Carbonated mantle peridotites represent a hidden sink for subducted CO_2

Domain: chemistry
Published: 2026-02-27
Editorial outcome: minor_revision

Abstract
Subduction of carbon rich sediments and crust at convergent plate boundaries exerts a crucial control on Earth’s mantle chemistry and surface habitability. Recent attention has focused on exposures of fully-carbonated mantle rocks as these may attest to an overlooked sink for subducted carbon not sampled by arc volcanism. However, even in the best-studied example, the Semail Ophiolite, Oman, the setting for carbonation remains highly contentious, with conflicting inferences from geochemistry and geochronology. We approach this problem by combining microanalysis of halogens and detailed petrography to fingerprint the origins of carbonating fluids. Fluids were derived from both sedimentary pore fluid expulsion and deep slab decarbonation reactions in a subduction zone setting. Through mass balance modelling we show that CO fluxes into the forearc from deep decarbonation (1.7–3.4 x 1013 gyr-1 C) could represent up to 90% of the global flux entering subduction zones, indicating that carbonated mantle peridotites likely represent a major sink for subducted CO which may have varied through geological time.

Introduction
Subduction of altered oceanic lithosphere rich in volatile elements including carbon, water and the halogens, exerts a major control on the long-term evolution of the atmosphere and oceans, and ultimately the habitability of Earth and other planets1–3. Establishing the magnitude of volatile fluxes into the deep Earth or of their return to the atmosphere requires evaluating mass balance between inputs to the subduction system – via altered oceanic lithosphere – and outputs – via slab dehydration and arc volcanism4–6. Recently attention has turned to the potential for hidden outfluxes not sampled by volcanism and unaccounted for in current estimates6–10. Slab-parallel return of subducted CO into the forearc mantle represents one such potential hidden flux.

Mantle peridotites react readily with CO to form listvenites – fully-carbonated rocks in which silicate phases such as olivine and pyroxene are replaced by magnesite (MgCO ), dolomite (CaMg[CO ] ) and quartz11. The net result of listvenite formation is to bind aqueously-dissolved CO into solid, inert minerals which are stable over geological timescales. Their occurrence as natural products of mineral carbonation provide an invaluable opportunity to evaluate the extent to which CO -metasomatism of the shallow mantle impacts global carbon cycling at subduction zones.

Carbonation reactions at convergent margins have been indirectly implicated by the carbon isotopic composition of forearc spring gases8. However, although listvenites are documented in numerous ophiolites worldwide12–14 there are few whose formation is unambiguously linked to contemporaneous subduction11. In the Semail Ophiolite, Oman, arguably the best studied example of a supra-subduction zone ophiolite15, deposits of listvenite overlie the basal thrust of the ophiolite and are estimated to have naturally sequestered 1 GT of CO 4.

This study focuses on listvenites from Hole BT1b of the Oman Drilling Project16 drilled in Wadi Mansah (23.364374°N, 58.182693°E) in the Semail Ophiolite. Listvenites from BT1b and nearby exposures show elevated 87Sr/86Sr ratios (0.7092–0.7145) relative to uncarbonated peridotites (~0.7028), Cretaceous to modern seawater or groundwater (~0.707-0.709), but which overlap the isotopic composition of underlying allochthonous Hawasina metasediments (0.7082–0.7241)17,18. Clumped isotopes indicate carbonate formation temperatures between 50 and 250 °C with most <150 °C, corresponding to inferred depths of ~1-10 km17,19. Trace element geochemistry shows pronounced enrichment of fluid mobile elements in listvenites in a pattern mirroring that of forearc serpentinites20.

However, there appears to be a mismatch between interpretations based on trace element and isotope geochemistry, which both suggest a forearc subduction zone setting for listvenite formation17,18,20,21, and those from recent in-situ U-Pb geochronology on samples from localities near (<10 km) Hole BT1b22. The latter yielded dates from two dolomite veins and several imprecise dates from listvenites, all <60 Myr, postdating active subduction by at least 20 Myr. The dichotomy between these viewpoints is significant: in the former case, listvenite formation is a volumetrically appreciable and potentially ubiquitous element of carbon cycling at subduction zones, in the latter it is a product of some late-stage tectono-magmatic event in Oman’s geological history (e.g. elevated heatflow related to minor Paleogene magmatism23) and of regional rather than global significance.

In order to resolve this question, we analysed halogen geochemistry in a suite of progressively-carbonated serpentinites and listvenites from the Semail Ophiolite. The heavy halogens (Cl, Br, I) are hydrophilic and make ideal tracers for interrogating fluid processes in ophiolites24–26. Combining, secondary ionisation mass-spectrometry and neutron-irradiation noble gas mass-spectrometry to analyse all four halogens in situ and in bulk, we are able to fingerprint fluid provenance as well as discriminate between the effects of fluid-mineral partitioning and fluid evolution. We identify two petrographically and chemically distinct carbonation events and suggest the latter of these likely correlates with young ages from geochronology while the main stage of carbonation has a subduction origin. We show that the halogen compositions of these fluids have been modified by Cl release associated with carbonation. Mass balance modelling of that fluid evolution indicates significant extra input of CO transported by lateral flow from zones of decarbonation deeper in the subduction zone is required, suggesting carbonation at the leading edge of the mantle wedge may be a significant and overlooked part of the global carbon and other volatile element cycles (e.g., H O, Cl).

Results
Geological context and sample selection

The ~96 Ma Semail Ophiolite, Oman, represents the largest known and arguably best-studied ophiolite in the world (Fig. 1a). The basal thrust of the ophiolite tectonically juxtaposes mantle peridotites on top of allochthonous metasediments and metabasalts of the Hawasina and Haybi complexes which are themselves underlain by autochthonous shelf carbonates29,30. A greenschist to amphibolite grade metamorphic sole formed by subduction and exhumation of allochthonous lithologies is discontinuously exposed along the basal thrust30,31. The mantle section has recently been sampled by diamond core drilling, as part of the Oman Drilling Project16, yielding an exceptionally well-characterised sample set.

The core from Hole BT1B consists of 300 m of listvenites, serpentinites, ophicarbonates and metasedimentary/metabasaltic rocks with 100% recovery (Fig. 1b). It is divided into an upper and lower listvenite, the latter characterised by several broad zones containing the chromium-rich mica fuchsite and by elevated concentrations of many fluid-mobile trace elements20. Within the upper listvenite a 20 m thick lens of serpentinite and ophicarbonate (partially-carbonated serpentinite) is preserved (80-100 m depth) and affords almost complete preservation across a ~5 m thick reaction front between the serpentinite protolith and fully-carbonated listvenite (Fig. 1e). This interval shows a sharp compositional gradient over which the primary mineralogical variation is due to the degree of interaction with the carbonating fluid.

This study comprises a detailed microanalytical investigation of halogen geochemistry in six samples spaced across this reaction zone, supplemented by whole rock analyses of a broader sample set including representative examples of ophicarbonate, listvenites (±fuchsite) and metamorphic lithologies (Fig. 1).

Petrography of reaction zone samples

The reaction zone shows a continuous macroscopic progression in colour, from very dark through increasingly-pale green ophicarbonate (samples 43z4, 44z1, 44z2, 44z3) to pale green and then red listvenite (44z4, 45z1) (Fig. 1e).

Petrographic observations and mineralogical analysis by powder X-ray diffraction (XRD) reveal that ophicarbonates consist predominantly of mesh-textured lizardite with variable replacement by carbonate and talc (Fig. 1c, d, Fig. 2). Serpentine mineral stoichiometry from electron probe microanalysis (EPMA) indicates variable mixtures of lizardite with increasing proportions of talc with increasing carbonation; samples 43z4 and 44z1 plot almost entirely between lizardite and brucite compositions while samples 44z2 and 44z3 extend towards talc to much higher Si and lower Fe + Mg (Fig. 3a). The mineralogy of the samples thus reflects carbonation via the intermediate reactions:

2Mg Si O (OH) + 3CO → 3MgCO + Mg Si O (OH) + 3H O (1)
[lizardite] 2 [magnesite] [talc] 2

Mg Si O (OH) + 3CO → 3MgCO + 4SiO + 3H O (2)
[talc] 2 [magnesite] [quartz] 2

There is a systematic relationship between stoichiometry and Cl content in the ophicarbonates with the highest Cl concentrations closest to stoichiometric lizardite and lower Cl contents with increasing talc or brucite content, respectively (Fig. 3b). This suggests that Cl is principally hosted in lizardite and is less compatible in other minerals32.

Carbonate is common in all four ophicarbonate samples and appears as scattered small (<10 µm) round growths of magnesite (Fig. 2a-c); larger patches (from 50 µm to several mm across) showing a rough globular or botryoidal habit; and as veins (Fig. 2d-i).

In the listvenites, magnesite is the principal constituent, with subordinate quartz, dolomite and, in samples from the lower listvenite (56z4, 60z3, 64z2), fuchsite. The listvenite samples from the reaction zone (44z4 and 45z1) both consist of a network of closely spaced subparallel veins. The EPMA mapping shows these typically have a symmetrical composite structure with a thin Fe-poor medial line of magnesite surrounded by an Fe-rich layer of either ferro-magnesite or Fe-oxyhydroxides, which typically grade into or are overgrown by Fe-poor magnesite at the edges of the veins (Fig. 4). The listvenite groundmass typically consists of small granular crystals of magnesite together with larger, often lozenge-shaped, dolomite crystals. Textural evidence from this and other studies indicates that dolomite is a late forming phase19,33–35; it primarily occurs either overgrowing/crosscutting zoned magnesite-ferromagnesite veins or as an interstitial phase (Fig. 4). Interstices between the carbonates are either open voids or filled by a Cl-rich, poorly crystalline silicate phase whose composition departs from stoichiometric serpentine ± talc (Fig. 3b). Chlorine abundance is high in this phase (<2 wt.%) but its overall abundance in the samples is low (<4 vol.%), representing concentration of Cl into a minor residual phase as it is progressively excluded during carbonation.

Chemical changes accompanying carbonation

Bulk Cl, Br and I, show the highest concentrations in ophicarbonates, reflecting their relative compatibility in serpentine minerals (Cl = 264 ± 160 [1σ] ppm, Br = 428 ± 112 ppb, I = 32 ± 4 ppb, n=5; Supplementary Data 1). Listvenites show much lower abundances but higher Br/Cl and I/Cl (Cl = 22 ± 12 ppm, Br = 42 ± 13 ppb, I = 7 ± 5 ppb, n=7). Whole rock F concentrations show the opposite trend and are lowest in the reaction zone ophicarbonates (F = 28 ± 16 ppm, n=4) and slightly higher in listvenites (F = 35 ± 12 ppm, n=7). Metamorphic sole samples are highly varied, with relatively high F (F = 672 ± 583 ppm, n=4), low Cl and Br, but with high I/Cl ratios (Cl = 14 ± 4 ppm, Br = 21 ± 4 ppb, I = 51 ± 73 ppb, n=3).

In situ data from secondary ionisation mass spectrometry (SIMS) similarly show moderate to high halogen concentrations in serpentine (F = 172 ± 116 ppm, Cl = 614 ± 559 ppm, n=95; Supplementary Data 4). Carbonates from both ophicarbonates and listvenites show generally slightly lower F and much lower Cl concentrations than serpentine (F = 117 ± 174 ppm, Cl = 121 ± 133 ppm; n=108). However, dolomite and some magnesite from the listvenites have markedly elevated F contents (dolomite F =240 ± 172 ppm, Cl = 90.7 ± 249 ppm; n=20).

The bulk halogen contents of the ophicarbonates show strong negative correlations with bulk CO showing expulsion of volatiles from the samples with progressive carbonation (Fig. 5). For Cl there is rapid initial loss followed by a more gradual decrease. We interpret this as a two-stage process, but the data can also be fit by an exponential curve (Cl = a⋅e-k CO2) which yields a similar or higher rate of early Cl loss (Supplementary Text 1, Supplementary Data 8). By contrast, bromine, iodine and water show a relatively linear loss with CO content, without any inflection in the slope. Chlorine is initially lost at much higher rates than Br or I, likely due to incompatibility of Cl in talc32 (Fig. 3). This results in expulsion of a fluid with very low Br/Cl and I/Cl (2 x 10-4 and 1.6 x 10-5, respectively; see Supplementary Text 1), during the early part of the carbonation reaction (Fig. 5). Addition of this expelled fluid thus changes the composition of the original carbonating fluid during the course of reaction.

Fingerprinting the origin of carbonating fluids

Serpentine and carbonate from ophicarbonate samples are compositionally similar in F–Cl space (F/Cl = 0.1-1; Fig. 6). Carbonates from listvenites are also mostly colinear with serpentine compositions, although a subset show higher F concentrations. Serpentine halogen compositions show a progressive shift across the reaction zone mirroring visual, mineralogical and chemical indications of increasing equilibration with CO -bearing fluids (Figs. 1-3, 4).

Bromine/Cl and I/Cl in individual spot analyses of serpentine in ophicarbonates form a correlated array (R2 = 0.52) extending from very low Br/Cl and I/Cl (Br/Cl ≥ 2.50 x 10-4; I/Cl ≥ 2.37 x 10-5) over two orders of magnitude to higher ratios (Br/Cl ≤ 7.71 x 10-3; I/Cl ≤ 1.35 x 10-3; Fig. 7). The high Br/Cl and I/Cl end of this trend intersects the trend of sedimentary pore fluids36. The bulk compositions of both ophicarbonate and listvenite samples are also colinear with this array, with the mean listvenite composition plotting at the high Br/Cl and I/Cl end. These combined observations indicate that both carbonate and serpentine compositions in the ophicarbonates are controlled by a common process and compositional endmembers. This suggests that the composition of serpentine across the reaction zone has been variably influenced by interaction with carbonating fluids and can therefore provide insight into their origin.

Serpentine Br/Cl and I/Cl are lowest in the samples furthest from the listvenite front (43z4 and 44z1) and increase towards the mean bulk composition of listvenites in the samples nearer the listvenite front (44z2, 44z3; Fig. 7). Both bulk Br/Cl and individual spots measured by SIMS show statistically significant correlations with bulk CO (R2 = 0.81, 0.16, respectively), the latter affected by intrasample scatter but nonetheless statistically significant (p = 1.4 x 10-7, n = 165). The low Br/Cl and I/Cl ratios observed in the ophicarbonates are below the seawater Br/Cl ratio (3.47 x10-3) but above seawater I/Cl (3.04 x10-6) and show distinctly lower Br/Cl than seafloor or forearc serpentinites24,37 (Fig. 7b). We interpret the correlation between the two ratios as a binary mixing trend between a low I/Cl endmember, representing the uncarbonated serpentinite (or a fluid it has equilibrated with) and a high I/Cl endmember representing the composition of the carbonating fluid.

The main control on Br/Cl and I/Cl in serpentine is addition or loss of Cl; Br/I is nearly invariant across ophicarbonate samples to within analytical uncertainty (Br/I = 13.29 ± 2.35, n = 5; bulk Br/I = 9.64 ± 5.11) and this ratio therefore likely reflects the source of carbonating fluid. The in-situ average serpentine Br/I is consistent with sedimentary pore fluids which cover a wide range of Br/I with a mean of 3.0 ± 9.836. In comparison, the underlying metamorphic sole shows lower Br/I of 1.77 ± 1.42 (n = 3) while altered oceanic crust shows a highly variable Br/I with a mean of 74 ± 144 (n = 108)26,32,38,39, with most samples plotting to lower I/Cl than the ophicarbonate trend. The mean Br/I of the ophicarbonates is towards the higher end of the range of sedimentary pore fluids, suggesting fluids were derived from sediments with low I and therefore relatively low organic contents40. This is consistent with evidence from carbon isotopes in the listvenites which indicate an inorganic sedimentary source for the carbon17.

However, the unusual low Br/Cl and low I/Cl serpentine endmember requires further explanation since it is significantly distinct from seafloor37,41, forearc37 and ophiolitic serpentinites24, all of which have Br/Cl close to or higher than seawater (Fig. 7b).

Carbonation-inducted fractionation of halogens explains exotic serpentine compositions

It is possible that the unusual composition of serpentine in the least carbonated samples reflects an atypical serpentine chemistry characteristic of the Oman mantle section. However, serpentinites sampled far from the basal thrust are more typical of seafloor or forearc serpentinites, with Br/Cl close to seawater43 (Fig. 7b). This strongly implies that the particular composition of the reaction zone ophicarbonates relates to carbonation.

Carbonation of serpentinite results in an expulsion of Cl-rich fluid (Fig. 5). As a result, fluids involved in the reaction should evolve to lower CO contents, higher salinity and lower F/Cl, Br/Cl and I/Cl in a transient pulse preceding the carbonation front. Fluids fractionated in this manner therefore provide a potential source for the unusual halogen composition of the ophicarbonates, either by equilibration/recrystallisation of serpentine with the fractionated fluid or by direct serpentinisation. Notably, a similar trend of correlated Br/Cl and I/Cl, is shown by variably carbonated Isua serpentinites, with low Br/Cl compositions (>7 x 10-4) suggested to arise from fractionation of fluids during water-rock reaction42.

Mass balance would suggest the existence of a complementary high Br/Cl component. This may be represented by a handful of serpentine data and one bulk listvenite which have very high Br/Cl and/or I/Cl, suggesting such a component is volumetrically small and potentially poorly preserved. Such a component will only form at low fluid/rock ratios however (see below) and we therefore expect the average composition of the main body of listvenite to largely reflect that of the carbonating fluid (potentially with some contribution from the uncarbonated serpentinite protolith).

Modelling fluid evolution to constrain its carbon and chlorine contents

Serpentinites are thought to faithfully reflect the relative halogen abundance of the fluids from which they form37. We therefore take the average composition of serpentine as reflecting the composition of a fractionated fluid. Since addition of carbon to the rock causes the release of Cl which modifies the chemistry of the fluid, fractionation at a given fluid/rock ratio will be more significant at lower fluid Cl concentrations and at higher CO concentrations. Modelling the fluid fractionation required to produce the serpentinite signature therefore allows us to place compositional constraints on the carbon and Cl content of the fluid prior to carbonating reaction-driven fractionation. Since carbonate growing in reaction zone ophicarbonates is magnesite-dominated and serpentinite and most magnesite from all samples are colinear in F-Cl space (Fig. 6), the chemical evolution of the ophicarbonate serpentinites is taken to be related to the main stage of magnesite dominated carbonation (rather than a later phase of dolomite-dominated carbonation, see below).

Modelling the evolution of carbonating fluid compositions can reproduce the data trend and halogen composition of the least carbonated samples’ serpentine. However, we show that to do so requires the initial fluid to have high CO /Cl ratios.

If we define R as:

R = (∆[Cl]rock) / (∆[CO ]rock) (3)

And, equivalently, define R and R, the Br/Cl of fluids expelled from the serpentinite during carbonation can be calculated as:

(Br/Cl)out = (Br/Cl)R / R (4)

It can then be shown that the minimum CO /Cl ratio needed to explain a given final fluid Br/Cl ratio (Br/Cl) from its initial composition (Br/Cl) is given by:

[CO ]fluid = [(Br/Cl)f - (Br/Cl)i] / [(Br/Cl)i / (Br/Cl)out - 1] (5)

An equivalent expression can be derived by substituting I/Cl for Br/Cl (for full derivation see Supplementary Text 1). Since the least carbonated serpentinites have been affected by interaction with fractionated, Cl-rich fluid, this approach assumes that the compositional gradients across the reaction zone had reached a steady state (i.e. less carbonated samples evolve to resemble more carbonated samples; see Supplementary Text 1). If this is not the case it is possible that the Br/Cl calculated is an underestimate and the initial CO /Cl of the carbonating fluid required would be even higher.

The preceding calculation yields a minimum CO /Cl since the exact fluid/rock ratio is not known; instead the minimum possible fluid/rock ratio required to explain the degree of carbonation observed is assumed (~1 to 23; Supplementary Data 8). For higher fluid/rock ratios the degree of fractionation in the final fluid would be less at constant CO /Cl and vice versa a higher CO /Cl ratio would be needed to explain a given degree of fractionation.

It is reasonable, however, to suppose that fluid rock ratios are likely to have been relatively low within the lens of remnant, partially carbonated serpentinites and ophicarbonates whose edge is transected by the reaction zone18; if these rocks had experienced high fluid/rock ratios they would have been carbonated like the rest of the mantle section at Site BT1b. Low fluid/rock ratios aren’t necessarily representative of the entire succession. Rather, the reaction zone provides a fortuitous window into fluid fractionation at low fluid/rock ratios which allow us to constrain the fluid composition.

To calculate the CO /Cl ratio of the fluid we take the average serpentine composition measured from sample 44z1 as representative of the most fractionated final fluid composition (Br/Cl = 1.1 x 10-3, I/Cl = 1.0 x 10-4). For an initial fluid composition close to that of sedimentary pore fluid with Br/Cl of 3.0 x 10-3 and I/Cl of 3.1 x 10-4, the model predicts very similar minimum CO /Cl ratios of 394 and 407 to match the target Br/Cl and I/Cl, respectively during Part 1 of the reaction (Fig. 5; <12% CO ). Our calculation assumes a perfect closed batch reaction. However, Mg/Si ratios show evidence for mesoscale mobility of elements during carbonation20. Minor halogen redistribution may therefore explain the more extreme low Br/Cl and I/Cl ratios from individual spot measurements (Fig. 7).

These calculations indicate that carbonation was driven by fluids with markedly higher CO /Cl than sedimentary pore fluid (400 vs. 0.02; assuming seawater salinity, 19,000 ppm44 and Cretaceous CO contents, 360 ppm for the latter45) but with initially pore fluid-like halogen abundance ratios. This in turn implies an extra input of relatively carbon-rich and/or low salinity fluid. Mixing of pore fluid with a carbon-rich endmember (XCO of 0.01 to 0.2) can provide a fluid which fulfils the geochemical constraints (Supplementary Data 8).

The mixed carbonating fluid XCO is predicted to have been approximately 10-1 to 10-2, consistent with the upper end of estimates of the minimum XCO required to stabilise magnesite + quartz at 50 MPa and 100 °C, which range from ~10-4 to 10-2.5 [11,17,46]. The mineral assemblage of the listvenites does not provide a strong constraint on the XCO of the carbonating fluid provided it is above this minimum11. However, fluid-vapour immiscibility will occur at increasingly high CO contents, above XCO ~0.1 (at 275 °C and 0.1 GPa47). Our calculations meanwhile suggest XCO ~0.05 as an absolute minimum required to produce the fractionation seen. These two bounds therefore constrain the fluid XCO to 0.05-0.10, notably similar to the fluid CO content (20,000 ppm, equivalent to XCO ~ 0.03) calculated by Kelemen et al.21 from thermodynamic considerations.

The relatively low I contents of the carbonating fluid and lack of light C isotopic signatures rule out organic matter as the carbon-rich endmember17,18. Similarly, the maximum CO content of low salinity meteoric waters (CO /Cl < 6) is too low to explain the degree of fractionation seen48. Suitable CO -rich fluids could, however, derive from carbonate dissolution or decarbonation in the slab.

Expelled slab fluids evolve to greater XCO with depth in the subduction zone where shallow expulsion of pore fluid is followed by devolatilisation of water and then carbonate49,50. Carbon saturation in fluids in equilibrium with carbonate-bearing sediments at the approximate P-T conditions of listvenite formation has been estimated at 50-500 ppm4. However, at higher pressures the solubility of CO increases such that fluids may contain up to 3 wt.% CO (XCO ~0.01) at eclogite facies conditions4. Migration of such fluids into the shallow forearc could supply high CO /Cl fluids. Similarly, decarbonation reactions are capable of supplying significantly more carbon-rich fluids (XCO ≤ 0.2)51.

For a fluid resulting from dissolution of carbonate with 3 wt.% CO our calculation only yields a solution for very low Cl contents in the carbon rich endmember (Supplementary Data 8). Low salinity waters are found in forearc settings, derived from the breakdown of clay minerals in the slab52. However, breakdown of clay with 20 ppm Cl and 7% water32 would be expected to generate fluids with ~300 ppm Cl, significantly in excess of those required by the model. Moreover, final fluid Cl is predicted to be only 35-75 ppm which would be expected to result in very low serpentine Cl contents (cf. mean Cl = 665 ppm). Fluids from decarbonation reactions (XCO = 0.05–0.2) yield model solutions with higher Cl contents (285-915 ppm) more consistent with those in serpentine. The latter appears to be the more consistent scenario with respect to the available geochemical constraints although we do not rule out high pressure carbonate dissolution.

Whether the involvement of a carbon-rich fluid from metamorphic decarbonation necessitates significant lateral movement of fluids depends on the depth of both carbonation and decarbonation reactions, both of which are uncertain. The PT conditions of listvenite formation are poorly constrained due to a lack of pressure-sensitive mineral parageneses, but can be broadly constrained to 0.3 to 1.4 GPa (~10-50 km)18,29,53. Pressure estimates based on clumped isotope temperatures17,19 rely on assumptions of geothermal gradient but suggest similar maximum depths of 10-50 km with lower temperatures potentially representing a protracted cooling history19,21.

Modelling of devolatilisation reactions in the downgoing slab generally indicate that the main metamorphic release of both CO and H O occurs in a pulse between 2.2 and 2.6 GPa, corresponding to depths of ~60-80 km in the forearc51,54,55. This would imply lateral flow of fluids into the forearc. However, the depth of decarbonation will also depend on the thermal structure of the subduction zone, with warmer subduction favouring decarbonation, commencing as shallow as 40 km55–57. The Semail subduction zone appears, at least initially, to have been hot as indicated by sediment-derived granitoid melts generated at granulite facies conditions58. This may have favoured metamorphic decarbonation at shallower levels. Overall the mixing of sedimentary pore fluid with carbon-rich fluids to produce the carbonating fluid implies lateral migration of fluids, since most pore fluid expulsion occurs relatively shallowly during subduction49,50. That said, the exact distance between these loci of devolatilisation and carbonation is difficult to constrain and may have been relatively small under a warm subduction geotherm.

However, despite this geochemical evidence requiring input of decarbonation-derived CO -rich fluids, a subduction origin for carbonating fluids is contradictory to interpretations from recent geochronology which has dated dolomite veins in Oman listvenites to 60 Ma and younger22, significantly after the cessation of subduction at ~80 Ma 59,60. Some resolution to this dichotomy is therefore needed.

Resolving the dichotomy between geochemistry and geochronology

While most serpentine and carbonate analyses plot in a single array of consistent F/Cl, a subset of analyses show distinctly higher F contents (<1000 ppm) which are decoupled from Cl contents (Fig. 6). Samples 43z4 and 44z1 fall entirely within the lower F/Cl array; however, the remaining samples (44z2, 44z3, 44z4, 45z1) straddle the two, with analyses in both groups. Subdividing the carbonate data by mineral (e.g., dolomite, magnesite) and texture (Fig. 6b), reveals that nearly all dolomite analyses plot in the higher F/Cl array whereas all analyses from the early-forming ferromagnesite vein cores are restricted to the lower F/Cl array. Combined with the sample petrography, these observations provide robust temporal constraints on fluid composition; dolomite clearly overgrows the zoned ferromagnesite vein network and otherwise is typically an interstitial phase (Fig. 4). This indicates that an early low F/Cl fluid was responsible for the main phase of magnesite-dominated carbonation (Stage 1) and interacted with all the ophicarbonate samples to some degree, while a second, higher F/Cl fluid resulted in the growth of dolomite (Stage 2), cross-cutting earlier carbonate growth in the listvenites and affecting serpentine compositions only in ophicarbonate samples 44z2 and 44z3 (Fig. 6). The average composition of the listvenites lies close to the Stage 1 array but slightly offset towards higher F contents (Figs. 6), reflecting the small proportion of Stage 2 fluids which have contributed to their composition. The strong correlation of Br/Cl and I/Cl for all analyses (Fig. 7) suggests that the two fluid sources had similar Br/I ratios although there is a weak correlation between I/Cl and F/Cl which suggests the Stage 2 fluids were slightly more I-rich compared to Stage 1.

Average Ca contents of listvenites and Oman mantle20 recalculated and normalised as Ca, Fe or Mg carbonate give 8.7 and 1.3 wt.% CaCO , respectively, suggesting that Ca metasomatism accounts for <10 wt.% of the total carbonate. Otherwise, the similarity of mean listvenite Mg:Si:Ca ratios to those of uncarbonated Oman peridotites indicates that carbonation was largely isochemical for the major elements17,20.

The occurrence of a second stage of carbonation with a different halogen signature and clear petrographic evidence indicating it postdates the main stage of magnesite-dominated carbonation is notable and provides a potential resolution to the dichotomy between geochemistry and geochronology. The dating of Scharf et al. 22 relies primarily on analyses of dolomite. Our observation that dolomite is a late-forming phase is corroborated by numerous other studies of listvenites from BT1b and nearby localities19,33,35. The identification of BT1b dolomites as belonging to a distinct carbonation event with a markedly different fluid composition therefore suggests that the young dates from other sites may represent minimum ages for the main phase (Stage 1) of carbonation, albeit with some inherent uncertainty in extrapolating results between localities. This would suggest that the bulk of carbonation occurred before 60 Ma, coincident with subduction, as indicated by a range of geochemical proxies – not least the sedimentary pore fluid halogen signature and carbon isotopes18 of CO bearing fluids.

It should also be noted that the bulk chemical characteristics of the BT1b listvenites suggest that they are highly unsuitable for U-Pb dating. All the listvenites and associated serpentinites have very low U/Pb ratios and lie at the extreme end of a trend defined by forearc serpentinite compositions reflecting low fluid U contents under low fO conditions20,61. A priori it would be expected that any attempt to date such materials by U-Pb methods would be inherently biased towards later perturbations under more oxic conditions where U would be more soluble in fluids. We tentatively suggest that, unless their listvenite bulk geochemistry differs significantly from that at BT1b, a sequence of later fluid infiltration events may be what was dated by Scharf et al.22.

Rather than a single event we envisage formation of the listvenites as a multi-stage process: prior to carbonation the mantle peridotite protolith appears to have been partially serpentinised possible in a seafloor setting43 (Fig. 7b). Subsequently the BT1b mantle section was incorporated into the forearc of a young subduction zone (Fig. 8a). Water- and carbon- rich fluids derived, respectively, from sediment compaction and decarbonation or dissolution reactions deeper in the subduction zone reacted with the serpentinites forming talc and eventually carbonate. This released Cl from serpentine, lowering the Br/Cl and I/Cl ratios of the fluids. These fractionated fluids resulted in additional serpentinisation (or re-equilibration with existing serpentinites) further along their flowpath, producing unusually low Br/Cl and I/Cl serpentine (e.g. 43z4, 44z1; Fig. 7). Where Stage 1 carbonation reached completion, removal of aqueous carbon and release of Cl ceased and the carbon content and Br/Cl and I/Cl of the fluids ‘downstream’ of them increased, such that moving fronts of carbonation and serpentinisation were established (Fig. 8). Later, Stage 2 dolomite and magnesite crystallisation occurred from a Ca-rich fluid with higher F/Cl and I/Cl ratios. This stage is only dominant in some horizons and microstructurally crosscuts earlier carbonates. Fluids derived from low temperature metamorphism of autochthonous shelf carbonates during obduction53 are a plausible source for the F- and I-rich Stage 2 fluids, especially given the marked enrichment of Ca in the lowermost listvenites overlying the basal thrust20. Their similarity in halogen composition to the metamorphic sole may relate to interaction with overlying allochthonous units, the protoliths for the metamorphic sole20,31. Equally, Stage 2 carbonates could be derived from interaction with another F-rich reservoir (e.g. fluid circulation associated with later igneous activity23,62). Implicitly this suggests that Stage 2 dolomite-dominated carbonation likely occurred at equal or lower pressure (~0.3 GPa53) than much of Stage 1, although this is difficult to confirm.

Overall these results are unsurprising if ophiolitic mantle sections are viewed as a patchwork of overprinting metasomatic events (e.g. ref [24]) not all of which may be apparent or detectable at all scales or at all localities. It is likely that multiple stages of fluid infiltration affected the ophiolite prior to, during and since obduction and may have overprinted some chemical or isotopic systems. Furthermore, the extent of such overprinting may have varied spatially resulting in different conclusions from studies at different sites (e.g., ref. [17] cf. [21]). Nevertheless, our data indicate a strong subduction affinity which has escaped such overprinting and attest to fluxing and carbonation of the shallow forearc by CO -rich slab derived fluids.

Implications for subducted carbon fluxes

Having demonstrated strong evidence for a subduction origin for the bulk of carbonation expressed at Site BT1b we may consider the implications for subducted CO fluxes (Fig. 8a).

Listvenites are widespread although not continuously exposed throughout the ophiolite (Fig. 1a). The metamorphic sole is also not ubiquitously preserved and in numerous places late brittle thrusting has juxtaposed relatively unmetamorphosed allochthonous sediments against the mantle section31. This suggests that listvenites might also be prone to removal during obduction and may therefore have relatively poor preservation potential, especially in less laterally extensive ophiolites. In the broader geological record there are numerous examples of listvenites associated with the basal thrusts of supra-subduction zone (SSZ) ophiolites although often with some ambiguity as to the timing of their formation relative to obduction11. Nonetheless they are not ubiquitous in SSZ ophiolites with exposed metamorphic soles, which suggests either that they are poorly preserved or that specific conditions are required for their genesis.

Modelling of fluid evolution (see above) indicates that pore-fluid provides 48-87% of the Cl in the carbonating fluid (Supplementary Data 8). This allows us to constrain the deep CO flux through mass balance and independent estimates of pore-fluid fluxes in subduction zones. Much of the initial pore fluid contained in subducting sediment is lost at very shallow depths and expelled through the accretionary wedge. By 10 km depth the instantaneous flux of pore fluid from sediment is of the order of 1 x 1013 kg/yr globally63. Mass balance of the composition of the carbonating fluid, assuming the proportion of Cl from pore fluid (48-87%), a pore fluid chlorinity of 0.11-0.15 wt.% and pore-fluid flux of 1 x 1013 kg/yr into the forearc63, yields a deep Cl flux of 4.5 x 1010–1.6 x 1011 g/yr. Combined with calculated CO /Cl ratios of CO -rich fluids (Supplementary Data 8) this corresponds to a deeply sourced carbon flux into the forearc of 1.6 x 1013– 3.4 x 1013 g C/yr. It is possible that expelled pore-fluids will not be perfectly mixed with deeper-sourced fluids so this likely represent an upper bound. Nonetheless the magnitude of the potential flux is a significant proportion (22-86%) of estimates of the total subducted carbon flux, which range 4.0 x 1013 – 1.5 x 1014 g/yr [4,63,64]. Combined with estimates of diffuse and volcanic arc outfluxes4,64 this suggests limited deep subduction of CO , unless subduction inputs are towards the upper end of their estimated range. If carbonation is ubiquitous in the shallow forearc at subduction zones, as has been suggested on the basis of gases in forearc springs8, it would constitute a significant proportion of the global input CO flux which is neither subducted to the mantle, nor returned to the atmosphere.

It should be noted, however, that listvenites have not been directly observed in dredges or drilling at modern forearcs such as Mariana and Tonga which allow fluid compositions at slab depths to be inferred. This highlights key differences between those and the Semail subduction zone which may promote listvenites genesis in the latter, including: subduction geotherm (cold65 vs hot29), sedimentary input (dominantly siliceous, 2.6% CO 66 vs mixed volcanic-carbonate-siliciclastic, 8.6% CO 18,67), and even redox state and the relative timing of serpentinisation and carbonation68 (see Supplementary Text 2). This raises the important possibility that the magnitude of the forearc carbon sink may vary spatially and temporally in response to changes in oceanic sedimentation, subduction zone thermal regimes and redox state over geological time and could therefore play an important role in fluctuations in the long-term carbon cycle.

Methods
Sample preparation

For the samples investigated here, two immediately adjacent billets were sawn from each sample and one polished as a thick (~130 µm) section for in situ analyses (EPMA, SIMS, see below) and one powdered for bulk rock halogen and mineralogical analyses. Powders were produced from ~10 g of rock, which was first ultrasonicated in distilled water, coarsely crushed in a porcelain pestle and mortar and then powdered by hand in an agate pestle and mortar to <125 µm particle size. Thin sections were set in epoxy resin, then polished using water.

Bulk halogens by pyrohydrolysis

A split of whole rock powders was analysed for bulk halogens (F, Cl, Br, I) at Institut de Physique du Globe de Paris, (IPGP), Paris, France. Full details of method and analytical procedures are given in ref. [69]. A ~500 mg aliquot of powdered sample was mixed with the same amount of V O and placed in a platinum crucible. The sample was directly introduced into a quartz combustion tube and heated at 1200°C in a H O-vapour stream driven by a nitrogen gas flux. The H O-vapour containing halogen acids extracted by hydrolysis was condensed via a cooling system and trapped into a vial containing 10 mL NaOH solution (25 mMol L-1). Vapour flux was adjusted in order to complete extraction in ~45 min to 1 hr with a final solution volume ~80-100 mL. The mean dilution factor (mass of solution/mass of sample) was ~150 to 200. F and Cl were determined by ion chromatography and Br and I by ICP-MS. Both methods were conducted with standard calibration curves bracketing the range of halogen contents in sample and rock standard solutions. For IC these were prepared by dilution of commercial standard in deionised water with 2% v/v HNO3. For ICP-MS the calibration solutions were likewise prepared from commercial standards and matrix matched to the final solutions from pyrohydrolysis (5% v/v NaOH, 2% v/v HNO ) to avoid any potential matrix effects69. In both cases calibrations were linear for all elements analysed. Repeat analyses of rock standards (BHVO-2, GS-N, JG-1, JR-1, JB-1b, JB-2) indicate mean relative errors of ~10% for F and Cl, ~30% for Br and ~60% for I, and yields of >95% for all halogens69. Detection limits in solution were well below sample concentrations for F and Cl (<100 µg/L) and were approximately 10ppb and 1ppb for Br and I, respectively. For extraction of 500 mg of sample, producing 100 ml of pyrohydrolysis solution, detection limits in rock samples are estimated to be 20 mg kg−1 for F and Cl and 2 μg kg−1 for Br and I. The mean analytical errors on the studied composition ranges were estimated at 10 mg kg−1 for F and Cl, 100 μg kg−1 for Br and 25 μg kg−1 for I. No blank correction was necessary as procedural blanks (both tube and tube + platinum crucible) were always below detection limits. The batch of V O used was tested prior to analysis to ensure it did not contain detectible halogens. Repeat analyses were conducted on three samples (44z1, 56z4, 60z3) to assess repeatability. Relative standard deviation (RSD) between the repeats ranged from 3-10% for F, Cl and SO 2-. RSD for Br and I varied, with one sample (44z1) showing 7% variation, one showing Br and I below detection (56z4) and one showing 70-80% RSD suggesting either intra-sample heterogeneity or less reliable analytical reproduction of Br and I. A summary of all whole rock halogen data is given in Supplementary Data 1 and the pyrohydrolysis dataset is provided in Supplementary Data 2.

Bulk halogens by the noble gas method

Heavy halogens (Cl, Br, I) were also measured using the noble gas method70,71. Prior to irradiation, ~20 mg of finely crushed (<250 µm) sample was wrapped in aluminium foil and sealed in an evacuated quartz tube. Standards were spaced throughout the tube to monitor neutron conversion of K, Cl, Br and I. Samples were irradiated for 48 hours in the flooded reflector area of the Missouri University Research Reactor Center (MURR), Missouri, USA from 23rd-24th March 2022, at a predicted flux of 8 x 1013 n cm-2s-1. Combined data from monitors spaced along the irradiated tube were used to account for minor vertical variations in neutron flux. A Lagrange polynomial fit to Hb3Gr monitor data was used to interpolate noble gas–element conversion factors for each sample based on their vertical positions in the irradiation tube.

Halogens were determined from their proxy noble gas isotope abundances (38Ar , 80,82Kr , 128Xe) measured using an ARGUS VI multi-collector noble gas mass spectrometer at the University of Manchester. Prior to analysis, 1-5 mg of each sample was loaded into an aluminium sample holder baked at ~120 °C for at least 12 hours to achieve high vacuum. Samples were heated to fusion in a single step under vacuum using a Photon Machines CO fusion laser (Model C-55L Laser HD), with a 3 mm diameter defocused beam. Where possible, two repeat analyses were made for each sample. Laser powder was gradually ramped from 0.7-13.2W over 180s to achieve fusion without overly vigorous degassing of samples. A subset of 3 samples was analysed by crushing in vacuo; ~20 mg of sample was loaded into stainless steel manual valve-type crusher, baked as above and the gas released by crushing in 1 or 2 steps. The crushed residues were retained and analysed by laser heating as above. Released sample gases were purified for 10 minutes by exposure to SAES NP10 and ST172 getters at room temperature and ~300 °C, respectively, and the purified noble gases were then expanded into the mass spectrometer. During the analytical session blanks were measured at the start of the day and then between every 1-2 analyses. A calibrated aliquot of air was measured daily to monitor instrument performance, mass discrimination and sensitivity. Due to the elapsed period between irradiation and analysis, 37Ar had entirely decayed away and a small correction for the decay of 37Ar to 36Ar was instead made based on the Ca content measured by XRF. Details of data processing steps were otherwise all as in refs. 18 and 48. A summary of key halogen and noble gas data is given in Supplementary Data 1 and the full NI-NG-MS halogen and noble gas dataset including all data processing steps is provided in Supplementary Data 3.

Secondary ionisation mass-spectrometry (SIMS)

In situ concentrations of halogens (F, Cl, Br, I) and carbon were determined by secondary ionisation mass spectrometry (SIMS) using a Cameca IMS 1270 (NERC Ion Microprobe Facility, University of Edinburgh, UK). Sections were re-polished with a 1 µm diamond paste to remove carbon coating from EPMA analyses, cut into rounds, ultrasonicated in distilled water, dried and finally sputter-coated with a ~30 nm layer of gold.

The SIMS analyses were carried out over two sessions, first for F and Cl and then for Br and I. Fluorine and Cl were determined in serpentine, carbonates and a range of veins from all six samples (totalling 208 spot analyses). A Cl-bearing interstitial silicate phase in samples 44z4 and 45z1 could not be analysed due to its poor polish giving surface irregularity and electric field gradients that can lead to increased mobility of halogens and the risk of electrical arcing within the instrument.

Bromine and I could only be determined in serpentine and high Cl veins from samples 43z4, 44z1, 44z2, 44z3 (92 spots total) due to low concentrations in carbonate and the lack of a reliable peak centring species (a result of the variable carbonate chemistry). Bromine and I were determined adjacent to the F-Cl pit using petrographic images and EPMA maps to ensure significant heterogeneities were avoided and the second pit was as much as possible within the same grain/mineral domain as the first. A subset of points in samples 44z1 and 44z2 were analysed at a third adjacent point to determine carbon together with repeat F and Cl measurements (59 spots total).

The ion microprobe was operated with a 10 kV Cs+ primary ion beam with a negative secondary accelerating voltage of -10 kV resulting in a nominal beam size of 1 µm at ~1 nA. Fluorine and Cl were determined simultaneously with a 60 s pre-sputter, rastered over a 15x15 µm square, to remove gold coat and any surface contaminants and 10 cycles of analysis measuring 18O, 19F, 30Si, 35Cl, on an electron multiplier with count times of 2 s, 4 s, 2 s and 4 s, and settling times of 1 s, 1 s, 9 s and 2 s, respectively. Backgrounds were measured at masses 17.5 and 17.8 during the magnet settling time, with a total time (including settling) of 15 s. A 2 mm field aperture was used. Analysis for C, F and Cl was identical except for the addition of 12C with 2 s counting and 1 s wait times, and background/settling steps at masses 11.2 and 11.5 for 6.5 s each.

The 19F and 35Cl signals were internally normalised to both 18O and 30Si but 18O gave better stability throughout the analytical session and better agreement with Cl contents determined independently by EPMA. Fluorine and Cl concentrations were calibrated against laboratory glass standard T1G (F 321 ppm, Cl 113 ppm, SiO 58.60 wt.%) using the 19F and 35Cl relative ion yields (RIY) to 18O determined on 5 repeat analyses each morning. It should be noted that some serpentine analyses had Cl significantly above that of the primary standard. If signal response was not linear with increasing Cl this could result in large systematic errors. However, linearity of the signal was assessed by analysis of two natural glasses with different SiO contents and Cl contents (Supplementary Figs. 1-2; T1G, 58.6 wt% SiO , ATHO, 75.6 wt% SiO ). Processing data from each standard using the other as the primary standard gives F and Cl abundances that are within 10% of the accepted value for each. The standard deviation on the calculated relative ion yields was propagated together with the analytical error on each point and includes the increase in uncertainty involved with extrapolating beyond the calibration range. Moreover, the SIMS Cl data can be checked against EPMA data from the same points and show a strong linear corelation with a slope of 1.04 (Supplementary Fig. 3). The combined evidence therefore points to the reliability of these data. Nonetheless, the potential for unaccounted-for systematic error in the highest Cl analyses should be noted.

Bromine and I were analysed under similar conditions as above, but at a mass resolution of ~22,000 and with a 1.5 mm field aperture. The surface was pre-sputtered for 30 s and 10 cycles of 30SiO , 79Br, 81Br and 127I measured on an electron multiplier with count times of 2 s, 5 s, 5 s, and 5 s and wait times of 2.5, 1.5, 2.5, and 1 s, respectively. Backgrounds were measured at masses 76 and 126 for 60 s and 40 s, respectively, to allow the magnet to stabilise after large mass jumps. Analyses were internally normalised to 30SiO . Reported F/Cl, Br/Cl and I/Cl ratios use the 18O-normalised Cl values as discussed above. The linearity of the Br signal over a range of concentrations was established by repeat analyses of a range of halogen-bearing standards (Supplementary Figure 4; scapolites, BB1, BB2, SY; Kendrick et al. 71; USGS synthetic glasses GSD, GSE; Myers et al., 72; Marks et al. 73). Some analyses of samples showed significantly higher 81Br/79Br ratios than their natural isotope ratio (0.977) suggesting an isobaric interfering species on 81Br in some analyses. No samples showed unusually low 81Br/79Br, implying the measurements of 79Br are unaffected by such an interference. As a result, Br contents have been calculated from 79Br only and scaled to 100% from its natural abundance (50.57%). As I is monoisotopic, similar assessment of possible interferences could not be made. Bromine and I concentrations were calibrated to the composition of USGS synthetic glass GSE (267 ppm Br, 25.06 ppm I, 53.7 wt% SiO 73,74), which provided the closest SiO matrix match to our serpentine samples in a reference material with known I and Br abundances. Relative ion yields of Br and I were determined from three repeat analyses at the start of each analytical session and their standard deviation propagated with the analytical uncertainty. For serpentine, median relative uncertainties on measured abundances were 2.5%, 2.4%, 7.4% and 17% for F, Cl, Br and I and on ratios were 4.7%, 8.4% and 17.3% for F/Cl, Br/Cl and I/Cl, respectively. The uncertainties on carbonate analyses were slightly higher with median values of 4.2%, 5.4% and 6.9% for F, Cl and F/Cl respectively. For both carbonate and serpentine, all analyses had relative uncertainties below 15%, 23%, 26% and 25% for F, Cl, Br and I respectively and below 26%, 26% and 35% for F/Cl, Br/Cl and I/Cl, respectively. The largest analytical errors were from analyses of thin high-Cl veins, likely due to these typically sampling some surrounding material. Median uncertainties were 59%, 35%, 31% and 36% for F, Cl, Br and I, respectively, and 69%, 48% and 50% for F/Cl, Br/Cl and I/Cl. All errors are plotted as 1 sigma unless otherwise stated.

Matrix effects are a potential issue in SIMS analysis75. These effects are difficult to assess or correct for since there are no widely available matrix-matched standards available for halogens in serpentine or magnesite. Nonetheless, the good agreement of SIMS, EPMA and NI-NG-MS data (see below, Supplementary Fig. 3) lends confidence that any matrix effects are relatively minor as do the linear responses of F, Cl and Br across a range of standard matrices in this study (Supplementary Figs. 1-2, 4) and others in the literature76. Furthermore, differences in matrix effects are likely to be minimised, although not absent, in analyses of the same mineral, meaning that relative differences in compositions of a given mineral can be compared with some confidence. The ratio of F and Cl relative ion yields for two natural glasses with a wide range of SiO contents are within 20% of one another suggesting matrix effects cannot explain the large variations seen in data (carbonate F/Cl = 0.02 to 398. serpentine F/Cl = 0.02 to 1.48). Furthermore, within carbonates, the principal control on F/Cl appears to be textural rather than mineralogical with interstitial dolomite and magnesite both showing high F/Cl (Fig. 6b). Nonetheless, it is impossible to discount that some matrix effects may affect the data and particularly absolute concentrations. As a result, care should be taken in making direct comparison of concentrations between different minerals.

A summary of SIMS halogen data is provided in Supplementary Data 4 and the full dataset is given in Supplementary Data 5. SIMS standard data are provided in Supplementary Data 9.

Comparison of halogen datasets

Comparison of bulk halogen data collected by NI-NG-MS and pyrohydrolysis reveals some notable differences. Chlorine abundances generally agree reasonably well however, the abundance of Br and I is lower in most pyrohydrolysis analyses suggesting low yields during pyrohydrolytical extraction, possibly related to the unusual sample matrix. Monitoring sample-by-sample yield requires the use of hazardous radioisotope tracers (e.g. 125I [77]) and was not performed in this instance, so this cannot be confirmed.

It is difficult to directly compare SIMS and NI-NG-MS data since these were measured on individual minerals and bulk rocks chips, respectively. Nonetheless, the Br/Cl and I/Cl ratios are all within error of the mean of SIMS data from serpentine (which is likely to dominate the whole rock signature). Moreover, bulk NI-NG-MS data and individual serpentine SIMS data are colinear in Br/Cl – I/Cl space suggesting any slight differences arise due to differences in the proportions of different serpentine endmember compositions analysed by SIMS versus the bulk composition of the sample. The SIMS chlorine data also agree well with results from EPMA (Supplementary Fig. 3). The correspondence of halogen data from NI-NG-MS, SIMS and EPMA shows the consistency of the data across multiple methods.

A significant proportion of listvenite samples had Br and I below detection by pyrohydrolysis. This makes calculation of a mean composition problematic and prone to systematic error depending on the assumptions made about the distribution of the data (e.g. log-normal, gaussian, uniform). In producing an overall bulk halogen dataset, the NI-NG-MS data are preferred for the heavy halogens (Cl, Br, I) since they provide a complete dataset for all samples analysed. These data are supplemented by the F abundance from pyrohydrolysis. To avoid any potential inaccuracy due to analytical differences between the two methods, the F/Cl ratio for each sample was calculated using the Cl abundance measured by pyrohydrolysis.

Analysis of major elements, total carbon and loss on ignition

Major elements were determined by X-Ray Fluorescence Spectroscopy (XRF) on fused glass beads at the Earth Surface Research Laboratory, Trinity College Dublin. Beads were prepared by mixing powdered sample and lithium borate flux (50:50 LiT:LiM) in a Pt crucible with a sample:flux ratio of 1:10, before fusion and casting using an automated Claisse LeNeo system at 1050°C.

Major element data were acquired on a Malvern Panalytical Zetium wavelength dispersive XRF spectrometer using a calibration based on WROXI synthetic standards and optimised using international certified reference materials. For full methodological details and accuracy and precision of analytical results determined by repeat analysis of secondary standards see Carter et al. 78.

Total carbon (TC) was determined by combustion on a separate sample aliquot using an Analytik Jena multi EA4000 element analyser at the Earth Surface Research Laboratory, Trinity College Dublin. A primary calibration curve was prepared using a blank boat and 6 aliquots of differing mass of NIST SRM 1632b (Trace elements in coal) and was optimised for the expected range of TC in samples (~0.1–12 wt%). Pure CaCO (Analytik Jena, 12 wt.% TC) was used as a secondary standard to monitor instrument performance; repeat analyses indicate relative accuracy of better than 3% and relative precision of better than 5%. Depending on expected sample TC abundance, 250-400 mg of sample powder was weighed into a clean ceramic boat, and the weights of both sample and total of sample + boat recorded. TC was determined by combustion in O at 1200°C for ~10 minutes and measurement of CO on a nondispersive infrared (NDIR) detector. Following TC analysis, loss on ignition (LOI) was determined from the combusted sample + boat by gravimetry. The full major element, LOI and TC dataset is provided in Supplementary Data 6.

X-Ray diffraction (XRD)

A split of each of the whole rock powders used for bulk halogen analysis was analysed for its mineral constituents by XRD at the University of Manchester. Powders were mixed with ~1 ml of amyl acetate and spectra were acquired using a Bruker D8 Advance Diffractometer. CuKα1 X-rays were produced from a copper target with a wavelength of 1.5406 Å and the diffracted spectrum scanned from 5-70° 2θ, with a step size of 0.02° and a count time of 0.2 s per step. The resulting spectra were matched to a database of reference mineral spectra using the EVA software package (version 4). The XRD spectra were then further processed to semi-quantitatively calculate the relative proportions of minerals in the samples. The full dataset mineral abundance is provided as Supplementary Data 7.

Electron probe microanalysis (EPMA)

In the six reaction zone samples (43z4, 44z1, 44z2, 44z3, 44z4, 45z1), target areas for in-situ analysis by SIMS were quantitatively mapped by EPMA using a Cameca SX100 Electron Microprobe at the University of Manchester. The operating conditions were 15 kV accelerating voltage and 200 nA beam current with a 5 µm electron beam. Maps of SiO , Al O , FeO, MgO, CaO, Cl, K O, Na O and Cr O were produced for 24 areas of 2.56x2.56 mm with a step (i.e. pixel) size of 5 µm. Fluorine was not mapped due to its high detection limits and the detrimental impact counting times would have had on the analysis of the other elements. Maps were fully matrix-corrected and quantified as both silicate normative (0.5 calculated O2- per X+ cation) and carbonate normative (0.5 calculated CO 2- per X+ cation) using the ProbeForEPMA software package.

All pixels (each corresponding to a single point analysis) were extracted from the maps using a script written in the R programming language and used to assess mineral chemistry and stoichiometry across the reaction zone. Pixels corresponding to serpentine or carbonate phases were extracted by filtering by the following compositional criteria (all in wt%, applied in order from top to bottom):

• SiO < 5 & Cl > 0.25: epoxy
• Total < 70 & SiO < 15: carbonate
• Total > 70 & Total < 92 & CaO < 1 & SiO > 35: serpentine
• Total > 90 & Cr O > 0.5 & SiO < 5: spinel
• SiO > 85 & CaO + MgO + FeO < 25: quartz
• (in listvenite samples) SiO > 20: residual silicate
• Everything else: other

Where other mostly consists of non-ideal mixed analyses along the boundaries between minerals.

Structural formulae, normalised to 7 oxygens, have been calculated for each pixel to interrogate serpentine stoichiometry. Given the EPMA beam diameter of 5 µm, intergrown microcrystalline minerals are not spatially resolvable and the analysis will represent a mixture of the individual constituents. Plotting mineral compositions as Si/7 oxygens against Fe+Mg/7 oxygens allows discrimination between lisardite, brucite, talc, quartz and chlorite and microcrystalline mixtures of these endmember components79.

Compositions for individual SIMS points (for internal normalisation to O or Si and validation of Cl data) were extracted by averaging the pixels (~60) in a polygon drawn round each point location in QGIS software. Where spots were analysed in areas not covered by an EPMA map, an average composition of petrographically similar areas was used for internal normalisation. Within each sample there was only minor variation between points with typical relative standard deviations of 4% on O and SiO in serpentine and 6% on O in carbonate.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a detailed mineralogic, petrologic, and geochemical (focused on C and halogens) of variably carbonated serpentinites from Oman (through a drill core to the metamorphic sole). The goal is to determine fluids involved in carbonation reaction, carbonation of the forearc, and the subsequent role of the forearc in the C cycle.\nThe authors have collected an impressive amount of data linking mineralogy (and different mineral generations) to geochemistry (particularly detailed in situ work). This is no small feat! In fact, determining the different chemistry of the dolomite is key to interpreting the data in context with previously determined geochronology (I thought this was a particularly nicely done part of the contribution that demonstrated the importance of detailed contextual work).\nMajor comments:\nI recognize that halogen work is extremely difficult, and particularly halogen concentrations via SIMS is limited. Therefore, some more details on methods (see comments below) would be helpful. Also given that some of the same data are collected by different methods, and in some cases have dramatically different results, these differences must be addressed and why some data are used and not others.\nThe discussion linking dehydration, carbonation, and depths of these reactions needs to be clarified and better linked to the proposed (2 step) model from the outset.\nNature Communications is targeted for a general geology audience. There are numerous places throughout (see comments below) in which a few more details would help the contribution be more accessible. In particular, including other reservoirs (e.g., serpentinites, AOC, mantle) on figures (Fig 7) would be helpful in following the interpretation/discussion.\nMinor comments:\nGiven that Nature Communications allows for 70 references and the authors only have 54 references, many more references can be included particularly in the introductory section. For example, on lines 31-32, the authors state “Recently attention has turned to the potential for hidden outfluxes not sampled by volcanism and unaccounted for in current estimates6,7.”\nNumerous additional references can be added to this, in which previous authors note commonly overlooked outfluxes via the forearc or backarc, as well as sequestration in volatiles in additional reservoirs (e.g., sub-continental lithospheric mantle).\nSome suggestions are:\nBekaert DV, Turner SJ, Broadley MW, Barnes JD, Halldórsson SA, Labidi J, Wade J, Walowski KJ, Barry PH. Subduction-driven volatile recycling: A global mass balance. Annual Review of Earth and Planetary Sciences. 2021 May 30;49(1):37-70.\nGibson, S. A., & McKenzie, D. (2023). On the role of Earth's lithospheric mantle in global volatile cycles. Earth and Planetary Science Letters, 602, 117946.\nBarnes, J. D., Manning, C. E., Scambelluri, M., & Selverstone, J. (2018). The behavior of halogens during subduction-zone processes. The role of halogens in terrestrial and extraterrestrial geochemical processes: Surface, crust, and mantle, 545-590.\nFoley, S. F., & Fischer, T. P. (2017). An essential role for continental rifts and lithosphere in the deep carbon cycle. Nature Geoscience, 10(12), 897-902.\nParagraph starting on line 48. At the start of the sentence clarify that you are discussing the listvenites from the Semail Ophiolite. Is this discussing the exact same samples that you studied, or just from the same area?\nLine 55: Geochronology based on what? More details are needed since this is related to one of the key motivations of the work.\nFig 1. Some of the small text in Fig 1a, 1d, and 1e is really hard to read. One could use mineral abbreviations in 1d and 1e to save space, thus allowing for a larger font size.\nFig 1. Show the location of all the samples in the figure, not just those in the reaction zone. Otherwise, it looks like there are only 6 samples.\nFig. 4. d-f and g-i are not labelled on the figure. It looks like it is the same sample in a row, but the caption reads like it is the same sample in a column?\nLine 201: Are these EMPA or SIMS data or an average of both?\nLine 231: Higher F contents than what? It looks like the serpentine from the ophicarbonates has just as high F concentrations as the listvenites. Or do you mean that some listvenites have higher F concentrations than other listvenites?\nLine 238: Since you have the space, you may want to add more references for sedimentary pore fluids. For example:\nMuramatsu, Y., Doi, T., Tomaru, H., Fehn, U., Takeuchi, R., and Matsumoto, R. (2007). Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates. Applied Geochemistry, 22(3), 534–556.\nLine 246: Showing some statistics to back up this statement would be helpful (e.g., box and whisker plots for the different samples) because I don’t see a trend. Overall, the data look highly scattered. For example, 43z4 has some of the highest and lowest Br/Cl values and some of the highest and lowest I/Cl values.\nLine 250: “seawater serpentinites”- reword. I assume you mean abyssal serpentinites hydrated by seawater? Put serpentinites (both abyssal and forearc) on Fig 7 for reference. “Plot away from” is vague. Be more specific (and adding serpentinites to the figure will help).\nLine 252: That would mean the serpentine protolith has a very, very low Br/Cl (lower that the reported mantle value of 2.8 ± 0.8 x 10-3; Kendrick et al., 2017). How is this reconciled?\nLine 258: Clarify the arrays. Do you mean samples with high F/Cl and with low F/Cl?\nLine 264: A plot of Br vs I (showing different reservoirs) would be helpful.\nLine 270: Add AOC onto Figure 7. AOC span a large range of I/Cl, many with high I/Cl values.\nFig 7. Given that the focus of the paper is on carbonation, it might be useful to add carbonate to this figure. Since iodate can substitute for carbonate, why is the I/Cl not higher (like as seen in some AOC)?\nFig 7. Why are there more whole rock data shown than samples? For example, there are 4 metamorphics listed on Table S1, but more than 4 metamorphic samples shown on Fig 7. Is this because Fig 7 is showing both Pyr and NI-NG-MS data? (If so, then there needs to be some discussion on why the different methods determine different concentrations- see below.)\nLine 297: The full reference for reference 31 is not given. What are the halogen values for other Oman serpentinites (and their tectonic relationship to your samples)? Can you add these data to your figures for comparison?\nLine 304: Since this is key to the presented interpretation, I suggest drawing this trend/arrow on the figure.\nLine 308: Can you expand on this since it is directly related to your interpretation? Adding those data onto figure 7 (with an arrow depicting the trend) would be helpful\nLine 335: And what is that fluid/rock ratio that is assumed?\nLine 369: Reference?\nLine 375: Clarify both scenarios? At shallower and deeper depths? What CO2 concentrations are used at each of those depths? The assumed parameters are not completely clear. Are these different scenarios related to parts 1 and 2 on figure 5?\nLine 399-440: The volatile release is also going to be a function of the thermal structure of the subduction zone. What is most reasonable for your study area?\nLine 403: Based on the discussion starting on lines 368, it is not clear exactly what the depths of carbonation are. This discussion linking dehydration, carbonation, and depths of these reactions should be streamlined and better linked to the steps outlined in the model later in the manuscript.\nLines 404-405: This sentence seems like an add on.\nLine 418: delete “etc” and add “e.g.,” before dolomite\nLine 462: Careful with the wording here because the serpentinites in this study do not have halogen ratios similar to those from other forearc regions (other regions have higher Br/Cl and I/Cl plotting up in the pore fluid range). (The subsequent explanation on the following lines on the evolving composition is much clearer here than previously in the manuscript. Some of these concepts need to be better introduced earlier to avoid confusion.)\nLine 478: Reference? This is interesting, but if going to be mentioned then should be introduced earlier in the discussion.\nLine 508: Define Prop ClPF\nLine 550: A few more details on the bulk rock pyrohydrolysis + IC/ICP-MS halogen concentrations are needed. For example, what are the yields (or concentrations) of halogens on rock standards (e.g., Kendrick et al, 2018)? What are the blanks (e.g., halogen blanks from NaOH, V2O5, etc.)? Are blanks corrected for or are they all below detection limits? What were the analytical protocols for the IC and ICP-MS- or at least a reference to where these are outlined?\nLine 584: Were rock standards analyzed via NI-NG-MS? Comments on the reproducibility of samples analyzed via NI-NG-MS? Given the poor reproducibility of Br and I on some samples via pyrohydrolysis and ICP-MS and the explanation was “sample heterogeneity or less reliable analytical reproduction of Br and I” it would be good to see if potential sample heterogeneity is also documented via NI-NG-MS.\nA comparison of Br and I concentration data collected by the two different methods shows stark differences. The Br concentrations via pyrohydrolysis are approximately 50% less than those of the noble gas method, except for one sample with ~470% more Br. The same is seen with I, in which I by pyrohydrolysis records much lower concentrations except for a couple with ~400-500% more I. Some mention of the poor comparison is needed.\nIn Table S2 is says that Cl, Br, and I values determined by NI-NG-MS are preferred. Why? But, then in Table S1, it says that F and F/Cl uses F and Cl data measured by Pyr-IC. Why is the Cl measured by Pyr-IC used if NI-NG-MS is preferred? And is the Cl Pyr-IC data only used for the F/Cl and not [Cl]? In Table S2, it is denoted that Cl is measured via ICP-MS (not IC).\nLine 617: Was only one glass standard used for Cl and F SIMS analyses? Is there concern about matrix effects given that serpentine, carbonate, and vein material was analyzed? On Table S5, both EMPA and SIMS Cl data are given. With the exception of some very high Cl concentrations via EMPA, there is reasonable agreement. This would be good to point out.\nTable S1: What does “shallowater-corrected data used for I abundance and I/Cl ratios” mean?", "reviewer_2": "This manuscript focuses on the carbon cycle in subduction zones, exploring the role of carbonated mantle peridotites in sequestering subducted CO2 through a study of the Oman ophiolite. The research is of great significance, aiming to address the long - standing debate regarding the origin of carbonating fluids and their impact on the global carbon cycle. The approach combines multiple analytical techniques, which is innovative and promising. In general, I think this manuscript is well written.I don't have special comments for the authors. I would suggest acceptance for this manuscript.", "reviewer_3": "Carter et al. present a well written and illustrated paper integrating careful petrography with multiple geochemical techniques, several of which are innovative in their application to the topic. The paper presents a new, untapped line of evidence (halogens) and a compelling petrographic case for a two stage listvenite formation in the SE Oman ophiolite. This conclusion does the best job so far in the literature of reconciling the controversial mismatch between previous geochronological and geochemical interpretations. The authors take this conclusion and point out its implications for a hidden carbon sink in forearc mantle, which would represent an important component of subduction systems and the deep C cycle.\nThese are the now the most studied listvenites in the world and will doubtless continue to receive attention from the subduction zone cycling community if a forearc wedge carbonation interpretation is partly true. The rocks and reactions are also highly relevant to recent efforts to engineer peridotite carbonation as a means to permanently sequester CO2. Halogens as important fingerprints, ligands, and catalysts in such reactions are an important but overlooked part of this effort.\nThe subject matter is of vibrant interest and the conclusions are of relevance to several different swaths of our community: those working on listvenites and ophiolite, those on subduction zone cycling, and those on global biogeochemical cycling.\nI have a few concerns below and many minor comments, that should hopefully be possible to address with minor/moderate revisions.\nGeneral comments\nThe main argument I see against the principle thrust of the paper – that forearcs are a hidden carbon sink, is that listvenites are discontinuous along the metamorphic sole in Oman, and rarely present in other ophiolites with exposed soles (?), and have not been dredged from forearc sections in the Marianas or Tonga trenches. Can this be considered, explained or conclusions tempered to reflect this reality? Showing the locations of all the listvenites described in the ophiolite in a more detailed Fig 1a would assist this discussion.\nIts important to note that Scharf et al. worked on listvenites from another locality, and that this study focuses only on listvenites from BT1B. While the drilled localituy is the among the largest listvenite bodies in the ophiolite, it is not the only one. If there were indeed two pulses of carbonating fluids, then it would follow that they have different spatial foci/extents, which might explain differing conclusions from workers in different parts of the ophiolite.\nThe conclusions of this paper rely heavily on the petrographic evidence for dolomite overprinting magnesite. I don’t find the evidence presented very clear/convincing. Can it be expanded on or shown in higher resolution/different imaging types?\nThe final panel of Fig 8c and the accompanying metamorphic sole F-rich fluid source interpretation doesn’t make sense. By the time of this F rich dol precipitation (~60 Ma by this model), the sole had already been metamorphosed to amphibolite facies (and exhumed). Exhumation and retrogression of these rocks cannot produce a fluid at shallow depths – they are almost dry following their trip into the mantle. The source must be tectonically deeper than the sole in lower grade rocks. There are also several nappes beneath the ophiolite that are omitted here, as ‘Arabian continent margin’ evokes the autochthonous continental basement. I recommend consulting the cross sections in Searle (2007, Geoarabia) to get the tectonostratigraphy correct, and this may help with your interpretation of F rich fluid source options.\nFigure comments:\nFig. 1. The intervening met. sole close up section (Fig 1c) breaks the flow of the figure from left to right. Is it really necessary considering it has been well described and no samples in this study come from it? If you deem it so, it may be better to either side of the full 0–300 m stratigraphy.\nFig. 2. Annotate the side of the fig. with an arrow indicating carbonation intensity.\nFig. 4. I don’t find the dol x-cutting particularly convincing, at least at the resolution in the merged PDF. Dol doesn’t have euhedral faces, it looks like the Mgs is corroding early dol. The red vs white arrows don’t seem to have differing significance?\nHow closely does the drill hole depth approximate the true pseudostratigraphic depth? This should be mentioned in the caption or annotation.\nFig. 8. IN panel a, it looks like the mantle wedge is dehydrating and decarbonating.\nLine by line comments:\nL45. The tectonic setting of the ophiolite does still attact controversy, a reference here would serve to illustrate which of the various models the authors prefer to contextualize this work.\nL50. Adding specifics of the Sr ratios in parentheses would be helpful here. Also, for transparency, note that they overlap the Hawasina metasediments, but also other crustal materials present in the sub-ophiolitic sequence, i.e. they are also likely intermediate between the protolith and the more radiogenic autocthonous basement.\nL55. There is certainly a mismatch in the interpretations of the trace elements and geochronology, the key point to made/tone to be struck is that the uncertainty/apparent nature of the dichotomy is not a fault of the data, but interpretation of them on one or both sides of the argument. Set out the timing constraints specifically here, as it is crucial context.\nL63. State which halogens.\nL78. What other volatile cycles does it have a bearing on?\nL94. The origin of those metasediments/metabasalts is of importance for context here, as is the intervening deeply subducted and exhumed met. sole. See Searle’s papers for reference.\nL96. Reference for the Oman Drilling Project?\nL97-101. *More tectonostratigraphically complete. This sequence is not a stratigraphy.\nTectonostratigraphy/pseudostratigraphy are sometimes used in this situation. Or otherwise avoid the term altogether.\nL107. How is the minimal impact of protolith heterogeneity established for these rocks? Sharp compositional variations between dunite/harzburgite are common near the sole.\nL110. Reaction zone at the base of the upper listvenite? Or where?\nL133. Along the reaction zone? This needs clarification (further into the reaction zone? With increasing carbonation?)\nL132. Don’t all three serpentine minerals have similar stoichiometry?\nL217-219. Would it? This is not a closed system process, the reaction proceeds because fresh CO2 rich fluids are introduced and spent ones (necessarily) removed, maintaining high CO2 activity and high fluid/rock ratios. The implication would be Cl addition to serpentinites downstream. Is this seen?\nL296. It is not surprising the Oman serpentinites have markedly different halogen compositions to seafloor serpentinites. Can a more relevant comparison be made to other ophiolitic serpentinites that would rule out the distinct possibility this is some common feature of deeply formed, ophiolitic serpentinites (or those formed by meteoric fluids?)?\nL306. What is meant by equilibration of a serpentine protolith? Recrystallization?\nL407. It is more than ‘somewhat’ in conflict, at least for a single event carbonation model, which is worth specifying. Also, it should be ordered 100–80 Ma; however, recent petrochronological work from e.g. Garber et al. has failed to reproduce the >100 Ma Guilmette et al prograde dates, so it may be safer to say from ‘at least 96 Ma to ~80 Ma’ or to focus on the cessation date, which is the relevant one here.\nL441. I agree this is significant, but the petrographic evidence as provided is not that clear/definitive. In line with my previous comment can this be bolstered?\nL446. These samples were from a different locality, so I would not say that they have been identified (with certainty implied) as belonging to a distinct carbonation event. Your data rather provides a compelling explanation as to why they may belong to a distinct carbonation event and potentially represent minimum ages.\nL451. It should be noted that this statement is limited in scope to BT1B, clearly some aspect of the lisvenites (primary or secondary) was suitable to dating to Scharf et al.\nL460. Is this parallel list not reversed?\nL479. It would be a pertinent place to note a fact all too ften forgoteen by studies of these cores, that these rocks were transposed under a ~15 km thick ophiolitic nappe over 100s of km of continental margin (Searle et al. 2007) before reaching their final resting place. Rocks so close to the basal thrust could hardly escape progressive overprinting by metamorphic fluids during this translation.\nL482: the ophiolite does not have 100 Myr subaerial exposure history, indeed, it is only 96 Ma and a large part of its life was underwater.\nL484. Last line could be more specifically geared toward carbonating fluids. There is nothing controversial about forearc slab fluid fluxing.\nLine 533: Spell out IPGP."}, "author_response": "Thank you for these positive comments and for the many constructive criticisms; addressing these has undoubtedly strengthened and focused the manuscript. We were particularly pleased to read praise for the petrographic and in situ geochemical approach taken.\nMajor comments:\nI recognize that halogen work is extremely difficult, and particularly halogen concentrations via SIMS is limited. Therefore, some more details on methods (see comments below) would be helpful. Also given that some of the same data are collected by different methods, and in some cases have dramatically different results, these differences must be addressed and why some data are used and not others.\nFurther analytical details for halogen work have been added to the methods along with a section comparing the results between different methods and accounting for any discrepancies. See also detailed responses to individual comments below.\nThe discussion linking dehydration, carbonation, and depths of these reactions needs to be clarified and better linked to the proposed (2 step) model from the outset.\nThis section of the discussion (lines 384-457) has been rewritten to more clearly delineate the (admittedly relatively poor) constraints on the PT conditions of carbonation and decarbonation, how these may vary with different subduction zone geotherms, and what may be inferred in the case of Oman. The discussion here has also been reordered to more cleanly divide inferences about the fluid composition (based on the halogen geochemistry and modelling) from discussion about the possible source of such fluid(s). Additionally, clarification has been added to indicate that this section is focused on Stage 1 of the two-stage model (lines 341-345). Finally, a further note has been added to the following section to indicate that implicitly this model suggests Stage 2 must have occurred at similar or shallower pressures than Stage 1 (lines 519-521).\nNature Communications is targeted for a general geology audience. There are numerous places throughout (see comments below) in which a few more details would help the contribution be more accessible. In particular, including other reservoirs (e.g., serpentinites, AOC, mantle) on figures (Fig 7) would be helpful in following the interpretation/discussion.\nA large body of literature data (incl. forearc/alpine, seafloor/passive margin serpentinites, AOC, Isua talc-schists/serpentinites, and new annotations) has been added to Figure 7 to better contextualise the halogen analyses within the possible geochemical reservoirs. In addition, numerous changes have been made throughout in response to detailed comments which have been helpful in making manuscript more accessible and readable (e.g. including Sr isotope ratios).\nMinor comments:\nGiven that Nature Communications allows for 70 references and the authors only have 54 references, many more references can be included particularly in the introductory section. For example, on lines 31-32, the authors state “Recently attention has turned to the potential for hidden outfluxes not sampled by volcanism and unaccounted for in current estimates6,7.” Numerous additional references can be added to this, in which previous authors note commonly overlooked outfluxes via the forearc or backarc, as well as sequestration in volatiles in additional reservoirs (e.g., sub-continental lithospheric mantle). Some suggestions are:\nBekaert DV, Turner SJ, Broadley MW, Barnes JD, Halldórsson SA, Labidi J, Wade J, Walowski KJ, Barry PH. Subduction-driven volatile recycling: A global mass balance. Annual Review of Earth and Planetary Sciences. 2021 May 30;49(1):37-70.\nGibson, S. A., & McKenzie, D. (2023). On the role of Earth's lithospheric mantle in global volatile cycles. Earth and Planetary Science Letters, 602, 117946.\nBarnes, J. D., Manning, C. E., Scambelluri, M., & Selverstone, J. (2018). The behavior of halogens during subduction-zone processes. The role of halogens in terrestrial and extraterrestrial geochemical processes: Surface, crust, and mantle, 545-590.\nFoley, S. F., & Fischer, T. P. (2017). An essential role for continental rifts and lithosphere in the deep carbon cycle. Nature Geoscience, 10(12), 897-902.\nAgreed – further reference to the literature have been added throughout the manuscript: this includes several of those listed above (though not all could be included due to additions elsewhere) and more thorough referencing of the geological background literature on the Semail Ophiolite.\nParagraph starting on line 48. At the start of the sentence clarify that you are discussing the listvenites from the Semail Ophiolite. Is this discussing the exact same samples that you studied, or just from the same area?\nPrior work was not on the same samples but on samples from the same area (Falk & Kelemen, 2015) and from the same BT1b drill core (de Obeso et al. 2022). This has been clarified as suggested.\n“This study focuses on listvenites from Hole BT1b of the Oman Drilling Project16 drilled in Wadi Mansah (23.364374°N, 58.182693°E) in the Semail Ophiolite. Listvenites from BT1b and nearby exposures show elevated 87Sr/86Sr ratios” (lines 49-51)\nLine 55: Geochronology based on what? More details are needed since this is related to one of the key motivations of the work.\nThis has been clarified as suggested.\n“recent in-situ U-Pb geochronology on samples from localities near (<10 km) Hole BT1b. The latter yielded dates from two dolomite veins and several imprecise dates from listvenites, all <60 Myr, postdating active subduction by at least 20 Myr” (lines 61-64)\nFig 1. Some of the small text in Fig 1a, 1d, and 1e is really hard to read. One could use mineral abbreviations in 1d and 1e to save space, thus allowing for a larger font size.\nThis is a good point. Fig. 1 has been adjusted as suggested to use mineral abbreviations and increase font sizes for smaller elements.\nFig 1. Show the location of all the samples in the figure, not just those in the reaction zone. Otherwise, it looks like there are only 6 samples.\nSample locations have been added to panel b of Figure 1 as suggested\nFig. 4. d-f and g-i are not labelled on the figure. It looks like it is the same sample in a row, but the caption reads like it is the same sample in a column?\nFigure 4 has been redrafted to address comments from Reviewer 3; all panels are now labelled on the new version.\nLine 201: Are these EMPA or SIMS data or an average of both?\nThese are SIMS data. This has been clarified in the text\n“In situ data from secondary ionisation mass spectrometry (SIMS) similarly show...” (line 213)\nLine 231: Higher F contents than what? It looks like the serpentine from the ophicarbonates has just as high F concentrations as the listvenites. Or do you mean that some listvenites have higher F concentrations than other listvenites?\nThe meaning was the latter – within the listvenites some carbonates have higher F than others. This sentence has been rewritten to be clearer.\n“Carbonates from listvenites are also mostly colinear with serpentine compositions, although a subset show higher F concentrations” (lines 241-242)\nLine 238: Since you have the space, you may want to add more references for sedimentary pore fluids. For example:\nMuramatsu, Y., Doi, T., Tomaru, H., Fehn, U., Takeuchi, R., and Matsumoto, R. (2007). Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates. Applied Geochemistry, 22(3), 534–556.\nGiven the additions of various other references and since there is already a source for the composition of sedimentary pore fluid there wasn’t space to fit this in. I read it with interest for future work thought, especially as it provides halogen data for solid phases in sediments.\nLine 246: Showing some statistics to back up this statement would be helpful (e.g., box and whisker plots for the different samples) because I don’t see a trend. Overall, the data look highly scattered. For example, 43z4 has some of the highest and lowest Br/Cl values and some of the highest and lowest I/Cl values.\nIndeed, there is a lot of variation between individual spots within a sample but these do show a progressive trend from the least to the most carbonated ophicarbonate. A boxplot of Br/Cl by sample has been added to Fig. 7 to illustrate this more clearly.\nLine 250: “seawater serpentinites”- reword. I assume you mean abyssal serpentinites hydrated by seawater? Put serpentinites (both abyssal and forearc) on Fig 7 for reference. “Plot away from” is vague. Be more specific (and adding serpentinites to the figure will help).\nA range of endmembers (upper and lower AOC, forearc/alpine serpentinites, abyssal/passive margin serpentinites), none of which provide a convincing match for the low Br/Cl endmember in the BT1b data, have been added as an inset to Fig. 7. It was not possible to include these on the main panel of Fig. 7 without it becoming unreasonably cluttered. Seawater serpentinites was a mistake and should have read seafloor serpentinites (i.e. abyssal or passive margin). The sentence has been rephrased to be clearer:\n“The low Br/Cl and I/Cl ratios observed in the ophicarbonates are below the seawater Br/Cl ratio (3.47 x10-3) but above seawater I/Cl (3.04 x10-6) and show distinctly lower Br/Cl than seafloor or forearc serpentinites24,37 (Fig. 7b)” (Lines 259-262)\nLine 252: That would mean the serpentine protolith has a very, very low Br/Cl (lower that the reported mantle value of 2.8 ± 0.8 x 10-3; Kendrick et al., 2017). How is this reconciled?\nThis passage is intended to refer to the halogen composition of the uncarbonated serpentinite prior to carbonation not the fresh peridotite protolith of both. The halogen composition of serpentinite is dominated by that of the serpentinising fluid rather than peridotite protolith except at very low degrees of alteration (e.g. Kendrick et al. 2022 Contrib. Min. Pet.). The use of the word protolith was meant to refer to the protolith for carbonation but we acknowledge that is potentially confusing and the sentence has been rewritten:\n“We interpret the correlation between the two ratios as a binary mixing trend between a low I/Cl endmember, representing the uncarbonated serpentinite (or a fluid it has equilibrated with) and a high I/Cl endmember representing the composition of the carbonating fluid.” (Lines 262-265)\nLine 258: Clarify the arrays. Do you mean samples with high F/Cl and with low F/Cl?\nFig. 6 has been annotated to show these more clearly and the caption clarified: “The data fall into two arrays: one with fairly constant F/Cl (~0.01-1) and one with elevated F and variable F/Cl (1-1000), labelled I and II respectively” (Lines 270-1)\nLine 264: A plot of Br vs I (showing different reservoirs) would be helpful.\nAs this information is implicitly available in Figure 7 (i.e. lines of constant Br/I form linear arrays on this plot) we felt that the discussion in the text was sufficient to get the point across.\nLine 270: Add AOC onto Figure 7. AOC span a large range of I/Cl, many with high I/Cl values.\nAOC has been added to Fig. 7. Upper AOC would be a more likely fluid source given the proximity of volcanics from the Haybi and Hawasina complexes but this is incapable of supplying the low Br/Cl signature seen.\nFig 7. Given that the focus of the paper is on carbonation, it might be useful to add carbonate to this figure. Since iodate can substitute for carbonate, why is the I/Cl not higher (like as seen in some AOC)?\nBased on the coexistence of graphite and haematite, f was relatively low during O2 carbonation (log[f ] < -40; Kelemen et al. 2020 JGR:S). This strongly suggests iodine will O2 be speciated as iodide only (e.g. Eh-pH diagram, Fuge and Johnson, 1986, Env. Geochem. & Health) which would preclude compatible behaviour of iodate having an influence. Since this is not thought to be a relevant effect we have not included an oxidative carbonate endmember on Figure 7 as doing so would add to an already rather busy figure.\nFig 7. Why are there more whole rock data shown than samples? For example, there are 4 metamorphics listed on Table S1, but more than 4 metamorphic samples shown on Fig 7. Is this because Fig 7 is showing both Pyr and NI-NG-MS data? (If so, then there needs to be some discussion on why the different methods determine different concentrations- see below.)\nWhere sample material allowed, the whole rock samples were run in duplicate and all these data have been plotted on this figure. This was judged to be the most appropriate approach since the aim is to show the overall trends in comparison to the in-situ data and both intra- and inter-sample are valid measures of this. This has been clarified in the caption: “Bulk data, including duplicate analyses, are shown for listvenites, ophicarbonates and metamorphic samples as inverted triangles and the mean composition for each lithology as large triangles.” (Lines 294-297)\nLine 297: The full reference for reference 31 is not given. What are the halogen values for other Oman serpentinites (and their tectonic relationship to your samples)? Can you add these data to your figures for comparison?\nReference has been updated to include full details. These data from four serpentine veins and one whole rock serpentinite are to our knowledge the only halogen data from “normal” Oman mantle section serpentinites. These come from transects through the full ophiolite stratigraphy at Wadi Tayin and Wadi Haymiliyah with the mantle section sampled near to the MOHO (0-500m below) and far (>10 km) from the basal thrust (D’Andres, 2021, Figs 2.1, 2.2). These are therefore taken as being representative of the composition of serpentinites in the mantle section (e.g. the “main mantle section” of Godard et al. 2021, JGR:SE), in the absence of carbonation processes.\nThe field defined by these data has been added to Fig. 7b and the text rewritten to specify the context of these data:\n“It is possible that the unusual composition of serpentine in the least carbonated samples reflects an atypical serpentine chemistry characteristic of the Oman mantle section. However, serpentinites sampled far from the basal thrust are more typical of seafloor or forearc serpentinites, with Br/Cl ¬close to seawater43 (Fig. 7b).” (Lines 312-315)\nLine 304: Since this is key to the presented interpretation, I suggest drawing this trend/arrow on the figure.\nAnnotated arrow has been added to Fig. 7 as suggested\nLine 308: Can you expand on this since it is directly related to your interpretation?\nThis passage has been expanded upon slightly as suggested: “Notably, a similar trend of correlated Br/Cl and I/Cl, is shown by variably carbonated Isua serpentinites, with low Br/Cl compositions (>7x10-4) suggested to arise from fractionation of fluids during water-rock reaction” (Lines 324-326)\nAdding those data onto figure 7 (with an arrow depicting the trend) would be helpful\nIsua serpentinites and talc-schist have been added to Fig. 7 with an arrow indicating the apparent vector of carbonation for both this study and Isua data.\nLine 335: And what is that fluid/rock ratio that is assumed?\nDepending on the CO content of the CO -rich fluid component, a min. fluid/rock ratio of 1 2 2 to 23 is required to explain the fractionation seen. The text has been updated to include this: “the minimum possible fluid/rock ratio required to explain the degree of carbonation observed is assumed (~1 to 23; Supplementary Table S8)” (Line 355-256)\nLine 369: Reference?\nMissing reference (Kelemen and Manning, 2015) has been added (Line 403)\nLine 375: Clarify both scenarios? At shallower and deeper depths? What CO2 concentrations are used at each of those depths? The assumed parameters are not completely clear. Are these different scenarios related to parts 1 and 2 on figure 5?\nThis section of the discussion has been substantially redrafted in response to one of the major points about to give a more thorough overview on the constraints around PT conditions of carbonation and decarbonation reactions. We feel this has also clarified the specific point raise here, specifically in this passage:\n“For a fluid resulting from dissolution of carbonate with 3 wt.% CO2 our calculation only yields a solution for very low Cl contents in the carbon rich endmember (Supplementary Table S8). Low salinity waters are found in forearc settings, derived from the breakdown of clay minerals in the slab52. However, breakdown of clay with 20 ppm Cl and 7% water32 would be expected to generate fluids with \"300 ppm Cl, significantly in excess of those required by the model. Moreover, final fluid Cl is predicted to be only 35-75 ppm which would be expected to result in very low serpentine Cl contents (cf. mean Clserpentine = 665 ppm). Fluids from decarbonation reactions (XCO2 = 0.05–0.2) yield model solutions with higher Cl contents (285-915 ppm) more consistent with those in serpentine. The latter appears to be the more consistent scenario with respect to the available geochemical constraints although we do not rule out high pressure carbonate dissolution.” (Lines 407-418).\nLines 399-440: The volatile release is also going to be a function of the thermal structure of the subduction zone. What is most reasonable for your study area?\nThis is a good point. This is partly addressed in the redrafted discussion referred to in the previous point, specifically:\n“Modelling of devolatilisation reactions in the downgoing slab generally indicate that the main metamorphic release of both CO2 and H2O occurs in a pulse between 2.2 and 2.6 GPa, corresponding to depths of \"60-80 km in the forearc51,54,55. This would imply lateral flow of fluids into the forearc. However, the depth of decarbonation will also depend on the thermal structure of the subduction zone, with warmer subduction favouring decarbonation, commencing as shallow as 40 km55–57. The Semail subduction zone appears, at least initially, to have been hot as indicated by sediment-derived granitoid melts generated at granulite facies conditions58. This may have favoured metamorphic decarbonation at shallower levels. Overall the mixing of sedimentary pore fluid with carbon-rich fluids to produce the carbonating fluid implies lateral migration of fluids, since most pore fluid expulsion occurs relatively shallowly during subduction49,50. That said, the exact distance between these loci of devolatilisation and carbonation is difficult to constrain and may have been relatively small under a warm subduction geotherm.” (Lines 427-440)\nThis point is also further discussed under Implications for subducted carbon fluxes in response to comments from Reviewer 3. See lines 546-591 and Supplementary Text S2.\nLine 403: Based on the discussion starting on lines 368, it is not clear exactly what the depths of carbonation are. This discussion linking dehydration, carbonation, and depths of these reactions should be streamlined and better linked to the steps outlined in the model later in the manuscript.\nWe agree this was a deficiency in the original draft. This section of the discussion has been substantially rewritten to streamline and clarify things as suggested. See comments immediately above and in response to major point.\nRelevant changes too long to reproduce here, see lines 377-445.\nLines 404-405: This sentence seems like an add on.\nAgreed, sentence was removed during rewriting\nLine 418: delete “etc” and add “e.g.,” before dolomite\nAmended as suggested\nLine 462: Careful with the wording here because the serpentinites in this study do not have halogen ratios similar to those from other forearc regions (other regions have higher Br/Cl and I/Cl plotting up in the pore fluid range). (The subsequent explanation on the following lines on the evolving composition is much clearer here than previously in the manuscript. Some of these concepts need to be better introduced earlier to avoid confusion.)\nAgreed – the BT1b serpentinites don’t show evidence for a forearc setting for serpentinisation. This passage has been rephrased.\n“Rather than a single event we envisage formation of the listvenites as a multi-stage process: prior to carbonation the mantle peridotite protolith may have been partially serpentinised in a seafloor setting43. Subsequently the BT1b mantle section was incorporated into the forearc of a young subduction zone.” (Lines 498-501)\nLine 478: Reference? This is interesting, but if going to be mentioned then should be introduced earlier in the discussion.\nThis has been included as a possible driving force for carbonation in the introduction: “in the latter it is a product of some late-stage tectono-magmatic event in Oman’s geological history (e.g. elevated heatflow related to minor Paleogene magmatism23) and of regional rather than global significance.” (Lines 66-69)\nLine 508: Define Prop ClPF\nThis sentence has been rephrased to be clearer: “Mass balance of the composition of the carbonating fluid, assuming the proportion of Cl from pore fluid (48-87%), a pore fluid chlorinity of 0.11-0.15 wt.% and pore-fluid flux of 1x1013 kg/yr in the forearc63, yields a deep Cl flux of 4.5x1010–1.6x1011 g/yr.” (Lines 567-569)\nLine 550: A few more details on the bulk rock pyrohydrolysis + IC/ICP-MS halogen concentrations are needed. For example, what are the yields (or concentrations) of halogens on rock standards (e.g., Kendrick et al, 2018)? What are the blanks (e.g., halogen blanks from NaOH, V2O5, etc.)? Are blanks corrected for or are they all below detection limits? What were the analytical protocols for the IC and ICP-MS- or at least a reference to where these are outlined?\nFurther details of the method have been added and the reference for the full analytical protocols for IC and ICP-MS (Balcone-Boissard et al. 2009). Yields are difficult to distinguish from other sources of error (e.g. inhomogeneity of standards for halogens, analytical error) but typical uncertainties have been added in the text. Blanks were not subtracted as they were always below detection limits. This has been clarified in the methods.\n“A split of whole rock powders was analysed for bulk halogens (F, Cl, Br, I) at Institut de Physique du Globe de Paris, (IPGP), Paris, France. Full details of method and analytical procedures are given in [69]. [...] Repeat analyses of rock standards indicate mean relative errors of \"10% for F and Cl, \"30% for Br and \"60% for I [69]. Detection limits were well below sample concentrations for F and Cl (<100 g/L) and were approximately 10ppb and 1ppb for Br and I, respectively. No blank correction was necessary as procedural blanks were always below detection limits.” (Lines 602-604, 612-616)\nLine 584: Were rock standards analyzed via NI-NG-MS? Comments on the reproducibility of samples analyzed via NI-NG-MS? Given the poor reproducibility of Br and I on some samples via pyrohydrolysis and ICP-MS and the explanation was “sample heterogeneity or less reliable analytical reproduction of Br and I” it would be good to see if potential sample heterogeneity is also documented via NI-NG-MS.\nA comparison of Br and I concentration data collected by the two different methods shows stark differences. The Br concentrations via pyrohydrolysis are approximately 50% less than those of the noble gas method, except for one sample with \"470% more Br. The same is seen with I, in which I by pyrohydrolysis records much lower concentrations except for a couple with \"400-500% more I. Some mention of the poor comparison is needed.\nSecondary rock standards were not analysed however several primary standards and flux monitors are included in each irradiation tube and the results are reported relative to those standards (see Kendrick, 2012 Chem Geol, and Ruzie-Hamilton et al. 2016 Chem Geol, for detailed discussion of the method). The main reason secondary standards are not included is the general lack of well-characterised homogeneous and matrix matched standards (e.g. for serpentine). A further issue is the limited space in an irradiation tube and the large proportion of the tube taken up by primary standards/monitors. Given the significant costs associated with irradiation, adding secondary standards too would be prohibitive.\nThis approach is common across the labs engaged in halogen analyses of these type (see e.g. papers from Kendrick’s group). We also note that similar issues have been faced trying to identify rock standards for Ar-Ar with inhomogeneity of potential standard materials a common pitfall (e.g. Alexander and Davis, 1974, GCA) and these too are commonly reported without additional secondary rock standards.\nRegarding comparison of datasets, a new section has been added to the methods to specifically comment on the agreement or lack thereof between different halogen analyses (pyrohydrolysis, NI-NG-MS, SIMS, EPMA) and explain the choice of NI-NG-MS over pyrohydrolysis data (in response to comment below).\nWe note that there is good agreement between SIMS and NI-NG-MS results and suggest that low abundance of Br and I in pyrohydrolysis may be due to low yields:\n“Comparison of bulk halogen data collected by NI-NG-MS and pyrohydrolysis reveals some notable differences. Chlorine abundances generally agree reasonably well however, the abundance of Br and I is lower in most pyrohydrolysis analyses suggesting low yields during pyrohydrolytical extraction, possibly related to the unusual sample matrix. Monitoring sample-by-sample yield requires the use of hazardous radioisotope tracers (e.g. 125I [76]) and was not performed in this instance, so this cannot be confirmed.\nIt is difficult to directly compare SIMS and NI-NG-MS data since these were measured on individual minerals and bulk rocks chips, respectively. Nonetheless, the Br/Cl and I/Cl ratios are all within error of the mean of SIMS data from serpentine (which is likely to dominate the whole rock signature). Moreover, bulk NI-NG-MS data and individual serpentine SIMS data are colinear in Br/Cl – I/Cl space suggesting any slight differences arise due to differences in the proportions of different serpentine endmember compositions analysed by SIMS versus the bulk composition of the sample. The SIMS chlorine data also agree well with results from EPMA. The correspondence of halogen data from NI-NG-MS, SIMS and EPMA shows the consistency of the data across multiple methods.” (Lines 738-753)\nIn Table S2 is says that Cl, Br, and I values determined by NI-NC-MS are preferred. Why? But, then in Table S1, it says that F and F/Cl uses F and Cl data measured by Pyr-IC. Why is the Cl measured by Pyr-IC used if NI-NC-MS is preferred? And is the Cl Pyr-IC data only used for the F/Cl and not [Cl]? In Table S2, it is denoted that Cl is measured via ICP-MS (not IC).\nThis is now explicitly discussed in an additional section to the method referred to immediately above. In short, there were a large number of samples below detection for Br and I by pyrohydrolysis which made it complicated to calculate a meaningful mean composition. Producing a complete whole rock dataset for these elements was the main motivation for the NI-NC-MS analyses.\nF/Cl was calculated using Cl from pyrohydrolysis because of the potential for intrasample heterogeneity, meaning a ratio of two analyses made on same sample aliquot is likely to be more accurate.\n“A significant proportion of listvenite samples had Br and I below detection by pyrohydrolysis. This makes calculation of a mean composition problematic and prone to systematic error depending on the assumptions made about the distribution of the data (e.g. log-normal, gaussian, uniform). In producing an overall bulk halogen dataset, the NI-NC-MS data are preferred for the heavy halogens (Cl, Br, I) since they provide a complete dataset for all samples analysed. These data are supplemented by the F abundance from pyrohydrolysis. To avoid any potential inaccuracy due to analytical differences between the two methods, the F/Cl ratio for each sample was calculated using the Cl abundance measured by pyrohydrolysis.” (Lines 754-762)\nTable S2 – this was an error. Cl was measured by IC. The table has been corrected\nLine 617: Was only one glass standard used for Cl and F SIMS analyses? Is there concern about matrix effects given that serpentine, carbonate, and vein material was analyzed? On Table S5, both EMPA and SIMS Cl data are given. With the exception of some very high Cl concentrations via EMPA, there is reasonable agreement. This would be good to point out.\nYes indeed matrix effects are a potential concern however this is difficult to address as there simply aren’t matrix matched standards available. This would certainly be an area of interest to improve the method.\nHowever, we do not believe this affects our conclusion. The most important interpretations are based on a) ratios (e.g. F/Cl) where matrix effects are likely to at least partly cancel out and b) on relative differences between samples of the same matrix (e.g. comparison of carbonate compositions). For instance, although there might be some difference in matrix effect between magnesite and dolomite this is unlikely to result in a 100x increase in the apparent abundance of F in the later. Moreover, we see some dolomite with low F and some magnesite with high F (Fig. 6) clearly indicating that matrix effects alone cannot explain the variation seen.\nAs well as the good agreement between Cl measured by SIMS and EPMA, we also note the good agreement between NI-NG-MS and SIMS data for serpentine (see Fig. 7) wherein all SIMS data and bulk ophicarbonate data lie colinear in Br/Cl – I/Cl space. Given the large degree of scatter which typifies most heavy halogen datasets, even from a single locality (e.g. Fig. 7b; Carter et al. 2021 GCA) this array is strikingly tightly defined and gives us good confidence in the robustness of both datasets.\nThe potential for matrix effects is now explicitly commented on in the methods:\n“Matrix effects are a potential issue in SIMS analysis75. These effects are difficult to assess or correct for since there are no widely available matrix-matched standards available for halogens in serpentine or magnesite. Nonetheless, the good agreement of SIMS, EPMA and NI-NG-MS data (see below) lends confidence that any matrix effects are relatively minor. Furthermore, differences in matrix effects are likely to be minimised, although not absent, in analyses of the same mineral, meaning that relative differences in compositions of a given mineral can be compared with some confidence.” (Lines 728-734)\nTable S1: What does “shallowater-corrected data used for I abundance and I/Cl ratios” mean?\nThe production of 128Xe from I during irradiation can be monitored using either scapolite I mineral standards (which are also used to monitor 80,82Kr Br production from Br) or from Shallowater meteorite irradiated in the tube with the samples. Generally, Shallowater-corrected data is preferred as it has lower intrinsic uncertainty, although the two values are almost always within uncertainty of one-another (and indeed offer a helpful check on the internal consistency of the data). Both values can be compared in Table S5.\nReviewer #2 (Remarks to the Author):\nThis manuscript focuses on the carbon cycle in subduction zones, exploring the role of carbonated mantle peridotites in sequestering subducted CO2 through a study of the Oman ophiolite. The research is of great significance, aiming to address the long - standing debate regarding the origin of carbonating fluids and their impact on the global carbon cycle. The approach combines multiple analytical techniques, which is innovative and promising. In general, I think this manuscript is well written. I don't have special comments for the authors. I would suggest acceptance for this manuscript.\nThank you for the positive comments it is nice to read that you found the approach innovative and the manuscript well-written.\nReviewer #3 (Remarks to the Author):\nCarter et al. present a well written and illustrated paper integrating careful petrography with multiple geochemical techniques, several of which are innovative in their application to the topic. The paper presents a new, untapped line of evidence (halogens) and a compelling petrographic case for a two stage listvenite formation in the SE Oman ophiolite. This conclusion does the best job so far in the literature of reconciling the controversial mismatch between previous geochronological and geochemical interpretations. The authors take this conclusion and point out its implications for a hidden carbon sink in forearc mantle, which would represent an important component of subduction systems and the deep C cycle.\nThese are the now the most studied listvenites in the world and will doubtless continue to receive attention from the subduction zone cycling community if a forearc wedge carbonation interpretation is partly true. The rocks and reactions are also highly relevant to recent efforts to engineer peridotite carbonation as a means to permanently sequester CO2. Halogens as important fingerprints, ligands, and catalysts in such reactions are an important but overlooked part of this effort.\nThe subject matter is of vibrant interest and the conclusions are of relevance to several different swaths of our community: those working on listvenites and ophiolite, those on subduction zone cycling, and those on global biogeochemical cycling.\nThank you for your thoughtful comments, both positive and constructively critical. These have been addressed in full, and this has certainly improved the manuscript. We were particularly pleased to read the positive comments regarding our attempts to reconcile of geochemistry and geochronology.\nI have a few concerns below and many minor comments, that should hopefully be possible to address with minor/moderate revisions.\nGeneral comments\nThe main argument I see against the principle thrust of the paper – that forearcs are a hidden carbon sink, is that listvenites are discontinuous along the metamorphic sole in Oman, and rarely present in other ophiolites with exposed soles (?), and have not been dredged from forearc sections in the Marianas or Tonga trenches. Can this be considered, explained or conclusions tempered to reflect this reality? Showing the locations of all the listvenites described in the ophiolite in a more detailed Fig 1a would assist this discussion.\nThis is an important comment, and we accept that there is more work to be done to fully delineate the controls on listvenite formation and whether this might be favoured in certain subduction zones or at certain times in geological history due to factors such as thermal structure, sedimentary input. In particular we note that the Marianas and Tonga forearcs while probably the best studied in terms of dredges and drilling into mud volcanoes also may have poor potential for listvenite carbonation since they have low carbonate content in sedimentary input (e.g. GLOSS-II; Plank, 2014) and cold geotherms neither of which favour generation of CO -rich fluids, especially at relatively 2 shallow depths (e.g. <50km). Production of CO -rich fluids is favoured by hot 2 subduction geotherms and mixed carbonate-siliciclastic sediment input (Stewart and Ague, 2020, Nat. Comms.) both which appear to have been true of Oman (e.g. production of plagiogranites by melting of Haybi complex, Cox et al. 1999, Contrib. Min. Pet.). Where CO -rich fluids are allowed to migrate without re-equilibration (e.g. 2 channelised along the slab interface) they may promote significant carbonation. It is also notable that the Central American forearc which shows strong evidence for carbonation occurring at depth (Barry et al., 2019, Nature) is subducting sediments of intermediate composition (~25% CO ; Plank, 2014).\n“However, it should be noted that listvenites have not been directly observed in dredges or drilling at modern forearcs such as Mariana and Tonga which allow fluid compositions at slab depths to be inferred. This highlights key differences between those and the Semail subduction zone which may promote listvenites genesis in the latter, including: subduction geotherm (cold65 vs hot29), sedimentary input (dominantly siliceous, 2.6% CO266 vs mixed volcanic-carbonate-siliciclastic, 8.6% CO2¬18,67), and even redox state and the relative timing of serpentinisation and carbonation68 (see Supplementary Text S2). This raises the important possibility that the magnitude of the forearc carbon sink may vary spatially and temporally in response to changes in oceanic sedimentation, subduction zone thermal regimes and redox state over geological time and could therefore play an important role in fluctuations in the long-term carbon cycle.” (Lines 581-591)\nListvenites are widely distributed throughout the Semail ophiolite near to the basal thrust. We also note that the metamorphic sole is also discontinuously exposed along the basal thrust having been removed by late-stage brittle thrusting (Searle et al. 2022, J. Struc. Geol.). It is not unreasonable to imagine a similar fate for listvenites both in Oman and other ophiolite localities. Finally, although listvenites are not present in all SSZ ophiolites they are nonetheless relatively common as demonstrated by the recent review of Menzel et al. (2024; Earth Sci. Rev.). The lack of listvenites in some ophiolites may relate to poor preservation potential during obduction or to differences in formation potential as discussed above; more work is needed to clarify these important points.\n“Listvenites are widespread although not continuously exposed throughout the ophiolite (Fig. 1a). The metamorphic sole is also not ubiquitously preserved and in numerous places late brittle thrusting has juxtaposed relatively unmetamorphosed allochthonous sediments against the mantle section31. This suggests that listvenites might also be prone to removal during obduction and may therefore have relatively poor preservation potential, especially in less laterally extensive ophiolites. In the broader geological record there are numerous examples of listvenites associated with the basal thrusts of supra-subduction zone (SSZ) ophiolites although often with some ambiguity as to the timing of their formation relative to obduction11. Nonetheless they are not ubiquitous in SSZ ophiolites with exposed metamorphic soles which suggests either that they are poorly preserved or that specific conditions are required for their genesis.” (Lines 549-560)\nFig. 1 has been updated to include published listvenite localities (Nasir et al., 2007, Geochem.; Boudier and Nicolas, 2018, Tectonics).\nIts important to note that Scharf et al. worked on listvenites from another locality, and that this study focuses only on listvenites from BT1B. While the drilled localituy is the among the largest listvenite bodies in the ophiolite, it is not the only one. If there were indeed two pulses of carbonating fluids, then it would follow that they have different spatial foci/extents, which might explain differing conclusions from workers in different parts of the ophiolite.\nAgreed – the key passage has been updated to explicitly suggest this as a reason for divergence of opinion in the literature:\n“Overall these results are unsurprising if ophiolitic mantle sections are viewed as a patchwork of overprinting metasomatic events (e.g. ref 24) not all of which may be apparent or detectable at all scales or at all localities. It is likely that multiple stages of fluid infiltration affected the ophiolite prior to, during and since obduction and may have overprinted some chemical or isotopic systems. Furthermore, the extent of such overprinting may have varied spatially resulting in different conclusions from studies at different sites (e.g., ref. [17] cf. [21]). Nevertheless, our data indicate a strong subduction affinity which has escaped such overprinting and attest to fluxing and carbonation of the shallow forearc by CO2-rich slab derived fluids.” (Lines 523-531)\nThe conclusions of this paper rely heavily on the petrographic evidence for dolomite overprinting magnesite. I don’t find the evidence presented very clear/convincing. Can it be expanded on or shown in higher resolution/different imaging types?\nWe accept that the original version of Figure 4 didn’t show the petrographic evidence in high enough resolution. This figure has been redrafted with 4 insets added showing key areas at higher magnification and annotated with our interpretation of cross-cutting relationships. We have also highlighted in the text that many other studies have interpreted dolomite as a late forming phase.\n“Our observation that dolomite is a late-forming phase is corroborated by numerous other studies of listvenites from BT1b and nearby localities19,33,35.” (Lines 479-481)\nThe final panel of Fig 8c and the accompanying metamorphic sole F-rich fluid source interpretation doesn’t make sense. By the time of this F rich dol precipitation (~60 Ma by this model), the sole had already been metamorphosed to amphibolite facies (and exhumed). Exhumation and retrogression of these rocks cannot produce a fluid at shallow depths – they are almost dry following their trip into the mantle. The source must be tectonically deeper than the sole in lower grade rocks. There are also several nappes beneath the ophiolite that are omitted here, as ‘Arabian continent margin’ evokes the autochthonous continental basement. I recommend consulting the cross sections in Searle (2007, Geoarabia) to get the tectonostratigraphy correct, and this may help with your interpretation of F rich fluid source options.\nWe agree that the original version of this figure was potentially misleading. The discussion and figure have been updated to better reflect the relative timing of events and geological relationships. We suggest the F-rich Stage 2 fluids may have derived either from metamorphism of underlying shelf carbonates during obduction (e.g. Grobe et al. 2019, Solid Earth) or from fluid circulation associated with Paleogene magmatism (e.g. Wilde et al. 2002, J. Virtual Explor.; Scharf et al. 2020, J. Afr. Earth Sci.). Their similarity in composition to the metamorphic sole lithologies is suggested to derive from interaction with allochthonous sediments/volcanics from which the metamorphic sole formed although it could be co-incidental (e.g. reflecting an F-rich magmatic fluid source).\nDiscussion has been redrafted to more clearly explain proposed model:\n“Later, Stage 2 dolomite and magnesite crystallisation occurred from a Ca-rich fluid with higher F/Cl and I/Cl ratios. This stage is only dominant in some horizons and microstructurally crosscuts earlier carbonates. Fluids derived from low temperature metamorphism of autochthonous shelf carbonates during obduction53 are a plausible source for the F- and I-rich Stage 2 fluids, especially given the marked enrichment of Ca in the lowermost listvenites overlying the basal thrust20. Their similarity in halogen composition to the metamorphic sole may relate to interaction with overlying allochthonous units, the protoliths for the metamorphic sole20,31. Equally, Stage 2 carbonates could be derived from interaction with another F-rich reservoir (e.g. fluid circulation associated with later igneous activity23,62).” (Lines 511-520)\nFigure 8 has been redrafted to clearly show the nature of underlying autochthonous and allochthonous units and to show more explicitly the two possible formation mechanisms for Stage 2 carbonation.\nFigure comments:\nFig. 1. The intervening met. sole close up section (Fig 1c) breaks the flow of the figure from left to right. Is it really necessary considering it has been well described and no samples in this study come from it? If you deem it so, it may be better to either side of the full 0–300 m stratigraphy.\nFig 1c (now Fig. 1e) does not show the metamorphic sole but the reaction zone between listvenite and serpentinite (~100 m depth) which this manuscript focuses on. We do accept, however, that its position broke the flow from Stratigraphy to Mineralogy. This has now been moved to rightmost.\nFig. 2. Annotate the side of the fig. with an arrow indicating carbonation intensity.\nAnnotation has been added as suggested\nFig. 4. I don’t find the dol x-cutting particularly convincing, at least at the resolution in the merged PDF. Dol doesn’t have euhedral faces, it looks like the Mgs is corroding early dol. The red vs white arrows don’t seem to have differing significance?\nThe first formed magnesite veins have a Fe-rich medial lines and grade outwards to Fe-poor magnesite. This structure can also be seen in Sample 44z3 (Fig. 2b). This structure is clearly crosscut in numerous places by dolomite. Dolomite also appears to be growing interstitially between magnesite with globular and lozenge like habits (Fig. 4f). We are not the first to observe that dolomite forms as a late phase really to most magnesite-dominated carbonation; Beinlich et al. (2020, JGR:SE), Menzel et al. (2022, Solid Earth) and Decrausaz (2023, Eur. J. Min.) all note the formation of dolomite principally as a late phase in BT1b listvenites (and 2 further sites in the latter reference). Menzel et al. (2022) likewise note the common occurrence of Fe-rich medial lines in the earliest forming magnesite. These indicate that our petrographic observations are robust and are confirmed in study of a wide-range of samples from BT1b (and beyond).\nWith the exception of Menzel et al. (2022), these studies were already cited in the original manuscript:\n“Textural evidence from this and other studies indicates that dolomite is a late forming phase19,33–35; it primarily occurs either overgrowing/crosscutting zoned magnesite-ferromagnesite veins or as an interstitial phase (Fig. 4).” (Lines 184-187)\nVery few of the carbonate phases show idiomorphic or euhedral forms, and have been taken to suggest formation under disequilibrium conditions (Beinlich et al. 2020).\nHowever, some of the dolomite does show subhedral lozenge-like habit which may reflect free growth into porosity following a phase of dissolution.\nThe red and white arrows in Fig. 4 highlight dolomite cross-cutting and growing interstitially between magnesite crystals respectively. This is stated in the figure caption.\nHow closely does the drill hole depth approximate the true pseudostratigraphic depth? This should be mentioned in the caption or annotation.\nHole BT1b was drilled at an incline so depth in Hole is close to true pseudostratigraphic depth. Caption to Fig. 1 has been updated to reflect this:\n“The hole was drilled sub-perpendicular to lithological boundaries so depth is approximately true thickness16.” (Lines 93-94)\nFig. 8. IN panel a, it looks like the mantle wedge is dehydrating and decarbonating.\nAgreed. Figure has been amended to be clearer that sediments are dehydrating\nLine by line comments:\nL45. The tectonic setting of the ophiolite does still attact controversy, a reference here would serve to illustrate which of the various models the authors prefer to contextualize this work.\nA reference (Goodenough et al., 2014) has been added as suggested (Line 46)\nL50. Adding specifics of the Sr ratios in parentheses would be helpful here. Also, for transparency, note that they overlap the Hawasina metasediments, but also other crustal materials present in the sub-ophiolitic sequence, i.e. they are also likely intermediate between the protolith and the more radiogenic autocthonous basement.\nSpecifics of Sr isotope ranges mentioned have been added as suggested. However, we are not aware of any work which suggests involvement of autochthonous basement in listvenite formation, we therefore feel that reference to this would be somewhat distracting and potentially misleading.\n“This study focuses on listvenites from Hole BT1b of the Oman Drilling Project16 drilled in Wadi Mansah (23.364374°N, 58.182693°E) in the Semail Ophiolite. Listvenites from BT1b and nearby exposures show elevated 87Sr/86Sr ratios (0.7092–0.7145) relative to uncarbonated peridotites (\"0.7028), Cretaceous to modern seawater or groundwater (\"0.707-0.709), but which overlap the isotopic composition of underlying allochthonous Hawasina metasediments (0.7082–0.7241)17,18.” (Lines 49-54)\nL55. There is certainly a mismatch in the interpretations of the trace elements and geochronology, the key point to made/tone to be struck is that the uncertainty/apparent nature of the dichotomy is not a fault of the data, but interpretation of them on one or both sides of the argument. Set out the timing constraints specifically here, as it is crucial context.\nThis is a fair point. The sentence has been slightly rephrased to put the emphasis on the interpretation rather than the data. The timing constraints have also been set out in more detail:\n“However, there appears to be a mismatch between interpretations based on trace element and isotope geochemistry which suggest a forearc subduction zone setting for listvenite formation17,18,20,21 and those from recent in-situ U-Pb geochronology on samples from localities near (<10 km) Hole BT1b. The latter yielded dates from two dolomite veins and several imprecise dates from listvenites, all <60 Myr, postdating active subduction by at least 20 Myr22.” (Lines 59-64)\nL63. State which halogens.\nAmended as suggested: “The heavy halogens (Cl, Br, I) are hydrophilic and make ideal tracers for interrogating fluid processes in ophiolites24–26” (Lines 71-73)\nL78. What other volatile cycles does it have a bearing on?\nAmended to include examples: “may be a significant and overlooked part of the global carbon and other volatile element cycles (e.g., H2O, Cl).” (Lines 84-85)\nL94. The origin of those metasediments/metabasalts is of importance for context here, as is the intervening deeply subducted and exhumed met. sole. See Searle’s papers for reference.\nFurther geological context for units underlying basal thrust has been added as suggested:\n“The basal thrust of the ophiolite tectonically juxtaposes mantle peridotites on top of allochthonous metasediments and metabasalts of the Hawasina and Haybi complexes which are themselves underlain by autochthonous shelf carbonates29,30. A greenschist to amphibolite grade metamorphic sole formed by subduction and exhumation of allochthonous lithologies is discontinuously exposed along the basal thrust¬30,31.” (Lines 108-113)\nL96. Reference for the Oman Drilling Project?\nReference (Kelemen et al. 2020, Proc. Oman Drilling Project) added as suggested (Line 116)\nL97-101. *More tectonostratigraphically complete. This sequence is not a stratigraphy.\nTectonostratigraphy/pseudostratigraphy are sometimes used in this situation. Or otherwise avoid the term altogether.\nAccepted. The term has been omitted here and replaced with pseudostratigraphy where necessary.\nL107. How is the minimal impact of protolith heterogeneity established for these rocks? Sharp compositional variations between dunite/harzburgite are common near the sole.\nWe accept this point. Such protolith heterogeneity is very unlikely to impact the halogens which will have low concentrations. Nonetheless we don’t have a strong constraint on the degree of protolith heterogeneity and so reference to this has been omitted:\n“This interval shows a sharp compositional gradient over which the primary mineralogical variation is due to the degree of interaction with the carbonating fluid.” (Lines 123-125)\nL110. Reaction zone at the base of the upper listvenite? Or where?\nThe reaction zone is near the base of the upper listvenite but within it (Fig. 1b). The boundary between upper and lower listvenite is made on the basis of trace element geochemistry at \"110 m depth (Godard et al., 2021, JGR:SE). The original version of Figure 1 was based on the original shipboard logging. Fig. 1 has now been updated to align with the geochemical division of Godard et al. (2021) and clearly shows the location of the reaction zone.\nThe depth of the serpentinite body in question is now indicated in the text to clarify this:\n“Within the upper listvenite a 20 m thick lens of serpentinite and ophicarbonate (partially-carbonated serpentinite) is preserved (80-100 m depth) and affords almost complete preservation across a \"5 m thick reaction front between the serpentinite protolith and fully-carbonated listvenite (Fig. 1e).” (Lines 120-123)\nL133. Along the reaction zone? This needs clarification (further into the reaction zone? With increasing carbonation?)\nClarified as suggested: “Serpentine mineral stoichiometry from electron probe microanalysis (EPMA) indicates variable mixtures of lizardite with increasing proportions of talc with increasing carbonation” (Lines 147-149)\nL132. Don’t all three serpentine minerals have similar stoichiometry?\nYes indeed. Lizardite and chrysotile are true polymorphs, antigorite has very slightly higher Si content. Based on petrographic observations, mesh textured lizardite is the main serpentine phase present (cf. bladed forms typical of antigorite, e.g. Carter et al. 2021, GCA). Similar observations were made by Beinlich et al. (2020, JGR:SE). The main point being made here is that (whatever the polymorph) the serpentine is accompanied by variable proportions of talc in a microcrystalline mixture reflecting variable extent of carbonation.\nSome extra detail of petrographic observations added to preceding paragraph:\n“Petrographic observations and mineralogical analysis by powder X-ray diffraction (XRD) reveal that ophicarbonates consist predominantly of mesh-textured lizardite with variable replacement by carbonate and talc (Fig. 1c, d, Fig. 2).” (Lines 145-147)\nL217-219. Would it? This is not a closed system process, the reaction proceeds because fresh CO2 rich fluids are introduced and spent ones (necessarily) removed, maintaining high CO2 activity and high fluid/rock ratios. The implication would be Cl addition to serpentinites downstream. Is this seen?\nThis isn’t necessarily an instantaneous process but incrementally yes: each time some proportion of serpentinite is converted to talc or carbonate it takes in CO and expels Cl 2 which must be lost to the fluid. There is potentially some petrographic evidence of this in the form of Cl-rich veins which increase in abundance with carbonation intensity (e.g. Fig. 2).\nIndeed, we would expect to see Cl increase “downstream”. It is hard to assess if this is happening – it could have contributed to the higher Cl contents of sample 44z1 (the least carbonated). However, more detailed sampling extending further from the front of listvenite formation would be necessary to really map out the presence of such an effect.\nL296. It is not surprising the Oman serpentinites have markedly different halogen compositions to seafloor serpentinites. Can a more relevant comparison be made to other ophiolitic serpentinites that would rule out the distinct possibility this is some common feature of deeply formed, ophiolitic serpentinites (or those formed by meteoric fluids?)?\nA direct comparison of the reaction zone serpentine with seafloor, forearc and ophiolitic serpentinites has been added. However, it does in fact it appears that serpentinites in Oman far from the basal thrust have similar halogen compositions to seafloor serpentinites (D’Andres, 2021, thesis; Fig. 7).\n“It is possible that the unusual composition of serpentine in the least carbonated samples reflects an atypical serpentine chemistry characteristic of the Oman mantle section. However, serpentinites sampled far from the basal thrust are more typical of seafloor or forearc serpentinites, with Br/Cl close to seawater43 (Fig. 7b).” (Lines 312-315)\nL306. What is meant by equilibration of a serpentine protolith? Recrystallization?\nChemical exchange could occur in the solid state but admittedly this might be quite inefficient at low temperatures and with low porosities. Recrystallisation is another possibility, especially given the abundant mineralogical transformations the rocks have undergone. Given the pseudomorphic preservation of mesh texture even in fully carbonated rocks, this may not have left clear texture evidence.\nPassage has been rephrased: “either by equilibration/recrystallisation of serpentine with the fractionated fluid or by direct serpentinisation” (Lines 322-323)\nL407. It is more than ‘somewhat’ in conflict, at least for a single event carbonation model, which is worth specifying. Also, it should be ordered 100–80 Ma; however, recent petrochronological work from e.g. Garber et al. has failed to reproduce the >100 Ma Guilmette et al prograde dates, so it may be safer to say from ‘at least 96 Ma to \"80 Ma’ or to focus on the cessation date, which is the relevant one here.\nWe agree with both suggestions. Passage has been rephrased:\n“However, despite this geochemical evidence requiring input of decarbonation-derived CO2-rich fluids, a subduction origin for carbonating fluids is contradictory to interpretations from recent geochronology which has dated dolomite veins in Oman listvenites to 60 Ma and younger22, significantly after the cessation of subduction at \"80 Ma 59,60.” (Lines 441-445)\nL441. I agree this is significant, but the petrographic evidence as provided is not that clear/definitive. In line with my previous comment can this be bolstered?\nSee comments above in response to major point. Although they are mentioned earlier, we have added another reference here to other studies which corroborate late dolomite formation:\n“The dating of Scharf et al. 22 relies primarily on analyses of dolomite. Our observation that dolomite is a late-forming phase is corroborated by numerous other studies of listvenites from BT1b and nearby localities19,33,35.” (Lines 478-481)\nL446. These samples were from a different locality, so I would not say that they have been identified (with certainty implied) as belonging to a distinct carbonation event. Your data rather provides a compelling explanation as to why they may belong to a distinct carbonation event and potentially represent minimum ages.\nThis is fair point. The passage has been slightly rewritten to highlight the difference in locality between the studies and to more appropriately qualify the uncertainty associated with extrapolating results between sites:\n“The dating of Scharf et al. 22 relies primarily on analyses of dolomite. Our observation that dolomite is a late-forming phase is corroborated by numerous other studies of listvenites from BT1b and nearby localities19,33,35. The identification of BT1b dolomites as belonging to a distinct carbonation event with a markedly different fluid composition therefore suggests that the young dates from other sites may represent minimum ages for the main phase (Stage 1) of carbonation, albeit with some inherent uncertainty in extrapolating results between localities. This would suggest that the bulk of carbonation occurred before 60 Ma, coincident with subduction, as indicated by a range of geochemical proxies – not least the sedimentary pore fluid halogen signature and carbon isotopes18 of CO2 bearing fluids.” (Lines 478-488)\nL451. It should be noted that this statement is limited in scope to BT1B, clearly some aspect of the lisvenites (primary or secondary) was suitable to dating to Scharf et al.\nWe have added a qualifying that we are discussing the bulk geochemistry of BT1b listvenites and qualified the final statement with the proviso that it may not apply if bulk geochemistry of the samples dated by Scharf et al. (2022) differs significantly from that at BT1b. There is not to our knowledge any bulk geochemical data available for these samples. However, there is no particular reason to believe that these listvenites are of completely different (i.e. high U/Pb) composition and so we feel that this is a reasonable statement to make given the available evidence.\n“It should also be noted that the bulk chemical characteristics of the BT1b listvenites suggest that they are highly unsuitable for U-Pb dating. All the listvenites and associated serpentinites have very low U/Pb ratios and lie at the extreme end of a trend defined by forearc serpentinite compositions reflecting low fluid U contents under low fO2 conditions20,61. A priori it would be expected that any attempt to date such materials by U-Pb methods would be inherently biased towards later perturbations under more oxic conditions where U would be more soluble in fluids. We tentatively suggest that, unless their listvenite bulk geochemistry differs significantly from that at BT1b, a sequence of later fluid infiltration events may be what was dated by Scharf et al.22.” (Lines 489-497)\nL460. Is this parallel list not reversed?\nYes indeed this was a mistake and has been amended:\n“Water- and carbon- rich fluids derived, respectively, from sediment compaction and decarbonation or dissolution reactions deeper in the subduction zone reacted with the serpentinites forming talc and eventually carbonate.” (Lines 501-504)\nL479. It would be a pertinent place to note a fact all too ften forgoteen by studies of these cores, that these rocks were transposed under a ~15 km thick ophiolitic nappe over 100s of km of continental margin (Searle et al. 2007) before reaching their final resting place. Rocks so close to the basal thrust could hardly escape progressive overprinting by metamorphic fluids during this translation.\nThis is a very good point and we have incorporated this into a more nuanced model for generation of the Stage 2 carbonating fluids:\n“Fluids derived from low temperature metamorphism of autochthonous shelf carbonates during obduction53 are a plausible source for the F- and I-rich Stage 2 fluids, especially given the marked enrichment of Ca in the lowermost listvenites overlying the basal thrust20.” (Lines 513-516)\nL482: the ophiolite does not have 100 Myr subaerial exposure history, indeed, it is only 96 Ma and a large part of its life was underwater.\nThis was imprecise language on our part and indeed misleading; the intended meaning was ~100 Myr near-surface history, or words to that effect. This passage has been rephrased:\n“It is likely that multiple stages of fluid infiltration affected the ophiolite prior to, during and since obduction and may have overprinted some chemical or isotopic systems.” (Lines 525-527)\nL484. Last line could be more specifically geared toward carbonating fluids. There is nothing controversial about forearc slab fluid fluxing.\nAgreed. Sentence has been rephrased:\n“Nevertheless, our data indicate a strong subduction affinity which has escaped such overprinting and attest to fluxing and carbonation of the shallow forearc by CO2-rich slab derived fluids.” (Line 529-531)\nLine 533: Spell out IPGP.\nDone"}, {"round": 2, "reviewer_comments": {"reviewer_3": "The Authors have done an exceptionally thorough job of addressing the comments, leaving tighter and more balanced/considered manuscript than previously. I congratulate them and recommend it for immediate publication.", "reviewer_4": "The authors have adequately addressed my concerns in the revised manuscript and supplement. I recommend that this manuscript for publication."}, "author_response": "We are pleased to read that the reviewer found our approach novel and worthy of publication and thank them for their well-considered comments which have helped clarify and strengthen several important areas of discussion.\nComments on Main Manuscript:\nLines 222-223:\nThe negative correlation between bulk halogen content and bulk CO2 content is clearly demonstrated by Figure 5, but the two-stage process for Cl loss is an interpretation of the data. This could also be a single-stage process with a non-linear correlation between Cl and CO2. The wording here needs to be clear that this is an interpretation.\nThis is true and indeed we did calculate an exponential fit to the data (Cl = 0.1088exp[-0.12399CO ]) and from this calculated an initial rate of Cl loss (dCl/dCO = -0.12399Cl) 2 2 even higher than that from the two-stage linear model (-1.35 x 10-2 @ CO = 0). This was 2 included in Table S8 for information but not further discussed in the text.\nSince our calculated initial Cl loss involved projecting the exponential curve to zero CO , 2 beyond our ophicarbonate data (min CO = 7%) this was judged to be potentially 2 inaccurate as the evolution of the system in the very earliest stages of carbonation (0-7% CO ) is not well-constrained. Calculating a rate of Cl loss from the exponential curve 2 for CO = 7.06 (i.e. the least carbonated point where it is constrained) gives RCl of -5.62 2 x 10-3 very similar to the part 1 linear value of -6.43 x 10-3. Given the similarity of these results, we favour the linear model as being simpler. We have now clarified in the main text that the two-stage model is an interpretation and highlighted the alternate exponential model. This is further expanded upon in the supplementary text to make the above arguments explicit.\nLines 222-226: “For Cl there is rapid initial loss followed by a more gradual decrease. We interpret this as a two-stage process but the data can also be fit by an exponential curve (Cl = a⋅e-k CO2) which yields a similar or higher rate of early Cl loss (Supplementary Text S1, Table S8).”\nLines 233-235: “Change in bulk halogen abundance accompanying carbonation. (a) Cl versus CO showing rapid initial Cl loss interpreted as two distinct stages of reaction 2 with different slopes”\nSupplementary text p.3: “Alternatively, Cl loss can be fitted by exponential function as:\n𝐶𝑙 = 𝑎∙𝑒−𝑘𝐶𝑂2 (10)\nWhere a is 0.1088 wt% and k = 0.12399 (with CO in wt%). The rate of Cl loss against 2 CO is then itself dependent on CO and given by:\n2 2\n𝑑𝐶𝑙\n𝑅 = = −𝑘𝑎∙𝑒−𝑘𝐶𝑂2 (11)\n𝐶𝑙 𝑑𝐶𝑂\n2\n𝑅 = −𝑘∙𝐶𝑙 (12)\n𝐶𝑙\n[…]\nSince calculating an initial rate of Cl loss from the exponential model involves projecting beyond the sample data to zero CO , there is a risk this may not accurately reflect the 2 behaviour of the system in the very earliest stage of the reaction (CO = 0–7 wt%). 2 Calculated at the CO content of the least carbonated sample (CO = 7.06 wt%), the 2 2 exponential model gives a very similar R to part 1 of the two-stage linear model. Given Cl this similarity and in the interests of simplicity, we use only the linear model hereafter.”\nLines 257-259 and Figure 7c:\nI don’t find this claim very convincing. Serpentine Br/Cl and I/Cl from all ophiocarbonate samples seem to span most of the range of Br/Cl and I/Cl. The boxplot in Figure 7c tries to show the trend with bulk CO2, but I don’t see much of a trend in the data. However, a statistical test of correlation or differences in the Br/Cl in different samples would be the best way to show this.\nBromine/Cl measured in both bulk samples and individual SIMS spots correlates positively with bulk CO for ophicarbonate samples. A linear regression of the former 2 gives an R2 of 0.8 and p-value of 0.006 (n = 7), while the latter gives a relatively low R2 of 0.16 but with a p-value of 1.4 x 10-7 (n = 165) indicates that correlation is statistically highly significant (p <0.05) despite a large degree of scatter resulting from intrasample heterogeneity (as seen e.g. on Fig. 7a). The text has been amended to refer to these statistical tests of correlation.\nLines 261-264: “Both bulk Br/Cl and individual spots measured by SIMS show statistically significant correlations with bulk CO (R2 = 0.81, 0.16, respectively), the 2 latter affected by intrasample scatter but nonetheless statistically significant (p = 1.4 x 10-7, n = 165).”\nLines 343-355:\nTo use the equation on line 353 to determine the CO2/Cl of the fluid, this paper makes assumptions about the Br/Cl of the starting fluid, the final fluid composition, and how Cl and Br change with changing CO2 content during Stage 1 of the model. In the model presented, an initial fluid with a sedimentary pore fluid-like composition alters the peridotite forming serpentine, which then reacts with CO2 to form carbonates, preferentially expelling Cl over Br and resulting in the expulsion of a low-Br/Cl fluid. This fluid then travels through the peridotite farther from the CO2 reaction front, altering it and imparting the low Br/Cl signature of the fluid onto those rocks. For the Br/Cl composition of the initial fluid in the equation on line 353, a sedimentary pore fluid-like composition is used because the high-Br/Cl-I/Cl end of the sample array in Figure 7 has a composition similar to sedimentary pore fluids, correct? But wouldn’t mass balance require that expulsion of a fractionated, low-Br/Cl fluid result in the partially-carbonated partially-dehydrated residue having higher Br/Cl than the initial fluid that first formed the serpentine? So, the highest Br/Cl values observed in the samples would actually be higher than the composition of the initial fluid. Of course, exactly how fractionated the Br/Cl of the residual partially carbonated samples are would depend on the extent of halogen loss, but it could be quite extensive in the most CO2-rich samples. Unless I am misunderstanding the assumption made about the intial fluid Br/Cl composition, then this pitfall needs to be addressed.\nThis is a good point. However, to perform a consistent mass balance requires that all elements of the system have been preserved and this is not necessarily the case.\nEstimating the expelled fluid composition from the changing gradient of Cl and Br contents versus CO (notwithstanding the comment below, see further answers) 2 indicates that the expelled fluid would have higher Br/Cl in the latter stages of carbonation and indeed that the final residue would have high Br/Cl. It is possible that these fluids/solids are poorly preserved and have mostly been lost from the system. We do see a handful of very high Br/Cl points both from SIMS measurements on serpentine and NI-NG-MS on bulk listvenites (Fig. 7a). These extend to Br/Cl > 1x10-2 and I/Cl > 4x10-3, consistent with a component of the mass balance which is small in volume (and consequently rarely found), but highly fractionated. A further uncertainty which prevents a full mass balance is that the reaction profile between pristine uncarbonated serpentinite and the least carbonated serpentinite is not preserved. As a result, we cannot speculate about the fractionations which may have occurred in the earliest stages of the reaction and which may have been overprinted in the rocks we see.\nPerhaps more importantly, however, the large fractionations seen within the reaction zone are only possible at low water/rock ratios (which have permitted preservation of partially carbonated serpentinites; as argued in lines 381-387). At the higher water/rock ratios which characterise the formation of fully carbonated listvenites, the carbonating fluid will have been little affected by the addition of the expelled fluid. We thus expect the bulk composition of the listvenites to be close to that of the carbonating fluid (and indeed the mean listvenite composition plots close to sedimentary pore fluid).\nLastly, the pivot point for mass balance of Br/Cl should also include the contribution of any volatiles in the partially serpentinised protolith. Since our best estimate of the composition of “normal” Oman mantle (D’Andres, 2021) has Br/Cl slightly below that of seawater or pore fluid (1.8–3.2 x 10-3, mean 2.4 ± 0.5; Fig. 7b) this will tend to shift the pivot point to slightly lower than the carbonating fluid.\nA passage has been added to the main text to summarise the arguments made above and make the important clarification that despite fractionation within the reaction zone, elsewhere the bulk composition of the listvenites should largely reflect that of the carbonating fluid.\nLines 333-339: “Mass balance would suggest the existence of a complementary high Br/Cl component. This may be represented by a handful of serpentine data and one bulk listvenite which have very high Br/Cl and/or I/Cl, suggesting such a component is volumetrically small and potentially poorly preserved. Such a component will only form at low fluid/rock ratios however (see below) and we therefore expect the average composition of the main body of listvenite to largely reflect that of the carbonating fluid (potentially with some contribution from the uncarbonated serpentinite protolith).”\nSimilarly, the calculation of (Br/Cl)out essentially determines the Br/Cl of the fluid expelled from the rock during carbonation by determining the the change in bulk rock Cl and Br contents for a given change in bulk CO2 (ΔCl/Δ CO2; ΔBr/Δ CO2) and then taking the ratio of these two ratios, correct? However, the inherent assumption made by this argument and in Figure 5 is that the ophicarbonates with the lowest CO2 and highest Cl, Br, and I contents have bulk Cl, Br, and I contents representative of the halogen contents originally in the higher CO2 ophicarbonates and listvenites prior to their more extensive carbonation and halogen loss. However, it is later argued that the ophicarbonates with the lowest CO2 contents have interacted with a fractionated fluid that has higher Cl and lower Br than the fluid that initially serpentinized rocks closer to the CO2 reaction front. In that case, the Cl contents of the samples farther from the listvenites would be higher than the initial Cl contents of the samples closer to the listvenites, whereas the opposite would be true for Br. This would result in the RCl on line 347 being overestimated and RBr being underestimated, which would make the calculated value of (Br/Cl)out lower than it should be. Again, perhaps I am misunderstanding an aspect of the assumptions made, but if I am not mistaken then the potential impact of this should be addressed.\nThis is a very salient point. The assumptions outlined above are correct. However, the implicit assumption in our modelling which wasn’t perhaps made clear enough is that we assume that a very similar profile of ophicarbonate Cl and Br existed upstream prior to further carbonation such that the listvenite front is preceded by a moving front of high Cl fluids which interact to form high Cl, low Br/Cl serpentinites. As long as such a front continues to migrate, the Br/Cl of the composition of expelled fluids will evolve as we have modelled. The implication of this is that in the earliest instance the composition of uncarbonated and unmodified serpentinites rocks and fluids likely evolved differently.\nBut since our only evidence is the remaining rocks, we can only base our modelling on what is preserved in these. We acknowledge that the conceptual model of a steady state moving front is an assumption and have made this clear in the text. If incorrect, this could result in underestimation of the Br/Cl of the expelled fluid as suggested. However, it can be reasoned that within the reaction zone a steady state is likely to have developed as our modelling assumes. Decoupling of rock and fluid Br/Cl vs CO 2 behaviour requires high fluid/rock ratios to supply excess of Cl to the least carbonated serpentinites. However, high fluid/rock ratios would also serve to dilute the low Br/Cl fluid from the rock increasing the fluid Br/Cl ratio from that seen. Conversely, decreasing water/rock ratios both increase the Br/Cl ratio of the fluid and tend towards equilibrium between fluids and rocks as there is not excess fluid to continually resupply halogens and CO . The tendency of these countervailing effects will naturally be 2 towards a steady state of the type we assume. Furthermore, the formation of low Br/Cl fluid is only possible under low fluid/rock ratios suggesting that the potential for large spikes of excess fluid-derived Cl is limited. Instead, we envisage that low Br/Cl developed incrementally as a transient front at the low fluid/rock limit of carbonation. In this scenario, the CO /Cl constraints we calculate are those necessary for maintenance 2 and continued propagation of this front.\nIt is also possible that the composition of the least carbonated serpentinites only partially reflects that of the most evolved carbonating fluid, since the mantle section was very likely partially or fully serpentinised prior to carbonation (e.g. Beinlich et al. 2020, JGR: SE). Given this, the composition of the low Br/Cl endmember represents a maximum fluid composition and might represent an intermediate composition between the serpentinite protolith and a more fractionated (lower Br/Cl) fluid. Carried through to calculating CO /Cl of the carbonating fluid the effect of this would tend to cancel out 2 any potential overestimation of Br/Cl resulting from the considerations above out suggesting the modelling represents a reasonable estimate based on the available constraints.\nThe text has been modified to clearly indicate this important assumption the potential impact of any divergence from this. Further justification of this assumption including the arguments above has been included at the start of Supplementary Text S1 preceding the derivation of modelling equations.\nLines 368-374: “Since the least carbonated serpentinites have been affected by interaction with fractionated, Cl-rich fluid, this approach assumes that the compositional gradients across the reaction zone had reached a steady state (i.e. less carbonated samples evolve to resemble more carbonated samples; see Supplementary Text S1). If this is not the case it is possible that the Br/Cl calculated is an out underestimate and the initial CO /Cl of the carbonating fluid required would be even 2 higher.”\nSupplementary text p.1: “It should be noted that the least carbonated ophicarbonates have been affected by interaction with fractionated low Br/Cl fluids. A key assumption in the following calculations is therefore that compositional gradients of halogens and CO content across the reaction zone (main text Figure 5) were in steady state, such that 2 all samples analysed evolved (or would have evolved) along the same compositional gradient. This is consistent with the view of the reaction zone as a transient compositional front and implies that it migrated with an unchanging compositional profile with respect to halogens and CO . If this is not the case, then the rate of Cl loss 2 during carbonation may be overestimated relative to other halogens, resulting in underestimation of CO /Cl through the following calculations. That said, it is reasonable 2 to think that such a steady state should have evolved since low Br/Cl fluids can only be maintained at low fluid/rock ratios, limiting their ability to supply excess Cl to uncarbonated rocks downstream. Conversely, increasing fluid/rock ratios increases the capacity of the fluids to deliver excess Cl downstream rocks but at the same time increases the Br/Cl of those fluids. These existence of low Br/Cl serpentinites affected by a fractionated fluid therefore suggests a balance between these countervailing tendencies. It is also possible that the least carbonated ophicarbonates are compositionally intermediate between their serpentinite protolith and the evolved carbonating fluid, since they were most likely serpentinised prior to carbonation1. The effect of this would tend to cancel out any potential overestimation of Br/Cl and out suggests that these assumptions and the modelling approach which follow are both reasonable given the available constraints.”\nFigure 6:\nThere are error bars with no associated data in both a) and b).\nThere are a few samples with larger than normal errors (for most data points they are smaller than the symbol). These appear asymmetric due to the logarithmic scale used.\nThe caption has been amended to make this clear and to state that the plot is log-log.\nLines 279-281: “Error bars are smaller than the symbols for most data points and, where visible, appear as asymmetric due to the logarithmic scale.”\nComments on Methods Section:\nLines 604-605:\nThere are many potential problems with measuring halogens by pyohydrolysis-IC-ICP-MS that need to be clear to the reader. As an example, if the calibration standard solutions are not appropriately matrix matched to the same NaOH concentration as the samples, there can significant matrix effects on the apparent Br content of the samples. There is a reference to Balcone-Boissard et al., (2009) for the method, but there needs to be some additional detail given here. Nop all the details of the method need to be added, but I would like to know at least some details of the ion chromatography and ICP-MS methods (e.g., what standard concentrations were used, were they matrix matched to the standards for ICP-MS).\nAnalyses were made using calibration curves across the range of halogen concentrations in sample/rock standard solutions and for ICP-MS were matrix matched to avoid potential bias. These and several other important details have been added to the description of the methods.\nLines 633-649: “Both methods were conducted with standard calibration curves bracketing the range of halogen contents in sample and rock standard solution. For IC these were prepared by dilution of commercial standard in deionised water with 2% v/v HNO3. For ICP-MS the calibration solutions were likewise prepared from commercial standards and matrix matched to the final solutions from pyrohydrolysis (5% v/v NaOH, 2% v/v HNO ) to avoid any potential matrix effects69. In both cases calibrations were 3 linear for all elements analysed. Repeat analyses of rock standards (BHVO-2, GS-N, JG-1, JR-1, JB-1b, JB-2) indicate mean relative errors of ~10% for F and Cl, ~30% for Br and ~60% for I, and yields of >95% for all halogens69. Detection limits in solution were well below sample concentrations for F and Cl (<100 µg/L) and were approximately 10ppb and 1ppb for Br and I, respectively. For extraction of 500 mg of sample, producing 100 ml of pyrohydrolysis solution, detection limits in rock samples are estimated to be 20 mg kg−1 for F and Cl and 2 μg kg−1 for Br and I. The mean analytical errors on the studied composition ranges were estimated at 10 mg kg−1 for F and Cl, 100 μg kg−1 for Br and 25 μg kg−1 for I. No blank correction was necessary as procedural blanks (both tube and tube + platinum crucible) were always below detection limits. The batch of V O used 2 5 was tested prior to analysis to ensure it did not contain detectible halogens.”\nLine 606:\nWas the V2O5 heat treated prior to fusion? I know that Balcone-Boissard et al. (2009) indicated that the V2O5 did not contain significant quantities of halogens, but Segee-Wright et al. (2023) indicated that different V2O5 batches (even with the same product number and from the same manufacturer) can be have starkly different halogen contents, particularly for Br and I. Unless this study used the exact same batch of V2O5 used by Balcone-Boissard et al. (2009), then there needs to be some concern about contamination from the V2O5. Line 617 mentions that procedural blanks were run. Did those procedural blanks include V2O5? This is something that can be quickly addressed.\nEach batch of V O accepted into the lab, including that used for these analyses, was 2 5 checked for purity in the same manner as described by Balcone-Boissard et al. (2009) and no detectable halogens were found. Procedural blanks included both tube and tube + Pt crucible. The method has been amended to include these details.\nSee lines 633-649 above.\nLines 613-614:\nHere, the names of the rock standards measured need to be reported, as well as the calculated yields are for each halogen. Without this information, it is challenging to assess the quality of this data.\nBHVO-2, GS-N, JG-1, JR-1, JB-1b, JB-2 were run as standards and indicated yields of >95% for all halogens. This information has been added to the method.\nSee lines 633-649 above.\nLine 615:\nIs this supposed to say <100 g/L? Because the is equivalent to 100,000 μg/mL or 10 wt% F or Cl, which is impossibly high for 0.5 grams of fluxed rock at ~80 mL of solution extracted. I assume this is a typo of some sort. What was this supposed to say?\nYes indeed this was a typo and should have read <100 µg/L. The text has been amended.\nSee lines 633-649 above.\nLine 616:\nIs this in the solution or propagated to the rock? Either way, both should be reported for every halogen.\nThese were for solution analysis. This has been clarified and the corresponding detection limits and analytical errors in rock are now included in the methods.\nSee lines 633-649 above.\nLine 684:\n18O and 30Si were measured on an electron multiplier in serpentines that have high O and Si contents? Wouldn’t the counts be extremely high and damage the EM over time? Faraday cups weren’t used for measuring those elements? I’m not saying this is wrong, I’m just checking to make sure this is correct because it is different from the volatile measurement procedures I’ve used at other SIMS facilities.\nHaving checked the original condition files, both 18O and 30Si were indeed measured on the electron multiplier.\nLines 689-691:\nShow the good correlation between EPMA and SIMS Cl measurements in a supplementary figure.\nThis is now included in the supplementary text along with figures demonstrating the linearity of SIMS signals for Cl and Br over a range of standard matrices and halogen concentrations.\nSupplementary text p.10\nLines 690-692:\nWas only one standard used to make F and Cl calibration curves? I don’t like the idea of using only one calibration standard for Cl and F, particularly when the standard is not matrix matched. This is a problem for both F and Cl, but at least the F contents of most samples are lower than that of the standard, so their values can be interpolated between the standard and the origin. For Cl, the standard has 113 ppm Cl, but the samples extend to 900-1100 ppm Cl. The error associated with extrapolating a calibration curve to 9 times higher than the one standard will result in enormous errors. Do the errors reported for the SIMS data take this extrapolation into account? If they don’t, they need to. Why wasn’t a higher Cl standard chosen?\nA single primary standard was indeed used for both F and Cl. In choosing this we were led by the judgement of the staff at the Edinburgh Ion Microprobe Facility where the analyses were conducted. Uncertainty in the relative ion yield determined from standards will scale linearly with increasing sample Cl/O and such uncertainties are already propagated into the errors quoted for Cl etc. If the sensitivity of the SIMS for Cl deviated from linear this could indeed impart large systematic errors in the determination of Cl extrapolated beyond the standard values. However, two lines of evidence suggest this is not the case.\nFirstly, the rhyolitic glass ATHO was also run during method set up. Processing T1G data using ATHO as the primary standard and vice versa for ATHO gives F and Cl abundances that are within 10% of the accepted value for each standard and thus within typical propagated errors, despite ATHO having 3 and 4 times higher F and Cl, respectively, than T1G. These data are now provided as Supplementary Table S9 and discussed in the methods.\nFurther demonstration of the validity of the data for Cl is provided by the strong linear correlation between Cl abundance measured by SIMS and EPMA (see above comments) with a slope of 1.04 and R2 of 0.81. The data show no indication of systematic deviation from this trend to suggest any non-linearity in SIMS signal at high Cl contents. We also note that the correlation between our Cl datasets appears highly comparable or indeed somewhat better than similar data available in the literature (e.g. Cassidy et al. 2022, Am. Mineral. 107, supp. material). Nonetheless we recognise that there may be some unaccounted-for uncertainty in the highest Cl values and have added discussion of the above evidence for linearity and a cautionary note to the methods section in this regard.\nLines 727-740: “It should be noted that some serpentine analyses had Cl significantly above that of the primary standard. If signal response was not linear with increasing Cl this could result in large systematic errors. However, linearity of the signal was assessed by analysis of two natural glasses with different SiO contents and Cl contents 2 (Supplementary Figs. S1-2; T1G, 58.6 wt% SiO , ATHO, 75.6 wt% SiO ). Processing data 2 2 from each standard using the other as the primary standard gives F and Cl abundances that are within 10% of the accepted value for each. The standard deviation on the calculated relative ion yields was propagated together with the analytical error on each point and includes the increase in uncertainty involved with extrapolating beyond the calibration range. Moreover, the SIMS Cl data can be checked against EPMA data from the same points and show a strong linear corelation with a slope of 1.04 (Supplementary Fig. S3). The combined evidence therefore points to the reliability of these data. Nonetheless, the potential for unaccounted-for systematic error in the highest Cl analyses should be noted.”\nLines 704-707:\nInstead of telling the reader that there is a linear relationship, show this in a supplementary figure. This is especially important since these measurements are not routine. If scapolites and glasses of different compositions also fall along a single calibration line, that could be used as an argument against significant matrix effects for these measurements.\nThese data indeed show a single calibration line for scapolites and synthetic glasses over a wide range of Br contents. A plot of these data is now included in the Supplementary Text as Fig. S4. We also note that a very similar and linear Br SIMS response was reported by Cassidy et al. (2022, Am. Mineralogist) and they similarly argued that matrix effects were likely to be minimal. The linearity of signals in this and the study of Cassidy et al. are now referenced as evidence against significant matrix effects as suggested.\nLines 775-779: “Nonetheless, the good agreement of SIMS, EPMA and NI-NG-MS data (see below, Supplementary Fig. S3) lends confidence that any matrix effects are relatively minor as do the linear responses of F, Cl and Br across a range of standard matrices in this study (Supplementary Figs. S1-2, S4) and others in the literature76”\nSupplementary Text p8-11:\nLines 729-737:\nAlthough I do not like that there are no matrix matched standards or that only one calibration standard was run, the reasoning in the section is decent. I don’t think the problems I have raised regarding the methods will have a large impact on the main conclusions of this paper.\nHowever, the claim that matrix effects are minimal when comparing two different elements within the same mineral needs to be justified, either through a literature reference or by making the argument in this section. This could be demonstrated by comparing SIMS calibrations of two different elements (F and Cl for example, but it could really be any two elements) in two sets of standard materials with very different matrices that have been previously published. If the ratio of the slope of the Cl calibration line to the slope of the F calibration line is similar for both standard sets, then one could argue that the matrix will have no significant impact on the measured F/Cl ratio.\nThe reliability of the F/Cl ratio is demonstrated by the strong linearity of F and Cl signals across a wide range of glass SiO contents. The ratios of the slope of the calibration 2 lines for F and Cl (i.e. RIY /RIY ) for both standards are within 20% of each other. This F Cl strongly suggest that the large variation seen within minerals species (carbonate F/Cl = 0.02 to 398, serpentine F/Cl = 0.02 to 1.48) cannot be explained by matrix effects.\nFurther argument against an important role for matrix effects is the occurrence of vein margin magnesite with high F/Cl (<108) and a few grains of dolomite with low F/Cl ratios (<1; see Fig. 6). The key distinction controlling the F/Cl ratio measured appears to be textural rather than mineralogical suggesting it is indeed geological factors not matrix effect that gives rise to these distinctions. A passage has been added to the method section to make this line of argument explicit.\nLines 782-790: “The ratio of F and Cl relative ion yields for two natural glasses with a wide range of SiO contents are within 20% of one another suggesting matrix effects 2 cannot explain the large variations seen in data (carbonate F/Cl = 0.02 to 398. serpentine F/Cl = 0.02 to 1.48). Furthermore, within carbonates, the principal control on F/Cl appears to be textural rather than mineralogical with interstitial dolomite and magnesite both showing high F/Cl (Fig. 6b). Nonetheless, it is impossible to discount that some matrix effects may affect the data and particularly absolute concentrations. As a result, care should be taken in making direct comparison of concentrations between different minerals.”\nSee also Supplementary plots above\nEven if matrix effects don’t change F/Cl, Br/Cl, or I/Cl values by orders of magnitude, matrix effects will almost certainly have some systematic effect on the absolute concentrations of all halogens that will differ between different phases. Although the halogen ratios are unlikely to change enough to alter the interpretations of the manuscript, the concentrations could be systematically offset by >50-100%. There needs to be mention of the deficiencies of the method and the resulting deficiencies of the data in respect to absolute halogen concentrations.\nWe acknowledge this is a fair point and have added a note to the methods cautioning against direct comparisons of concentrations between samples.\nSee Lines 782-790 above"}]

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