideas would be to actually measure the Ba isotopes in these sediments. That would\nstrengthen the argument a lot.\nLine 149: As detailed above, that is incorrect\nLine 166-168: As outlined above this is with imaginary mixing lines using a sediment component that is unconstrained and\nmost likely wrong\nLine 169-176: Same problem as for the Izu arc, the authors here invent the sediment Ba isotope composition (that is almost\ncertainly incorrect) to suit their interpretation."}, "author_response": "Response: Thank you for your comments. We have carefully considered your comments and suggestions and revised the manuscript accordingly. We believe that the manuscript has been substantially improved.\n\nResponse: Good question. We agree with the reviewer that a mélange of similar composition along the subduction channel cannot be excluded. In fact, what we proposed is exactly the mélange formed at the slab-mantle interface.This point has been explicitly stated to avoid any ambiguities. However, the slab-mantle interface beneath the Izu arc is usually too cold (<800 ˚C; Syracuse et al., 2010) for the mélange at the subduction channel to directly melt to form the arc lavas (Till et al., 2012). Thus, if such a mélange is formed along the subduction channel, it should be transported to the hot mantle wedge via some processes like corner flow or diapirism. That is why we prefer the mélange diapir model. Indeed, the density considerations and numerical modeling results support our proposed model. Following the suggestion, we have incorporated other geophysical studies that support this model and added such results to the text (e.g., Gerya et al., 2006; Lin et al., 2021; Sang et al., 2023).In addition, during revision, we have also incorporated the discussion on the possibility of mélange melting formed along the subduction channel, although we still prefer the mélange diapir model. In this way, we present a more balanced and open discussion on the formation processes. (Line 223-247)\n\nResponse: Thank you for your appreciation.\n\nResponse: The precision of Ba isotope measurements is high, which can be routinely obtained in our lab.As you can see below (Fig. A1), the latest precision [two-times standard deviation (2SD)] of δ138/134Ba for the long-term measurement of 9 years in our laboratory is only ±0.04‰. This represents the maximum uncertainty of the measurements. The internal precision, i.e., 2SD/√n, can be significantly lower than ±0.01‰ (2SE). Nevertheless, to be conservative, we only report the 2SD of the standards for the uncertainty of the data. Notably, the δ138/134Ba variation of the samples covers a range of ~0.2‰, but it represents the largest range for the currently reported data of basalts, which is significant considering the long-term externaluncertainty,especially sincewe reported themaximum uncertainty of thedata.\n\nResponse: Rock sample powders were decomposed using a 1:3 mixture of concentrated HNO and HF for one to two days. Subsequently, they were dried down and fluxed with aqua regia overnight to remove fluorides. Samples were then dried down and refluxed with 2 mL3 mol·L−1 HCl to convert nitrate compounds to chloride. Finally, the samples were dried down and dissolved in 1 mL of 3 mol·L−1 HCl for further purification. The above digestion and evaporation procedures were conducted on the hotplate with temperatures of 120 to 140°C. Following the suggestion, more details on the sample preparation are provided in the Methods section (Line 281-287).\n\nResponse: Yes,this isastandard procedure that yielded good data quality.\n\nResponse: Thank you.\n\nResponse: Thank you for your suggestion. Yes, we analyzed Ba isotopes under “dry” plasma using an Aridus III desolvator (CETAC Technologies). We have added more description and somecitations(Nan et al., 2015;Zeng et al., 2018) tothetext(Line 292-293).\n\nResponse: Following the suggestion, we have added the literature values for BCR-2 and RGM-1 in the “Supplementary Table S5”. For JB-2, there are Ba isotope measurements on it in the literature (Miyazaki et al., 2014; Lin et al., 2019), which have reported consistent δ138/134Ba values to ours. We have also included these published data in the supplementary TableS5.\n\nResponse: Following the suggestion, we have incorporated a quantitative model (Line 228-232). First, we have calculated a hypothetical melt source formed by hybridizing DMM with different proportions of bulk sediments. Then, we calculated the composition of a partial melt of the hybridized mantle source (mélange) that is subsequently fluxed by slab-derived fluids. The modeling details are listed in Supplementary Text S1 and Table S4 and are plotted in Supplementary Figure S9. As you can see from Figure A2 (Fig. S9 in Supplementary Materials), the melt shows similar trace element abundances and patterns to the Izu arc lavas. Using the same set of parameters, we also quantitatively modeled the Sr–Nd–Ba isotopic compositions of the lavas, as presented in Figure 4. Therefore, such a quantitative model can well reproduce both the trace element and Sr-Nd-Ba isotope data of Izu arc lavas. Thus, it indeed strengthens our arguments presented in themanuscript.\n\nResponse: Thank you for your suggestions. During the revision process, we have compared our results with previous work on B isotopes (Line 166-188), and we agree that such a comparison is very helpful. We note that the across-arc trends (decrease) of B/X ratios (e.g., B/Nb) and B isotope ratios in arc lavas have been observed in many previous studies, and the δ11B of these arc lavas also shows roughly linear correlations with B/Nb as well as Sr or Pb isotopes (Figure A3 in response). As the reviewer suggested, B isotope fractionation during progressive fluid loss from the slab may also occur simultaneously, but such a process should not affect Sr-Pb isotopes. Therefore, an across-arc decrease in the relative proportion of slab-derived contributions (typically fluids) compared to the mantle is still required to explain theacross-arc decrease in δ11B.\nUnfortunately, B isotope data were not reported for our samples (the samples used in Ishikawa and Nakamura (1994) are different from ours), so adirect comparison is not feasible. Nevertheless, the across-arc trends in Ba/X ratios (e.g., Ba/Th) and Ba isotopic composition of the Izu arc lavas exhibit patterns similar to the B isotope system. Barium isotopes also correlate with Sr and Nd isotopes (Fig. A4), supporting the influence of slab-derived fluids. Therefore, our interpretation of this scenario aligns with the model for B isotopes that the relative contribution of slab-derived fluids decreases with increasing slab depth, leading to a decrease inδ138/134Ba across the Izu arc.\nNevertheless, it is noted that Ba isotope system also provides us with new insights into arc magma formation. Previous studies have linked the decrease in δ11B to more mantle-like Sr and Pb isotope ratios, and suggested that the low δ11B end-member points to a mantle source with B isotopic compositions consistent with a DMM signature (Figure A4 in response). However, such a DMM component cannot explain the Ba isotopic composition of the mantle end-member in the Izu Arc, which has a much lighter Ba isotopic composition (< -0.10‰) than DMM (0.03–0.14‰) and most EMM (0.00–0.05‰) (Figure A4 in response). The light Ba isotopic signature requires the addition of a sedimentary component to the DMM-type mantle prior to the release of slab fluids. Therefore, we believe the best explanation is that mélanges containing sediment ascend as diapirs at forearc depths, where they subsequently underwent fluxed melting upon addition of AOC-derived fluids. Although the melting of the subduction-zone mélange at the slab-mantle interface represents another possibility, the temperature of the slab-mantle interface at forearc depths in the Izu arc is generally considered too low to trigger melting of mélange (or sediment) (Syracuse et al., 2010;Tillet al., 2012).\nDuring revision, following the suggestion, we have incorporated the comparative discussion of B isotope systematics by citing the previous studies mentioned by the reviewer, as well as the Ba isotope systematics in arc lavas. The above considerations of cross-arc Ba isotope composition variations have been added to the text. By doing so, it indeed helps to deepen our understanding of fluid-mobile elements in subduction zones as well as the formation mechanism of arc magmas.\n\nResponse: As replied above, we agree with the reviewer that a mélange of similar composition along the subduction channel cannot be completely excluded. In fact, what we proposed is exactly the mélange formed at the slab-mantle interface. This point has been explicitly stated to avoid any ambiguities. However, the slab-mantle interface beneath the Izu arc is usually too cold (<800 ˚C; Syracuse et al., 2010) for the mélange at the subduction channel to directly melt to form the arc lavas (Till et al., 2012). Thus, if such a mélange is formed along the subduction channel, it should be transported to the hot mantle wedge via some processes like corner flow or even diapirism. That is why we prefer the mélange diapir model. Indeed, the density considerations and numerical modeling results support our proposed model. Following the suggestion, we have incorporated other geophysical studies and added such results to the text (e.g., Gerya et al., 2006; Lin et al., 2021; Sang et al., 2023). In addition, during revision, we have also incorporated the discussion on the possibility of mélange melting formed along the subduction channel, although we still prefer the mélange diapir model. In this way, we present a more balanced and open discussion on the formation processes. (Line 223-247)\n\nResponse: Yes, during revision, following your suggestion, we have incorporated the references of B isotope studies on arc lavas, and discussed them compared to the Ba isotope data. (Line 166-188)\n\nResponse: We agree with your opinion and have made a thorough revision according to your comments.\n\nResponse: Thank you for your suggestion.Wehave revised the text to make this point clearer. Arecent study suggested that Ba/Th variation in global arc magmas was ascribed to sediment melts formed at different temperatures (Li et al., 2022). Specifically, high Ba/Th in arc magmas are attributed to sediment melts generated at relatively lower temperatures (750–800 °C), rather than AOC-derived aqueous fluid as traditionally believed. The model is based on high-pressure/high-temperature experimental results (Hermann and Rubatto, 2009), which showed that the Ba/Th of sediment melt decreases with higher temperatures (750–1050 °C).As such, sediment melt with varying temperatures may explain the across-arc variation of Ba/Th oftheIzu arc lavas. However,fortheexpected temperature range (<100 °C) from the frontal arc to the rear-arc regions of the Izu arc, the corresponding decrease in Ba/Th would be minor. This contrasts sharply with the wide range of Ba/Th (100–1000) of Izu arc lavas that is comparable to the global range for arc magmas. Moreover, although the increase in melting temperatures of sediment may reduce Ba/Th in sediment melts, partial melting under high temperatures (>700 °C) is anticipated to have a limited effect on the fractionation of Ba isotopes insediment melts (Xiao et al., 2013).Therefore, we didnot emphasizethat low Ba/Th and low δ138/134Ba in our samples are not linked through high-temperature sediment melts. Instead, we argue that the large variation of δ138/134Ba and Ba/Th in Izu arc lavas cannot simply be ascribed to differences in sediment melting temperatures, but are closely linked to thefluid contribution.(Line 123-141)\nMoreover, as Reviewer #2 suggested, the reason that sediments have Ba isotope compositions similar to or lighter than the pre-subduction protoliths is largely a result of slab dehydration, rather than isotopic fractionation during the sediment melting. Therefore, we have removed therelated descriptions.\n\nResponse: Yes, the relevant references have been added here (Pearce & Peate, 1995; Elliot et al.,1997;Hawkesworth et al., 1997) (Line 164).\n\nResponse: Here, we intended to discuss the isotope fractionation between AOC fluid and residue. The light Ba isotopes tend to be preserved in the residue. Considering the relevance, wehave restructured this sentence,and theoriginal onehas been deleted inthetext.\n\nResponse: Yes, here we compiled the global data for comparison. The term \"backarc lavas\" indeed describes the lavas in association with a spreading center, which are used differently from the \"rear arc\" as for the Izu arc. So, this is consistent. To avoid any confusion, we have clarified themeaning of“backarc”in therevised text. (Line 213-218)\n\nResponse: Here, we proposed that the AOC fluid metasomatized the mélange (bulk sediment-mantleperidotitemixture).Thesentence has been revised. (Line 254)\n\nResponse: Rocksamples were decomposed using a1:3mixture ofconcentrated HNO and HF forone to two days. Subsequently,they were dried downand fluxed with aqua regia overnight toremove fluorides.Wehave added suchdetails tothe Methods part. (Line 281-287)\n\nResponse: Yes,we agree.The horizontalaxes have been revised to bethesame.\n\nResponse: Thank you for your suggestion. We have unified the scale of the Mariana and Tonga-Kermadec plots (see Fig. A5 below). However, considering that combining all six panels into a figure would make the figure cluttered and less legible, we have kept the Izu arc data and modeling (Figure 4) separate from the other arc systems (Figure 5) to maintain optimalclarity.\n\nResponse: Good point. Following the suggestions, we have moved this figure into the supplementary file, as it may serve as supplementary evidence. What we emphasize here is the deviation of arc magmas from the array defined by backarc lavas. Backarc lavas are generally distributed along the DMM-bulk sediment mixing line. In contrast, arc lavas mostly plotonthe right sideofthis mixingline, indicating that the extraaddition ofAOC fluid causes the deviation of Sr–Nd isotopes. The curvature of the mantle‐sediment mixing line is affected by the compositions, which may cause uncertainties. However, the key observation is that the deviation of Sr–Nd isotopes of arc lavas is defined relative to the “backarc array”, rather than the mantle–sediment mixing line, although the two trends are usually consistent. Moreover, as arc lavas deviate farther from the backarc array, their Ba/Th increase (Figs. S8). This further indicates that anAOC fluid is added separately toa sediment-hybridized mantlesource.\n\nResponse: We adopted the average value of AOC in the Northwest Pacific (IODP 442B, Shikoku Basin, the Philippine Sea) near the Izu arc (average ca. +0.15‰, not 0.015‰ as mentioned by the reviewer; Nielsen et al., 2018). It is worth noting that this value is also consistent with the estimated δ138/134Ba of AOC derived from the linear correlation between δ138/134Ba and Sr-Nd isotopes.The δ138/134Ba value oftheAOC endmember for the Mariana arc is also based on the composition of this Northwest PacificAOC (IODP442B). Wehave added an explanation inthe SupplementaryTextS1.\n\nResponse:Yes,we have added the longitudedata inTableS1.\n\nResponse:Thank you for your appreciation and constructivecomments.\n\nResponse: We have carefully considered your comments and revised accordingly. We believe thatthe manuscript was substantially improved.\n\nResponse: Great suggestions. Yes, we have reconsidered the Ba isotope compositions of the sediments. We agree with the reviewer that Ba isotopes in sediments correlate with the nutrient availability of the water column above the sediment. Following the suggestion, we have measured the Ba isotope composition of near-trench sediments in the Izu arc system (IODP Site 1149), and found that they have high δ138/134Ba values of -0.02 to +0.08‰ (except a radiolarite sample with remarkably low δ138/134Ba of -0.12‰), which are essentially similar to those reported in otherWestern Pacific sediments (DSDP442, 443, 444) (-0.02 to +0.15‰; Nielsen et al., 2020). A figure (Fig. S2) has been added to the Supplementary Materials. See also Fig.A6 in theresponse below.\nWeagree with thereviewer that the direct adoption of the δ138/134Ba value of sediments in the Izu arc system as the endmember cannot explain the patterns of arc lavas. Considering previous studies showed that significant fractionation may occur for Ba isotopes during sediment subduction, following the suggestion, we have added the discussion on the Ba isotope fractionation during sediment dehydration. This process can result in metasediments with low δ138/134Ba values of ca. -0.1‰, as used for the mixing with the DMM.As suggested, this is the same Ba isotope pattern that has been observed for all other arcs published to date: Ryukyu, Aleutians, South Sandwich, and Tonga. Moreover, this low δ138/134Ba value is consistent with previous suggestions of a metasediment component with low δ138/134Ba values down to ca. -0.10 to -0.20‰, based on the Ba isotope compositions of OIBs (Bai et al., 2022; Jiang et al., 2024; Cheng et al., 2025). The process is also supported by low δ138/134Ba values (–0.16 to –0.05‰) of phengites in eclogites from ultrahigh-pressure metamorphic belts (Gu et al., 2021; Xu et al., 2023). Therefore, it is highly likely that there is a significant fractionation during sediment subduction.Wehave added theabove discussions tothe text. (Line 192-203)\n\nResponse: During revision, we have considered the controversial issue of Ba isotope compositions of upper mantle to evaluate the robustness of our proposed model. We admit that there are controversial views of DMM endmember,i.e., 0.10to 0.14 (Nielsen et al.,2018) versus 0.03 to 0.05 (Wu et al., 2025). In order to accommodate all views of the Ba isotope composition of DMM, we have revised the DMM range of δ138/134Ba values as from +0.03 to +0.14‰ rather than a single point as we used to represent Ba isotope compositions of DMM. The chosen DMM range essentially covers almost all the observed Ba isotope variation of MORB reported in Nielsen et al. (2018) and is close to that (+0.03 to +0.12‰) used in Shu et al.(2025).\nThen, we used this range for the calculations in Figs. 4 and 5. As shown in the revised Fig. 4 (Fig. A7 in the response below), the low-δ138/134Ba endmember continues to display much lighter Ba isotope compositions than the DMM, because DMM range extends to higher values. Moreover, the isotopic mixing patterns between DMM and sediment do not change significantly, due to the significant difference in Ba content between them. Thus, the consideration of heterogeneous Ba isotope compositions of upper mantle still supports our proposed model, which strengthens ourconclusions.\n\nResponse: Good suggestions. In fact, in our proposed model, the mantle source for both the frontal arc and rear arc contains mélange components, which is shown in Figs. 4-5. In Figs. 4-5, the mixing of sediment with DMM to form a mélange and then the infiltration of AOC fluids into such mélange can explain the Ba, Sr, and Nd isotope compositions (see Fig.A7 in this response).As pointed out by the reviewer in Fig. 6, there is a positivecorrelation between Nd and Sr isotopes. The high Ba/Th samples point towards a Sr-Nd isotope composition similar to what is expected for AOC. Notably, the other end of that trend does not point towards a mantle wedge composition, but rather points towards a mélange composition that is composed of mixing between sediment and mantle wedge peridotite. In fact, the process explainedbythe revieweris exactly what weintended to interpret in thetext.\nIn the proposed model, a mélange component is involved in both frontal arc and rear-arc lavas. The frontal arc lavas contain a higher proportion of AOC fluids compared to the rear-arc lavas, which is reasonable. For the arc and back-arc (note this is different from rear-arc lavas) shown in Fig. 6a, indeed, as stated by the reviewer, the intersection of the bulk sediment-mantle mixing line and theAOC fluid-mélange mixing lines indicates the amount of sediment present in the mélange, which is higher for the arc samples (that plot far away from the DMM along the DMM-sediment mixing line) compared to the back-arc lavas. It is expected that the slab contains less sediment once material has been removed during arc magma formation (Note the usage of frontal arc, rear-arc, and back-arc; both frontal-arc and rear-arc are used for the arc lavas, but the back-arc is used for the lavas with a spreading center system).\nThus, the argument by the reviewer is essentially the same as the one we meant in the text. Nevertheless, we have clarified the argument as well as figures (Figs. 4 and 5) to avoid any ambiguities.\n\nResponse: Yes,we agree. But forclarity,we would liketo maintain ouroriginal statement.\n\nResponse: This is a fair point. Previous studies have shown that serpentinization can also occur at the seafloor or the shallow mantle wedge (e.g., Deschamps et al., 2013). Abundant meta-serpentinites representing the hydrated oceanic lithospheric mantle occur in the western and central Alps (e.g., Scambelluri et al., 2004, 2014). Whether it can occur at the bending faults remains a matter of debate, although the study (Korenaga, 2017) reviewer mentioned didnot favor such a process.Therefore,we havemaintained theoriginal statement.\n\nResponse: Wehave revised therange to 0.03to 0.14‰,as replied above.\n\nResponse: Following your suggestion, wehaverevised therange to 0.03to0.14‰.\n\nResponse: We have provided a plot of Th/Rb vs. Th/Cs following the work of Shu et al. (2025). Compared with data from other subaerial arc lavas that do not record significant alteration, we do not observe higher ratios for any samples in our study (Supplementary Fig. S5; see Fig.A8 below in response). Combined with the fact that all samples investigated here are relatively young and fresh arc lavas, the weathering effect on them is insignificant. (Line 116-118)\n\nResponse: Thank you for the suggestions. Following these references and the suggestion in your initial comment, we think that the reason that subducted metasediments have lighter Ba isotope compositions compared to the pre-subduction protoliths is largely due to the slab dehydration process, rather than isotopicfractionation during thesediment melting.Therefore, thedescription and citationhere are inappropriate, andwe havedeleted them.\n\nResponse: Following the suggestion, we have measured the sediments near the Izu arc system. As discussed above, wehave integrated these resultsinto thediscussion during revision.\n\nResponse: This sentence has been revised.\n\nResponse: As noted in our responses above, following your suggestion, we have considered the realistic Ba isotope composition in the Izu arc system by measuring the sediments there, and have included the discussion of the Ba isotope fractionation during sediment dehydration. By doing sucha revision, thearguments can bestrengthened.\n\nResponse: As noted in our responses above, following the suggestion by the reviewer, we have included the discussion of Ba isotope fractionation during the subduction of sediment and considered the potential Ba isotope composition of subducted metasediments for each arc system. As the Ba isotope compositions of offshore sediments subducting beneath these arcs are not reported, we estimated the Ba isotope compositions of these metasediment endmembers by fitting the data points of these arc lavas having enriched Nd isotope compositions.Webelieve that thisrevision can strengthen ourarguments."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This revision of the paper is a significant improvement, and the authors have addressed most of my concerns to my satisfaction. Indeed, I appreciate the acknowledgement that there is some ambiguity as to whether mélanges melt along the subduction channel vs. as diapirs within the mantle wedge.\nPrior to publication of this manuscript, I have some points that I suggest the authors consider:\n1. One of the reasons the authors prefer the mélange diapir model is the assertion that at sub-arc depth, the slab surface is too cold (~750 °C) for sediments to melt. Nichols et al. (1994—Nature) found a wet solidus hovering around 650 °C for pelagic sediments. The classic and widely cited sediment melting experiments of Hermann and Rubatto (2009--ChemGeol) and Skora and Blundy (2010--JoP) find sediments melting at temperatures as low as 750 °C. So although Syracuse et al. (2010) predict that the Izu slab will not intersect the wet solidus for sediment, it seems plausible (based on the range given by the studies cited above) that the Izu slab surface may be hot enough for some fluid-fluxed sediment melting to occur along the subduction channel. I would suggest the authors not lean too heavily on a firm statement that the slab-mantle interface is too cold for sediments to melt (though admittedly it would be at the cooler end of the temperature range for sediment melting).\n2. In the model preferred by the authors, the AOC-derived fluids that trigger melting must travel through a few tens of km of peridotite in the mantle wedge , prior to reaching the mélange diapirs. Only then does melting begin, at least as considered by this model. However, the thermal gradient in the mantle wedge is likely to be high enough for wet melting of peridotite (e.g. Grove et al., 2006--EPSL). This may not produce a melt that is all that different from what the authors envision, and as a widely accepted process probably does not warrant much discussion, though it is surprising that it is not mentioned at all in the authors’ description of their model.\nOther specific comments:\nIn the Izu data set, samples HJ36 and HJ37 are listed as containing unusually high TiO2 (21 and 91 wt%, respectively).\nOther data (both Izu samples and sediment samples) look fine. Nothing stands out as suspect or erroneous.\nL 111-114: These sentences should be rephrased. It is not the fact that the 1149 core has been assumed by other studies to represent Izu arc sediments that makes it a reasonable representation of Izu sediments; rather, it is the core’s proximity to the trench and its position on the Pacific plate. This is why the other studies you cite used it to represent Izu sediments.\nL 121: Minor issue - “All samples are fresh, with no clay minerals.” Should this be revised now that sediment samples are included in the study? E.g. “All volcanic samples are fresh…”", "reviewer_2": "This is the revised version of a manuscript that I have previously reviewed on Ba isotopes in the Izu Bonin arc. In my original review I pointed out several concerns, most notably regarding the assumed Ba isotope compositions for sediments. I also had some additional less important comments relating to the Ba isotope composition of the mantle and details of the final interpretation the authors preferred.\nI think the authors have done a really nice job of revising their manuscript. I am particularly pleased that they took the time to analyze sediments from outboard of the Izu arc, which as predicted yielded results contrary to what the authors initially assumed. However, I believe that the authors have done a very nice job of modifying their interpretations so they are consistent with these new data as well as previous studies of Ba isotopes in arcs.\nWhile I am less sure whether the proposed model is really generally applicable to many subduction zones, I do think the authors have presented a consistent interpretation for their own data set as well as a possible means by which this interpretation could be applied elsewhere. Hence, I have no further major concerns regarding the publication of this manuscript."}, "author_response": "Reply: Thank you for bringing this important point. We fully agree that there is a spectrum of experimental data and interpretations regarding the potential for sediment melting at the slab-mantle interface temperature conditions beneath the Izu arc, as highlighted by the key studies you cited (e.g., Hermann & Rubatto, 2009; Skora & Blundy, 2010).\nUpon careful consideration of your comment, we have revised the relevant section of our manuscript. We have revised the firm statement that the interface is \"too cold for sediments to melt\" and have adopted a more balanced discussion, as stated below\n\"Therefore, although the melting of mélange or sediment along the subduction channel may occur at forearc depths, the mélange diapir is the most feasible scenario to explain our Ba isotope data.\" (Line 237-238)\n\nReply: We have complemented the description concerning the possible melting of mélange before the arrival of AOC fluids as stated below: “Such mélanges might have experienced the melting to some extent in the hot mantle wedge prior to the addition of AOC-derived fluids (Grove et al., 2006; Nielsen and Marschall, 2017). However, this process may not produce a melt that is significantly different in composition from the melt we envisioned.” (Line 252-255)\n\nReply: Thanks for pointing out this issue. We have checked the data and corrected the mistake about the unusually high TiO of samples HJ36 and HJ37 (Supplementary Data 2).\n\nReply: We have revised the associated contents as “Ba isotope data of offshore sediments from IODP Site 1149 in the western Pacific Plate are presented in Table S2. The core is located at a proximal geographic position to the Izu-Bonin trench (Fig. 1), likely rendering it an ideal representation of the Ba isotope input of sediments to the Izu arc”. (Line 99-102)\n\nReply: We have revised “All samples…” to “All volcanic samples…” (Line 109)\n\nReply: Thank you very much for your great effort on reviewing our manuscript and lots of insightful comments."}]

Decision letter